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4.7: Internal Structures of Eukaryotic Cells - Biology

4.7: Internal Structures of Eukaryotic Cells - Biology


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4.7: Internal Structures of Eukaryotic Cells

Eukaryotic cell: Structure and organelles

The cell is the smallest functional unit within a living organism, which can function independently. It is made up of several types of organelles that allow the cell to function and reproduce. There are two general classes of cells that exist: the self-sustaining simple cells known as prokaryotic (bacteria and archaea) and the more complex dependent cells known as eukaryotic. The eukaryotic cells types are generally found in animals, plants, algae, and fungi. For the purpose of this article, the primary focus will be the structure and histology of the animal cell. The major differences between animal and plant cells will be explored as well.

As previously stated, the fundamental components of a cell are its organelles. These organelles are made up of varying combinations of atoms and molecules. The organelles drive different functions of the cell from metabolism, to energy production and subsequently to replication. Cells with particular functions come together to form organs (i.e. lung parenchyma). Organs with interrelated functions work together within a system (i.e. respiratory system). These systems, although of different functions, work in synergy to allow the organism (i.e. human) to survive. Every aspect of a cell is important for it to survive.

Rough - has ribosomes bound to its surface, stores proteins, and is the extension of the nuclear membrane

Smooth - lacks ribosomes, is a collection of independent sacs or a continuation of the rough ER, and synthesizes lipids, steroids, and phospholipids

Shape - animal cells are irregular, plant cells are rectangular

Cellulose - absent in animal cells, surrounds the plasma membrane in plant cells

ATP production - mitochondria in animal cells, chloroplasts in plant cells

Cillia - present in animal cells, absent in plant cells


Eukaryotic Cells: Types and Structure (With Diagram)

The eukaryotic cells occur in all eukaryotes votes like protists, plants, fungi and animals.

The eukaryotic cells are too complex than prokaryotic cells and evolved from them about 1.5 billion years ago (BYA).

Eukaryotic cell size varies greatly from 10 mm to 500 mm. Ostrich egg is the largest eukaryotic cell known measuring 170 mm X150 mm. In plants, the longest cells are the sclerenchyma fibers of Ramie (Boehmeria nivea) of Utricaceae. But in animals, neurons are the longest cells reaching up to 3 mt. in elephants and whales.

The size of a cell depends upon:

i. Surface area to volume ratio

ii. Nucleo-cytoplasmic or kern-plasmic ratio

iii. Size and number of chromosomes.

The shape of eukaryotic cell may be variable or fixed. Variable shape occurs in Amoeba and white blood cells. Fixed shape of cell occurs in most plants and animals. In unicellular organisms, the cell shape is maintained by plasma membrane and exoskeleton (e.g. Polystomella or Elphidium).

In multicellular organisms, the shape of a cell depends mainly on its functional adaptations and partly by external pressure, surface tension, internal stress, viscosity of protoplasm and cytoskeleton. Thus, the eukaryotic cells have diverse shapes such as spherical, elongated, spindle-shaped, discoidal, polyhederal, branched, oval and so on.

Types of Eukaryotic Cells:

In multicellular organisms cells can be classified into three main types — Undifferentiated, differentiated and. dedifferentiated cells.

1. Undifferentiated Cells:

These unspecialized cells are capable of undergoing division and development. For example, Zygote, stem cells (in animals) and meristematic cells (in plants)

2. Differentiated cells (- Post-mitotic cells):

These are specialized cells which perform a specific function and exhibit division of labour. For example, mesophyll cells carry out photosynthesis, RBCs transport O2 and CO2 etc. However, in animals the cellular differentiation is an irreversible phenomenon.

3. Dedifferentiated cells:

Sometimes, the differentiated cells revert back to undifferentiated cells and carry out cell division. The process by which differentiated cells lose their specialization is called dedifferentiation. For example, parenchyma dedifferentiated into meristematic cells for wound healing, regeneration and secondary growth. Tissue culture and colonial propagation depends on dedifferentiation of cells. If any dedifferentiated cells again become specialize then process is called redifferentiation.

Ultrastructure of Eukaryotic cell:

Though eukaryotic cells vary in shape, size and functions, all show some basic structural plan. A generalized/typical eukaryotic cell as seen under Electron Microscope (EM) consists of cell wall (absent in animal cells and some protists), plasma membrane. Cytoplasm, nucleus, other organells and inclusions. Except nucleus and plastids all other cytoplasmic structures can be seen under EM. Eukaryotic cell is, by definition, possess a nucleus containing nuclear material enclosed by a double layered nuclear envelope.

EM has revealed that the cytoplasm has extensive compartmentalization due to presence of membrane bound organelles. In addition, eukaryotic cells are supported by cytoskeleton and have locomotory appendages (cilia or flagella) in motile cells. Since the functions of ER, golgi complex, lysosomes and vacuoles are coordinated, these are considered together as an endomembrane system.


Biology 171

By the end of this section, you will be able to do the following:

  • Describe the structure of eukaryotic cells
  • Compare animal cells with plant cells
  • State the role of the plasma membrane
  • Summarize the functions of the major cell organelles

Have you ever heard the phrase “form follows function?” It’s a philosophy that many industries follow. In architecture, this means that buildings should be constructed to support the activities that will be carried out inside them. For example, a skyscraper should include several elevator banks. A hospital should have its emergency room easily accessible.

Our natural world also utilizes the principle of form following function, especially in cell biology, and this will become clear as we explore eukaryotic cells ( (Figure)). Unlike prokaryotic cells, eukaryotic cells have: 1) a membrane-bound nucleus 2) numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others and 3) several, rod-shaped chromosomes. Because a membrane surrounds eukaryotic cell’s nucleus, it has a “true nucleus.” The word “organelle” means “little organ,” and, as we already mentioned, organelles have specialized cellular functions, just as your body’s organs have specialized functions.

At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. Before turning to organelles, let’s first examine two important components of the cell: the plasma membrane and the cytoplasm.



If the nucleolus were not able to carry out its function, what other cellular organelles would be affected?

