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In the highlighted paragraph from NCERT textbook, they mention that cellular organisation is the defining property of life forms while metabolism without exception is also the defining property of life forms. My doubt is whether the term 'metabolism' also includes the metabolism reaction in vitro condition too or metabolism only means all the chemical reactions occuring in the cell excluding in vitro condition. If what I mean is true, does cellular organisation and metabolism both mean the same thing? But then again if both are the same thing, metabolism also should be the defining property of life forms, but it is not the case as mentioned by my textbook. So that means both metabolism and cellular organisation are different things. Is this right?
This paragraph appears to draw a distinction between "the sum total of all reactions in cells" and "some isolated reactions that occur in cells". The former is "metabolism" and one defining trait of life; the latter are "metabolic reactions" and can occur in a test tube, and if so they do not fit tidily in a living/nonliving distinction (the paragraph talks about "not living things" but "surely living reactions", which is one way of illustrating the confusion I guess).
As to whether "metabolism" and "cellular structure" are the same thing, I think it's not so much that they're literally the same thing but they're not really separable, are they. The cellular structure is made by "all the chemical reactions in the cell", and "all the chemical reactions in the cell" are mediated by the cellular structure. It's a bit like trying to separate out "a race" from "the people running in the race". You can have one or two metabolic reactions happening in vitro, but if you had all of them in your test tube, happening the same way they do in the cell, you'd have a cell.
The reality of course is that there isn't a single all-encompassing definition of "life", so don't try and nitpick things too much. Metabolism is important because it's what allows living things to do what living things do, thermodynamically speaking. Cellular structure is important because it happens to be the way all living things implement their metabolism, and arguably it is a necessary requirement for metabolism to occur (there are arguments for why you need a small space enclosed by a membrane for example). Other things are also important like reproduction (and other near-universal feature of life that is a basic thing living things do, and allows Darwinian evolution), being subject to Darwinian evolution (because that is the process that leads life to being optimized in the way it is, and able to do the things it does to begin with), sensing and reacting to its environment (all this stuff about "what life does", well, those are universal things life does)… That means you can have arguable examples like viruses, which I would expect your textbook to call "nonliving" because they do not have metabolism or a cellular structure, but some others might call "living" because they have genetic information, replicate, are subject to Darwinian evolution and share a common ancestor with all other known life. The labels are less important than the features one is discussing at any one time. I expect your textbook probably says something like this at some point.
The cell (from Latin cella, meaning "small room"  ) is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to as the "building blocks of life". The study of cells is called cell biology, cellular biology, or cytology.
Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids.  Most plant and animal cells are only visible under a light microscope, with dimensions between 1 and 100 micrometres.  Electron microscopy gives a much higher resolution showing greatly detailed cell structure. Organisms can be classified as unicellular (consisting of a single cell such as bacteria) or multicellular (including plants and animals).  Most unicellular organisms are classed as microorganisms.
The number of cells in plants and animals varies from species to species it has been estimated that humans contain somewhere around 40 trillion (4×10 13 ) cells. [a]  The human brain accounts for around 80 billion of these cells. 
Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery.   Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells.  Cells emerged on Earth at least 3.5 billion years ago.   
What You Need to Know About Metabolism and Homeostasis
The metabolism is comprised of two opposing processes: anabolism and catabolism. Anabolism is a set of synthesis reactions that transform simpler compounds into organic molecules, normally consuming energy. Catabolism is a set of reactions that break down organic molecules into simpler and less complex substances, normally releasing energy. The energy released in catabolism may be used in vital processes of the body, including anabolism.
The Definition of Homeostasis
3. What is homeostasis? What are the sensors, controllers and effectors of homeostasis?
Homeostasis comprises the processes through which the body maintains adequate intra and extracellular conditions so that the metabolism can carry out its normal reactions.
Homeostatic sensors are structures that detect environmental information inside and outside the body. These sensors may be nervous receptor cells, cytoplasmic or membrane proteins or other specialized molecules. Controllers are structures responsible for processing and interpreting information received from the sensors. In general, controllers are specialized regions of the central nervous system. However, they also exist on the molecular level, like in the case of DNA, a molecule that can receive information from proteins to inhibit or boost the expression of certain genes. Effectors are elements commanded by the controllers that have the function of carrying out actions that in fact regulate and maintain the equilibrium of the body, including in muscles, glands, cellular organelles, etc., as well as structures that participate in genetic translation, production of proteins, etc., on the molecular level.
