Everything about Metabolic totally explained
Metabolism is the set of
chemical reactions that occur in living
organisms in order to maintain
life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories.
Catabolism breaks down large molecules, for example to harvest energy in
cellular respiration.
Anabolism, on the other hand, uses energy to construct components of cells such as
proteins and
nucleic acids.
The chemical reactions of metabolism are organized into
metabolic pathways, in which one chemical is transformed into another by a sequence of
enzymes. Enzymes are crucial to metabolism because they allow organisms to drive desirable but
thermodynamically unfavorable reactions by
coupling them to favorable ones. Enzymes also allow the
regulation of metabolic pathways in response to changes in the
cell's environment or
signals from other cells.
The metabolism of an organism determines which substances it'll find
nutritious and which it'll find
poisonous. For example, some
prokaryotes use
hydrogen sulfide as a nutrient, yet this gas is poisonous to animals. The speed of metabolism, the metabolic rate, also influences how much food an organism will require.
A striking feature of metabolism is the similarity of the basic metabolic pathways between even vastly different species. For example, the set of
carboxylic acids that are best known as the intermediates in the
citric acid cycle are present in all organisms, being found in species as diverse as the
unicellular bacteria Escherichia coli and huge
multicellular organisms like
elephants. These striking similarities in metabolism are most likely the result of the high efficiency of these pathways, and of their early appearance in evolutionary history.
Key biochemicals
Most of the structures that make up animals, plants and microbes are made from three basic classes of
molecule:
amino acids,
carbohydrates and
lipids (often called
fats). As these molecules are vital for life, metabolism focuses on making these molecules, in the construction of cells and tissues, or breaking them down and using them as a source of energy, in the digestion and use of food. Many important biochemicals can be joined together to make
polymers such as
DNA and
proteins. These
macromolecules are essential parts of all living organisms. Some of the most common biological polymers are listed in the table below.
Amino acids and proteins
Proteins are made of
amino acids arranged in a linear chain and joined together by
peptide bonds. Many proteins are the
enzymes that
catalyze the chemical reactions in metabolism. Other proteins have structural or mechanical functions, such as the proteins that form the
cytoskeleton, a system of
scaffolding that maintains the cell shape. Proteins are also important in
cell signaling,
immune responses,
cell adhesion,
active transport across membranes and the
cell cycle.
Lipids
Lipids are the most diverse group of biochemicals. Their main structural uses are as part of
biological membranes such as the
cell membrane, or as a source of energy. The
fats are a large group of compounds that contain
fatty acids and
glycerol; a glycerol molecule attached to three fatty acid
esters is a
triacylglyceride. Several variations on this basic structure exist, including alternate backbones such as
sphingosine in the
sphingolipids, and
hydrophilic groups such as
phosphate in
phospholipids.
Steroids such as
cholesterol are another major class of lipids that are made in cells.
Carbohydrates
Carbohydrates are straight-chain
aldehydes or
ketones with many
hydroxyl groups that can exist as straight chains or rings. Carbohydrates are the most abundant biological molecules, and fill numerous roles, such as the storage and transport of
energy (
starch,
glycogen) and structural components (
cellulose in plants,
chitin in animals).
Nucleotides
The polymers
DNA and
RNA are long chains of
nucleotides. These molecules are critical for the storage and use of genetic information, through the processes of
transcription and
protein biosynthesis. RNA in
ribozymes such as
spliceosomes and
ribosomes is similar to enzymes as it can catalyze chemical reactions. Individual
nucleosides are made by attaching a
nucleobase to a
ribose sugar. These bases are
heterocyclic rings containing nitrogen, classified as
purines or
pyrimidines. Nucleotides also act as coenzymes in metabolic group transfer reactions.
Coenzymes
Metabolism involves a vast array of chemical reactions, but most fall under a few basic types of reactions that involve the transfer of
functional groups. This common chemistry allows cells to use a small set of metabolic intermediates to carry chemical groups between different reactions.