The Plasma Membrane

Like prokaryotes, eukaryotic cells have a plasma membrane ((Figure)), a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane.


The plasma membranes of cells that specialize in absorption fold into fingerlike projections that we call microvilli (singular = microvillus) ((Figure)). Such cells typically line the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function.
People with celiac disease have an immune response to gluten, which is a protein in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.


The Cytoplasm

The cytoplasm is the cell’s entire region between the plasma membrane and the nuclear envelope (a structure we will discuss shortly). It is comprised of organelles suspended in the gel-like cytosol , the cytoskeleton, and various chemicals ((Figure)). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are also there. Ions of sodium, potassium, calcium, and many other elements also dissolve in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.

The Nucleus

Typically, the nucleus is the most prominent organelle in a cell ((Figure)). The nucleus (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins. Let’s look at it in more detail ((Figure)).


The Nuclear Envelope

The nuclear envelope is a double-membrane structure that constitutes the nucleus’ outermost portion ((Figure)). Both the nuclear envelope’s inner and outer membranes are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus.

Chromatin and Chromosomes

To understand chromatin, it is helpful to first explore chromosomes , structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight.
Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins attach to chromosomes, and they resemble an unwound, jumbled bunch of threads. We call these unwound protein-chromosome complexes chromatin ((Figure)). Chromatin describes the material that makes up the chromosomes both when condensed and decondensed.



The Nucleolus

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.

Ribosomes

Ribosomes are the cellular structures responsible for protein synthesis. When we view them through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may be attached to the plasma membrane’s cytoplasmic side or the endoplasmic reticulum’s cytoplasmic side and the nuclear envelope’s outer membrane ((Figure)). Electron microscopy shows us that ribosomes, which are large protein and RNA complexes, consist of two subunits, large and small ((Figure)). Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA transcribes into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.


Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), there are ribosomes in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.

Mitochondria

Scientists often call mitochondria (singular = mitochondrion) the cell’s “powerhouses” or “energy factories” because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the cell’s short-term stored energy. Cellular respiration is the process of making ATP using the chemical energy in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need considerable energy to keep your body moving. When your cells don’t get enough oxygen, they do not make much ATP. Instead, producing lactic acid accompanies the small amount of ATP they make in the absence of oxygen.

Mitochondria are oval-shaped, double membrane organelles ((Figure)) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. We call the area surrounded by the folds the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.


Peroxisomes

Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide, H2O2, which would be damaging to cells however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into oxygen and water.) For example, peroxisomes in liver cells detoxify alcohol. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars. Plant cells contain many different types of peroxisomes that play a role in metabolism, pathogene defense, and stress response, to mention a few.

Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them. Vesicle membranes can fuse with either the plasma membrane or other membrane systems within the cell. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. The vacuole’s membrane does not fuse with the membranes of other cellular components.

Animal Cells versus Plant Cells

At this point, you know that each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some, vacuoles, but there are some striking differences between animal and plant cells. While both animal and plant cells have microtubule organizing centers (MTOCs), animal cells also have centrioles associated with the MTOC: a complex we call the centrosome. Animal cells each have a centrosome and lysosomes whereas, most plant cells do not. Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole whereas, animal cells do not.

The Centrosome

The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to each other ((Figure)). Each centriole is a cylinder of nine triplets of microtubules.


The centrosome (the organelle where all microtubules originate) replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the centriole’s exact function in cell division isn’t clear, because cells that have had the centrosome removed can still divide, and plant cells, which lack centrosomes, are capable of cell division.

Lysosomes

Animal cells have another set of organelles that most plant cells do not: lysosomes. The lysosomes are the cell’s “garbage disposal.” In plant cells, the digestive processes take place in vacuoles. Enzymes within the lysosomes aid in breaking down proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are active at a much lower pH than the cytoplasm’s. Therefore, the pH within lysosomes is more acidic than the cytoplasm’s pH. Many reactions that take place in the cytoplasm could not occur at a low pH, so again, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.

The Cell Wall

If you examine (Figure), the plant cell diagram, you will see a structure external to the plasma membrane. This is the cell wall , a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and some protistan cells also have cell walls. While the prokaryotic cell walls’ chief component is peptidoglycan, the major organic molecule in the plant (and some protists’) cell wall is cellulose ((Figure)), a polysaccharide comprised of glucose units. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That’s because you are tearing the celery cells’ rigid cell walls with your teeth.


Chloroplasts

Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals. Plants (autotrophs) are able to make their own food, like sugars, while animals (heterotrophs) must ingest their food.

Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs we call thylakoids ((Figure)). Each thylakoid stack is a granum (plural = grana). We call the fluid enclosed by the inner membrane that surrounds the grana the stroma.


The chloroplasts contain a green pigment, chlorophyll , which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle.

Endosymbiosis We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation.

Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = “within”) is a mutually beneficial relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. We have already mentioned that microbes that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and from drying out, and they receive abundant food from the environment of the large intestine.

Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just like mitochondria and chloroplasts. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts.

The Central Vacuole

Previously, we mentioned vacuoles as essential components of plant cells. If you look at (Figure)b, you will see that plant cells each have a large central vacuole that occupies most of the cell’s area. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the plant’s cell walls results in the wilted appearance.

The central vacuole also supports the cell’s expansion. When the central vacuole holds more water, the cell becomes larger without having to invest considerable energy in synthesizing new cytoplasm.

Section Summary

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning a membrane surrounds its DNA), and has other membrane-bound organelles that allow for compartmentalizing functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleus’s nucleolus is the site of ribosome assembly. We find ribosomes either in the cytoplasm or attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria participate in cellular respiration. They are responsible for the majority of ATP produced in the cell. Peroxisomes hydrolyze fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.

Animal cells also have a centrosome and lysosomes. The centrosome has two bodies perpendicular to each other, the centrioles, and has an unknown purpose in cell division. Lysosomes are the digestive organelles of animal cells.

