Molecular landscape of a cell via Digizyme.


Scanning electron micrograph of blood cells. From left to right: human erythrocyte, thrombocyte (platelet), leukocyte.

A cell is a microscopic mass of protoplasm_. [1] For example, adipocytes, muscle fibers, red blood cells, and white blood cells.

Cells are the smallest structural and functional unit of an organism. Cells are the smallest unit of life that can replicate independently.


1   Etymology

"Cell" comes from Latin cella, meaning "small room". Robert Hooke, who discovered the cell in 1665, named the biological unit for its resemblance to the cells inhabited by Christian monks in a monastery.

2   Study

The study of cells is called cell biology.

3   Substance


Diagram of a cell. Henry Gray. Anatomy of the Human Body.

Cells essentially consist of a cell membrane and cytoplasm.

3.1   Cell membrane

The cell membrane (= plasma membrane) is a biological membrane that separates the interior of cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells.

Membranes are composed two back-to-back layers of lipids.

The plasma membrane is composed primarily of proteins and lipids, especially phospholipids. The lipids occur in two layers (a bilayer). Proteins embedded in the bilayer appear to float within the lipid, so the membrane is constantly in flux. The membrane is therefore referred to as a fluid mosaic structure. Within the fluid mosaic structure, proteins carry out most of the membrane's functions.

3.2   Cytoplasm


"The structure of cytoplasm" from Molecular Biology of the Cell. A representation of how crowded cells really are, with blue RNAs, green ribosomes, and red proteins.

Calls are often depicted as being mostly empty space, but actually they are crammed full of stuff.

Cytoplasm is a thick solution that fills each cell and is enclosed by the cell membrane. It is mainly composed of water, salts, and proteins.

is a material probably of variable constitution during life, but yielding on its disintegration bodies chiefly of proteid nature. Lecithin and cholesterin are constantly found in it, as well as inorganic salts, chief among which are the phosphates and chlorides of potassium, sodium, and calcium. It is of a semifluid, viscid consistence, and probably colloidal in nature. The living cytoplasm appears to consist of a homogeneous and structureless ground-substance in which are embedded granules of various types.

The cytoplasm consists of cytosol. The cytoplasm is about 80% water and usually colorless.

All of the contents of the cells of prokaryotes_ are contained within the cytoplasm. In eukaryotic cells, the cytoplasm includes all of the material inside the cell and outside of the nucleus. The inner, granular mass is called the endoplasm and the outer, clear and glassy layer is called the cell cortex or the ectoplasm.

Most cellular activities occur within the cytoplasm, such as many metabolic pathways including glycolysis, and processes such as `cell division`_.

3.2.1   Cytosol

Cytosol is the gel-like substance enclosed within the cell membrane.

Cytosol is the aqueous phase of the cytoplasm which surrounds cellular organelles.

3.2.2   Ribosome

A ribosome is an organelle.

The ribosome is a large and complex molecular machine, found within all living cells, that serves as the site of biological protein synthesis (translation). Ribosomes link amino acids together in the order specified by messenger RNA (mRNA) molecules. Ribosomes consist of two major components — the small ribosomal subunit which reads the RNA, and the large subunit which joins amino acids to form a polypeptide chain.

4   Classification

There are two types of cells, eukaryotes (from Greek eu "good" + karyon "kernel"), which contain a nucleus, and prokaryotes_ (from Greek pro "before" + karyon), which do not. Prokaryotic cells are usually single-celled

organisms, while eukaryotic cells can be either single-celled or part of multicellular organisms.