4. How do antagonistic mechanisms produce homeostatic regulation?
The homeostatic maintenance of the body mostly occurs by means of alternating antagonistic compensatory mechanisms. Some of these regulators lower pH while others increase it. Furthermore, there are effectors whose function is to increase body temperature and others that lower it. Likewise, there exist hormones that reduce the level of glucose in the blood, for example, and others that increase the glucose levels. The use of antagonistic mechanisms is an evolutionary strategy to solve the problem of the maintenance of the equilibrium in the body.
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6.1 Energy and Metabolism
In this section, you will explore the following questions:
- What are metabolic pathways?
- What are the differences between anabolic and catabolic pathways?
- How do chemical reactions play a role in energy transfer?
Connection for AP ® Courses
All living systems, from simple cells to complex ecosystems, require free energy to conduct cell processes such as growth and reproduction.
Organisms have evolved various strategies to capture, store, transform, and transfer free energy. A cell’s metabolism refers to the chemical reactions that occur within it. Some metabolic reactions involve the breaking down of complex molecules into simpler ones with a release of energy (catabolism), whereas other metabolic reactions require energy to build complex molecules (anabolism). A central example of these pathways is the synthesis and breakdown of glucose.
The content presented in this section supports the Learning Objectives outlined in Big Idea 1 and Big Idea 2 of the AP ® Biology Curriculum Framework listed below. The AP ® Learning Objectives merge Essential Knowledge content with one or more of the seven Science Practices. These objectives provide a transparent foundation for the AP ® Biology course, along with inquiry-based laboratory experiences, instructional activities, and AP ® exam questions.
|Big Idea 1||The process of evolution drives the diversity and unity of life.|
|Enduring Understanding 1.B||Organisms are linked by lines of descent from common ancestry.|
|Essential Knowledge||1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today.|
|Science Practice||3.1 The student can pose scientific questions.|
|Learning Objective||1.14 The student is able to pose scientific questions that correctly identify essential properties of shared, core life processes that provide insight into the history of life on Earth.|
|Essential Knowledge||1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today.|
|Science Practice||7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.|
|Learning Objective||1.15 The student is able to describe specific examples of conserved core biological processes and features shared by all domains or within one domain of life, and how these shared, conserved core processes and features support the concept of common ancestry for all organisms.|
|Essential Knowledge||1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today.|
|Science Practice||6.1 The student can justify claims with evidence.|
|Learning Objective||1.16 The student is able to justify the scientific claim that organisms share many conserved core processes and features that evolved and are widely distributed among organisms today.|
|Big Idea 2||Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.|
|Enduring Understanding 2.A||Growth, reproduction and maintenance of living systems require free energy and matter.|
|Essential Knowledge||2.A.1 All living systems require a constant input of free energy.|
|Science Practice||6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.|
|Learning Objective||2.1 The student is able to explain how biological systems use free energy based on empirical data that all organisms require constant energy input to maintain organization, to grow and to reproduce.|
The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.1][APLO 2.3][APLO 4.3][APLO 4.15][APLO 4.17][APLO 2.21]
Starting with the definition of metabolism as the total chemical activity of an organism, ask students for examples of processes that fit. Tally the examples on a board or screen and expand on them as appropriate.
The concepts of anabolism and catabolism may be difficult to keep straight. Use the example of anabolic steroids as a way (inappropriate and dangerous) to build up the body, therefore, any anabolic process builds macromolecules and the opposite, catabolic, breaks them down.
Scientists use the term bioenergetics to discuss the concept of energy flow (Figure 6.2) through living systems, such as cells. Cellular processes such as the building and breaking down of complex molecules occur through stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed. Just as living things must continually consume food to replenish what has been used, cells must continually produce more energy to replenish that used by the many energy-requiring chemical reactions that constantly take place. All of the chemical reactions that take place inside cells, including those that use energy and those that release energy, are the cell’s metabolism .