Plant cells and plant-like cells each have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives the cell shape. Photosynthesis takes place in chloroplasts. The central vacuole can expand without having to produce more cytoplasm.

Art Connections

(Figure) If the nucleolus were not able to carry out its function, what other cellular organelles would be affected?

(Figure) Free ribosomes and rough endoplasmic reticulum (which contains ribosomes) would not be able to form.

Free Response

You already know that ribosomes are abundant in red blood cells. In what other cells of the body would you find them in great abundance? Why?

Ribosomes are abundant in muscle cells as well because muscle cells are constructed of the proteins made by the ribosomes.

What are the structural and functional similarities and differences between mitochondria and chloroplasts?

Both are similar in that they are enveloped in a double membrane, both have an intermembrane space, and both make ATP. Both mitochondria and chloroplasts have DNA, and mitochondria have inner folds called cristae and a matrix, while chloroplasts have chlorophyll and accessory pigments in the thylakoids that form stacks (grana) and a stroma.

Why are plasma membranes arranged as a bilayer rather than a monolayer?

The plasma membrane is a bilayer because the phospholipids that create it are amphiphilic (hydrophilic head, hydrophobic tail). If the plasma membrane was a monolayer, the hydrophobic tails of the phospholipids would be in direct contact with the inside of the cell. Since the cytoplasm is largely made of water, this interaction would not be stable, and would disrupt the plasma membrane of the cell as the tails were repulsed by the cytoplasm (in water, phospholipids spontaneously form spherical droplets with the hydrophilic heads facing outward to isolate the hydrophobic tails from the water). By having a bilayer, the hydrophilic heads are exposed to the aqueous cytoplasm and extracellular space, while the hydrophobic tails interact with each other in the middle of the membrane.

Glossary


Cells

The first cells that arose about 3.5 billion years ago most likely resembled Bacteria or Archaea they had relatively simple structures and lacked nuclei or internal organelles. Most phylogenetic trees of life show Archaea and Bacteria diverging first from the Last Universal Common Ancestor (LUCA). We infer therefore, that the LUCA had a simple cell structure, with cytoplasm bounded by some type of phospholipid bilayer membrane, and no nuclei or internal membrane compartments or organelles.

Phylogenetic Tree of Life with 3 Domains, based on 16S rRNA sequences, from Wikimedia Commons

Bacteria and Archaea are classified as prokaryotes, meaning cells without nuclei, although some modern biologists dislike the term because prokaryotes appear not to form a monophyletic group.

Methanococcus janaschii, with many flagella, image courtesy of UC Museum of Paleontology, www.ucmp.berkeley.edu.

Bacteria and Archaea have diverse cell morphologies, but they all have some common structural features.

Prokaryotic cell structure, from Wikipedia

  • a single circular chromosome (a few species have two circular chromosomes)
  • a nucleoid region that contains the chromosomal DNA, with no surrounding membrane to separate it from the cytoplasm
  • small circular DNA molecules called plasmids dispersed in the cytoplasm.

In addition to their phospholipid bilayer cell membrane, they have cell walls that differ in composition between Bacteria and Archaea. Prokaryotic cells are generally smaller than eukaryotic cells. They have a rudimentary cytoskeleton and can have flagella for motility.

Relative scale of cell sizes, from Wikipedia

Evolution of eukaryotes

About 2.1-2.4 billion years ago, the first eukaryotic cells appear in the fossil record. This coincides with, or occurs soon after, the Great Oxygenation Event. Eukaryotic cell membranes have sterols, whose synthesis requires molecular oxygen. How did eukaryotes arise? One clue is that eukaryotic genes for proteins that replicate DNA and synthesize RNA in the nucleus are similar to Archaeal genes, whereas eukaryotic genes for energy metabolism and lipid biosynthesis in the cytoplasm resemble Bacterial genes. This observation led to the current hypothesis that eukaryotes evolved from an ancient endosymbiosis or cell fusion event between an Archaeon and a Bacterium.

Eukaryotic evolution required many innovations. One is endocytosis (taking in molecules bound to the plasma membrane by forming a small vesicle, a bubble-like structure made by a lipid bilayer sac enclosing internal fluid). Modern prokaryotes lack endocytosis or phagocytosis (taking particles into the cell by forming a large vesicle). But endocytosis or phagocytosis is essential for taking in and harboring endosymbionts within a membrane enclosure, and leads to formation of vesicles inside the cell. Invagination of the plasma membrane deep into the cytoplasm to surround the cell’s chromosomes can lead to the formation of a membrane envelope that separates the nuclear compartment from the rest of the cell, and simultaneous development of an endomembrane system.

Proteins required for endocytosis share structural similarities with nuclear pore proteins, suggesting a common evolutionary origin for the endomembrane system and the nucleus. Fig. 5 from Devos et al. 2004, PLoS Biology doi:10.1371/journal.pbio.0020380

Therefore, phagocytosis/endocytosis can account for the formation of the nucleus enclosed by a nuclear envelope, the endomembrane system, and the evolution of mitochondria and chloroplasts from endosymbiosis of aerobic bacteria and cyanobacteria, respectively.

Eukaryotic cell structure

Eukaryotic cell from Wikipedia

What should students in freshman biology know about the structure of a eukaryotic cell? Rather than trying to memorize details about the various organelles and cell structures, students should think about major cell systems.

Cytoplasm

The cytoplasm is the internal region of the cell bounded by the plasma membrane, excluding the interior of the nucleus and the interior regions of organelles and the endomembrane system. The cytoplasm contains ribosomes, tRNAs and mRNAs for protein synthesis, the cytoskeleton, many metabolic enzymes, and proteins that function in cell signaling. The cytoplasm is so crowded with macromolecules that it has the consistency of a hydrated gel much of the water molecules are associated with other molecules.

Endomembrane system

The endomembrane system includes the nuclear envelope, the endoplasmic reticulum (ER), the Golgi complex, lysosomes, transport vesicles, secretory vesicles, endosomes, and the plasma membrane. The double membrane of the nuclear envelope is contiguous with the ER.