4.1   Stem cells

A stem cell is a cell that has three essential properties. First, they are capable of proliferation (dividing and renewing themselves many times). Second, they are unspecialized, that is, they have no tissue-specific structures that allow them to perform specialized functions. Third, they can give rise to specialized cells through a process called differentiation, in which they go through several stages of becoming more and more specialized. [4]

Scientists primarily work with two kinds of stem cells from animals and humans: embyronic stem cells and adult stem cells. [4] An embryonic stem cell is a stem cell derived from an embyro_. [4] An adult stem cell is a stem cell that can maintains and repairs the tissue in which it is found. [4]

Scientists discovered ways to derive embryonic stem cells from early mouse embryos more than 30 years ago, in 1981. The detailed study of the biology of mouse stem cells led to the discovery, in 1998, of a method to derive stem cells from human embryos and grow the cells in the laboratory. These cells are called human embryonic stem cells. [4]

Stem cells are important for living organisms for many reasons. In the 3- to 5-day-old embryo, called a blastocyst_, the inner cells give rise to the entire body of the organism, including all of the many specialized cell types and organs such as the heart, lungs, skin, sperm, eggs and other tissues. In some adult tissues, such as bone marrow, muscle, and brain, discrete populations of adult stem cells generate replacements for cells that are lost through normal wear and tear, injury, or disease. [4]

It has taken scientists many years of trial and error to learn to derive and maintain stem cells in the laboratory without them spontaneously differentiating into specific cell types. For example, it took two decades to learn how to grow human embryonic stem cells in the laboratory following the development of conditions for growing mouse stem cells. [4]

Scientists are just beginning to understand the signals inside and outside cells that trigger each step of the differentiation process. The internal signals are controlled by a cell's genes. The external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules in the microenvironment. [4]

Adult stem cells typically generate the cell types of the tissue in which they reside. For example, a blood-forming adult stem cell in the bone marrow normally gives rise to the many types of blood cells. It is generally accepted that a blood-forming cell in the bone marrow—which is called a hematopoietic stem cell—cannot give rise to the cells of a very different tissue, such as nerve cells in the brain. [4]

5   Production

Drosophila cell division on Lattice light sheet microscope


Diagram showing the changes which occur in the centrosomes and nucleus of a cell in the process of mitotic division. (Schäfer.) I to III, prophase; IV, metaphase; V and VI, anaphase; VII and VIII, telophase.

All the tissues and organs of the body originate from a microscopic structure (the fertilized ovum), which consists of a soft jelly-like material enclosed in a membrane and containing a vesicle or small spherical body inside which are one or more denser spots. This may be regarded as a complete cell. All the solid tissues consist largely of cells essentially similar to it in nature but differing in external form. [1]

6   Metabolic pathways

Metabolism is the sum of all the chemical processes that occur in the body. One phase of metabolism is catabolism (catabol- "throwing down"), the breaking down of complex chemical substances into simpler ones. Another phase is anabolism (anabol- "raising up"), the building up of complex substances from smaller, simple ones.

ATP must be present for skeletal muscles to contract.

When an ATP molecule is combined with water the last of three phosphate groups splits apart and produces energy. This breakdown of ATP for muscle contraction results in adenosine diphosphate (ADP). The limited stores of ATP must be replenished for work to continue; so chemical reactions add a phosphate group back to ADP to make ATP. [3]

Conventionally, there are three energy systems that produce ATP: ATP-PC (high power, short duration), glycolytic (moderate power/short duration), and oxidative (low power/long duration). [3]

How does the body "burn" fat? When somebody loses weight, where does it go?

The chemical formula for the average fat molecule is C55 H104 O6. All have fat molecules have six oxygen atoms, but some molecules have more or less carbon and hydrogen atoms. [2]

The body turns fat and oxygen into carbon dioxide and water through one of the metabolic pathways [2]:

C55 H104 O6 + 78 O2 -> 55 CO2 + 52 H20

When fat is metabolized, the body exhales 84% of what was once fat as CO2 (5% of your breath is CO2) and excretes the remaining 16% as H20 (urine, feces, sweat, or tears):

# atomic mass units (from periodic table)
c = 12.011
h = 1.0079
o = 15.99

# oxygen leaves in the same ratio that they exist in the molecules (2:1)

a = (55 * c + 4 * o)
b = (104 * h + 2 * o)

a / (a + b) = 0.84

b / (a + b) = 0.16

Therefore, 10kg of fat becomes 8.4kg CO2 and 1.6kg of water. [2]

