StC L1

What is the main energy currency of the cell?

ATP (adenosine triphosphate).

How is ATP produced?

Through substrate-level and oxidative phosphorylation.

Why is glucose considered an excellent fuel?

Its complete oxidation releases a large amount of energy (ΔG = –2840 kJ/mol).

What happens to pyruvate under aerobic vs. anaerobic conditions?

• Aerobic: Converted to Acetyl-CoA and enters the TCA cycle (→ ~36 ATP).

• Anaerobic: Converted to lactate (→ 2 ATP).

Which tissues have an absolute requirement for glucose?

Brain, nerves, red blood cells, testes, and kidney medulla.

What are the main storage forms of glucose?

Glycogen (in liver and muscle) and triglycerides (in adipose tissue).

What triggers insulin release?

High blood glucose levels.

What triggers glucagon release?

Low blood glucose levels.

What are the main actions of insulin?

• Increases glucose uptake into fat and muscle.

• Increases glycogen synthesis in the liver.

• Inhibits gluconeogenesis.

What are the main actions of glucagon?

• Stimulates gluconeogenesis.

• Inhibits glycogen synthesis.

• Promotes glycogen breakdown and lipid breakdown.

• Signals glucose release into the blood.

How do insulin and glucagon act together?

They act reciprocally to maintain blood glucose homeostasis.

What is glycogen?

A polymer of glucose subunits used for glucose storage, mainly in liver and muscle cells.

What happens to glycogen in the fed state?

It is synthesized from glucose (storage).

What happens to glycogen in the fasted state?

It is broken down to release glucose.

What does “reciprocal regulation” mean in metabolism?

When one metabolic pathway (e.g., glycogen synthesis) is active, the opposing one (e.g., glycogen breakdown) is inhibited, and vice versa.

Why is reciprocal regulation important?

It allows rapid switching between storage and release of glucose in response to blood sugar changes.

What are the main ways enzymes can be regulated?

1. Changing enzyme levels (biosynthesis/degradation).

2. Changing activity (e.g., phosphorylation, allosteric modulation).

3. Changing location within the cell.

What is reversible covalent modification?

The rapid regulation of enzyme activity by adding or removing covalent groups, such as phosphate.

What is phosphorylation?

The covalent addition of a phosphate group from ATP by kinases, altering enzyme activity.

How is phosphorylation reversed?

By phosphatases, which remove the phosphate group.

How can phosphorylation affect enzyme activity?

It can turn an enzyme on or off by altering its 3D conformation through charged interactions.

What are the two main classes of kinases?

• Tyrosine kinases (phosphorylate tyrosine residues).

• Serine/threonine kinases (phosphorylate serine or threonine residues).

How do insulin and glucagon regulate carbohydrate metabolism?

They coordinate the flux through metabolic pathways (like glycogen synthesis and breakdown) via reciprocal regulation of key enzymes.