Endomembrane system from Wikipedia. The rough ER has ribosomes bound to the ER membrane. ER-bound ribosomes synthesize proteins into the ER membrane or lumen (internal space). Other ribosomes remain in the cytoplasm and synthesize proteins that remain in the cytoplasm or go to the nucleus or the mitochondria or chloroplasts.

That all these membranes comprise a single system becomes clear when we think about membrane biogenesis. For cells to grow, they have to make more membrane lipids and membrane proteins.

Membrane proteins for the endomembrane system and proteins for secretion are made in the rough ER (rER) by ribosomes docked to protein channels in the ER membrane. The polypeptide chain emerging from the ribosome passes through the channel into the ER lumen (the interior space of the ER) and begins to fold. Any parts of the chain that form hydrophobic alpha-helices remain embedded in the ER membrane, as transmembrane domains. The newly synthesized proteins in the rER membrane or lumen move to the smooth ER, where they are partially glycosylated (oligosaccharide groups are covalently bonded to particular amino acids). Membrane lipids (phospholipids, sterols) are also made in and added to the smooth ER. Transport vesicles containing membrane proteins and secreted proteins bud from the smooth ER and travel to the Golgi. These vesicles fuse with the Golgi, adding their membrane lipids and membrane proteins, as well as their internal contents, to the Golgi vesicles. In the Golgi, the membrane proteins and secreted proteins are sorted and processed via additional glycosylation. Lysosomal proteins are segregated to vesicles that pinch off and become lysosomes. Secreted proteins are packaged into secretory vesicles that pinch off and are transported to the cell periphery, where the secretory vesicles fuse with the plasma membrane, adding their lipid and protein to the plasma membrane and dumping their internal contents to the outside of the cell.
rough ER –> smooth ER –> transport vesicles –> Golgi –> secretory vesicles –> PM
Note that the endomembrane system does not include mitochondria nor chloroplasts, which are independent organelles and will be discussed later in the context of energy metabolism. Proteins destined for mitochondria or chloroplasts, as well as proteins destined for the interior of the nucleus, are made by free cytoplasmic ribosomes (undocked to any membrane). These proteins are then imported into the respective organelles via specialized protein import systems (mitochondria and chloroplasts) or via the nuclear pore complexes (nuclei). Of course, proteins that function in the cytoplasm are also made by free cytoplasmic ribosomes.

Cytoskeleton

The cytoskeleton is another cellular system. It consists of actin microfilaments, several types of intermediate filaments, and microtubules. These are dynamic structures required for cell shape, cell mobility, and organization and movement of materials inside the cell. Microfilaments are thinner, and form networks near the plasma membrane to either stabilize or change the shape of the cell, especially when parts of the membrane are extended outward. Microtubules (polymerized from dimers of alpha- and beta-tubulin) serve as tracks for movement of transport vesicles and secretory vesicles by motor proteins, and also for movement of chromosomes during cell division. In brief, microfilaments are for cell shape, microtubules are for moving stuff around inside the cell.

Extracellular matrix

Outside the cell, overlying the plasma membrane, is the extracellular matrix. In plants and yeast, this is the cell wall. In animal cells, this consists of collagen and other polymers of protein and polysaccharides.

Nucleus

The nucleus contains the cell’s chromosomes. All chromosomal DNA replication and transcription to make RNA occurs in the nucleus, as well as RNA processing. The enzymes that perform these tasks, the proteins that bind to DNA to form chromatin, indeed all proteins in the nucleus, are made by ribosomes in the cytoplasm, and then imported into the nucleus through the nuclear envelope pore complexes. Conversely, ribosomal and messenger RNAs are made in the nucleus and exit the nucleus via the same pore complexes, so they can function in cytoplasmic protein synthesis.

Cellular dynamics: Inner Life of the Cell molecular animation

Watch the Inner Life of the Cell video below, and see if you can identify the various components of the endomembrane system and narrate what is going on. This video is for more advanced students, but the middle of the video, starting with the plasma membrane, beautifully illustrates the dynamic interconnections between the cell structures.

The video begins with leukocytes (white blood cells) rolling along a blood vessel. Endothelial cells are the cells that form the inner lining of the blood vessel. Cell surface proteins on the white blood cell interact and bind to the cell surface proteins on the lining of the blood vessel to slow down and stop the white blood cell. From here the video dives into the cell.

The key parts to watch for:

  • The plasma membrane is a fluid mosaic of phospholipids and proteins.
  • Sphingolipids and cholesterol make parts of the plasma membrane rigid – these rigid parts are called lipid rafts, that are important for cell signaling.
  • The cell contains different types of cytoskeletal elements – the video shows spectrin, an intermediate filament actin microfilaments and microtubules. Let’s not worry about additional details mentioned in the video.
  • Motor proteins “walk” along the microtubules, transporting vesicles back and forth. The “walking” of these motor proteins is powered by ATP hydrolysis.
  • The nuclear envelope contains pores, and mRNA molecules exit the nucleus into the cytoplasm through the nuclear pores.
  • Free ribosomes in the cytoplasm translate and make proteins that stay in the cytoplasm, or partner with special proteins that deliver them to mitochondria and other organelles that are independent of the endomembrane system.
  • Free ribosomes also initiate translation of endomembrane system proteins and secreted proteins, but they stall until they are docked to a protein complex in the rER. The rER is “rough” because all the ribosomes located there gives this portion of the ER a rough appearance in electron micrographs. Membrane proteins are embedded in the ER membrane, whereas secreted proteins end up in the lumen.
  • The membrane and secreted proteins are transported in vesicles to the Golgi.
  • The Golgi completes the glycosylation of these proteins.
  • Secretory vesicles are transported from the Golgi to the plasma membrane, where they fuse.
  • You can ignore the rest of the video, although it’s really cool. It shows how white blood cells squeeze between the cells that line the blood vessel to get into the tissues at a site of infection and inflammation.


The hypothesis of symbiosis is widely accepted.

