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Lipids as an Energy Reserve:

Nearly all of the energy needed by the human body is provided by theoxidation of carbohydrates and lipids. Whereas carbohydrates provide areadily available source of energy, lipids function primarily as an energyreserve. The amount of lipids stored as an energy reserve far exceeds theenergy stored as glycogen since the human body is simply not capable of storing as much glycogen compared to lipids. Lipids yield 9 kcal of energy

per gram while carbohydrates and proteins yield only 4 kcal of energy pergram.It is interesting to compare the relative amounts of energy provided byvarious biochemicals in a typical 154 lb male. The free glucose in the bloodprovides only a 40 kcal energy reserve -- only enough to maintain bodyfunctions for a few minutes. Glycogen remaining stored in the liver andmuscles after an overnight fast, amounts to about 600 kcal energy. Glycogenreserves can maintain body functions for about one day without new inputsof food. Protein (mostly in muscle) contains a substantial energy reserve of about 25,000 kcal.Finally, lipid reserves containing 100,000 kcal of energy can maintain human

body functions without food for 30-40 days with sufficient water. Lipids orfats represent about 24 pounds of the body weight in a 154 pound male.Lipids provide the sole source of energy in hibernating animals and migratingbirds. Fortunately, lipids are more compact and contain more energy pergram than glycogen, otherwise body weight would increase approximately110 pounds if glycogen were to replace fat as the energy reserve.

Functions of Lipids:

Lipids or fats are stored in cells throughout the body principle in special kindsof connective tissue called adipose tissue or depot fat. Whereas many cells

contain phospholipids in the bilayer cell membranes, adipose tissue cellsconsist of fat globules of triglycerides which may occupy as much as 90% of the cell volume.In addition to energy storage, depot fat provides a number of otherfunctions. Fat serves as a protective cushion and provides structural supportto help prevent injury to vital organs such as the heart, liver, kidneys, andspleen. Fat insulates the body from heat loss and extreme temperaturechanges. At the same time, fat deposits under the skin may be metabolizedto generate heat in response to lower skin temperatures.

Lipids in the Blood:

Lipids ingested as food are digested in the small intestine where bile saltsare used to emulsify them and pancreatic lipase hydrolyzes lipids into fattyacids, glycerol, soaps, or mono- and diglycerides. There is still some disputeabout the lipid form that passes through the intestinal wall -- whether asfatty acids or as glycerides. In either case, triglycerides are found in thelymph system and the blood.Since lipids are not soluble in blood, they are transported as lipoproteinsafter reaction with water-soluble proteins in the blood. Fatty acids aregenerally transported in this form as well.

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 There is always a relatively constant supply of lipids in the blood, although of course, the concentration increases immediately following a meal. Lipids inthe blood are absorbed by liver cells to provide energy for cellular functions. The liver is responsible for providing the proper concentrations of lipids inthe blood. Some lipids are utilized by brain cells to synthesize brain andnerve tissue.Excess lipids in the blood are eventually converted into adipose tissue. If lipidlevels in the blood become too low, the body synthesizes lipids from other

foods, such as carbohydrates, or removes lipids from storage. The body alsoexcretes some lipids in the form of fats, soaps, or fatty acids as a normalcomponent of feces.Abnormally high levels of triglycerides and cholesterol are thought to beinvolved in hardening of the arteries. Lipids may be deposited on the walls of arteries as a partial consequence of their insolubility in the blood.

Muscle is a contractile tissue . Muscle cells contain contractile filaments that move past each other and

change the size of the cell. They are classified as skeletal, cardiac, or  smooth muscles. Their function is

to produce force and cause motion. Muscles provide strength, balance, posture, movement and heat for 

the body to keep warm. Muscles can cause either locomotion of the organism itself or movement

of internal organs. Cardiac and smooth muscle contraction occurs without conscious thought and is

necessary for survival. Examples are the contraction of the heart andperistalsis which pushes food

through the digestive system. Voluntary contraction of the skeletal muscles is used to move the body and

can be finely controlled. Examples are movements of the eye, or gross movements like the quadriceps

muscle of thethigh. There are two broad types of voluntary muscle fibers: slow twitch and fast twitch. Slow

twitch fibers contract for long periods of time but with little force while fast twitch fibers contract quickly

and powerfully but fatigue very rapidly.