One central coenzyme is
adenosine triphosphate (ATP), the universal energy currency of cells. This
nucleotide is used to transfer chemical energy between different chemical reactions. There is only a small amount of ATP in cells, but as it's continuously regenerated, the human body can use about its own weight in ATP per day.
Nicotinamide adenine dinucleotide (NADH), a derivative of vitamin B
3 (
niacin), is an important coenzyme that acts as a hydrogen acceptor. Hundreds of separate types of
dehydrogenases remove electrons from their substrates and
reduce NAD
+ into NADH. This reduced form of the coenzyme is then a substrate for any of the
reductases in the cell that need to reduce their substrates. Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD
+/NADH form is more important in catabolic reactions, while NADP
+/NADPH is used in anabolic reactions.
Minerals and cofactors
Inorganic elements play critical roles in metabolism; some are abundant (for example
sodium and
potassium) while others function at minute concentrations. About 99% of mammals' mass are the elements
carbon,
nitrogen,
calcium,
sodium,
chlorine,
potassium,
hydrogen,
phosphorus,
oxygen and
sulfur. The
organic compounds (proteins, lipids and carbohydrates) contain the majority of the carbon and nitrogen and most of the oxygen and hydrogen is present as water. Ions are also critical for
nerves and
muscles, as
action potentials in these tissues are produced by the exchange of electrolytes between the
extracellular fluid and the
cytosol. Electrolytes enter and leave cells through proteins in the cell membrane called
ion channels. For example,
muscle contraction depends upon the movement of calcium, sodium and potassium through ion channels in the cell membrane and
T-tubules.
The
transition metals are usually present as
trace elements in organisms, with
zinc and
iron being most abundant. These metals are used in some proteins as
cofactors and are essential for the activity of enzymes such as
catalase and oxygen-carrier proteins such as
hemoglobin. These cofactors are bound tightly to a specific protein; although enzyme cofactors can be modified during catalysis, cofactors always return to their original state after catalysis has taken place. The metal micronutrients are taken up into organisms by specific transporters and bound to storage proteins such as
ferritin or
metallothionein when not being used.
Catabolism
Catabolism is the set of metabolic processes that break down large molecules. These include breaking down and oxidising food molecules. The purpose of the catabolic reactions is to provide the energy and components needed by anabolic reactions. The exact nature of these catabolic reactions differ from organism to organism, with organic molecules being used as a source of energy in
organotrophs, while
lithotrophs use inorganic substrates and
phototrophs capture
sunlight as
chemical energy. However, all these different forms of metabolism depend on
redox reactions that involve the transfer of electrons from reduced donor molecules such as
organic molecules,
water,
ammonia,
hydrogen sulfide or
ferrous ions to acceptor molecules such as
oxygen,
nitrate or
sulfate. In animals these reactions involve complex
organic molecules being broken down to simpler molecules, such as
carbon dioxide and water. In
photosynthetic organisms such as plants and
cyanobacteria, these electron-transfer reactions don't release energy, but are used as a way of storing energy absorbed from sunlight.
The most common set of catabolic reactions in animals can be separated into three main stages. In the first, large organic molecules such as
proteins,
polysaccharides or
lipids are digested into their smaller components outside cells. Next, these smaller molecules are taken up by cells and converted to yet smaller molecules, usually
acetyl coenzyme A (CoA), which releases some energy. Finally, the acetyl group on the CoA is oxidised to water and carbon dioxide in the
citric acid cycle and
electron transport chain, releasing the energy that's stored by reducing the coenzyme
nicotinamide adenine dinucleotide (NAD
+) into NADH.
Digestion
Macromolecules such as starch, cellulose or proteins can't be rapidly taken up by cells and need to be broken into their smaller units before they can be used in cell metabolism. Several common classes of enzymes digest these polymers. These digestive enzymes include
proteases that digest proteins into amino acids, as well as
glycoside hydrolases that digest polysaccharides into monosaccharides.