Metabolism of Carbohydrates
The metabolism of sugar (a simple carbohydrate) is a classic example of the many cellular processes that use and produce energy. Living things consume sugar as a major energy source, because sugar molecules have a great deal of energy stored within their bonds. The breakdown of glucose, a simple sugar, is described by the equation:
Carbohydrates that are consumed have their origins in photosynthesizing organisms like plants (Figure 6.3). During photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas (CO2) into sugar molecules, like glucose (C6H12O6). Because this process involves synthesizing a larger, energy-storing molecule, it requires an input of energy to proceed. The synthesis of glucose is described by this equation (notice that it is the reverse of the previous equation):
During the chemical reactions of photosynthesis, energy is provided in the form of a very high-energy molecule called ATP, or adenosine triphosphate, which is the primary energy currency of all cells. Just as the dollar is used as currency to buy goods, cells use molecules of ATP as energy currency to perform immediate work. The sugar (glucose) is stored as starch or glycogen. Energy-storing polymers like these are broken down into glucose to supply molecules of ATP.
Solar energy is required to synthesize a molecule of glucose during the reactions of photosynthesis. In photosynthesis, light energy from the sun is initially transformed into chemical energy that is temporally stored in the energy carrier molecules ATP and NADPH (nicotinamide adenine dinucleotide phosphate). The stored energy in ATP and NADPH is then used later in photosynthesis to build one molecule of glucose from six molecules of CO2. This process is analogous to eating breakfast in the morning to acquire energy for your body that can be used later in the day. Under ideal conditions, energy from 18 molecules of ATP is required to synthesize one molecule of glucose during the reactions of photosynthesis. Glucose molecules can also be combined with and converted into other types of sugars. When sugars are consumed, molecules of glucose eventually make their way into each living cell of the organism. Inside the cell, each sugar molecule is broken down through a complex series of chemical reactions. The goal of these reactions is to harvest the energy stored inside the sugar molecules. The harvested energy is used to make high-energy ATP molecules, which can be used to perform work, powering many chemical reactions in the cell. The amount of energy needed to make one molecule of glucose from six molecules of carbon dioxide is 18 molecules of ATP and 12 molecules of NADPH (each one of which is energetically equivalent to three molecules of ATP), or a total of 54 ATP molecule equivalents required for the synthesis of one molecule of glucose. This process is a fundamental and efficient way for cells to generate the molecular energy that they require.
Ask the students where the energy used for metabolism comes from. Have them trace the energy back to the plants and light energy that the plants convert to sugars. Begin to introduce the interactions between carbohydrate metabolism, lipids, and proteins. Ask them what the ultimate end of the energy is (heat).
The processes of making and breaking down sugar molecules illustrate two types of metabolic pathways. A metabolic pathway is a series of interconnected biochemical reactions that convert a substrate molecule or molecules, step-by-step, through a series of metabolic intermediates, eventually yielding a final product or products. In the case of sugar metabolism, the first metabolic pathway synthesized sugar from smaller molecules, and the other pathway broke sugar down into smaller molecules. These two opposite processes—the first requiring energy and the second producing energy—are referred to as anabolic (building) and catabolic (breaking down) pathways, respectively. Consequently, metabolism is composed of building (anabolism) and degradation (catabolism).
Discuss the evolution of metabolic pathways as they probably developed on Earth. Using the Miller–Urey experiment discussed in Chapter 3, ask why there was no free oxygen in an early atmosphere. What pathways could develop under these conditions? How did this limit the development of organisms? What pathway created free oxygen as a waste that could permeate the atmosphere? Is this really a good idea for the organisms that existed? Why?
Evolution of Metabolic Pathways
There is more to the complexity of metabolism than understanding the metabolic pathways alone. Metabolic complexity varies from organism to organism. Photosynthesis is the primary pathway in which photosynthetic organisms like plants (the majority of global photosynthesis is done by planktonic algae) harvest the sun’s energy and convert it into carbohydrates. The by-product of photosynthesis is oxygen, required by some cells to carry out cellular respiration. During cellular respiration, oxygen aids in the catabolic breakdown of carbon compounds, like carbohydrates. Among the products of this catabolism are CO2 and ATP. In addition, some eukaryotes perform catabolic processes without oxygen (fermentation) that is, they perform or use anaerobic metabolism.
