Vedanā (Pāli and Sanskrit: वेदना) is an ancient term traditionally translated as either "feeling" or "sensation." In general, vedanā refers to the pleasant, unpleasant and neutral sensations that occur when our internal sense organs come into contact with external sense objects and the associated consciousness. Vedanā is identified as valence or "hedonic tone" in psychology.
Vedanā is identified within the Buddhist teaching as follows:
Feeling is the mental factor
which feels the object. It is the affective mode in which the object is
experienced. The Pali word vedanā does not signify emotion (which
appears to be a complex phenomenon involving a variety of concomitant
mental factors), but the bare affective quality of an experience, which may be either pleasant, painful or neutral....
Nina van Gorkom states:
When we study the Abhidhamma we learn that 'vedanā' is not the
same as what we mean by feeling in conventional language. Feeling is nāma, it experiences something. Feeling never arises alone; it accompanies citta and other cetasikas
and it is conditioned by them. Thus, feeling is a conditioned nāma.
Citta does not feel, it cognizes the object and vedanā feels...
All feelings have the function of experiencing the taste, the flavour of an object (Atthasālinī, I, Part IV, Chapter I, 109). The Atthasālinī
uses a simile in order to illustrate that feeling experiences the taste
of an object and that citta and the other cetasikas which arise
together with feeling experience the taste only partially. A cook who
has prepared a meal for the king merely tests the food and then offers
it to the king who enjoys the taste of it:
...and the king, being lord, expert, and master, eats
whatever he likes, even so the mere testing of the food by the cook is
like the partial enjoyment of the object by the remaining dhammas (the
citta and the other cetasikas), and as the cook tests a portion of the
food, so the remaining dhammas enjoy a portion of the object, and as the
king, being lord, expert and master, eats the meal according to his
pleasure, so feeling, being lord, expert and master, enjoys the taste of
the object, and therefore it is said that enjoyment or experience is
its function.
Thus, all feelings have in common that they experience the
'taste' of an object. Citta and the other accompanying cetasikas also
experience the object, but feeling experiences it in its own
characteristic way.
What is the absolutely specific characteristic of vedana? It is
to experience. That is to say, in any experience, what we experience is
the individual maturation of any positive or negative action as its
final result.
Mipham Rinpoche states:
Sensations are defined as impressions.
The aggregate of sensations can be divided into three: pleasant,
painful, and neutral. Alternatively, there are five: pleasure and mental
pleasure, pain and mental pain, and neutral sensation.
In terms of support, there are six sensations resulting from contact...
Alexander Berzin describes this mental factors as feeling (tshor-ba, Skt. vedanā) some level of happiness. He states:
When we hear the word “feeling” in a Buddhist context, it’s only
referring to this: feeling some level of happy or unhappy, somewhere on
the spectrum. So, on the basis of pleasant contacting awareness—it
comes easily to mind—we feel happy. Happiness is: we would like it to
continue. And, on the basis of unpleasant contacting awareness—it
doesn’t come easily to the mind, we basically want to get rid of it—we
feel unhappiness. “Unhappiness” is the same word as “suffering”
(mi-bde-ba, Skt. duhkha). Unhappiness is: I don’t want to continue this;
I want to be parted from this.
And neutral contacting awareness. We feel neutral about it—neither want to continue it nor to discontinue it...
Relation to "emotions"
Vedanā is the distinct valence or "hedonic tone" of emotional psychology, neurologically identified and isolated.
Contemporary teachers Bhikkhu Bodhi and Chögyam Trungpa Rinpoche clarify the relationship between vedanā (often translated as "feelings") and Western notions of "emotions."
Bhikkhu Bodhi writes:
"The Pali word vedanā does not signify emotion (which
appears to be a complex phenomenon involving a variety of concomitant
mental factors), but the bare affective quality of an experience, which may be either pleasant, painful or neutral."
Chögyam Trungpa Rinpoche writes:
"In case [i.e. within the Buddhist teachings] 'feeling' is not
quite our ordinary notion of feeling. It is not the feeling we take so
seriously as, for instance, when we say, 'He hurt my feelings.' This
kind of feeling that we take so seriously belongs to the fourth and
fifth skandhas of concept and consciousness."
