MCAT - Carbohydrate Metabolism

Pentose phosphate pathway step 1

Glucose-6-phosphate is oxidized to form lactone. NADPH is produced as a byproduct of this reaction as NADP‍  is reduced as glucose-6-phosphate is oxidized. Following the oxidation of glucose-6-phosphate, another reaction, catalyzed by a different enzyme, uses water to form 6-phosphogluconate, the linear product.

Pentose Phosphate pathway step 2

a carbon is removed (cleaved) and CO‍  is released. Once again, the electrons released from this cleavage is used to reduce NADP‍  to NADPH. This new 5-carbon molecule is called ribulose-5-phosphate

Pentose phosphate pathway step 3

Ribulose-5-phosphate can be converted into ribose-5-phosphate

Pentose phosphate pathway step 4

The ribose-5-phosphate from step 3 is combined with another molecule of ribose-5-phosphate to make one, 10-carbon molecule. The 10-carbon molecule is interconverted to create a 3-carbon molecule and a 7-carbon molecule. The 3-carbon product can be shipped over to glycolysis if it needs

Pentose Phosphate pathway step 5

The 3-carbon molecule and the 7-carbon molecule, from the interconversion above in step 4, interconvert again to make a new 4-carbon molecule and 6-carbon molecule. 4-carbon molecule is a precursor for amino acids, while the 6-carbon molecule can be used in glycolysi

Starch

primary energy storage carbohydrate in plants. It is composed of two types of glucose polymers: amylose and amylopectin

Amylose

linear polymer of glucose units linked by α(1→4) glycosidic bonds

Amylopectin

branched polymer of glucose units. It has α(1→4) glycosidic bonds along its linear chains, and branching occurs at α(1→6) glycosidic bonds approximately every 24 to 30 glucose units

Salivary and pancreatic amylases

cleave the α(1→4) glycosidic bonds of amylose and amylopectin to produce maltose (a disaccharide) and glucose

debranching enzyme

hydrolyzes the α(1→6) bonds in amylopectin to ensure complete glucose liberation

Glycogen

animal equivalent of starch and serves as the primary storage form of glucose in animals. contains α(1→4) glycosidic bonds for linear chains and α(1→6) glycosidic bonds at branch points

Liver

primary storage site for glycogen and helps regulate blood glucose levels

Muscle

glycogen is only used locally within the muscle and cannot be released into the bloodstream

Glycogenesis

glycogen synthase adds glucose units to a growing glycogen chain by forming α(1→4) glycosidic bonds, while the branching enzyme creates α(1→6) bonds to introduce branches

Glycogenolysis

glycogen phosphorylase cleaves α(1→4) glycosidic bonds to release glucose-1-phosphate. The debranching enzyme cleaves α(1→6) bonds to remove branches, allowing for further breakdown

Insulin

promotes glycogenesis When blood glucose levels are high

Glucagon

stimulates glycogenolysis When blood glucose levels are low

Epinephrine

stimulates glycogenolysis, particularly in muscle tissue, to provide rapid energy for physical activity

fermentation

glycolysis with some extra reactions; make lactic acid; energy from fuels anaerobically; he extra reactions in fermentation, then, is to regenerate the electron carrier ‍ NAD+ from the ‍  NADH produced in glycolysis

Anaerobic cellular respiration

electrons extracted from a fuel molecule are passed through an electron transport chain, driving ATP‍  synthesis.

lactic acid fermentation

NADH transfers its electrons directly to pyruvate, generating lactate as a byproduct

alcohol fermentation

NADH donates its electrons to a derivative of pyruvate, producing ethanol