This process generates adenosine triphosphate. Body "heat" is generated by the metabolism. Like all energy conversions, metabolism is rather inefficient, and around 60% of the available energy is converted to heat rather than to ATP. In most organisms, this heat is simply lost to the environment. However, endothermic homeotherms (the animals generally characterized as "warm-blooded") both produce more heat and have better ways to retain and regulate it than other animals. They have a higher basal metabolic rate, and also a greater capacity to increase their metabolic rate when engaged in strenuous activity. They usually have well-developed insulation in order to retain body heat, fur in the case of mammals and feathers in birds. When this insulation is insufficient to maintain body temperature, they may resort to shivering — rapid muscle contractions that quickly use up ATP, thus stimulating cellular metabolism to replace it and consequently produce more heat.


With every energy conversion process, there is a certain conversion efficiency. For the human body, only about 20% of all the potential energy stored in food is available for useful work. The remaining 80% takes the form of heat as a by-product of the conversion. This results in the continuous generation of heat within the body.

The rate of heat production within the body is known as the metabolic rate. The metabolic rate is the heat released from the body per unit skin area.

Metabolic rate can be measured in the following ways:

There are many factors affecting metabolic rate. The factors are:

The basal metabolic rate is the metabolic rate of a person measured under basal conditions; when a person is awake and in absolute physical and mental rest after 12 hours of absolute fasting and when the environmental temperature is 20-25 degrees Celsius.

6.1   Glycolysis

Glycolysis is a metabolic pathway that converts glucose into pyruvate_.

Glycolysis occurs, with variations, in nearly all organisms, both aerobic and anaerobic. The wide occurrence of glycolysis indicates that it is one of the most ancient known metabolic pathways. Indeed, the reactions that constitute glycolysis and its parallel pathway, the pentose phosphate pathway, occur metal-catalyzed under conditions of the Archean ocean also in the absence of enzymes.

The entire glycolysis pathway can be separated into two phases:

  1. The Preparatory Phase – in which ATP is consumed and is hence also known as the investment phase.
  2. The Pay Off Phase – in which ATP is produced.

Anaerobic glycolysis breaks down sugar without oxygen

The glycolytic system is the “next in line” tool after the ATP-PC system runs its course. Dietary carbohydrates supply glucose that circulates in the blood or is stored as glycogen in the muscles and the liver. Blood glucose and/or or stored glycogen is broken down to create ATP through the process of glycolysis. Like the ATP-PC system, oxygen is not required for the actual process of glycolysis (but it does play a role with the byproduct of glycolysis: pyruvic acid). [3]

First ~5-30 seconds (for convenience purposes) is creatine phosphate energy

Second ~1-10 (typically considered 5) minutes is considered fast glycolysis, where we use carbohydrates for energy.

Third ~10-90 minutes is slow glycolysis where we would use our fat storages as energy. It should also be noted that research estimates if you want to burn fat storages (which are by far the most efficient energy storing cells we have and it's not even close) that any aerobic cardio must exceed 12 minutes. The first 12 minutes of cardio would be burning our CP and fast glycolysis storages, it isn't until the12 minutes mark that our body starts using fat cells for ATP

The above is a bit reductionist: All of the major energy systems are constantly in use, they just contribute different proportions of energy to the activity.

6.2   Pentose phosphate pathway

ATP and phosphocreatine (PC) compose the ATP-PC system, also sometimes called the Phosphogen system. [3]

It is immediate and functions without oxygen. It allows for up to approximately 12 seconds (+ or -) of maximum effort. During the first few seconds of any activity, stored ATP supplies the energy. For a few more seconds beyond that, PC cushions the decline of ATP until there is a shift to another energy system. [3]

6.3   Oxidative pathway

The effort demand is low, but ATP in this system can be produced three ways: Krebs cycle, Electron Transport Chain, or Beta Oxidation. [3]

7   Motion

7.1   Hypertrophy

Hypertrophy (from hyper- + Greek -trophe "nourishment") is the increase in the volume of an organ or tissue due to the enlargement of its component cells.