Organisms probably evolved anaerobic metabolism to survive (living organisms came into existence about 3.8 billion years ago, when the atmosphere lacked oxygen). Despite the differences between organisms and the complexity of metabolism, researchers have found that all branches of life share some of the same metabolic pathways, suggesting that all organisms evolved from the same ancient common ancestor (Figure 6.4). Evidence indicates that over time, the pathways diverged, adding specialized enzymes to allow organisms to better adapt to their environment, thus increasing their chance to survive. However, the underlying principle remains that all organisms must harvest energy from their environment and convert it to ATP to carry out cellular functions.
- Oxygen is a byproduct of anaerobic respiration, so there was very little oxygen in the atmosphere until anaerobic organisms evolved.
- Oxygen is a byproduct of fermentation, so there was little oxygen in the atmosphere until prokaryotes appeared.
- Oxygen is a byproduct of aerobic respiration, so there was very little oxygen in the atmosphere until animals evolved.
- Oxygen is a byproduct of photosynthesis, so there was very little oxygen in the atmosphere until photosynthetic organisms evolved.
Anabolic and Catabolic Pathways
Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones. Synthesizing sugar from CO2 is one example. Other examples are the synthesis of large proteins from amino acid building blocks, and the synthesis of new DNA strands from nucleic acid building blocks. These biosynthetic processes are critical to the life of the cell, take place constantly, and demand energy provided by ATP and other high-energy molecules like NADH (nicotinamide adenine dinucleotide) and NADPH (Figure 6.5).
ATP is an important molecule for cells to have in sufficient supply at all times. The breakdown of sugars illustrates how a single molecule of glucose can store enough energy to make a great deal of ATP, 36 to 38 molecules. This is a catabolic pathway. Catabolic pathways involve the degradation (or breakdown) of complex molecules into simpler ones. Molecular energy stored in the bonds of complex molecules is released in catabolic pathways and harvested in such a way that it can be used to produce ATP. Other energy-storing molecules, such as fats, are also broken down through similar catabolic reactions to release energy and make ATP (Figure 6.5).
It is important to know that the chemical reactions of metabolic pathways don’t take place spontaneously. Each reaction step is facilitated, or catalyzed, by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.
Gibberellin Metabolism and its Regulation
Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. Biochemical, genetic, and genomic approaches have led to the identification of the majority of the genes that encode GA biosynthesis and deactivation enzymes. Recent studies have highlighted the occurrence of previously unrecognized deactivation mechanisms. It is now clear that both GA biosynthesis and deactivation pathways are tightly regulated by developmental, hormonal, and environmental signals, consistent with the role of GAs as key growth regulators. In some cases, the molecular mechanisms for fine-tuning the hormone levels are beginning to be uncovered. In this review, I summarize our current understanding of the GA biosynthesis and deactivation pathways in plants and fungi, and discuss how GA concentrations in plant tissues are regulated during development and in response to environmental stimuli.
Cell therapy and gene therapy are overlapping fields of biomedical research and treatment 6 . Both therapies aim to treat, prevent, or potentially cure diseases, and both approaches have the potential to alleviate the underlying cause of genetic diseases and acquired diseases 6 . But, cell and gene therapies work differently.
Cell therapy aims to treat diseases by restoring or altering certain sets of cells or by using cells to carry a therapy through the body 5 . With cell therapy, cells are cultivated or modified outside the body before being injected into the patient. The cells may originate from the patient (autologous cells) or a donor (allogeneic cells) 6 .
Gene therapy aims to treat diseases by replacing, inactivating or introducing genes into cells— either inside the body (in vivo) or outside of the body (ex vivo) 6 .
Some therapies are considered both cell and gene therapies. These therapies work by altering genes in specific types of cells and inserting them into the body.
We thank L. Keren for insightful discussions and invaluable feedback as well as the Nakamura lab at the Gladstone Institutes for access to their Seahorse XF analyzer. Further, we thank A. Tsai for advice and help with clinical samples. This study was supported by an EMBO Long-Term Fellowship ALTF 1141–2017 (to F.J.H.), the Novartis Foundation for Medical-Biological Research 16C148 (to F.J.H.) and the Swiss National Science Foundation SNF Early Postdoc Mobility P2ZHP3-171741 (to F.J.H.). In addition, we received support from National Institutes of Health 1DP2OD022550-01 (to S.C.B.), 1R01AG056287-01 (to S.C.B.), 1R01AG057915-01 (to S.C.B.) and 1U24CA224309-01 (to S.C.B.).