Attributes
In general, the Pali canon
describes vedanā in terms of three "modes" and six "classes." Some
discourses discuss alternate enumerations including up to 108 kinds.
neither pleasant nor unpleasant (adukkham-asukhā, "ambivalent", sometimes referred to as "neutral" in translation)
Elsewhere in the Pali canon it is stated that there are six classes
of vedanā, corresponding to sensations arising from contact (Skt: sparśa; Pali: phassa) between an internal sense organ (āyatana; that is, the eye, ear, nose, tongue, body or mind), an external sense object and the associated consciousness (Skt.: vijnana; Pali: viññāna). (See Figure 1.) In other words:
feeling arising from the contact of eye, visible form and eye-consciousness
feeling arising from the contact of ear, sound and ear-consciousness
feeling arising from the contact of nose, smell and nose-consciousness
feeling arising from the contact of tongue, taste and tongue-consciousness
feeling arising from the contact of body, touch and body-consciousness
feeling arising from the contact of mind (mano), thoughts (dhamma) and mind-consciousness
Two, three, five, six, 18, 36, 108 kinds
In a few discourses, a multitude of kinds of vedana are alluded to ranging from two to 108, as follows:
six kinds: one for each sense faculty (eye, ear, nose, tongue, body, mind)
18 kinds: explorations of the aforementioned three mental kinds of
feelings (mental pleasant, mental painful, equanimous) each in terms of
each of the aforementioned six sense faculties
36 kinds: the aforementioned 18 kinds of feeling for the householder and the aforementioned 18 kinds for the renunciate
108 kinds: the aforementioned 36 kinds for the past, for the present and for the future
In the wider Pali literature, of the above enumerations, the post-canonical Visuddhimagga highlights the five types of vedanā: physical pleasure (sukha); physical displeasure (dukkha); mental happiness (somanassa); mental unhappiness (domanassa); and, equanimity (upekkhā).
extinction (khaya) of the taints (āsava) [Arahantship]
Vedanā is a pivotal phenomenon in the following frequently identified frameworks of the Pali canon:
the "five aggregates"
the twelve conditions of "dependent origination"
the four "foundations of mindfulness"
Mental aggregate
Vedanā is one of the five aggregates (Skt.: skandha; Pali: khandha) of clinging (Skt., Pali: upādāna;
see Figure 2 to the right). In the canon, as indicated above, feeling
arises from the contact of a sense organ, sense object and
consciousness.
vedanā arises with contact (phassa) as its condition
vedanā acts as a condition for craving (Pali: taṇhā; Skt.: tṛṣṇā).
In the post-canonical 5th-century Visuddhimagga, feeling (vedana) is identified as simultaneously and inseparably arising from consciousness (viññāṇa) and the mind-and-body (nāmarūpa). On the other hand, while this text identifies feeling as decisive
to craving and its mental sequelae leading to suffering, the
conditional relationship between feeling and craving is not identified
as simultaneous nor as being karmically necessary.
Mindfulness base
Throughout the canon, there are references to the four "foundations of mindfulness" (satipaṭṭhāna): the body (kāya), feelings (vedanā), mind states (citta) and mental experiences (dhammā). These four foundations are recognized among the seven sets of qualities conducive to enlightenment (bodhipakkhiyādhammā). The use of vedanā and the other satipaṭṭhāna in Buddhist meditation practices can be found in the Satipaṭṭhāna Sutta and the Ānāpānasati Sutta.
Wisdom practices
Each mode of vedanā is accompanied by its corresponding underlying tendency or obsession (anusaya).
The underlying tendency for pleasant vedanā is the tendency toward
lust, for unpleasant, the tendency toward aversion, and for neither
pleasant nor unpleasant, the tendency toward ignorance.
In the Canon it is stated that meditating with concentration (samādhi) on vedanā can lead to deep mindfulness (sati) and clear comprehension (sampajañña) (see Table to the right). With this development, one can experience directly within oneself the reality of impermanence (anicca) and the nature of attachment (upādāna). This in turn can ultimately lead to liberation of the mind (nibbāna).
Alternate translations
Alternate translations for the term vedana are:
Feeling (Nina van Gorkom, Bhikkhu Bodhi, Alexander Berzin)
Feeling some level of happiness (Alexander Berzin)
Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, and metabolism.
Over the last decades of the 20th century, biochemistry has become
successful at explaining living processes through these three
disciplines. Almost all areas of the life sciences are being uncovered and developed through biochemical methodology and research. Biochemistry focuses on understanding the chemical basis that allows biological molecules to give rise to the processes that occur within living cells and between cells, in turn relating greatly to the understanding of tissues and organs as well as organism structure and function. Biochemistry is closely related to molecular biology, the study of the molecular mechanisms of biological phenomena.