7.2   Hyperplasia

Hyperplasia (from ancient Greek huper, "over" + plasis, "formation"), or hypergenesis, is an increase in the amount of organic tissue that results from cell proliferation.

8   History


In 1665 Robert Hooke published Micrographia, a book describing observations made with microscopes and telescopes, as well as some original work in biology. Hooke coined the term cell for describing biological organisms, the term being suggested by the resemblance of plant cells to cells of a honeycomb.

During the 1950s, scientists developed the concept that all organisms may be classified as prokaryotes or eukaryotes.

9   Tumors

The study of tumors is called oncology.

10   Further reading

11   References

[1](1, 2) Henry Gray. Anatomy of the Human Body: Embryology. http://www.bartleby.com/107/2.html
[2](1, 2, 3) Ruben Meerman. TEDxQUT. The mathematics of weight loss. https://www.youtube.com/watch?v=vuIlsN32WaE
[3](1, 2, 3, 4, 5, 6)

Tom Kelso. Understanding Energy Systems: ATP-PC, Glycolytic, and Oxidative. http://breakingmuscle.com/health-medicine/understanding-energy-systems-atp-pc-glycolytic-and-oxidative-oh-my

This is a low-quality source and should be removed.

[4](1, 2, 3, 4, 5, 6, 7, 8, 9) National Institutes of Health. Stem Cell Basics. https://stemcells.nih.gov/info/basics/1.htm
[8]A.K.Roy Choudhury, P.K. Majumdar, and C. Datta. 2011. Woodhead Publising Limited. Factors affecting comfort: human physiologic and the role of clothing.

So lets say we have a scalpel, right? Simplest medical device there is. There's a number of ways to make it totally(ish) sterile- gases, steam, dry heat, gamma radiation. But as you ask- the little bacterial corpses are still there.

The problem occurs when you stab someone with the scalpel, preferably in a medicinal way. The bodies immune system works by identifying certain chemical triggers in bacteria, and has no way to know that, for example, the lipopolysaccharide hanging around in someone's heart is not part of a bunch of living bacteria, but the floating corpses of dead bacteria.

The dead byproducts of bacteria are called "pyrogens" because they cause (among other things, such as death) fevers.

Where do they go? Nowhere. Bacteria are small enough that water has completely different properties on their level. Beyond rinsing off gross matter and reducing bacterial load, washing can't do much.

So for things like heart surgery scalpels, there will usually be a second step of "Depyrogenation" This is the process, not of killing bacteria, but of removing the bits left behind so they don't trigger an immune reaction. This varies widely in complexity depending on what you have to depyrogenate- steel scalpels are easier than an injectable drug, for example. Typically, the goal of the process is to so thoroughly break down the biological material left behind.

Pyrogens aren't much of a concern for eating. Your mouth is filled with bacteria, so is your digestive tract, so is your skin. Basically,your entire body is covered in and filled with teeming hordes of bacteria trying desperately to eat you alive, so your body is used to dealing with it. Pyrogen reactions are a concern when you put dead-germ bits into places that don't have germs- blood, brain... THE EXCEPTIONS are things like E. Coli, Salmonella ("I barely know Ella!") and botulism. In that case, what makes you poo/die is the toxins left behind by the bacteria. So if you have a piece of rotting meat, you can't just cook it until it is safe, because the toxins are what get you, not the live bacteria.

The size of cells is limited by the Square-Cube law.

This might sound obvious but because cells have to traffic things into and out of their surfaces in order to process nutrients, the ability to transport becomes a limiting factor as the cells can no longer uptake fuel and get rid of waste fast enough.

ou might notice that this is the reason why some organelles and cells will Ybecome wrinkled heavily (like mitochondria, or even organs like the brain) and that's to increase their surface area to volume ratio.