Cells were first seen in 17th century Europe with the invention of the compound microscope. In 1665, Robert Hooke termed the building block of all living organisms as "cells" after looking at a piece of cork and observing a cell-like structure,   however, the cells were dead and gave no indication to the actual overall components of a cell. A few years later, in 1674, Anton Van Leeuwenhoek was the first to analyze live cells in his examination of algae. All of this preceded the cell theory which states that all living things are made up of cells and that cells are the functional and structural unit of organisms. This was ultimately concluded by plant scientist, Matthias Schleiden  and animal scientist Theodor Schwann in 1838, who viewed live cells in plant and animal tissue, respectively.  19 years later, Rudolf Virchow further contributed to the cell theory, adding that all cells come from the division of pre-existing cells.  Although widely accepted, there have been many studies that question the validity of the cell theory. Viruses, for example, lack common characteristics of a living cell, such as membranes, cell organelles, and the ability to reproduce by themselves.  Scientists have struggled to decide whether viruses are alive or not and whether they are in agreement with the cell theory.
Modern-day cell biology research looks at different ways to culture and manipulate cells outside of a living body to further research in human anatomy and physiology, and to derive medications. The techniques by which cells are studied have evolved. Due to advancements in microscopy, techniques and technology have allowed for scientists to hold a better understanding of the structure and function of cells. Many techniques commonly used to study cell biology are listed below: 
- : Utilizes rapidly growing cells on media which allows for a large amount of a specific cell type and an efficient way to study cells.  : Fluorescent markers such as GFP, are used to label a specific component of the cell. Afterwards, a certain light wavelength is used to excite the fluorescent marker which can then be visualized.  : Uses the optical aspect of light to represent the solid, liquid, and gas phase changes as brightness differences.  : Combines fluorescence microscopy with imaging by focusing light and snap shooting instances to form a 3-D image.  : Involves metal staining and the passing of electrons through the cells, which will be deflected upon interaction with metal. This ultimately forms an image of the components being studied.  : The cells are placed in the machine which uses a beam to scatter the cells based on different aspects and can therefore separate them based on size and content. Cells may also be tagged with GFP-florescence and can be separated that way as well.  : This process requires breaking up the cell using high temperature or sonification followed by centrifugation to separate the parts of the cell allowing for them to be studied separately. 
There are two fundamental classifications of cells: prokaryotic and eukaryotic. Prokaryotic cells are distinguished from eukaryotic cells by the absence of a cell nucleus or other membrane bound organelle.  Prokaryotic cells are much smaller than eukaryotic cells, making them the smallest form of life.  Prokaryotic cells include Bacteria and Archaea, and lack an enclosed cell nucleus. They both reproduce through binary fission. Bacteria, the most prominent type, have several different shapes which include mainly spherical, and rod-shaped. Bacteria can be classed as either gram positive or gram negative depending on the cell wall composition. Bacterial structural features include a flagellum that helps the cell to move,  ribosomes for the translation of RNA to protein,  and a nucleoid that holds all the genetic material in a circular structure.  There are many process that occur in prokaryotic cells that allow them to survive. For instance, in a process termed conjugation, fertility factor allows the bacteria to possess a pilus which allows it to transmit DNA to another bacteria which lacks the F factor, permitting the transmittance of resistance allowing it to survive in certain environments. 
Eukaryotic cells can either be unicellular or multicellular  and include animal, plant, fungi, and protozoa cells which all contain organelles with various shapes and sizes. 