Much of biochemistry deals with the structures, functions, and interactions of biological macromolecules such as proteins, nucleic acids, carbohydrates, and lipids. They provide the structure of cells and perform many of the functions associated with life. The chemistry of the cell also depends upon the reactions of small molecules and ions. These can be inorganic (for example, water and metal ions) or organic (for example, the amino acids, which are used to synthesize proteins). The mechanisms used by cells to harness energy from their environment via chemical reactions are known as metabolism. The findings of biochemistry are applied primarily in medicine, nutrition, and agriculture. In medicine, biochemists investigate the causes and cures of diseases. Nutrition studies how to maintain health and wellness and also the effects of nutritional deficiencies. In agriculture, biochemists investigate soil and fertilizers with the goal of improving crop cultivation, crop storage, and pest control. In recent decades, biochemical principles and methods have been combined with problem-solving approaches from engineering
to manipulate living systems in order to produce useful tools for
research, industrial processes, and diagnosis and control of disease—the
discipline of biotechnology.
At its most comprehensive definition, biochemistry can be seen as a
study of the components and composition of living things and how they
come together to become life. In this sense, the history of biochemistry
may therefore go back as far as the ancient Greeks. However, biochemistry as a specific scientific discipline
began sometime in the 19th century, or a little earlier, depending on
which aspect of biochemistry is being focused on. Some argued that the
beginning of biochemistry may have been the discovery of the first enzyme, diastase (now called amylase), in 1833 by Anselme Payen, while others considered Eduard Buchner's first demonstration of a complex biochemical process alcoholic fermentation in cell-free extracts in 1897 to be the birth of biochemistry. Some might also point as its beginning to the influential 1842 work by Justus von Liebig, Animal chemistry, or, Organic chemistry in its applications to physiology and pathology, which presented a chemical theory of metabolism, or even earlier to the 18th century studies on fermentation and respiration by Antoine Lavoisier.
Many other pioneers in the field who helped to uncover the layers of
complexity of biochemistry have been proclaimed founders of modern
biochemistry. Emil Fischer, who studied the chemistry of proteins, and F. Gowland Hopkins, who studied enzymes and the dynamic nature of biochemistry, represent two examples of early biochemists.
The term "biochemistry" was first used when Vinzenz Kletzinsky
(1826–1882) had his "Compendium der Biochemie" printed in Vienna in
1858; it derived from a combination of biology and chemistry. In 1877, Felix Hoppe-Seyler used the term (biochemie in German) as a synonym for physiological chemistry in the foreword to the first issue of Zeitschrift für Physiologische Chemie (Journal of Physiological Chemistry) where he argued for the setting up of institutes dedicated to this field of study. The German chemistCarl Neuberg however is often cited to have coined the word in 1903, while some credited it to Franz Hofmeister.
Around two dozen chemical elements are essential to various kinds of biological life. Most rare elements on Earth are not needed by life (exceptions being selenium and iodine), while a few common ones (aluminum and titanium) are not used. Most organisms share element needs, but there are a few differences between plants and animals. For example, ocean algae use bromine, but land plants and animals do not seem to need any. All animals require sodium, but is not an essential element for plants. Plants need boron and silicon, but animals may not (or may need ultra-small amounts).
Just six elements—carbon, hydrogen, nitrogen, oxygen, calcium and phosphorus—make up almost 99% of the mass of living cells, including those in the human body (see composition of the human body
for a complete list). In addition to the six major elements that
compose most of the human body, humans require smaller amounts of
possibly 18 more.
Two of the main functions of carbohydrates are energy storage and providing structure. One of the common sugars known as glucose
is a carbohydrate, but not all carbohydrates are sugars. There are more
carbohydrates on Earth than any other known type of biomolecule; they
are used to store energy and genetic information, as well as play important roles in cell to cell interactions and communications.
The simplest type of carbohydrate is a monosaccharide, which among other properties contains carbon, hydrogen, and oxygen, mostly in a ratio of 1:2:1 (generalized formula CnH2nOn, where n is at least 3). Glucose (C6H12O6) is one of the most important carbohydrates; others include fructose (C6H12O6), the sugar commonly associated with the sweet taste of fruits, and deoxyribose (C5H10O4), a component of DNA. A monosaccharide can switch between acyclic (open-chain) form and a cyclic form. The open-chain form can be turned into a ring of carbon atoms bridged by an oxygen atom created from the carbonyl group of one end and the hydroxyl group of another. The cyclic molecule has a hemiacetal or hemiketal group, depending on whether the linear form was an aldose or a ketose.