Because mitochondria and chloroplasts are semi-autonomous structures with their own gene pool, the US microbiologist Lynn Margulis postulated in 1970 that they had appeared during evolution, following the ingestion—also referred to as endocytosis—of certain free-living bacteria by other, single-cell prokaryotic organisms. The free bacteria initially had respiratory (mitochondria) or photosynthesis (chloroplasts) capacity. This chloroplast endocytosis may have occurred some 1.5 billion years ago, possibly after that of mitochondria. The "ingested" bacteria were then maintained in their hosts thanks to a mutual cooperation that was beneficial to both parties. This is known as a symbiotic relationship: the bacterium supplied the host cell with its energy generation process, and the cell reciprocated by protecting the bacterium and feeding it the nutrients necessary for its functioning. It is this theory of endosymbiosis that is accepted today by the majority of biologists.


New theory suggests alternate path led to rise of the eukaryotic cell

As a fundamental unit of life, the cell is central to all of biology. Better understanding how complex cells evolved and work promises new revelations in areas as diverse as cancer research and developing new crop plants.

But deep thinking on how the eukaryotic cell came to be is astonishingly scant. Now, however, a bold new idea of how the eukaryotic cell and, by extension, all complex life came to be is giving scientists an opportunity to re-examine some of biology’s key dogma.

All complex life — including plants, animals and fungi — is made up of eukaryotic cells, cells with a nucleus and other complex internal machinery used to perform the functions an organism needs to stay alive and healthy. Humans, for example, are composed of 220 different kinds of eukaryotic cells — which, working in groups, control everything from thinking and locomotion to reproduction and immune defense.

Thus, the origin of the eukaryotic cell is considered one of the most critical evolutionary events in the history of life on Earth. Had it not occurred sometime between 1.6 and 2 billion years ago, our planet would be a far different place, populated entirely by prokaryotes, single-celled organisms such as bacteria and archaea.

For the most part, scientists agree that eukaryotic cells arose from a symbiotic relationship between bacteria and archaea. Archaea — which are similar to bacteria but have many molecular differences — and bacteria represent two of life’s three great domains. The third is represented by eukaryotes, organisms composed of the more complex eukaryotic cells.

Eukaryotic cells are characterized by an elaborate inner architecture. This includes, among other things, the cell nucleus, where genetic information in the form of DNA is housed within a double membrane mitochondria, membrane-bound organelles, which provide the chemical energy a cell needs to function and the endomembrane system, which is responsible for ferrying proteins and lipids about the cell.

Prevailing theory holds that eukaryotes came to be when a bacterium was swallowed by an archaeon. The engulfed bacterium, the theory holds, gave rise to mitochondria, whereas internalized pieces of the outer cell membrane of the archaeon formed the cell’s other internal compartments, including the nucleus and endomembrane system.

“The current theory is widely accepted, but I would not say it is ‘established’ since nobody seems to have seriously considered alternative explanations,” explains David Baum, a University of Wisconsin–Madison professor of botany and evolutionary biologist who, with his cousin, University College London cell biologist Buzz Baum, has formulated a new theory for how eukaryotic cells evolved. Known as the “inside-out” theory of eukaryotic cell evolution, the alternative view of how complex life came to be was published recently (Oct. 28, 2014) in the open access journal BMC Biology.

The inside-out theory proposed by the Baums suggests that eukaryotes evolved gradually as cell protrusions, called blebs, reached out to trap free-living mitochondria-like bacteria. Drawing energy from the trapped bacteria and using bacterial lipids — insoluble organic fatty acids — as building material, the blebs grew larger, eventually engulfing the bacteria and creating the membrane structures that form the cell’s internal compartment boundaries.

“The idea is tremendously simple,” says David Baum, who first began thinking about an alternate theory to explain the rise of the eukaryotic cell as an Oxford University undergraduate 30 years ago. “It is a radical rethinking, taking what we thought we knew (about the cell) and turning it inside-out.”

From time to time, David Baum dusted off his rudimentary idea and shared it with others, including the late Lynn Margulis, the American scientist who developed the theory of the origin of eukaryotic organelles. Over the past year, Buzz and David Baum refined and detailed their idea, which, like any good theory, makes predictions that are testable.

“First, the inside-out idea immediately suggested a steady stepwise path of evolution that required few cellular or molecular innovations. This is just what is required of an evolutionary model,” argues Buzz Baum, an expert on cell shape and structure. “Second, the model suggested a new way of looking at modern cells.”

“The current theory is widely accepted, but I would not say it is ‘established’ since nobody seems to have seriously considered alternative explanations.”

Modern eukaryotic cells, says Buzz Baum, can be interrogated in the context of the new theory to answer many of their unexplained features, including why nuclear events appear to be inherited from archaea while other features seem to be derived from the bacteria.

“It is refreshing to see people thinking about the cell holistically and based on how cells and organisms evolved,” says Ahna Skop, a UW–Madison professor of genetics and an expert on cell division. The idea is “logical and well thought out. I’ve already sent the paper to every cell biologist I know. It simply makes sense to be thinking about the cell and its contents in the context of where they may have come from.”

The way cells work when they divide, she notes, requires the interplay of molecules that have evolved over many millions of years to cut cells in two in the process of cell division. The same molecular functions, she argues, could be repurposed in a way that conforms to the theory advanced by the Baums. “Why spend the energy to remake something that was made thousands of years ago to pinch in a cell? The functions of these proteins just evolve and change as the organism’s structure and function change.”

Knowing more about how the eukaryotic cell came to be promises to aid biologists studying the fundamental properties of the cell, which, in turn, could one day fuel a better understanding of things like cancer, diabetes and other cell-based diseases aging and the development of valuable new traits for important crop plants.

“I have no idea if it is right or wrong, but they’ve done a good job of pulling in detail and providing testable hypotheses. That, in itself, is incredibly useful.”

One catch for fleshing out the evolutionary history of the eukaryotic cell, however, is that unlike many other areas of biology, the fossil record is of little help. “When it comes to individual cells, the fossil record is rarely very helpful,” explains David Baum. “It is even hard to tell a eukaryotic cell from a prokaryotic cell. I did look for evidence of microfossils with protrusions, but, not surprisingly, there were no good candidates.”

