Plants that undergo C4 respiration must also utilize the enzyme PEP carboxylase. This enzyme allows for carbon dioxide to be fixed to PEP to produce oxaloacetate. C4 respiration allows for lower levels of photorespiration, which can cause a decrease in photosynthetic efficiency.

Rubisco is the enzyme that adds carbon dioxide to ribulose bisphosphate to produce two molecules of 3-phosphoglycerate. Alkaline phosphatase is an enzyme that hydrolyzes phosphates from a variety of substrates under alkaline conditions. Kinases are enzyme which add phosphate groups to specific targets, such as fructose-1-phosphate.

CAM plants utilize C4 photosynthesis and keep their stomata closed during the day. By using the enzyme PEP carboxylase, CAM plants are able to store up enough fixed carbon during the night in order to keep their stomata closed during the day. The closure of the stomata prevents the loss of water from the plant. This is especially important in hot, dry climates.

When oxygen is not readily available in the blood, cells use fermentation as a means of anaerobic respiration. The process is used to generate NAD⁺, which can be used as a reactant in glycolysis to produce small amounts of ATP. Glycolysis still occurs in this environment, breaking glucose into pyruvate and producing two ATP per cycle.

Fermentation in humans converts pyruvate to lactate (lactic acid) and NADH to NAD⁺.

Under anaerobic conditions, fermentation follows the process of glycolysis. While glycolysis is responsible for creating ATP, fermentation allows the body to regenerate the NAD⁺ that is reduced during glycolytic processes. This key step allows glycolysis to continue, and more ATP to be made.

Fermentation is a form of anaerobic respiration where there is no oxygen available as the final electron acceptor to the electron transport chain. As such, pyruvate is reduced, yielding lactic acid and, in the presence of lactate decarboxylase, ethanol. However, the products of fermentation do not undergo the Krebs cycle nor electron transport chain. The purpose of fermentation is to regenerate NAD⁺ so that glycolysis can continue. The cell does this by transferring the electron from NADH to pyruvate. Fermentation is less efficient per glucose than is aerobic oxidation, generating a fraction of the ATP. Lactic acid and ethanol are actually quite toxic to the cell (in humans they are immediately sent to the liver to be detoxified).

Either ethanol or lactic acid can be produced through fermentation depending on the organism. Ethanol fermentation only occurs in bacteria and yeast. Lactic acid fermentation has been found to occur in multiple kingdoms. 

Fermentation allows for the regeneration of  which can allow for glycolysis to continue. Butanol is a four carbon alcohol and aspartic acid is an amino acid found in proteins. 

Glycogen is the common form for storing glucose in animal cells, while starch is the common form for storing glucose in plant cells. These saccharides are very similar, but glycogen is more easily digestable due to its structure. Liver and muscle cells are the primary storage places for glycogen. Adipose cells are primarily known for the storage of fats.

When blood glucose levels are elevated, the liver will convert available carbohydrates into glucose, and then convert the glucose into glycogen. This allows for glucose to be stored in the liver until blood glucose levels drop, and it needs to be released. The act of converting glucose to glycogen in the liver is called glycogenesis. Glycogenolysis is the opposite reaction, in which glycogen is converted to glucose once again.

Glycolysis is a step in cell metabolism in which glucose is cleaved into pyruvate. Gluconeogenesis is the production of glucose from non-carbohydrate molecules when glycogen is unavailable.