In these cyclic forms, the ring usually has 5 or 6 atoms. These forms are called furanoses and pyranoses, respectively—by analogy with furan and pyran, the simplest compounds with the same carbon-oxygen ring (although they lack the carbon-carbon double bonds of these two molecules). For example, the aldohexose glucose
may form a hemiacetal linkage between the hydroxyl on carbon 1 and the
oxygen on carbon 4, yielding a molecule with a 5-membered ring, called glucofuranose. The same reaction can take place between carbons 1 and 5 to form a molecule with a 6-membered ring, called glucopyranose. Cyclic forms with a 7-atom ring called heptoses are rare.
Two monosaccharides can be joined by a glycosidic or ester bond into a disaccharide through a dehydration reaction
during which a molecule of water is released. The reverse reaction in
which the glycosidic bond of a disaccharide is broken into two
monosaccharides is termed hydrolysis. The best-known disaccharide is sucrose or ordinary sugar, which consists of a glucose molecule and a fructose molecule joined. Another important disaccharide is lactose found in milk, consisting of a glucose molecule and a galactose molecule. Lactose may be hydrolysed by lactase, and deficiency in this enzyme results in lactose intolerance.
When a few (around three to six) monosaccharides are joined, it is called an oligosaccharide (oligo- meaning "few"). These molecules tend to be used as markers and signals, as well as having some other uses. Many monosaccharides joined form a polysaccharide. They can be joined in one long linear chain, or they may be branched. Two of the most common polysaccharides are cellulose and glycogen, both consisting of repeating glucose monomers. Cellulose is an important structural component of plant's cell walls and glycogen is used as a form of energy storage in animals.
Sugar can be characterized by having reducing or non-reducing ends. A reducing end of a carbohydrate is a carbon atom that can be in equilibrium with the open-chain aldehyde (aldose) or keto form (ketose). If the joining of monomers takes place at such a carbon atom, the free hydroxy group of the pyranose or furanose form is exchanged with an OH-side-chain of another sugar, yielding a full acetal.
This prevents opening of the chain to the aldehyde or keto form and
renders the modified residue non-reducing. Lactose contains a reducing
end at its glucose moiety, whereas the galactose moiety forms a full
acetal with the C4-OH group of glucose. Saccharose
does not have a reducing end because of full acetal formation between
the aldehyde carbon of glucose (C1) and the keto carbon of fructose
(C2).
Lipids comprise a diverse range of molecules and to some extent is a catchall for relatively water-insoluble or nonpolar compounds of biological origin, including waxes, fatty acids, fatty-acid derived phospholipids, sphingolipids, glycolipids, and terpenoids (e.g., retinoids and steroids). Some lipids are linear, open-chain aliphatic molecules, while others have ring structures. Some are aromatic (with a cyclic [ring] and planar [flat] structure) while others are not. Some are flexible, while others are rigid.
Lipids are usually made from one molecule of glycerol combined with other molecules. In triglycerides, the main group of bulk lipids, there is one molecule of glycerol and three fatty acids. Fatty acids are considered the monomer in that case, and may be saturated (no double bonds in the carbon chain) or unsaturated (one or more double bonds in the carbon chain).
Most lipids have some polar character and are largely nonpolar. In general, the bulk of their structure is nonpolar or hydrophobic ("water-fearing"), meaning that it does not interact well with polar solvents like water. Another part of their structure is polar or hydrophilic ("water-loving") and will tend to associate with polar solvents like water. This makes them amphiphilic molecules (having both hydrophobic and hydrophilic portions). In the case of cholesterol, the polar group is a mere –OH (hydroxyl or alcohol).
In the case of phospholipids, the polar groups are considerably larger and more polar, as described below.
Lipids are an integral part of our daily diet. Most oils and milk products that we use for cooking and eating like butter, cheese, ghee etc. are composed of fats. Vegetable oils are rich in various polyunsaturated fatty acids
(PUFA). Lipid-containing foods undergo digestion within the body and
are broken into fatty acids and glycerol, the final degradation products
of fats and lipids. Lipids, especially phospholipids, are also used in various pharmaceutical products, either as co-solubilizers (e.g. in parenteral infusions) or else as drug carrier components (e.g. in a liposome or transfersome).