Upon stimulation by an action potential, skeletal muscles perform a coordinated contraction by shortening

each sarcomere. The best proposed model for understanding contraction is the sliding filament model of 

muscle contraction. Actin and myosin fibers overlap in a contractile motion towards each other. Myosin

filaments have club-shaped heads that project toward the actin filaments.

Larger structures along the myosin filament called myosin heads are used to provide attachment points

on binding sites for the actin filaments. The myosin heads move in a coordinated style, they swivel toward

the center of the sarcomere, detach and then reattach to the nearest active site of the actin filament. This

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is called a rachet type drive system. This process consumes large amounts of adenosine

triphosphate (ATP).

Energy for this comes from ATP, the energy source of the cell. ATP binds to the cross bridges between

myosin heads and actin filaments. The release of energy powers the swiveling of the myosin head.

Muscles store little ATP and so must continuously recycle the discharged adenosine

diphosphate molecule (ADP) into ATP rapidly. Muscle tissue also contains a stored supply of a fast acting

recharge chemical, creatine phosphate which can assist initially producing the rapid regeneration of ADP

into ATP.

Calcium ions are required for each cycle of the sarcomere. Calcium is released from the sarcoplasmic

reticulum into the sarcomere when a muscle is stimulated to contract. This calcium uncovers the actin

binding sites. When the muscle no longer needs to contract, the calcium ions are pumped from the

sarcomere and back into storage in the sarcoplasmic reticulum.

Muscles are predominately powered by the oxidation of fats and carbohydrates, but anaerobic chemical

reactions are also used, particularly by fast twitch fibers. These chemical reactions produce adenosine

triphosphate (ATP) molecules which are used to power the movement of the myosin heads.

There are three types of muscle:

Skeletal muscle or "voluntary muscle" is anchored by tendons (or by aponeuroses at a few

places) to bone and is used to effect skeletal movement such as locomotion and in maintaining

posture. Though this postural control is generally maintained as a subconscious reflex, the muscles

responsible react to conscious control like non-postural muscles. An average adult male is made up

of 42% of skeletal muscle and an average adult female is made up of 36% (as a percentage of body

mass).[3]

Smooth muscle or "involuntary muscle" is found within the walls of organs and structures such as

the esophagus,stomach, intestines, bronchi, uterus, urethra, bladder , blood vessels, and the arrector 

pili in the skin (in which it controls erection of body hair). Unlike skeletal muscle, smooth muscle is not

under conscious control.

Cardiac muscle is also an "involuntary muscle" but is more akin in structure to skeletal muscle,

and is found only in the heart.

Cardiac and skeletal muscles are "striated" in that they contain sarcomeres and are packed into highly

regular arrangements of bundles; smooth muscle has neither. While skeletal muscles are arranged in

regular, parallel bundles, cardiac muscle connects at branching, irregular angles (called intercalated

discs). Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains

longer or even near-permanent contractions.

Skeletal muscle is further divided into several subtypes:

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Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich

in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry

more oxygen and sustain aerobic activity.

Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile

speed:[4]

Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and

appears red.

Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin.

This is the fastest muscle type in humans. It can contract more quickly and with a greater amount

of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before

muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid). N.B.

in some books and articles this muscle in humans was, confusingly, called type IIB.[5]

Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in

mitochondria and myoglobin. In small animals like rodents this is the major fast muscle type,

explaining the pale color of their flesh.

Muscular activity accounts for much of the body's energy consumption. All muscle cells produce

adenosine triphosphate (ATP) molecules which are used to power the movement of the myosin heads.