Microbes simply secrete digestive enzymes into their surroundings, while animals only secrete these enzymes from specialized cells in their
guts. The amino acids or sugars released by these extracellular enzymes are then pumped into cells by specific
active transport proteins.
Energy from organic compounds
Carbohydrate catabolism is the breakdown of carbohydrates into smaller units. Carbohydrates are usually taken into cells once they've been digested into
monosaccharides. Once inside, the major route of breakdown is
glycolysis, where sugars such as
glucose and
fructose are converted into
pyruvate and some ATP is generated. Pyruvate is an intermediate in several metabolic pathways, but the majority is converted to
acetyl-CoA and fed into the
citric acid cycle. Although some more ATP is generated in the citric acid cycle, the most important product is NADH, which is made from NAD
+ as the acetyl-CoA is oxidized. This oxidation releases
carbon dioxide as a waste product. In anaerobic conditions, glycolysis produces
lactate, through the enzyme
lactate dehydrogenase re-oxidizing NADH to NAD+ for re-use in glycolysis. An alternative route for glucose breakdown is the
pentose phosphate pathway, which reduces the coenzyme
NADPH and produces
pentose sugars such as
ribose, the sugar component of
nucleic acids.
Fats are catabolised by
hydrolysis to free fatty acids and glycerol. The glycerol enters glycolysis and the fatty acids are broken down by
beta oxidation to release acetyl-CoA, which then is fed into the citric acid cycle. Fatty acids release more energy upon oxidation than carbohydrates because carbohydrates contain more oxygen in their structures.
Amino acids are either used to synthesize proteins and other biomolecules, or oxidized to
urea and carbon dioxide as a source of energy. The oxidation pathway starts with the removal of the amino group by a
transaminase. The amino group is fed into the
urea cycle, leaving a deaminated carbon skeleton in the form of a
keto acid. Several of these keto acids are intermediates in the citric acid cycle, for example the deamination of
glutamate forms α-
ketoglutarate. The
glucogenic amino acids can also be converted into glucose, through
gluconeogenesis (discussed below).
Energy transformations
Oxidative phosphorylation
eukaryotes by a series of proteins in the membranes of mitochondria called the
electron transport chain. In
prokaryotes, these proteins are found in the cell's
inner membrane. These proteins use the energy released from passing electrons from
reduced molecules like NADH onto
oxygen to pump
protons across a membrane.
Pumping protons out of the mitochondria creates a proton
concentration difference across the membrane and generates an
electrochemical gradient. This force drives protons back into the mitochondrion through the base of an enzyme called
ATP synthase. The flow of protons makes the stalk subunit rotate, causing the
active site of the synthase domain to change shape and phosphorylate
adenosine diphosphate - turning it into ATP. reduced
sulfur compounds (such as
sulfide,
hydrogen sulfide and
thiosulfate),
ferrous iron (FeII) or
ammonia as sources of reducing power and they gain energy from the oxidation of these compounds with electron acceptors such as
oxygen or
nitrite. These microbial processes are important in global
biogeochemical cycles such as
acetogenesis,
nitrification and
denitrification and are critical for
soil fertility.
Energy from light
The energy in sunlight is captured by
plants,
cyanobacteria,
purple bacteria,
green sulfur bacteria and some
protists. This process is often coupled to the conversion of carbon dioxide into organic compounds, as part of photosynthesis, which is discussed below. The energy capture and carbon fixation systems can however operate separately in prokaryotes, as purple bacteria and green sulfur bacteria can use sunlight as a source of energy, while switching between carbon fixation and the fermentation of organic compounds.
The capture of solar energy is a process that's similar in principle to oxidative phosphorylation, as it involves energy being stored as a proton concentration gradient and this proton motive force then driving ATP synthesis.