The general structure of an α-amino acid, with the amino group on the left and the carboxyl group on the right
Proteins are very large molecules—macro-biopolymers—made from monomers called amino acids. An amino acid consists of an alpha carbon atom attached to an amino group, –NH2, a carboxylic acid group, –COOH (although these exist as –NH3+ and –COO− under physiologic conditions), a simple hydrogen atom, and a side chain commonly denoted as "–R". The side chain "R" is different for each amino acid of which there are 20 standard ones.
It is this "R" group that makes each amino acid different, and the
properties of the side chains greatly influence the overall three-dimensional conformation of a protein. Some amino acids have functions by themselves or in a modified form; for instance, glutamate functions as an important neurotransmitter. Amino acids can be joined via a peptide bond. In this dehydration synthesis, a water molecule is removed and the peptide bond connects the nitrogen of one amino acid's amino group to the carbon of the other's carboxylic acid group. The resulting molecule is called a dipeptide, and short stretches of amino acids (usually, fewer than thirty) are called peptides or polypeptides. Longer stretches merit the title proteins. As an example, the important blood serum protein albumin contains 585 amino acid residues.
Generic amino acids (1) in neutral form, (2) as they exist physiologically, and (3) joined as a dipeptideA schematic of hemoglobin. The red and blue ribbons represent the protein globin; the green structures are the heme groups.
Proteins can have structural and/or functional roles. For instance, movements of the proteins actin and myosin
ultimately are responsible for the contraction of skeletal muscle. One
property many proteins have is that they specifically bind to a certain
molecule or class of molecules—they may be extremely selective in what they bind. Antibodies
are an example of proteins that attach to one specific type of
molecule. Antibodies are composed of heavy and light chains. Two heavy
chains would be linked to two light chains through disulfide linkages between their amino acids. Antibodies are specific through variation based on differences in the N-terminal domain.
The enzyme-linked immunosorbent assay
(ELISA), which uses antibodies, is one of the most sensitive tests
modern medicine uses to detect various biomolecules. Probably the most
important proteins, however, are the enzymes. Virtually every reaction in a living cell requires an enzyme to lower the activation energy of the reaction. These molecules recognize specific reactant molecules called substrates; they then catalyze the reaction between them. By lowering the activation energy, the enzyme speeds up that reaction by a rate of 1011
or more; a reaction that would normally take over 3,000 years to
complete spontaneously might take less than a second with an enzyme. The
enzyme itself is not used up in the process and is free to catalyze the
same reaction with a new set of substrates. Using various modifiers,
the activity of the enzyme can be regulated, enabling control of the
biochemistry of the cell as a whole.
The structure of proteins is traditionally described in a hierarchy of four levels. The primary structure
of a protein consists of its linear sequence of amino acids; for
instance,
"alanine-glycine-tryptophan-serine-glutamate-asparagine-glycine-lysine-...".
Secondary structure
is concerned with local morphology (morphology being the study of
structure). Some combinations of amino acids will tend to curl up in a
coil called an α-helix or into a sheet called a β-sheet; some α-helixes can be seen in the hemoglobin schematic above. Tertiary structure
is the entire three-dimensional shape of the protein. This shape is
determined by the sequence of amino acids. In fact, a single change can
change the entire structure. The alpha chain of hemoglobin contains 146
amino acid residues; substitution of the glutamate residue at position 6 with a valine residue changes the behavior of hemoglobin so much that it results in sickle-cell disease. Finally, quaternary structure
is concerned with the structure of a protein with multiple peptide
subunits, like hemoglobin with its four subunits. Not all proteins have
more than one subunit.
Examples of protein structures from the Protein Data BankMembers of a protein family, as represented by the structures of the isomerasedomains
Ingested proteins are usually broken up into single amino acids or dipeptides in the small intestine
and then absorbed. They can then be joined to form new proteins.
Intermediate products of glycolysis, the citric acid cycle, and the pentose phosphate pathway
can be used to form all twenty amino acids, and most bacteria and
plants possess all the necessary enzymes to synthesize them. Humans and
other mammals, however, can synthesize only half of them. They cannot
synthesize isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Because they must be ingested, these are the essential amino acids. Mammals do possess the enzymes to synthesize alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, and tyrosine, the nonessential amino acids. While they can synthesize arginine and histidine,
they cannot produce it in sufficient amounts for young, growing
animals, and so these are often considered essential amino acids.