Muscles conserve energy in the form of creatine phosphate which is generated from ATP and can

regenerate ATP when needed with creatine kinase. Muscles also keep a storage form of glucose in the

form of glycogen. Glycogen can be rapidly converted to glucose when energy is required for sustained,

powerful contractions. Within the voluntary skeletal muscles, the glucose molecule can be metabolized

anaerobically in a process called glycolysis which produces two ATP and two lactic acid molecules in the

process (note that in aerobic conditions, lactate is not formed; instead pyruvate is formed and transmitted

through the citric acid cycle). Muscle cells also contain globules of fat, which are used for energy

during aerobic exercise. The aerobic energy systems take longer to produce the ATP and reach peak

efficiency, and requires many more biochemical steps, but produces significantly more ATP than

anaerobic glycolysis. Cardiac muscle on the other hand, can readily consume any of the three

macronutrients (protein, glucose and fat) aerobically without a 'warm up' period and always extracts the

maximum ATP yield from any molecule involved. The heart, liver and red blood cells will also consume

lactic acid produced and excreted by skeletal muscles during exercise.

The efficiency of human muscle has been measured (in the context of rowing and cycling) at 18%

to 26%. The efficiency is defined as the ratio of mechanical work output to the totalmetabolic cost, as can

be calculated from oxygen consumption. This low efficiency is the result of about 40% efficiency of 

generating ATP from food energy, losses in converting energy from ATP into mechanical work inside the

muscle, and mechanical losses inside the body. The latter two losses are dependent on the type of 

exercise and the type of muscle fibers being used (fast-twitch or slow-twitch). For an overal efficiency of 

20 percent, one watt of mechanical power is equivalent to 4.3 kcal per hour. For example, a manufacturer 

of rowing equipment shows burned calories as four times the actual mechanical work, plus 300 kcal per 

hour,[16] which amounts to about 20 percent efficiency at 250 watts of mechanical output. The mechanical

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energy output of a cyclic contraction can depend upon many factors, including activation timing, muscle

strain trajectory, and rates of force rise & decay. These can be synthesized experimentally using work

loop analysis.

Exercise is often recommended as a means of improving motor skills, fitness, muscle and bone

strength, and joint function. Exercise has several effects upon muscles, connective tissue, bone, and the

nerves that stimulate the muscles. One such effect is muscle hypertrophy, an increase in size. This is

used in bodybuilding.

Various exercises require a predominance of certain muscle fiber utilization over another. Aerobic

exercise involves long, low levels of exertion in which the muscles are used at well below their maximal

contraction strength for long periods of time (the most classic example being the marathon). Aerobic

events, which rely primarily on the aerobic (with oxygen) system, use a higher percentage of Type I (or 

slow-twitch) muscle fibers, consume a mixture of fat, protein and carbohydrates for energy, consume

large amounts of oxygen and produce little lactic acid. Anaerobic exercise involves short bursts of higher 

intensity contractions at a much greater percentage of their maximum contraction strength. Examples of 

anaerobic exercise include sprinting and weight lifting. The anaerobic energy delivery system usespredominantly Type II or fast-twitch muscle fibers, relies mainly on ATP or glucose for fuel, consumes

relatively little oxygen, protein and fat, produces large amounts of lactic acid and can not be sustained for 

as long a period as aerobic exercise. The presence of lactic acid has an inhibitory effect on ATP

generation within the muscle; though not producing fatigue, it can inhibit or even stop performance if the

intracellular concentration becomes too high. However, long-term training

causes neovascularization within the muscle, increasing the ability to move waste products out of the

muscles and maintain contraction. Once moved out of muscles with high concentrations within the

sarcomere, lactic acid can be used by other muscles or body tissues as a source of energy, or 

transported to the liver where it is converted back topyruvate. In addition to increasing the level of lactic

acid, strenuous exercise causes the loss of potassium ions in muscle and causing an increase in

potassium ion concentrations close to the muscle fibres, in the interstitium. Acidification by lactic acid may

allow recovery of force so that acidosis may protect against fatigue rather than being a cause of fatigue. [6]

Humans are genetically predisposed with a larger percentage of one type of muscle group over another.

