Cell Biology- Chapter 13

Biological oxidation captures energy from molecules in multiple steps

This allows for controlled energy release and efficient ATP production.

Three stages of food breakdown in animals

1. Mechanical and chemical digestion
2. Breakdown into monomers (amino acids, sugars, fatty acids)
3. Entry into cellular metabolism for energy extraction

Macronutrients vs. Micronutrients

• Macronutrients: Needed in large amounts, serve as energy sources

• Micronutrients: Needed in small amounts, often function as coenzymes

Nutrient conversion before cellular metabolism

• Proteins → Amino acids

• Polysaccharides → Simple sugars

• Fats → Fatty acids & glycerol

Cellular locations for metabolism

• Sugars: Cytosol

• Fats: Peroxisomes & Mitochondria

• Amino acids: Cytosol & Mitochondria

Two possible fates of nutrients

1. Used to synthesize needed macromolecules
2. Enter oxidative catabolism for ATP production

Oxygen dependency in oxidation

• Full oxidation requires oxygen

• Cells have anaerobic methods (e.g., glycolysis & fermentation)

Glycolysis: Splitting glucose

Converts glucose into two pyruvate molecules

• Requires 2 ATP and NAD+

Three main phases of glycolysis

1. Energy investment: Requires 2 ATP to create intermediates
2. 3-carbon sugar formation
3. Energy payoff: Produces 4 ATP

Steps of Glycolysis (1-5)

1. Glucose phosphorylation
2. Glucose-6-phosphate isomerization
3. Fructose phosphorylation
4. Fructose-1,6-bisphosphate lysis
5. Dihydroxyacetone phosphate isomerization

Steps of Glycolysis (6-10)

6. Glyceraldehyde-3-phosphate oxidation
7. 1,3-bisphosphoglycerate dephosphorylation (ATP synthesis)
8. 3-phosphoglycerate mutation
9. 2-phosphoglycerate dehydration (water removed by enolase)
10. Phosphoenolpyruvate dephosphorylation (ATP synthesis)

Fermentation and NAD+ supply

Ensures glycolysis continues when oxygen is low

• Regenerates NAD+ by reducing pyruvate to lactate (animals) or ethanol (yeast)

Lactate fermentation

Used by animals and some bacteria

• Produces lactate from pyruvate

• Used in yogurt production

Pyruvate to Acetyl-CoA conversion

Occurs in mitochondria via pyruvate dehydrogenase complex

• CO₂ is released, NAD+ is reduced, and Acetyl-CoA is formed

Fatty acid oxidation

Produces Acetyl-CoA

• Generates activated carriers for energy production

Citric Acid Cycle: Overview

Series of oxidation reactions

• Produces electron carriers (NADH, FADH₂)

• Releases CO₂

Steps of the Citric Acid Cycle

Back: 1. Acetyl-CoA + Oxaloacetate → Citrate
2. Citrate → Isocitrate
3. Isocitrate → α-Ketoglutarate (NADH, CO₂)
4. α-Ketoglutarate → Succinyl-CoA (NADH, CO₂)
5. Succinyl-CoA → Succinate (ATP or GTP)
6. Succinate → Fumarate (FADH₂)
7. Fumarate → Malate
8. Malate → Oxaloacetate (NADH)

Citric Acid Cycle & CO₂ release

Pyruvate contributes two carbons per cycle

• CO₂ is released at multiple steps

• Takes three cycles to fully release original pyruvate-derived carbons

Role of oxygen in ATP production

Acts as the final electron acceptor

• Essential for oxidative phosphorylation

• Most energy is stored in reduced electron carriers (NADH, FADH₂)

Citric Acid Cycle Discovery

Used a manometer to measure pressure changes

• Adding intermediates speeds up the entire cycle

Gluconeogenesis: Reverse Glycolysis

Converts metabolites into glucose

• Liver cells perform this process

• Ends with glucose-6-phosphate dephosphorylation to produce free glucose

Glycogen storage of glucose

• Muscles: Store glycogen for personal use

• Liver: Stores glycogen for body-wide glucose release

Long-term energy storage

Energy is stored long-term in fat reserves