In plants,
photosystem II uses light energy to remove electrons from water, releasing oxygen as a waste product. The electrons then flow to the
cytochrome b6f complex, which uses their energy to pump protons across the
thylakoid membrane in the
chloroplast. These protons move back through the membrane as they drive the ATP synthase, as before. The electrons then flow through
photosystem I and can then either be used to reduce the coenzyme NADP
+, for use in the
Calvin cycle which is discussed below, or recycled for further ATP generation.
Anabolism
Anabolism is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex molecules. In general, the complex molecules that make up cellular structures are constructed step-by-step from small and simple precursors. Anabolism involves three basic stages. Firstly, the production of precursors such as
amino acids,
monosaccharides,
isoprenoids and
nucleotides, secondly, their activation into reactive forms using energy from ATP, and thirdly, the assembly of these precursors into complex molecules such as
proteins,
polysaccharides,
lipids and
nucleic acids.
Organisms differ in how many of the molecules in their cells they can construct for themselves.
Autotrophs such as plants can construct the complex organic molecules in cells such as polysaccharides and proteins from simple molecules like
carbon dioxide and water.
Heterotrophs, on the other hand, require a source of more complex substances, such as monosaccharides and amino acids, to produce these complex molecules. Organisms can be further classified by ultimate source of their energy: photoautotrophs and photoheterotrophs obtain energy from light, whereas chemoautotrophs and chemoheterotrophs obtain energy from inorganic oxidation reactions.
Carbon fixation
Photosynthesis is the synthesis of carbohydrates from sunlight,
carbon dioxide (CO
2) and water, with oxygen produced as a waste product. This process uses the ATP and NADPH produced by the
photosynthetic reaction centres, as described above, to convert CO
2 into
glycerate 3-phosphate, which can then be converted into glucose. This carbon-fixation reaction is carried out by the enzyme
RuBisCO as part of the
Calvin – Benson cycle. Three types of photosynthesis occur in plants,
C3 carbon fixation,
C4 carbon fixation and
CAM photosynthesis. These differ by the route that carbon dioxide takes to the Calvin cycle, with C3 plants fixing CO
2 directly, while C4 and CAM photosynthesis incorporate the CO
2 into other compounds first, as adaptations to deal with intense sunlight and dry conditions.
In photosynthetic
prokaryotes the mechanisms of carbon fixation are more diverse. Here, carbon dioxide can be fixed by the Calvin – Benson cycle, a
reversed citric acid cycle, or the carboxylation of acetyl-CoA. Prokaryotic
chemoautotrophs also fix CO
2 through the Calvin – Benson cycle, but use energy from inorganic compounds to drive the reaction.
Carbohydrates and glycans
In carbohydrate anabolism, simple organic acids can be converted into
monosaccharides such as
glucose and then used to assemble
polysaccharides such as
starch. The generation of
glucose from compounds like
pyruvate,
lactate,
glycerol,
glycerate 3-phosphate and
amino acids is called
gluconeogenesis. Gluconeogenesis converts pyruvate to
glucose-6-phosphate through a series of intermediates, many of which are shared with
glycolysis.
Although fat is a common way of storing energy, in
vertebrates such as
humans the
fatty acids in these stores can't be converted to glucose through
gluconeogenesis as these organisms can't convert acetyl-CoA into
pyruvate; plants do, but animals do not, have the necessary enzymatic machinery. As a result, after long-term starvation, vertebrates need to produce
ketone bodies from fatty acids to replace glucose in tissues such as the brain that can't metabolize fatty acids. In other organisms such as plants and bacteria, this metabolic problem is solved using the
glyoxylate cycle, which bypasses the
decarboxylation step in the citric acid cycle and allows the transformation of acetyl-CoA to
oxaloacetate, where it can be used for the production of glucose. The polysaccharides produced can have structural or metabolic functions themselves, or be transferred to lipids and proteins by enzymes called
oligosaccharyltransferases.