An individual born with a greater percentage of Type I muscle fibers would theoretically be more suited to

endurance events, such as triathlons, distance running, and long cycling events, whereas a human born

with a greater percentage of Type II muscle fibers would be more likely to excel at anaerobic events such

as a 200 meter dash, or weightlifting.[citation needed ]

Delayed onset muscle soreness is pain or discomfort that may be felt one to three days after exercising

and subsides generally within two to three days later. Once thought to be caused by lactic acid buildup, a

more recent theory is that it is caused by tiny tears in the muscle fibers caused by eccentric contraction,

or unaccustomed training levels. Since lactic acid disperses fairly rapidly, it could not explain pain

experienced days after exercise.[7]

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Muscular, spinal and neural factors all affect muscle building. Sometimes a person may notice an

increase in strength in a given muscle even though only its opposite has been subject to exercise, such

as when a bodybuilder finds her left biceps stronger after completing a regimen focusing only on the right

biceps. This phenomenon is called cross education.

Anaerobic respiration for humans:

In general in humans it is the muscle tissue that respires anaerobically normally

during exercise, at which time the body cannot intake the required oxygen for

the cells to respire. This clearly indicates that enough energy is not made and

the muscles require more energy. So they achieve it in the absence of oxygen.

But when they have stopped exercising, commonly an oxygen debt has been

created, because of the large amount of lactic acid inside the muscles. It is

because of this fact the humans breath heavily after exercising to negate the

oxygen debt.

Respiration is one of the vital functionality of the body which are of critical

significance for all living organisms let it be humans or the microscopic bacteria.Generally the respiration processserves two fundamental purposes in living

organisms. The first is the removal of the electrons generated during catabolism

and the second is generation of ATP. This respiration mechanism is situated in

cell membranes of prokaryotes whereas it is located in the inner membranes of 

mitochondria for eukaryotes. The respiration needs a terminal electron acceptor.

Quite simply, the respiration process, that uses oxygen as the terminal electron

acceptor is called as aerobic respiration and the one that makes use of terminal

electron acceptors other than oxygen is termed as anaerobic respiration.

Beginning from the bio-chemical pathway used to utilize bio-molecules, to the

quantity of energy released in the respiration process, there are a lot of 

differences in aerobic and anaerobic respiration. The fundamental difference in

the between the two types of respiration is that aerobic respiration requires the

presence of oxygen. Also the process of anaerobic respiration is comparatively

less energy generating as compared to the aerobic respiration process. In the

process of alcoholic fermentation or the anaerobic respiration 2 molecules of 

ATP (energy) are produced for the each and every molecule of glucose used in

the reaction. In the same way for the lactate fermentation two molecules of ATPare produced for every molecule of glucose used. Thus in anaerobic respiration

the process breaks down one molecule of glucose to get two units of energy

storing ATP molecules.

 The anaerobic respiration can be defined as release of energy from a foodstuff 

in the form cells could utilize in the absence of oxygen. It is different from

aerobic respiration in the sense that it doesn’t need oxygen. The word equation

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for anaerobic respiration in humans is as follows:

Glucose -> Lactic acid

Glycolysis (it literally means glucose degradation) is a metabolic pathway which

changes glucose C6H12O6 into pyruvate CH3COCOO + H. The free energy

which is generated in this process is utilized to make high energy compounds

namely ATP (adenosine triphophate) and NADH (reduced nicotinamide adenine

dinucleotide).

It is a confirmed process of ten reactions with ten intermediate compounds (one

step has 2 intermediates). The intermediate steps supply the point of entry for

glycolysis. Now let’s consider one example, a lot of monosaccharide like

fructose, glucose and galactose, could be turned in to one of the intermediates.

 The intermediate in itself may directly be of use. Consider an example the

intermediate dihydroxyacetone phosphate; it is a source of glycerol which gets

together with fatty acids to create fat.

Glycolysis is recognized as the archetype of a universal metabolic pathway. This

process happens with some degree of variation in all the organisms both aerobic

and anaerobic. The frequent occurrence of glycolysis shows that it is one of the

older known metabolic pathways. The most commonly occurring glycolysis is the

Embden-Meyerhof pathway which was found out by Gustav Embden and Otto

Meyerhof.

