The pathways of metabolic energy conversion are different within different tissues.

The brain has only glucose for fuel, except during starvation when ketone bodies may be utilized, which minimizes protein breakdown.

Muscle has large glycogen stores (75% of body's glycogen supply), where pyruvate formed in glycolysis is converted to lactate or alanine and exported to the liver for gluconeogenesis. In resting muscle, ketone bodies (like acetoacetate) are a primary fuel.

Adipose tissue is specialized for the interconversion of fatty acids and triacylglycerols, but to form the glycerol 3-phosphate needed for triacylglycerol synthesis, adipose cells need glucose. In the reverse pathway within adipose tissue, cyclic AMP activates the breakdown of triglycerides to fatty acids and glycerol.

The liver is the first destination of most substances absorbed by the intestine. Liver is a major control center for both glucose and lipid metabolism. Fatty acids can either become incorporated into triglycerides and phospholipids or they can be converted into ketone bodies for use as fuel.

Insulin, glucagon, epinephrine and norepinephrine are important hormones in metabolic control. Insulin and glucagon are the most important regulators of fuel metabolism. Insulin signals the fed state. Insulin stimulates glycogen synthesis. Suppresses gluconeogenesis. Stimulates glycolysis in the liver (primarily for building blocks). Enables transport of glucose into muscle and adipose tissue, and of certain amino acids into muscle tissue. Glucagon is released in response to low blood sugar and targets the liver. Glucagon inhibits glycogen synthesis. Inhibits fatty acid synthesis. Stimulates gluconeogenesis. Blocks glycolysis in liver (preventing the formation of precursors).

The catecholamines epinephrine and norepinephrine are released in response to low blood glucose level. Unlike glucagon, which primarily affects the liver, epinephrine and norepinephrine are primarily targeted at muscle tissue. These hormones stimulate glucagon release. So they increase glucose release by the liver and decrease glucose utilization by muscle.

The liver can take up and release large amounts of glucose in response to hormonal signals. Glucose 6-phosphate is formed rapidly by the liver as blood glucose rises. Glucagon and insulin control the fate of glucose 6-phosphate. In addition to the endocrine effects, with its effects on phosphorylase a, high blood glucose allosterically shifts the glycogen system from a degradative to a synthetic mode.

In starvation, because the fatty acyl portions of triglycerides cannot be converted into glucose, organs which use this fuel exclusively must look to the glycerol portion and to proteins. To keep as much protein safe as possible, organs which do not absolutely need glucose use fatty acids and ketone bodies as fuel. Later in starvation, ketone bodies produced by the liver become a more significant energy source for the brain, so not as much protein must be degraded to supply the brain's needs for glucose.

The above discussion reflects a condensed, somewhat advanced version of priority MCAT material, so you might want to read it again.

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