You may wonder how things get around inside cells if they are so crowded. It turns out that molecules move unimaginably quickly due to thermal motion. A small molecule such as glucose is cruising around a cell at about 250 miles per hour, while a large protein molecule is moving at 20 miles per hour. Note that these are actual speeds inside the cell, not scaled-up speeds.

Because cells are so crowded, molecules can't get very far without colliding with something. In fact, a molecule will collide with something billions of times a second and bounce off in a different direction. Because of this, molecules are doing a random walk through the cell and diffusing all around. A small molecule can get from one side of a cell to the other in 1/5 of a second.

As a result of all this random motion, a typical enzyme can collide with something to react with 500,000 times every second. Watching the video, you might wonder how the different pieces just happen to move to the right place. In reality, they are covering so much ground in the cell so fast that they will be in the "right place" very frequently just by chance.

In addition, a typical protein is tumbling around, a million times per second. Imagine proteins crammed together, each rotating at 60 million RPM, with molecules slamming into them billions of times a second. This is what's going on inside a cell.

I'm not blaming the makers of "Inner Life of a Cell" for slowing down the action in a cell. If the video were totally realistic, you wouldn't see anything, since the action would be too fast to even see a blur. But keeping the real speed of the cell in mind can clear up a lot of things, such as how molecules find their way around.

The incredible speed and density of cells also helps explain why it's so difficult to simulate what's happening inside a cell. Even with a supercomputer, there's way too much going on inside a cell to simulate it without major simplifications. Even simulating a single ribosome is a huge computational challenge.


Another thing that surprised me about cells is how fast the motors inside cells move. Like a mechanical robot with two lumbering feet, a kinesin motor protein can be seen in the video at the 2 minute mark dragging a monstrous bag-like vesicle along a microtubule track. (This should be what you see in the YouTube preview frame at the top of the page.) These motor proteins move cargo through the cell if diffusion isn't fast enough to get things to their destination, which is especially important in extremely long cells such as neurons. Kinesin motors also help separate cells that are dividing.

It's remarkable enough that cells contain these mechanical walkers, but I recently learned that they aren't plodding along, but actually sprint at 100 steps per second. If you watch the video again, imagine it sped up to that rate.


Mitochondria also provide a fascinating look at just how fast things are inside cells. You may know that mitochondria are the power plants of cells; they take in food molecules, process it through the famous citric acid cycle, and then use oxygen to extract more energy, which is provided to the rest of the cell through molecules of ATP, the cell's "energy currency".

Mitochondria have many strange features - such as their own DNA separate from the cell's - but one of their strangest features is they use electric motors to produce ATP. Mitochondria use the energy from oxidizing food to pump protons out of the cell, creating a voltage of 170mV across the cell. This voltage causes a complex enzyme to spin, and the mechanical energy of this spinning enzyme creates the ATP molecules that energize the rest of the cell.

These enzymes spin at up to 700 revolutions per second, which is faster than a jet engine. As I said earlier, cells are really, really fast.

Human cells can replace themselves, but they need a scaffold to replace themselves on for them to be in the right place. This is why scars stick around forever.

Humans cells regeerate at different rates. Muscles replace themselves one every 10 years at birth, but this declines rapidly as you age. Your epidermal cells (outermost skin cells) are replaced every month.

When you get a minor cut that does not puncture the entire epidermis, the injury will generally heal completely because there are epidermal cells underneath that grow outwards. As the cells above are pushed upwards and outwards they will slough off revealing the newer epidermal cells beneath. The skin is as good as new after a month or so.

However a deeper wound heals differently. The wound fills with blood, platelets and clotting agents to form a clot. Next fibroblasts are attracted to the wound site and begin producing collagen. The epidermis then tries to cover the wound site with skin cells (this can only happen if the wound remains moist - if the wound dries out, the healing process slows immensely which is one of the main reasons why bandaging a wound is so important). Scar tissue forms by filling the wound with collagen. Once the wound has healed, the collagen remains.