Anaerobic Respiration:

One way of doing this is to just get the pyruvate to do oxidation; in this

procedure the pyruvate gets converted in to lactate (this is the conjugate baseof lactic acid) in a process which is called lactic acid fermentation. This process

can be represented in a word equation as:

pyruvate + NADH + H -> lactate + NADAnaerobic respiration in humans takes place when muscle undergoes extreme contraction as invigorous exercise.Anaerobic respiration takes place in 2 places 1. the muscle cell and 2. large intestineof undigested food, from the small intestine producing minerals and vitamins.When we exercise really vigorously, the muscles are trying to break down glucose faster than theheart and lungs can supply oxygen. So the cells can only get as far as breaking down the glucose intopyruvate, then convert that into lactate (also called lactic acid).

The process is called lactic acid fermentation. The muscle cells obtain very little energy from it

(only two ATP molecules are synthesized for each glucose molecule respired) but it enables themuscles to carry on exercising longer than otherwise.

 This reaction happens in the bacteria which are involved in making yogurt (lactic

acid makes the milk to curdle). This reaction also happens in animals which are

under hypoxic (or partially anaerobic) conditions, found for example in overused

muscles which are lacking oxygen, or in infracted heart muscle cells. In most

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tissues for cells this is the final resort for energy; most of the animal tissues

cannot maintain the anaerobic respiration over an extended period of time.

Some organisms like yeast turn NADH to NAD in a reaction called as ethanol

fermentation. In the reaction the pyruvate is turned first into acetaldehyde and

CO2, after this into ethanol.

 The lactic acid fermentation and ethanol fermentation can happen in the lack of 

oxygen presence. The anaerobic fermentation lets a lot of single celledorganisms to use glycolysis as their only source of energy. From the two

examples above regarding the fermentation, NADH is oxidized by sending 2

electrons to pyruvate. But anaerobic bacteria use a big range of compounds as

the terminal electron acceptors in the process of cellular respiration.

Remember oxygen is not an essential for the glycolysis to occur. In many

organisms such as C. tetani (this causes tetanus) or C. Perfringence (this causes

gangrine) called as obligate anaerobes, the oxygen presence will be lethal. In

the organisms that use glycolysis, absence of oxygen stops pyruvate from being

metabolized to CO2 and H2O through the citric acid cycle and the electron

transport chain (which relies on oxygen) doesn’t work. Fermentation will not

generate energy more that already generated from glycolysis (2 ATP’s) but

serves to re obtain NAD so the glycolysis can go on. There are useful end

products created such as lactate or ethanol.

Cellular RespirationAll living things in the world including plants require energy in order to function.

 This energy is obtained from the food we eat. The cells break down the energy

stored in the food through a unique system known as cellular respiration. In

plain language, cellular respiration means the procedure through which the food

is broken down by the cells of living beings in order to produce the energy which

is in the form of ATP molecules (also known as the Adenosine Tri Phosphate

molecules). Plants use a part of this ATP energy during photosynthesis for

producing sugar. The sugars are then broken down during cellular respiration.

 This cycle is continued again and again as long as the plant lives.

In cellular respiration there are three stages, which are known as (a) Glycolysis(b) Krebs Cycle and (c) Electron Transport Chain (Etc). Every cell of all living

creatures, which includes plants, carries cellular respiration, as it is very

essential for life. However, there is no fixed time or point in the timing of the

respiration. Cells, which are neighbors, also involve itself in cellular respiration

at different stages. The reaction that occurs during cellular respiration produces

the energy. During this process, the polymers are broken down in more small

and manageable pieces. The carbohydrate that is obtained during the

respiration is disassembled into molecules of glucose, which in turn is used to

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produce ATP molecules, which are rich in energy. In plain terms, we can say that

during cellular respiration, one glucose molecules and six oxygen molecules will

produce six carbon dioxide and six water molecules and in between 36 to 38

ATP molecules.