If the amino group is removed from an amino acid, it leaves behind a carbon skeleton called an α-keto acid. Enzymes called transaminases
can easily transfer the amino group from one amino acid (making it an
α-keto acid) to another α-keto acid (making it an amino acid). This is
important in the biosynthesis of amino acids, as for many of the
pathways, intermediates from other biochemical pathways are converted to
the α-keto acid skeleton, and then an amino group is added, often via transamination. The amino acids may then be linked together to form a protein.
A similar process is used to break down proteins. It is first hydrolyzed into its component amino acids. Free ammonia (NH3), existing as the ammonium
ion (NH4+) in blood, is toxic to life forms. A suitable method for
excreting it must therefore exist. Different tactics have evolved in
different animals, depending on the animals' needs. Unicellular organisms release the ammonia into the environment. Likewise, bony fish can release ammonia into the water where it is quickly diluted. In general, mammals convert ammonia into urea, via the urea cycle.
In order to determine whether two proteins are related, or in
other words to decide whether they are homologous or not, scientists use
sequence-comparison methods. Methods like sequence alignments and structural alignments are powerful tools that help scientists identify homologies between related molecules. The relevance of finding homologies among proteins goes beyond forming an evolutionary pattern of protein families. By finding how similar two protein sequences are, we acquire knowledge about their structure and therefore their function.
The structure of deoxyribonucleic acid (DNA); the picture shows the monomers being put together.
Nucleic acids, so-called because of their prevalence in cellular nuclei, is the generic name of the family of biopolymers. They are complex, high-molecular-weight biochemical macromolecules that can convey genetic information in all living cells and viruses. The monomers are called nucleotides, and each consists of three components: a nitrogenous heterocyclic base (either a purine or a pyrimidine), a pentose sugar, and a phosphate group.
Structural
elements of common nucleic acid constituents. Because they contain at
least one phosphate group, the compounds marked nucleoside monophosphate, nucleoside diphosphate and nucleoside triphosphate are all nucleotides (not phosphate-lacking nucleosides).
The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The phosphate group
and the sugar of each nucleotide bond with each other to form the
backbone of the nucleic acid, while the sequence of nitrogenous bases
stores the information. The most common nitrogenous bases are adenine, cytosine, guanine, thymine, and uracil. The nitrogenous bases of each strand of a nucleic acid will form hydrogen bonds
with certain other nitrogenous bases in a complementary strand of
nucleic acid. Adenine binds with thymine and uracil, thymine binds only
with adenine, and cytosine and guanine can bind only with one another.
Adenine, thymine, and uracil contain two hydrogen bonds, while hydrogen
bonds formed between cytosine and guanine are three.
Aside from the genetic material of the cell, nucleic acids often play a role as second messengers, as well as forming the base molecule for adenosine triphosphate
(ATP), the primary energy-carrier molecule found in all living
organisms. Also, the nitrogenous bases possible in the two nucleic acids
are different: adenine, cytosine, and guanine occur in both RNA and
DNA, while thymine occurs only in DNA and uracil occurs in RNA.
Glucose is an energy source in most life forms. For instance, polysaccharides are broken down into their monomers by enzymes (glycogen phosphorylase
removes glucose residues from glycogen, a polysaccharide).
Disaccharides like lactose or sucrose are cleaved into their two
component monosaccharides.
The metabolic pathway of glycolysis converts glucose to pyruvate via a series of intermediate metabolites. Each chemical modification is performed by a different enzyme. Steps 1 and 3 consume ATP and steps 7 and 10 produce ATP. Since steps 6–10 occur twice per glucose molecule, this leads to a net production of ATP.
Glucose is mainly metabolized by a very important ten-step pathway called glycolysis, the net result of which is to break down one molecule of glucose into two molecules of pyruvate. This also produces a net two molecules of ATP, the energy currency of cells, along with two reducing equivalents of converting NAD+
(nicotinamide adenine dinucleotide: oxidized form) to NADH
(nicotinamide adenine dinucleotide: reduced form). This does not require
oxygen; if no oxygen is available (or the cell cannot use oxygen), the
NAD is restored by converting the pyruvate to lactate (lactic acid) (e.g. in humans) or to ethanol plus carbon dioxide (e.g. in yeast). Other monosaccharides like galactose and fructose can be converted into intermediates of the glycolytic pathway.