Over time, the collagen around the edge of the wound may slowly get replaced by neighboring cells, but if the wound is simply too large the regeneration of neighboring cells isn't fast enough to replace the nonfunctional tissue with functional cells. Remember, the neighboring cells have to pull double duty. In order for the scar to disappear, neighboring cells have to replace not only themselves but also their neighbors. They can do this to a point, but it is ultimately limited. So the body keeps the collagen in place which prevents the scar from disappearing entirely.

Scurvy (vitamin c deficiency) actually shows that the collagen holding wounds together is either temporary and requires upkeep or is scavenged in an attempt to correct the deficiency, cause wounds un-heal.

Each cell line has a due date (apoptosis). For example, when a cell starts to lose membrane integrity it sends out signals to be replaced. Our immune system may help break a cell apart so it can be removed from the body via the spleen. Part of human feces is simply discarded cells. Some of the longest living cells are neurons and myocardial (heart muscle) cells.

Scars are not cells, they are extracellular (outside cells) materials, mainly 'collagen' formed when more severe injury can not be healed by simple cell replication. Some scars fade slowly over many years (depending on severity and location) because that dense collagen is reabsorbed by cells and replaced by living tissue (cells) again.

Going deeper we find skeletal muscle tissue. Skeletal muscle consists of very long conjoined muscle cells called myofibrils - each myofibril has a large amount of cell nuclei evenly distributed along the fiber, each one contributing metabolically. Additionally skeletal (!) muscle tissue has small adjoined cellls called satellite cells, which kind of act like ¨stem cells¨ for the skeletal muscle (but not really). Slight damage is mitigated by donation of cell nuclei from the satelllite cell or they may even migrate to rebuild the fiber itself. The regenerative capabilities of skeletal muscles are limited though, and thus even slightly larger muscle damage is met with scar tisssue, which is, again, non functional so a scarred muscle will always be weaker.

Next on the list are bones - bones are specialized connective tissue (as in they have large networks of collagen) with additional hard mineral deposits. Bones when fractured are very regenerable given the right circumstances. There are two kinds of fracture healing - primary healing (very fast on the magnitude of few days to weeks, the bone directly grows back) and secondary healing (quite slow on the magnitude of weeks to months, a multi step process involving constructing a faux bone like structure that is replaced with real bone). Whether one or the other happens depends on how close the fracture ends are to each other, and the geometry between the fracture ends decides the structure of the healed bone, which is why it is important to fix fractured limbs so the bones can grow back in the right geometry again (or worst case scenario they dont heal and become a non functional joint called pseudarthrosis).

Lets go deeper. Cardiac muscle of the heart. Cardiac muscle is special in that it has distinct muscle cells called myocytes that are interconnected via gaps, thus act kind of like the myofibrils of skeletal muscle. Cardiac muscle is a nightmare to regenerate though: its myocytes are almost completely inert to proliferation and there are no or almost no regenerative stem cells available. Add to that the fact that the cardiac muscle is relatively resource intensive (which is part of the reason why it is unfavorable for cardiac muscle to be proliferating in the first place, the other being that the thickness of the walls of the heart are very carefully adjusted so unwarranted additional cells could disturb the balance). Oh, some tiny branch of the coronary artery is blocked by a thromb? Infarction (tissue death) within little time of the event, and you will never recover the dead muscle tissue. The body just straight up replaces it with scar tissue, which is, again, very non-functional.