 There are three stages of cellular respiration, which are as follows:

• (a) Glycolysis

 This process breaks down the molecules of glucose from thecarbohydrates and converts it into pyruvate. The procedure occurs in the

cytosol, inside the cell and can carry its work without the requirement of 

oxygen. At the first stage of glycolysis, the phosphate is drawn from the

ATP and added to the molecule of the glucose, which makes the molecule

to become chemically reactive. This reaction changes the molecule into

isomer and fructose.

• (b) Krebs cycle

By Krebs cycle we mean a series of procedures, which gets catalyzed by

the enzymes, and oxidize the molecule of Acetyl-coA. It is actually an

aerobic procedure, which actually means that it requires oxygen for

functioning. Krebs cycle must complete two complete turns for producing

4 molecules of carbon dioxide, 6 molecules of NADH, 2 molecules of ATP

and 2 molecules of FADH2 which is an energy giving molecule.

• (c) Electron Transport Chain

During the process of glycolysis and Krebs cycle, very little energy is

produced. The energy that remains inside the original molecule of glucose

gets released through the electron transport chain. This chain is actually a

widespread network of electron carrying proteins, which are found inside

the inner membrane of the mitochondrion. The work of these proteins is

to transfer the electrons from one to another and finally adds itself with

the protons to the oxygen, which is known as the final electron acceptor.

 Though water is produced during this procedure, no ATP is produced. ATP

is produced later through a proton. Thus the work of the electron

transport chain is only to produce an ingredient from which ATP can be

produced.We should remember that cellular respiration could occur only if oxygen is

available. There are some organisms that live in anaerobic conditions. In such

cases, full cellular respiration is not possible for those organisms that are living

in anaerobic conditions. Glycolysis is the one and only cellular respiration

process for such type of organisms.

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 Thus we can safely say that all organisms use the sugar, which is available in

their food to turn it into energy in order to be able to live and perform the

necessary actions that are made by all living creatures.

In the aerobic respiration (with the use of oxygen) the glucose molecules are

broken totally generating all of the useful energy and producing CO2 and H2O

as waste products. The word equation for aerobic respiration shows:

Glucose + oxygen -> carbon dioxide + water + energyHowever in the anaerobic respiration the glucose molecules are only partly

broken so only a part of energy is released and instead of CO2 and H2O, the by-

products are either CO2 and ethanol or lactic acid. The equation for this is:

Glucose -> ethanol + carbon dioxide + energy

Glucose -> lactic acid + energy

 These symbol equations are represented as:

C6H12O6 -> 2CO2 + 2CH3-CH2-OH (ethanol)

C6H12O6 -> 2C3H6O3 (lactic acid)

So as in aerobic respiration one molecule of glucose can generate 38 molecules

of ATP, in anaerobic respiration about 2 molecules of ATP are released per one

molecule of glucose.

A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester  derived from glycerol and

three fatty acids.[1] There are many triglycerides, depending on the oil source, some are highly

unsaturated, some less so.

Saturated compounds are "saturated" with hydrogen. Unsaturated compounds have double bonds (C=C)

between carbon atoms. Saturated compounds have single bonds (C-C) between the carbon atoms, and

the other bond is bound to hydrogen atoms (for example =CH-CH=, -CH2-CH2-, etc.).

Unsaturated fats have a lower melting point and are more likely to be liquid. Saturated fats have a higher 

melting point and are more likely to be solid.

Triglycerides are the main constituents of vegetable oil (typically more unsaturated) and animal

fats (typically more saturated).[2] In humans, triglycerides are a mechanism for storing unused calories,

and their high concentrations in blood correlates with the consumption of starchy and fatty foods.

Triglycerides, as major components of very-low-density lipoprotein (VLDL) and chylomicrons, play an

important role in metabolism as energy sources and transporters of dietary fat. They contain more than

twice as much energy (9 kcal/g or 38 kJ/g ) as carbohydrates and proteins. In the intestine, triglycerides

are split into monoacylglycerol and free fatty acids in a process called lipolysis, with the secretion

of lipases and bile, which are subsequently moved to absorptive enterocytes, cells lining the intestines.