A potentially more fruitful avenue to explore, he suggests, would be to look for intermediate forms of cells with some, but not all, of the features of a full-blown eukaryote. “The implication is that intermediates that did exist went extinct, most likely because of competition with fully-developed eukaryotes.”

However, with a more granular understanding of how complex cells evolved, it may be possible to identify living intermediates, says David Baum: “I do hold out hope that once we figure out how the eukaryotic tree is rooted, we might find a few eukaryotes that have intermediate traits.”

“This is a whole new take (on the eukaryotic cell), which I find fascinating,” notes UW–Madison biochemistry Professor Judith Kimble. “I have no idea if it is right or wrong, but they’ve done a good job of pulling in detail and providing testable hypotheses. That, in itself, is incredibly useful.”


Cellular tensegrity

Tensegrity is a building principle that was first described by the architect R. Buckminster Fuller (Fuller,1961) and first visualized by the sculptor Kenneth Snelson(Snelson, 1996). Fuller defines tensegrity systems as structures that stabilize their shape by continuous tension or `tensional integrity' rather than by continuous compression (e.g. as used in a stone arch). This is clearly seen in the Snelson sculptures, which are composed of isolated stainless steel bars that are held in position and suspended in space by high tension cables(Fig. 1A). The striking simplicity of these sculptures has led to a description of tensegrity architecture as a tensed network of structural members that resists shape distortion and self-stabilizes by incorporating other support elements that resist compression. These sculptures and similar structures composed of wood struts and elastic strings (Fig. 1B) beautifully illustrate the underlying force balance, which is based on local compression and continuous tension(Fig. 2A) that is responsible for their stability. However, rigid elements are not required, because similar structures can be constructed from flexible springs that simply differ in their elasticity (Fig. 1C).

Tensegrity structures. (A) Triple crown, a tensegrity sculpture, by the artist Kenneth Snelson, that is composed of stainless steel bars and tension cables. Note that this structure is composed of multiple tensegrity modules that are interconnected by similar rules. (B) A tensegrity sphere composed of six wood struts and 24 white elastic strings, which mimics how a cell changes shape when it adheres to a substrate(Ingber, 1993b). (C) The same tensegrity configuration as in B constructed entirely from springs with different elasticities.

Tensegrity structures. (A) Triple crown, a tensegrity sculpture, by the artist Kenneth Snelson, that is composed of stainless steel bars and tension cables. Note that this structure is composed of multiple tensegrity modules that are interconnected by similar rules. (B) A tensegrity sphere composed of six wood struts and 24 white elastic strings, which mimics how a cell changes shape when it adheres to a substrate(Ingber, 1993b). (C) The same tensegrity configuration as in B constructed entirely from springs with different elasticities.

(A) A high magnification view of a Snelson sculpture with sample compression and tension elements labeled to visualize the tensegrity force balance based on local compression and continuous tension. (B) A schematic diagram of the complementary force balance between tensed microfilaments(MFs), intermediate filaments (IFs), compressed microtubules (MTs) and the ECM in a region of a cellular tensegrity array. Compressive forces borne by microtubules (top) are transferred to ECM adhesions when microtubules are disrupted (bottom), thereby increasing substrate traction.

(A) A high magnification view of a Snelson sculpture with sample compression and tension elements labeled to visualize the tensegrity force balance based on local compression and continuous tension. (B) A schematic diagram of the complementary force balance between tensed microfilaments(MFs), intermediate filaments (IFs), compressed microtubules (MTs) and the ECM in a region of a cellular tensegrity array. Compressive forces borne by microtubules (top) are transferred to ECM adhesions when microtubules are disrupted (bottom), thereby increasing substrate traction.

According to Fuller's more general definition, tensegrity includes two broad structural classes — prestressed and geodesic — which would both fail to act like a single entity or to maintain their shape stability when mechanically stressed without continuous transmission of tensional forces(Fuller, 1961 Fuller, 1979 Ingber, 1998 Chen and Ingber, 1999). The former hold their joints in position as the result of a `prestress'(pre-existing tensile stress or isometric tension) within the structure(Fig. 1). The latter triangulate their structural members and orient them along geodesics (minimal paths) to geometrically constrain movement. Our bodies provide a familiar example of a prestressed tensegrity structure: our bones act like struts to resist the pull of tensile muscles, tendons and ligaments, and the shape stability (stiffness) of our bodies varies depending on the tone (prestress)in our muscles. Examples of geodesic tensegrity structures include Fuller's geodesic domes, carbon-based buckminsterfullerenes (Bucky Balls), and tetrahedral space frames, which are popular with NASA because they maintain their stability in the absence of gravity and, hence, without continuous compression.

Some investigators use tensegrity to refer only to the prestressed `bar and cable' structures or particular subclasses of these (e.g. unanchored forms)(Snelson, 1996 Heidemann et al., 2000). Since Fuller defined the term tensegrity, I use his more general definition here. The existence of a common structural basis for these two different classes of structure is also supported by recent work by the mathematician Robert Connelly. He developed a highly simplified method to describe prestressed tensegrity configurations and then discovered that the same fundamental mathematical rules describe the closest packing of spheres(Connelly and Back, 1998),which also delineate the different geodesic forms(Fuller, 1965).

