AP Biology Unit 3

Cellular Energetics

Metabolism

The sum of all chemical reactions in an organism.

Catabolic pathways

Break down molecules, release energy (e.g., cellular respiration).

Anabolic pathways

Build molecules, require energy (e.g., photosynthesis).

First law of thermodynamics

Energy cannot be created or destroyed, only transformed.

Second law of thermodynamics

Energy transfer increases entropy (disorder) of the universe.

Enzymes

• Proteins that speed up reactions by lowering activation energy.

• Remain unchanged after the reaction.

How Do Enzymes Work?

1. Substrate binds to the active site.

2. Enzyme-substrate complex forms.

3. Reaction occurs, and the product is released.

Temperature’s effects on enzymes

Optimal range (too high = denaturation)

pH’s effects on enzymes

Enzymes have an optimal pH (e.g., pepsin in the stomach works best at pH 2).

Substrate concentration’s effect on enzymes

More substrate = faster reaction until enzyme saturation.

Competitive inhibition

Inhibitor binds to the active site, blocking the substrate.

Noncompetitive inhibition

Inhibitor binds elsewhere, changing enzyme shape.

Allosteric regulation

Regulatory molecules activate/inhibit enzymes.

ATP

(Adenosine Triphosphate) stores energy in phosphate bonds.

Hydrolysis of ATP

ATP + H2O → ADP + Pi + Energy

Provides energy for cellular processes.

Cellular Respiration goal

Convert glucose into ATP.

Cellular Respiration formula

C₆H₁₂O₆ + 6O₂ --> 6CO₂ + 6H₂O + Chemical Energy (in ATP)

where does Cellular Respiration occur?

Occurs in mitochondria (eukaryotes).

Glycolysis

Occurs in glucose.

• Breaks glucose (6C) into 2 pyruvate (3C).

• Net ATP gain: 2 ATP.

• Produces 2 NADH.

Pyruvate Oxidation

Occurs in the mitochondria

• Pyruvate → Acetyl-CoA (2C) + CO₂.

• Produces 2 NADH.

Krebs Cycle (Citric Acid Cycle)

Occurs in the mitochondria

• Acetyl-CoA enters cycle → Produces CO₂, ATP, NADH, FADH₂.

• Net: 2 ATP, 6 NADH, 2 FADH₂.

Electron Transport Chain (ETC) & Oxidative Phosphorylation

Occurs in the Inner Mitochondrial Membrane

• NADH & FADH₂ donate electrons to ETC.

• Oxygen is the final electron acceptor, forming H₂O.

• Protons (H⁺) are pumped across the membrane, creating a gradient.

• ATP Synthase uses this gradient to make ATP (chemiosmosis).

• Net: ~32-34 ATP.

Anaerobic Respiration

Occurs when oxygen is unavailable, which means Glycolysis still occurs (producing 2 ATP)

Lactic Acid Fermentation

Animals & anaerobic

Pyruvate → Lactic acid.

Alcoholic Fermentation

Yeast & anaerobic

Pyruvate → Ethanol + CO₂

Photosynthesis

Convert light energy into chemical energy (glucose).

Photosynthesis equation

6CO2+6H2O→C6H12O6+6O2

where does Photosynthesis occur?

Occurs in chloroplasts (plants, algae).

Light-Dependent Reactions

Occurs in Thylakoid Membrane/chloroplasts.

• Uses sunlight to produce ATP & NADPH.

• Water (H₂O) is split, releasing O₂.

Photosystem II

Photosynthesis; absorbs light → ETC → ATP (via chemiosmosis).

Photosystem I

Photosynthesis; absorbs light → NADPH

Calvin Cycle (Light-Independent Reactions)

Occurs in the stroma

• Uses ATP & NADPH from light reactions.

• Fixes CO₂ into glucose.

Carbon Fixation

Calvin Cycle; RuBisCO enzyme captures CO₂.

Reduction

Calvin Cycle; Forms G3P (precursor to glucose).

Regeneration of RuBP

Calvin Cycle; ensuring the cycle can continue to fix carbon dioxide

Photosynthesis organisms

plants, algae

Photosynthesis organelle

chloroplasts

Photosynthesis energy source

sunlight

Photosynthesis electron carrier

NADPH

Photosynthesis Final Electron Acceptor

NADP+

Photosynthesis Byproducts

O2

Cellular Respiration organism

All organisms

Cellular Respiration organelle

mitochondria

Cellular Respiration energy source

glucose

Cellular Respiration Electron Carrier

NADH, FADH2

Cellular Respiration final electron acceptor

O2

Cellular Respiration byproducts

CO2

Photorespiration

Photosynthesis

• RuBisCO mistakenly binds O₂ instead of CO₂, reducing efficiency.

• Occurs in hot, dry conditions.

C4 Plants

(e.g., corn, sugarcane) Adaptations to Reduce Photorespiration

• Physically separate CO₂ fixation and Calvin Cycle.

• Uses PEP carboxylase, which only binds CO₂.

CAM Plants

(e.g., cacti, succulents) Adaptations to Reduce Photorespiration

• Stomata open at night to capture CO₂.

• CO₂ is stored and used during the day.

Feedback Inhibition

Products regulate enzyme activity.

Key Takeaways

• Enzymes speed up reactions by lowering activation energy.

• Cellular respiration converts glucose → ATP.

• Fermentation occurs without oxygen.

• Photosynthesis converts light → glucose.

• Plants adapt to prevent photorespiration.