Fatty acids, isoprenoids and steroids
Fatty acids are made by
fatty acid synthases that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the actyl group, reduce it to an alcohol,
dehydrate it to an
alkene group and then reduce it again to an
alkane group. The enzymes of fatty acid biosynthesis are divided into two groups, in animals and fungi all these fatty acid synthase reactions are carried out by a single multifunctional type I protein, while in plant
plastids and bacteria separate type II enzymes perform each step in the pathway.
Terpenes and
isoprenoids are a large class of lipids that include the
carotenoids and form the largest class of plant
natural products. These compounds are made by the assembly and modification of
isoprene units donated from the reactive precursors
isopentenyl pyrophosphate and
dimethylallyl pyrophosphate. These precursors can be made in different ways. In animals and archaea, the
mevalonate pathway produces these compounds from acetyl-CoA, while in plants and bacteria the
non-mevalonate pathway uses pyruvate and
glyceraldehyde 3-phosphate as substrates. Lanosterol can then be converted into other steroids such as
cholesterol and
ergosterol.
Amino acids are made into proteins by being joined together in a chain by
peptide bonds. Each different protein has a unique sequence of amino acid residues: this is its
primary structure. Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked in varying sequences to form a huge variety of proteins. Proteins are made from amino acids that have been activated by attachment to a
transfer RNA molecule through an
ester bond. This aminoacyl-tRNA precursor is produced in an
ATP-dependent reaction carried out by an
aminoacyl tRNA synthetase. This aminoacyl-tRNA is then a substrate for the
ribosome, which joins the amino acid onto the elongating protein chain, using the sequence information in a
messenger RNA.
Nucleotide synthesis and salvage
Nucleotides are made from amino acids, carbon dioxide and
formic acid in pathways that require large amounts of metabolic energy. Consequently, most organisms have efficient systems to salvage preformed nucleotides.
Purines are synthesized as
nucleosides (bases attached to
ribose). Both
adenine and
guanine are made from the precursor nucleoside
inosine monophosphate, which is synthesized using atoms from the amino acids
glycine,
glutamine, and
aspartic acid, as well as
formate transferred from the
coenzyme tetrahydrofolate.
Pyrimidines, on the other hand, are synthesized from the base
orotate, which is formed from glutamine and aspartate.
Xenobiotics and redox metabolism
All organisms are constantly exposed to compounds that they can't use as foods and would be harmful if they accumulated in cells, as they've no metabolic function. These potentially damaging compounds are called
xenobiotics. Xenobiotics such as
synthetic drugs,
natural poisons and
antibiotics are detoxified by a set of xenobiotic-metabolizing enzymes. In humans, these include
cytochrome P450 oxidases,
UDP-glucuronosyltransferasess, and
glutathione S-transferases. This system of enzymes acts in three stages to firstly oxidize the xenobiotic (phase I) and then conjugate water-soluble groups onto the molecule (phase II). The modified water-soluble xenobiotic can then be pumped out of cells and in multicellular organisms may be further metabolized before being excreted (phase III). In
ecology, these reactions are particularly important in microbial
biodegradation of pollutants and the
bioremediation of contaminated land and oil spills. Many of these microbial reactions are shared with multicellular organisms, but due to the incredible diversity of types of microbes these organisms are able to deal with a far wider range of xenobiotics than multicellular organisms, and can degrade even
persistent organic pollutants such as
organochloride compounds.
A related problem for
aerobic organisms is
oxidative stress. Here, processes including
oxidative phosphorylation and the formation of
disulfide bonds during
protein folding produce
reactive oxygen species such as
hydrogen peroxide. These damaging oxidants are removed by
antioxidant metabolites such as
glutathione and enzymes such as
catalases and
peroxidases.