The triglycerides are rebuilt in the enterocytes from their fragments and packaged together 

with cholesterol and proteins to form chylomicrons. These are excreted from the cells and collected by the

lymph system and transported to the large vessels near the heart before being mixed into the blood.

Various tissues can capture the chylomicrons, releasing the triglycerides to be used as a source of 

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energy. Fat and liver cells can synthesize and store triglycerides. When the body requires fatty acids as

an energy source, the hormone glucagon signals the breakdown of the triglycerides by hormone-

sensitive lipase to release free fatty acids. As the brain cannot utilize fatty acids as an energy source

(unless converted to a ketone), the glycerol component of triglycerides can be converted into glucose, 

via glycolysis by conversion into Dihydroxyacetone phosphate and then into Glyceraldehyde 3-

phosphate, for brain fuel when it is broken down. Fat cells may also be broken down for that reason, if the

brain's needs ever outweigh the body's.

Triglycerides cannot pass through cell membranes freely. Special enzymes on the walls of blood vessels

called lipoprotein lipases must break down triglycerides into free fatty acids and glycerol. Fatty acids can

then be taken up by cells via the fatty acid transporter (FAT).

Cholesterol is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce

hormones and cell membranes and is transported in the blood plasma of all mammals.[2] It is an essential

structural component of mammalian cell membranes and is required to establish proper membrane

permeability and fluidity. In addition, cholesterol is an important component for the manufacture of bile

acids,steroid hormones, and vitamin D. Cholesterol is the principal sterol synthesized by animals;

however, small quantities can be synthesized in other  eukaryotes such as plants and fungi. It is almost

completely absent among prokaryotes including bacteria.[3] Although cholesterol is important and

necessary for mammals, high levels of cholesterol in the blood have been linked to damage to arteries

and are potentially linked to diseases such as those associated with the cardiovascular system (heart

disease).[4]

The name cholesterol originates from the Greek chole- (bile) and stereos (solid), and the chemical suffix -

ol for an alcohol. François Poulletier de la Salle first identified cholesterol in solid form in gallstones, in

1769. However, it was only in 1815 that chemist Eugène Chevreul named the compound "cholesterine".[

Since cholesterol is essential for all animal life, it is primarily synthesized from simpler substances within

the body. However, high levels in blood circulation, depending on how it is transported within lipoproteins, 

are strongly associated with progression of atherosclerosis. For a person of about 68 kg (150 pounds),

typical total body cholesterol synthesis is about 1 g (1,000 mg) per day, and total body content is about 35

g. Typical daily additional dietary intake in the United States is 200–300 mg.[citation needed ] The body

compensates for cholesterol intake by reducing the amount synthesized.

Cholesterol is recycled. It is excreted by the liver via the bile into the digestive tract. Typically about 50%

of the excreted cholesterol is reabsorbed by the small bowel back into the bloodstream. Phytosterols can

compete with cholesterol reabsorption in the intestinal tract, thus reducing cholesterol reabsorption.[

Animal fats are complex mixtures of triglycerides, with lesser amounts of  phospholipids and cholesterol.

As a consequence, all foods containing animal fat contain cholesterol to varying extents.[13] Major dietary

sources of cholesterol include cheese, egg yolks, beef , pork, poultry, fish, andshrimp.[14]

Human breast milk also contains significant quantities of cholesterol.[15]

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Eukaryotes have areas inside the cell separated off from the rest of the cell by membranes, likethe cell membrane (see below). These areas include the nucleus, numerous mitochondria and

other organelles such as the golgi body, and or chloroplasts within each of their cells. These

areas are made distinct from the main mass of the cells cytoplasm by their own membrane in

order to allow them to be more specialised. You can think of them as separate rooms within your house. The nucleus contains all the cell's DNA, the Mitochondria are where energy is generated,

chloroplasts are where plants trap the suns energy in photosynthesis. There are exceptions to

every rule of course, and in this case the most obvious two are the red blood cells of animals and

the sieve tube elements of plants, which, though living, have no nucleus and no DNA, normallythese cells to do not live very long.