Aerobic
In aerobic cells with sufficient oxygen, as in most human cells, the pyruvate is further metabolized. It is irreversibly converted to acetyl-CoA, giving off one carbon atom as the waste product carbon dioxide, generating another reducing equivalent as NADH. The two molecules acetyl-CoA (from one molecule of glucose) then enter the citric acid cycle, producing two molecules of ATP, six more NADH molecules and two reduced (ubi)quinones (via FADH2
as enzyme-bound cofactor), and releasing the remaining carbon atoms as
carbon dioxide. The produced NADH and quinol molecules then feed into
the enzyme complexes of the respiratory chain, an electron transport system
transferring the electrons ultimately to oxygen and conserving the
released energy in the form of a proton gradient over a membrane (inner mitochondrial membrane in eukaryotes). Thus, oxygen is reduced to water and the original electron acceptors NAD+ and quinone
are regenerated. This is why humans breathe in oxygen and breathe out
carbon dioxide. The energy released from transferring the electrons from
high-energy states in NADH and quinol is conserved first as proton
gradient and converted to ATP via ATP synthase. This generates an
additional 28 molecules of ATP (24 from the 8 NADH + 4 from the 2
quinols), totaling to 32 molecules of ATP conserved per degraded
glucose (two from glycolysis + two from the citrate cycle).
It is clear that using oxygen to completely oxidize glucose provides an
organism with far more energy than any oxygen-independent metabolic
feature, and this is thought to be the reason why complex life appeared
only after Earth's atmosphere accumulated large amounts of oxygen.
In vertebrates, vigorously contracting skeletal muscles (during weightlifting or sprinting, for example) do not receive enough oxygen to meet the energy demand, and so they shift to anaerobic metabolism, converting glucose to lactate.
The combination of glucose from noncarbohydrates origin, such as fat and proteins. This only happens when glycogen supplies in the liver are worn out. The pathway is a crucial reversal of glycolysis from pyruvate to glucose and can use many sources like amino acids, glycerol and Krebs Cycle. Large scale protein and fat catabolism usually occur when those suffer from starvation or certain endocrine disorders. The liver regenerates the glucose, using a process called gluconeogenesis.
This process is not quite the opposite of glycolysis, and actually
requires three times the amount of energy gained from glycolysis (six
molecules of ATP are used, compared to the two gained in glycolysis).
Analogous to the above reactions, the glucose produced can then undergo
glycolysis in tissues that need energy, be stored as glycogen (or starch
in plants), or be converted to other monosaccharides or joined into di-
or oligosaccharides. The combined pathways of glycolysis during
exercise, lactate's crossing via the bloodstream to the liver,
subsequent gluconeogenesis and release of glucose into the bloodstream
is called the Cori cycle.
Relationship to other "molecular-scale" biological sciences
Researchers in biochemistry use specific techniques native to
biochemistry, but increasingly combine these with techniques and ideas
developed in the fields of genetics, molecular biology, and biophysics. There is not a defined line between these disciplines. Biochemistry studies the chemistry required for biological activity of molecules, molecular biology studies their biological activity, genetics studies their heredity, which happens to be carried by their genome. This is shown in the following schematic that depicts one possible view of the relationships between the fields:
Biochemistry is the study of the chemical substances and vital processes occurring in live organisms. Biochemists focus heavily on the role, function, and structure of biomolecules.
The study of the chemistry behind biological processes and the
synthesis of biologically active molecules are applications of
biochemistry. Biochemistry studies life at the atomic and molecular
level.
Genetics is the study of the effect of genetic
differences in organisms. This can often be inferred by the absence of a
normal component (e.g. one gene). The study of "mutants" – organisms that lack one or more functional components with respect to the so-called "wild type" or normal phenotype. Genetic interactions (epistasis) can often confound simple interpretations of such "knockout" studies.
Molecular biology is the study of molecular
underpinnings of the biological phenomena, focusing on molecular
synthesis, modification, mechanisms and interactions. The central dogma of molecular biology, where genetic material is transcribed into RNA and then translated into protein,
despite being oversimplified, still provides a good starting point for
understanding the field. This concept has been revised in light of
emerging novel roles for RNA.
Chemical biology seeks to develop new tools based on small molecules
that allow minimal perturbation of biological systems while providing
detailed information about their function. Further, chemical biology
employs biological systems to create non-natural hybrids between
biomolecules and synthetic devices (for example emptied viral capsids that can deliver gene therapy or drug molecules).
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