Thermodynamics of living organisms
Living organisms must obey the
laws of thermodynamics, which describe the transfer of
heat and
work. The
second law of thermodynamics states that in any
closed system, the amount of
entropy (disorder) will tend to increase. Although living organisms' amazing complexity appears to contradict this law, life is possible as all organisms are
open systems that exchange matter and energy with their surroundings. Thus living systems are not in
equilibrium, but instead are
dissipative systems that maintain their state of high complexity by causing a larger increase in the entropy of their environments. The metabolism of a cell achieves this by coupling the
spontaneous processes of catabolism to the non-spontaneous processes of anabolism. In
thermodynamic terms, metabolism maintains order by creating disorder.
Regulation and control
As the environments of most organisms are constantly changing, the reactions of metabolism must be finely
regulated to maintain a constant set of conditions within cells, a condition called
homeostasis. Metabolic regulation also allows organisms to respond to signals and interact actively with their environments. Two closely-linked concepts are important for understanding how metabolic pathways are controlled. Firstly, the
regulation of an enzyme in a pathway is how its activity is increased and decreased in response to signals. Secondly, the
control exerted by this enzyme is the effect that these changes in its activity have on the overall rate of the pathway (the
flux through the pathway). For example, an enzyme may show large changes in activity (
for example it's highly regulated) but if these changes have little effect on the flux of a metabolic pathway, then this enzyme isn't involved in the control of the pathway.
There are multiple levels of metabolic regulation. In intrinsic regulation, the metabolic pathway self-regulates to respond to changes in the levels of substrates or products; for example, a decrease in the amount of product can increase the
flux through the pathway to compensate. Extrinsic control involves a cell in a multicellular organism changing its metabolism in response to signals from other cells. These signals are usually in the form of soluble messengers such as
hormones and
growth factors and are detected by specific
receptors on the cell surface. These signals are then transmitted inside the cell by
second messenger systems that often involved the
phosphorylation of proteins.
A very well understood example of extrinsic control is the regulation of glucose metabolism by the hormone
insulin. Insulin is produced in response to rises in
blood glucose levels. Binding of the hormone to
insulin receptors on cells then activates a cascade of
protein kinases that cause the cells to take up glucose and convert it into storage molecules such as fatty acids and
glycogen. The metabolism of glycogen is controlled by activity of
phosphorylase, the enzyme that breaks down glycogen, and
glycogen synthase, the enzyme that makes it. These enzymes are regulated in a reciprocal fashion, with phosphorylation inhibiting glycogen synthase, but activating phosphorylase. Insulin causes glycogen synthesis by activating
protein phosphatases and producing a decrease in the phosphorylation of these enzymes.
Evolution
The central pathways of metabolism described above, such as glycolysis and the citric acid cycle, are present in all
three domains of living things and were present in the
last universal ancestor. The retention of these ancient pathways during later
evolution may be the result of these reactions being an optimal solution to their particular metabolic problems, with pathways such as glycolysis and the citric acid cycle producing their end products highly efficiently and in a minimal number of steps.
Many models have been proposed to describe the mechanisms by which novel metabolic pathways evolve. These include the sequential addition of novel enzymes to a short ancestral pathway, the duplication and then divergence of entire pathways as well as the recruitment of pre-existing enzymes and their assembly into a novel reaction pathway. The relative importance of these mechanisms is unclear, but genomic studies have shown that enzymes in a pathway are likely to have a shared ancestry, suggesting that many pathways have evolved in a step-by-step fashion with novel functions being created from pre-existing steps in the pathway. An alternative model comes from studies that trace the evolution of proteins' structures in metabolic networks, this has suggested that enzymes are pervasively recruited, borrowing enzymes to perform similar functions in different metabolic pathways (evident in the
MANET database) These recruitment processes result in an evolutionary enzymatic mosaic. A third possibility is that some parts of metabolism might exist as "modules" that can be reused in different pathways and perform similar functions on different molecules.