Structure of eukaryotic cells Edit
Eukaryotic cells are composed of the following organelles:
- : This functions as the genome and genetic information storage for the cell, containing all the DNA organized in the form of chromosomes. It is surrounded by a nuclear envelope, which includes nuclear pores allowing for transportation of proteins between the inside and outside of the nucleus.  This is also the site for replication of DNA as well as transcription of DNA to RNA. Afterwards, the RNA is modified and transported out to the cytosol to be translated to protein. : This structure is within the nucleus, usually dense and spherical in shape. It is the site of ribosomal RNA (rRNA) synthesis, which is needed for ribosomal assembly. : This functions to synthesize, store, and secrete proteins to the golgi apparatus.  : This functions for the production of energy or ATP within the cell. Specifically, this is the place where the Krebs cycle or TCA cycle for the production of NADH and FADH occurs. Afterwards, these products are used within the electron transport chain (ETC) and oxidative phosphorylation for the final production of ATP.  : This functions to further process, package, and secrete the proteins to their destination. The proteins contain a signal sequence which allows the golgi apparatus to recognize and direct it to the correct place.  : The lysosome functions to degrade material brought in from the outside of the cell or old organelles. This contains many acid hydrolases, proteases, nucleases, and lipases, which breakdown the various molecules. Autophagy is the process of degradation through lysosomes which occurs when a vesicle buds off from the ER and engulfs the material, then, attaches and fuses with the lysosome to allow the material to be degraded.  : Functions to translate RNA to protein. : This functions to anchor organelles within the cells and make up the structure and stability of the cell. : The cell membrane can be described as a phospholipid bilayer and is also consisted of lipids and proteins.  Because the inside of the bilayer is hydrophobic and in order for molecules to participate in reactions within the cell, they need to be able to cross this membrane layer to get into cell via osmotic pressure, diffusion, concentration gradients, and membrane channels.  : Function to produce spindle fibers which are used to separate chromosomes during cell division.
Eukaryotic cells may also be composed of the following molecular components:
- : This makes up chromosomes and is a mixture of DNA with various proteins. : They help to propel substances and can also be used for sensory purposes. 
Cell metabolism Edit
Cell metabolism is necessary for the production of energy for the cell and therefore its survival and includes many pathways. For cellular respiration, once glucose is available, glycolysis occurs within the cytosol of the cell to produce pyruvate. Pyruvate undergoes decarboxylation using the multi-enzyme complex to form acetyl coA which can readily be used in the TCA cycle to produce NADH and FADH2. These products are involved in the electron transport chain to ultimately form a proton gradient across the inner mitochondrial membrane. This gradient can then drive the production of ATP and H2O during oxidative phosphorylation.  Metabolism in plant cells includes photosynthesis which is simply the exact opposite of respiration as it ultimately produces molecules of glucose.
Cell signaling Edit
Cell signaling is important for cell regulation and for cells to process information from the environment and respond accordingly. Signaling can occur through direct cell contact or endocrine, paracrine, and autocrine signaling. Direct cell-cell contact is when a receptor on a cell binds a molecule that is attached to the membrane of another cell. Endocrine signaling occurs through molecules secreted into the bloodstream. Paracrine signaling uses molecules diffusing between two cells to communicate. Autocrine is a cell sending a signal to itself by secreting a molecule that binds to a receptor on its surface. Forms of communication can be through:
- : Can be of different types such as voltage or ligand gated ion channels. The allow for the outflow and inflow of molecules and ions. (GPCR): Is widely recognized to contain 7 transmembrane domains. The ligand binds on the extracellular domain and once the ligand binds, this signals a guanine exchange factor to convert GDP to GTP and activate the G-α subunit. G-α can target other proteins such as adenyl cyclase or phospholipase C, which ultimately produce secondary messengers such as cAMP, Ip3, DAG, and calcium. These secondary messengers function to amplify signals and can target ion channels or other enzymes. One example for amplification of a signal is cAMP binding to and activating PKA by removing the regulatory subunits and releasing the catalytic subunit. The catalytic subunit has a nuclear localization sequence which prompts it to go into the nucleus and phosphorylate other proteins to either repress or activate gene activity.  : Bind growth factors, further promoting the tyrosine on the intracellular portion of the protein to cross phosphorylate. The phosphorylated tyrosine becomes a landing pad for proteins containing an SH2 domain allowing for the activation of Ras and the involvement of the MAP kinase pathway. 
