Carbohydrate catabolism

aerobic and anaerobic

. In aerobic respiration, oxygen is required. Using oxygen increases ATP production from 4 ATP molecules to about 30 ATP molecules. In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.

There are several different types of carbohydrates: polysaccharides (e.g., starch, amylopectin, glycogen, cellulose), monosaccharides (e.g., glucose, galactose, fructose, ribose) and the disaccharides (e.g., sucrose, maltose, lactose).

Monosaccharides, also known as simple sugars, are the most basic, fundamental unit of a carbohydrate. These are simple sugars with the general chemical structure of C6H12O6.

Disaccharides are a type of carbohydrate. Disaccharides consist of compound sugars containing two monosaccharides with the elimination of a water molecule with the general chemical structure C12H22O11.

Oligosaccharides are carbohydrates that consist of a polymer that contains three to ten monosaccharides linked together by glycosidic bonds.

Glucose reacts with oxygen in the following reaction, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Carbon dioxide and water are waste products, and the overall reaction is

exothermic

.

The reaction of glucose with oxygen releasing energy in the form of molecules of ATP is therefore one of the most important biochemical pathways found in living organisms.

Glycolysis

Krebs cycle and oxidative phosphorylation

. [1]

Fermentation

Even if there is no oxygen present, glycolysis can continue to generate ATP. However, for glycolysis to continue to produce ATP, there must be NAD+ present, which is responsible for oxidizing glucose. This is achieved by recycling NADH back to NAD+. When NAD+ is reduced to NADH, the electrons from NADH are eventually transferred to a separate organic molecule, transforming NADH back to NAD+. This process of renewing the supply of NAD+ is called fermentation, which falls into two categories.^[1]

Alcohol Fermentation

In alcohol fermentation, when a glucose molecule is oxidized,

organic molecule that is responsible for renewing the NAD+ supply in this type of fermentation is the pyruvate from glycolysis. Each pyruvate releases a carbon dioxide molecule, turning into acetaldehyde. The acetaldehyde is then reduced by the NADH produced from glycolysis, forming the alcohol waste product, ethanol, and forming NAD+, thereby replenishing its supply for glycolysis to continue producing ATP.^[1]

Lactic Acid Fermentation

In

muscle cells as a means of generating ATP during strenuous exercise where oxygen consumption is higher than the supplied oxygen. As this process progresses, the surplus of lactate is brought to the liver, which converts it back to pyruvate.^[1]

Respiration

The Citric acid cycle (also known as the Krebs cycle)

If oxygen is present, then following glycolysis, the two pyruvate molecules are brought into the

electrons, is used to reduce NAD+ and FAD to NADH and FADH₂, respectively. NADH and FADH₂ contain the stored energy harnessed from the initial glucose molecule and is used in the electron transport chain where the bulk of the ATP is produced.^[1]

Oxidative phosphorylation

The last process in aerobic respiration is

exergonic flow of electrons throughout the membrane. These electrons are supplied by NADH and FADH₂ as they transfer their potential energy. Once the H+ concentration gradient is established, a proton-motive force is established, which provides the energy to convert ADP to ATP. The H+ ions that were initially forced to one side of the mitochondrion membrane now naturally flow through a membrane protein called ATP synthase

, a protein that converts ADP to ATP with the help of H+ ions. [1]

References

^
ISBN 978-0321558237
.

[Campbell-1] 
ISBN 978-0321558237
.

[1]

v t e Metabolism, catabolism, anabolism
General	Metabolic pathway Metabolic network Primary nutritional groups
Aerobic respiration	Glycolysis → Pyruvate decarboxylation → Citric acid cycle → Oxidative phosphorylation (electron transport chain + ATP synthase)
Anaerobic respiration	Electron acceptors other than oxygen
Fermentation	Glycolysis → Substrate-level phosphorylation ABE Ethanol Lactic acid