As well as the evolution of new metabolic pathways, evolution can also cause the loss of metabolic functions. For example, in some
parasites metabolic processes that are not essential for survival are lost and preformed amino acids, nucleotides and carbohydrates may instead be scavenged from the
host. Similar reduced metabolic capabilities are seen in
endosymbiotic organisms.
Investigation and manipulation
Classically, metabolism is studied by a
reductionist approach that focuses on a single metabolic pathway. Particularly valuable is the use of
radioactive tracers at the whole-organism, tissue and cellular levels, which define the paths from precursors to final products by identifying radioactively-labelled intermediates and products. The enzymes that catalyze these chemical reactions can then be
purified and their
kinetics and responses to
inhibitors investigated. A parallel approach is to identify the small molecules in a cell or tissue; the complete set of these molecules is called the
metabolome. Overall, these studies give a good view of the structure and function of simple metabolic pathways, but are inadequate when applied to more complex systems such as the metabolism of a complete cell.
An idea of the complexity of the
metabolic networks in cells that contain thousands of different enzymes is given by the figure showing the interactions between just 43 proteins and 40 metabolites to the right: the sequences of genomes provide lists containing anything up to 45,000 genes. However, it's now possible to use this genomic data to reconstruct complete networks of biochemical reactions and produce more
holistic mathematical models that may explain and predict their behavior. These models are especially powerful when used to integrate the pathway and metabolite data obtained through classical methods with data on
gene expression from
proteomic and
DNA microarray studies. Using these techniques, a model of human metabolism has now been produced, which will guide future drug discovery and biochemical research.
A major technological application of this information is
metabolic engineering. Here, organisms such as
yeast,
plants or
bacteria are genetically-modified to make them more useful in
biotechnology and aid the production of
drugs such as
antibiotics or industrial chemicals such as
1,3-propanediol and
shikimic acid. These genetic modifications usually aim to reduce the amount of energy used to produce the product, increase yields and reduce the production of wastes.
History
The term
metabolism is derived from the
Greek Μεταβολισμός – "Metabolismos" for "change", or "overthrow". The history of the scientific study of metabolism spans several centuries and has moved from examining whole animals in early studies, to examining individual metabolic reactions in modern biochemistry. The concept of metabolism dates back to
Ibn al-Nafis (1213-1288), who stated that "the body and its parts are in a continuous state of dissolution and
nourishment, so they're inevitably undergoing permanent change." The first controlled
experiments in human metabolism were published by
Santorio Santorio in 1614 in his book
Ars de statica medecina. He described how he weighed himself before and after eating, sleeping, working, sex, fasting, drinking, and excreting. He found that most of the food he took in was lost through what he called "insensible perspiration".
In these early studies, the mechanisms of these metabolic processes hadn't been identified and a
vital force was thought to animate living tissue. In the 19th century, when studying the
fermentation of sugar to
alcohol by
yeast,
Louis Pasteur concluded that fermentation was catalyzed by substances within the yeast cells he called "ferments". He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells." This discovery, along with the publication by
Friedrich Wöhler in 1828 of the chemical synthesis of
urea, proved that the organic compounds and chemical reactions found in cells were no different in principle than any other part of chemistry.
It was the discovery of
enzymes at the beginning of the 20th century by
Eduard Buchner that separated the study of the chemical reactions of metabolism from the biological study of cells, and marked the beginnings of
biochemistry. The mass of biochemical knowledge grew rapidly throughout the early 20th century. One of the most prolific of these modern biochemists was
Hans Krebs who made huge contributions to the study of metabolism. He discovered the urea cycle and later, working with
Hans Kornberg, the citric acid cycle and the glyoxylate cycle.
[ Modern biochemical research has been greatly aided by the development of new techniques such as chromatography, X-ray diffraction, NMR spectroscopy, radioisotopic labelling, electron microscopy and molecular dynamics simulations. These techniques have allowed the discovery and detailed analysis of the many molecules and metabolic pathways in cells.]
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