The cellular tensegrity model proposes that the whole cell is a prestressed tensegrity structure, although geodesic structures are also found in the cell at smaller size scales. In the model, tensional forces are borne by cytoskeletal microfilaments and intermediate filaments, and these forces are balanced by interconnected structural elements that resist compression, most notably, internal microtubule struts and extracellular matrix (ECM) adhesions(Fig. 2B). However, individual filaments can have dual functions and hence bear either tension or compression in different structural contexts or at different size scales (e.g. rigid actin filament bundles bear compression in filopodia). The tensional prestress that stabilizes the whole cell is generated actively by the contractile actomyosin apparatus. Additional passive contributions to this prestress come from cell distension through adhesions to the ECM and other cells, osmotic forces acting on the cell membrane, and forces exerted by filament polymerization. Intermediate filaments that interconnect at many points along microtubules,microfilaments and the nuclear surface provide mechanical stiffness to the cell through their material properties and their ability to act as suspensory cables that interconnect and tensionally stiffen the entire cytoskeleton and nuclear lattice. In addition, the internal cytoskeleton interconnects at the cell periphery with a highly elastic, cortical cytoskeletal network directly beneath the plasma membrane. The efficiency of mechanical coupling between this submembranous structural network and the internal cytoskeletal lattice depends on the type of molecular adhesion complex that forms on the cell surface. The entire integrated cytoskeleton is then permeated by a viscous cytosol and enclosed by a differentially permeable surface membrane.


BIO 140 - Human Biology I - Textbook

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Chapter 7

Eukaryotic Cells

By the end of this section, you will be able to:

  • Describe the structure of eukaryotic cells
  • Compare animal cells with plant cells
  • State the role of the plasma membrane
  • Summarize the functions of the major cell organelles

Have you ever heard the phrase &ldquoform follows function?&rdquo It&rsquos a philosophy practiced in many industries. In architecture, this means that buildings should be constructed to support the activities that will be carried out inside them. For example, a skyscraper should be built with several elevator banks a hospital should be built so that its emergency room is easily accessible.

Our natural world also utilizes the principle of form following function, especially in cell biology, and this will become clear as we explore eukaryotic cells ( Figure 1). Unlike prokaryotic cells, eukaryotic cells have: 1) a membrane-bound nucleus 2) numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others and 3) several, rod-shaped chromosomes. Because a eukaryotic cell&rsquos nucleus is surrounded by a membrane, it is often said to have a &ldquotrue nucleus.&rdquo The word &ldquoorganelle&rdquo means &ldquolittle organ,&rdquo and, as already mentioned, organelles have specialized cellular functions, just as the organs of your body have specialized functions.

At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. Before turning to organelles, let&rsquos first examine two important components of the cell: the plasma membrane and the cytoplasm .

Figure 1: These figures show the major organelles and other cell components of (a) a typical animal cell and (b) a typical eukaryotic plant cell. The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole&mdashstructures not found in animal cells. Plant cells do not have lysosomes or centrosomes.

The Plasma Membrane

Like prokaryotes, eukaryotic cells have a plasma membrane (Figure 2), a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane.

Figure 2: The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.

The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (singular = microvillus) (Figure 3). Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function. People with celiac disease have an immune response to gluten, which is a protein found in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.

Figure 3: Microvilli, shown here as they appear on cells lining the small intestine, increase the surface area available for absorption. These microvilli are only found on the area of the plasma membrane that faces the cavity from which substances will be absorbed. (credit "micrograph": modification of work by Louisa Howard)

The Cytoplasm

The cytoplasm is the entire region of a cell between the plasma membrane and the nuclear envelope (a structure to be discussed shortly). It is made up of organelles suspended in the gel-like cytosol , the cytoskeleton, and various chemicals (Figure 1). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there, too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.

The Nucleus

Typically, the nucleus is the most prominent organelle in a cell (Figure 1). The nucleus (plural = nuclei) houses the cell&rsquos DNA and directs the synthesis of ribosomes and proteins. Let&rsquos look at it in more detail (Figure 4).

Figure 4: The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The boundary of the nucleus is called the nuclear envelope. It consists of two phospholipid bilayers: an outer membrane and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus.

The Nuclear Envelope

The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 4). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus.

Chromatin and Chromosomes

To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nuclei of its body&rsquos cells. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins are attached to chromosomes, and they resemble an unwound, jumbled bunch of threads. These unwound protein-chromosome complexes are called chromatin (Figure 5) chromatin describes the material that makes up the chromosomes both when condensed and decondensed.

Figure 5: (a) This image shows various levels of the organization of chromatin (DNA and protein). (b) This image shows paired chromosomes. (credit b: modification of work by NIH scale-bar data from Matt Russell)

The Nucleolus

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.

Ribosomes

Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may be attached to the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum and the outer membrane of the nuclear envelope (Figure 1). Electron microscopy has shown us that ribosomes, which are large complexes of protein and RNA, consist of two subunits, aptly called large and small (Figure 6). Ribosomes receive their &ldquoorders&rdquo for protein synthesis from the nucleus where the DNA is transcribed into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.

Figure 6 Ribosomes are made up of a large subunit (top) and a small subunit (bottom). During protein synthesis, ribosomes assemble amino acids into proteins.

Because proteins synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), ribosomes are found in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.

Mitochondria

Mitochondria (singular = mitochondrion) are often called the &ldquopowerhouses&rdquo or &ldquoenergy factories&rdquo of a cell because they are responsible for making adenosine triphosphate (ATP), the cell&rsquos main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells don&rsquot get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid.

Mitochondria are oval-shaped, double membrane organelles (Figure 7) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.

Figure 7. This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes is called the intermembrane space, and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane. (credit: modification of work by Matthew Britton scale-bar data from Matt Russel

Peroxisomes

Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide, H2O2, which would be damaging to cells however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into oxygen and water.) For example, alcohol is detoxified by peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.

Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them: The membranes of vesicles can fuse with either the plasma membrane or other membrane systems within the cell. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. The membrane of a vacuole does not fuse with the membranes of other cellular components.

Animal Cells versus Plant Cells

At this point, you know that each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some, vacuoles, but there are some striking differences between animal and plant cells. While both animal and plant cells have microtubule organizing centers (MTOCs), animal cells also have centrioles associated with the MTOC: a complex called the centrosome. Animal cells each have a centrosome and lysosomes, whereas plant cells do not. Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole, whereas animal cells do not.

The Centrosome

The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to each other (Figure 8). Each centriole is a cylinder of nine triplets of microtubules.