Now here is the interesting one: Nerval tissue. Do nerves regenerate? Yeah, kind of! ... but not everywhere. See, there are two compartments to the nervous system: the central nervous system (all parts of the nervous system that are embedded in bone cavities, i.e. the skull and the vertebral column) and the periphery nervous system (all nerves that exit the central nervous system, i.e. all the nerves in the limbs, the torso and the head outside of the skull). Most nerves are embedded in a substance called myelin - basically like cable insulation, increases the conduction speed. That is not all they can do though: the cells that build the myelin - called glia cells - also nurture and maintain the actual nerve fibers. The periphery and the central nervous system has different kinds of glia cells. Central has a variation like oligodendrocytes and others, periphery has schwann cells. schwann cells are capable of repairing lesions in nerval tissue and grow periphery nerves back together, given that they are still in the same place of course. Central nerves and their glia cells... well, simply put they just dont. Have a damage in your spinal cord or your brain? Tough luck, those glia cells just cant. Additionally there are usually no real inflammatory processes in the brain that could kickstart a healing process (and inflammation in the brain is... well, pretty much lethal, so nature kind of had to do it like that). There ARE small stem cell populations in the brain that supposedly can regenerate damaged nerves, but they dont seem to do a significant enough job because spinal cord damage and strokes are pretty much permanent.

Biochemistry. 5th edition. Section 30.2 2002, W. H. Freeman and Company.

The metabolic patterns of the brain, muscle, adipose tissue, kidney, and liver are different.


Glucose is virtually the sole fuel for the human brain, except during prolonged starvation. The brain lacks fuel stores and hence requires a continuous supply of glucose. It consumes about 120 g daily, which corresponds to an energy input of about 420 kcal (1760 kJ), accounting for some 60% of the utilization of glucose by the whole body in the resting state.

Fatty acids do not serve as fuel for the brain, because they are bound to albumin in plasma and so do not traverse the blood-brain barrier. In starvation, ketone bodies generated by the liver partly replace glucose as fuel for the brain.


The major fuels for muscle are glucose, fatty acids, and ketone bodies. Muscle differs from the brain in having a large store of glycogen (1200 kcal, or 5000 kJ). In fact, about three-fourths of all the glycogen in the body is stored in muscle. This glycogen is readily converted into glucose 6-phosphate for use within muscle cells. Muscle, like the brain, lacks glucose 6-phosphatase, and so it does not export glucose. Rather, muscle retains glucose, its preferred fuel for bursts of activity.

Unlike skeletal muscle, heart muscle functions almost exclusively aerobically, as evidenced by the density of mitochondria in heart muscle. Moreover, the heart has virtually no glycogen reserves. Fatty acids are the heart's main source of fuel, although ketone bodies as well as lactate can serve as fuel for heart muscle.

Adipose tissue

The triacylglycerols stored in adipose tissue are an enormous reservoir of metabolic fuel. In a typical 70-kg man, the 15 kg of triacylglycerols have an energy content of 135,000 kcal (565,000 kJ).

In human beings, the liver is the major site of fatty acid synthesis.

The kidney
The major purpose of the kidney is to produce urine, which serves as a vehicle for excreting metabolic waste products and for maintaining the osmolarity of the body fluids. The blood plasma is filtered nearly 60 times each day in the renal tubules. Most of the material filtered out of the blood is reabsorbed; so only 1 to 2 liters of urine is produced. Water-soluble materials in the plasma, such as glucose, and water itself are reabsorbed to prevent wasteful loss. The kidneys require large amounts of energy to accomplish the reabsorption. Although constituting only 0.5% of body mass, kidneys consume 10% of the oxygen used in cellular respiration.

The metabolic activities of the liver are essential for providing fuel to the brain, muscle, and other peripheral organs. Indeed, the liver, which can be from 2% to 4% of body weight, is an organism's metabolic hub. Most compounds absorbed by the intestine first pass through the liver, which is thus able to regulate the level of many metabolites in the blood.

The liver can produce glucose for release into the blood by breaking down its store of glycogen and by carrying out gluconeogenesis.

John Ernest Walker is a British chemisty who shared the Nobel Prize for Chemisty in 1997 for his pioneering work on how the enzyme ATP synthase catalyses the formation of the "high-energy" compound adenosine triphosphate (ATP). These molecules of ATP function as a carrier of energy in all living organisms, whether simple bacteria, fungus or plant life, or higher animals and humans. ATP takes in the chemical energy released when nutrients are metabolized, and carries that energy to the various reactions that require energy. Such reactions include cell-building, the contraction of muscle fibers, or nerve signals.