Carbohydrate metabolism is the whole of the biochemical processes responsible for the metabolic formation, breakdown, and interconversion of carbohydrates in living organisms.

Carbohydrates are central to many essential metabolic pathways. Plants synthesize carbohydrates from carbon dioxide and water through photosynthesis, allowing them to store energy absorbed from sunlight internally. When animals and fungi consume plants, they use cellular respiration to break down these stored carbohydrates to make energy available to cells.

While carbohydrates are essential to human biological processes, consuming them is not essential for humans. There are healthy human populations that do not consume carbohydrates.

In humans, carbohydrates are available directly from consumption, from carbohydrate storage, or by conversion from fat components including fatty acids that are either stored or consumed directly.

Metabolic pathways

thumb|660x660px|Overview of connections between metabolic processes.

Glycolysis

Glycolysis is the process of breaking down a glucose molecule into two pyruvate molecules, while storing energy released during this process as adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH). There are various enzymes that are used throughout glycolysis. The enzymes upregulate, downregulate, and feedback regulate the process.

thumb|660x660px|The Glycolysis pathway diagram illustrates the metabolic reactions that allow for the breakdown of glucose into pyruvate, often as preparation for further catabolic reactions.

Gluconeogenesis

Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly in the liver and, to a lesser extent, in the cortex of the kidneys. It is one of two primary mechanisms – the other being degradation of glycogen (glycogenolysis) – used by humans and many other animals to maintain blood sugar levels, avoiding low levels (hypoglycemia). In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc. In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.

In humans, substrates for gluconeogenesis may come from any non-carbohydrate sources that can be converted to pyruvate or intermediates of glycolysis (see figure). For the breakdown of proteins, these substrates include glucogenic amino acids (although not ketogenic amino acids); from breakdown of lipids (such as triglycerides), they include glycerol, odd-chain fatty acids (although not even-chain fatty acids, see below); and from other parts of metabolism they include lactate from the Cori cycle. Under conditions of prolonged fasting, acetone derived from ketone bodies can also serve as a substrate, providing a pathway from fatty acids to glucose. Although most gluconeogenesis occurs in the liver, the relative contribution of gluconeogenesis by the kidney is increased in diabetes and prolonged fasting.

The gluconeogenesis pathway is highly endergonic until it is coupled to the hydrolysis of ATP or guanosine triphosphate (GTP), effectively making the process exergonic. For example, the pathway leading from pyruvate to glucose-6-phosphate requires 4 molecules of ATP and 2 molecules of GTP to proceed spontaneously. These ATPs are supplied from fatty acid catabolism via beta oxidation.

Glycogenolysis

Glycogenolysis refers to the breakdown of glycogen. In the liver, muscles, and the kidney, this process occurs to provide glucose when necessary.

Pentose phosphate pathway

The pentose phosphate pathway is an alternative method of oxidizing glucose. This pathway is regulated through changes in the activity of glucose-6-phosphate dehydrogenase. The cofactors NAD<sup>+</sup> and FAD are sometimes reduced during this process to form NADH and FADH<sub>2</sub>, which drive the creation of ATP in other processes.

{| class="wikitable"

|+Energy produced during metabolism of one glucose molecule

!Pathway

!ATP input

!ATP output

!Net ATP

!NADH output

!FADH<sub>2</sub> output

!ATP final yield

|-

|Glycolysis (aerobic)

|2

|4

|2

|2

|0

|5-7

|-

|Citric-acid cycle

|0

|2

|2

|6

|2

|17-25

|}

Typically, the complete breakdown of one molecule of glucose by aerobic respiration (i.e. involving glycolysis, the citric-acid cycle and oxidative phosphorylation, the last providing the most energy) is usually about 30–32 molecules of ATP. Glucose (blood sugar) is distributed to cells in the tissues, where it is broken down via cellular respiration, or stored as glycogen. Insulin and glucagon are the primary hormones involved in maintaining a steady level of glucose in the blood, and the release of each is controlled by the amount of nutrients currently available. In humans, insulin is made by beta cells in the pancreas, fat is stored in adipose tissue cells, and glycogen is both stored and released as needed by liver cells. Regardless of insulin levels, no glucose is released to the blood from internal glycogen stores from muscle cells.

Carbohydrates as storage

Carbohydrates are typically stored as long polymers of glucose molecules with glycosidic bonds for structural support (e.g. chitin, cellulose) or for energy storage (e.g. glycogen, starch). However, the strong affinity of most carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water-carbohydrate complex. In most organisms, excess carbohydrates are regularly catabolised to form acetyl-CoA, which is a feed stock for the fatty acid synthesis pathway; fatty acids, triglycerides, and other lipids are commonly used for long-term energy storage. The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates. Gluconeogenesis permits glucose to be synthesized from various sources, including lipids.

In some animals (such as termites) and some microorganisms (such as protists and bacteria), cellulose can be disassembled during digestion and absorbed as glucose.

Human diseases

  • Diabetes mellitus
  • Lactose intolerance
  • Fructose malabsorption
  • Galactosemia
  • Glycogen storage disease

See also

  • Inborn errors of carbohydrate metabolism
  • Hitting the wall (glycogen depletion)
  • Second wind (increased ATP from fatty acids after glycogen depletion)

References

  • BBC - GCSE Bitesize - Biology | Humans | Glucoregulation
  • Sugar4Kids

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