Figure 8. The centrosome consists of two centrioles that lie at right angles to each other. Each centriole is a cylinder made up of nine triplets of microtubules. Nontubulin proteins (indicated by the green lines) hold the microtubule triplets together.

The centrosome (the organelle where all microtubules originate) replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the exact function of the centrioles in cell division isn&rsquot clear, because cells that have had the centrosome removed can still divide, and plant cells, which lack centrosomes, are capable of cell division.

Lysosomes

Animal cells have another set of organelles not found in plant cells: lysosomes. The lysosomes are the cell&rsquos &ldquogarbage disposal.&rdquo In plant cells, the digestive processes take place in vacuoles. Enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, so again, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.

The Cell Wall

If you examine Figure 1b, the diagram of a plant cell, you will see a structure external to the plasma membrane called the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and protistan cells also have cell walls. While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose (Figure 9), a polysaccharide made up of glucose units. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That&rsquos because you are tearing the rigid cell walls of the celery cells with your teeth.

Figure 9. Cellulose is a long chain of &beta-glucose molecules connected by a 1-4 linkage. The dashed lines at each end of the figure indicate a series of many more glucose units. The size of the page makes it impossible to portray an entire cellulose molecule.

Chloroplasts

Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals plants (autotrophs) are able to make their own food, like sugars, while animals (heterotrophs) must ingest their food.

Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast&rsquos inner membrane is a set of interconnected and stacked fluid-filled membrane sacs called thylakoids (Figure 10). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane that surrounds the grana is called the stroma.

Figure 10. The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana. The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid inside the inner membrane, which is called the stroma. Chloroplasts also have their own genome, which is contained on a single circular chromosome.

The chloroplasts contain a green pigment called chlorophyll , which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle.

Evolution Connection

Endosymbiosis

We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation.

Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = &ldquowithin&rdquo) is a mutually beneficial relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. We have already mentioned that microbes that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and from drying out, and they receive abundant food from the environment of the large intestine.

Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just as mitochondria and chloroplasts do. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts.

The Central Vacuole

Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 1b, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. The central vacuole plays a key role in regulating the cell&rsquos concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That&rsquos because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant.

The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm.

Section Summary

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleus&rsquos nucleolus is the site of ribosome assembly. Ribosomes are either found in the cytoplasm or attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria participate in cellular respiration they are responsible for the majority of ATP produced in the cell. Peroxisomes hydrolyze fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.

Animal cells also have a centrosome and lysosomes. The centrosome has two bodies perpendicular to each other, the centrioles, and has an unknown purpose in cell division. Lysosomes are the digestive organelles of animal cells.

Plant cells and plant-like cells each have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole can expand without having to produce more cytoplasm.


Eukaryotic Cell Structure

The following article provides information regarding the structure and functions of various cell organelles belonging to the eukaryotic cell.

The following article provides information regarding the structure and functions of various cell organelles belonging to the eukaryotic cell.

Eukaryotic cells are present in complex living organisms like animals, humans, and plants. They formed as a result of evolutionary changes that took lace in the prokaryotic cells. You can refer to the following image for understanding the cell structure.

Structure and Functions

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If you happen to check the structure of eukaryotic cells under the microscope, you will find that they are made up of a number of cell organelles, which help in the smooth functioning of the overall cell. Essentially a part of all the plants, animals, fungi, algae, and protozoans, these diploid cells are 5 micrometers or more in diameter, and characterized by the presence of a nucleus, which is absent in the prokaryotic organisms.

Cell Wall

It is a distinguishing part of plant cells, and is absent in animals. It imparts rigidity. Its material is different for different plant species with cell shapes being elongated, oval, round, rectangular, or square-shaped.

Cell Membrane

The outermost part of the cell is the cell membrane, which encloses all the cell organelles. Protecting the cell, providing rigidity, and controlling the flow of nutrients within the cells are important functions of the cell membrane.

Cell Cytoplasm

This liquid gel-like substance is called matrix, within which the cell organelles float and/or are embedded. It provides the right environment to carry out all the metabolic reactions.

Nucleus

Eukaryotic cells are considered advanced and complex. The nucleus is made up of genetic material, i.e., the DNA (Deoxyribonucleic acid) and the chromosomes, owing to which it is considered as the brain of the cell. It basically controls all the cell functions, and guides it properly.

Nucleolus

The interior of the nucleus has a dark stained area called the nucleolus, which is responsible for protein formation.

Nuclear Membrane

Peculiar to the eukaryotic cells, the main function of this membrane is to protect the nucleus by formation of a protective sheath around it.

Nucleoplasm

Nucleus is filled with this dense fluid that contains chromatin fibers, chromosomes, and genes that carry the genetic information.

Mitochondria

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They are among the largest cell organelles present in the eukaryotic cells. They are characterized by their own Mitochondrial DNA, RNA, and ribosomes and hence, can self-replicate. It is the key site for production of energy in the form of ATP molecules, and thus aids photosynthesis and respiration.

Plastids

Another peculiar organelle present in eukaryotic plant cells are the plastids. Photosynthesis is the unique process, by which plants prepare their own food with the aid of these organelles. Plants generally contain chloroplasts that are characterized by the presence of a green colored pigment called chlorophyll.

Ribosomes

They are essential for protein synthesis, which includes transcription and translation. All the ribosomes are of 80S type, except the one from mitochondria and plastids, which is of the 70S type.

Lysosomes

They mainly help to undertake phagocytosis, and promote intracellular digestion. They are also responsible for secretion of enzymes, which are necessary for breaking down the cell debris.

Centrosomes

Centrioles contained within the centrosomes are important for the process of initiation of cell division, the result being either mitosis or meiosis.

Endoplasmic Reticulum (ER)

These interconnecting flattened tubular tunnels are of two types: Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER). In combination with the ribosomes, they help in functions related to protein transport. ER is regarded as one of the most important cell organelles after mitochondria.

Golgi Apparatus

Their function includes protein processing so that active protein chains are released whenever required.

Vacuoles

Alike in plants and animals, vacuoles are water-filled organelles responsible for storage.

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