Cell cycle Edit
The growth process of the cell does not refer to the size of the cell, but the density of the number of cells present in the organism at a given time. Cell growth pertains to the increase in the number of cells present in an organism as it grows and develops as the organism gets larger so does the number of cells present. Cells are the foundation of all organisms and are the fundamental unit of life. The growth and development of cells are essential for the maintenance of the host and survival of the organism. For this process, the cell goes through the steps of the cell cycle and development which involves cell growth, DNA replication, cell division, regeneration, and cell death. The cell cycle is divided into four distinct phases: G1, S, G2, and M. The G phase – which is the cell growth phase – makes up approximately 95% of the cycle. The proliferation of cells is instigated by progenitors. All cells start out in an identical form and can essentially become any type of cells. Cell signaling such as induction can influence nearby cells to differentiate determinate the type of cell it will become. Moreover, this allows cells of the same type to aggregate and form tissues, then organs, and ultimately systems. The G1, G2, and S phase (DNA replication, damage and repair) are considered to be the interphase portion of the cycle, while the M phase (mitosis) is the cell division portion of the cycle. Mitosis is composed of many stages which include, prophase, metaphase, anaphase, telophase, and cytokinesis, respectively. The ultimate result of mitosis is the formation of two identical daughter cells.
The cell cycle is regulated by a series of signaling factors and complexes such as cyclins, cyclin-dependent kinase, and p53. When the cell has completed its growth process and if it is found to be damaged or altered, it undergoes cell death, either by apoptosis or necrosis, to eliminate the threat it can cause to the organism's survival. 
Cell mortality, cell lineage immortality Edit
The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to the origin of life. It is not actually cells that are immortal but multi-generational cell lineages.  The immortality of a cell lineage depends on the maintenance of cell division potential. This potential may be lost in any particular lineage because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death (apoptosis) during development. Maintenance of cell division potential over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly DNA damage. In sexual organisms, continuity of the germline depends on the effectiveness of processes for avoiding DNA damage and repairing those DNA damages that do occur. Sexual processes in eukaryotes, as well as in prokaryotes, provide an opportunity for effective repair of DNA damages in the germ line by homologous recombination.  
The scientific branch that studies and diagnoses diseases on the cellular level is called cytopathology. Cytopathology is generally used on samples of free cells or tissue fragments, in contrast to the pathology branch of histopathology, which studies whole tissues. Cytopathology is commonly used to investigate diseases involving a wide range of body sites, often to aid in the diagnosis of cancer but also in the diagnosis of some infectious diseases and other inflammatory conditions. For example, a common application of cytopathology is the Pap smear, a screening test used to detect cervical cancer, and precancerous cervical lesions that may lead to cervical cancer.
The simplest unit of life is the cell. In fact, some organisms like bacteria are nothing more than a single cell. The human body contains approximately 30 trillion cells and that is without considering all the unicellular bacteria that naturally colonize the digestive tract. Scientists estimate there are approximately 200 unique types of cells in the human body.
Groups of cells organized together for a specific function form tissues. There are four basic types of tissue in the human body: epithelial, muscle, nerve and connective. Epithelial tissue covers the exterior of the body as well as the linings of the organs and cavities of the body. Muscle tissue contains cells that are sometimes called “excitable” because they are able to contract and enable movement. Nerve tissue conducts electrical impulses and send signals through the body. Connective tissue holds the body together and includes both bones and blood.
Difference between Metabolic Alkalosis and Respiratory Alkalosis | Acid-Base Regulation
4. Depression of respiratory centre and hyperventilation leading to retention of CO2.
5. In renal mechanism, there is increased NH3 formation and H + – Na + exchange, increased K + excretion, decreased reabsorption, retention of CI – .
6. Urine shows alkaline, decreased NH3, and decreased titratable acidity.
7. Low Ca ++ leading to tetany, hypokalemia, ketosis and ketonuria, degenerative changes in tubules leading to nitrogen retention.
8. Causes are excessive loss of HCI, high intestinal obstruction, pyloric obstruction, alkali ingestion, excessive loss of K + , X-ray therapy, ultra violet radiation.
Difference # Respiratory Alkalosis:
4. In renal mechanism, there is decreased H + -Na + exchange, decreased excretion of acid and ammonia, increased excretion of HCO3 – and K + , retention of CI – .
5. Hypoventilation due to respiratory high pH and low PCO2 and increase in H2CO3.
6. Urine show’s alkaline, decreased NH3 and decreased titratable acidity.
7. Low Ca ++ leading to tetany, hypokalemia, ketosis and ketonuria, kidney damage leading to nitrogen retention.
8. Causes are CNS diseases like meningitis and encephalitis, salicylate poisoning, hyperpyrexia, hysteria, high altitude ascending, apprehensive blood donors, injudicious use of respirator, some cases of hepatic coma.