Cellular Energy: Study Guide for General Biology

Cellular Energy

Redox Reactions and Energy Basics

Cellular energy processes rely on the transfer of electrons and the transformation of energy through oxidation-reduction (redox) reactions. These reactions are fundamental to metabolism and energy production in cells.

• Oxidation-Reduction (Redox) Reactions: Involve the transfer of electrons between molecules. Oxidation is the loss of electrons, while reduction is the gain of electrons.

• Electron Carriers: Molecules such as NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) accept electrons during metabolic reactions, becoming reduced to NADH and FADH2.

• Energy Coupling: ATP (adenosine triphosphate) is the main energy currency of the cell, coupling exergonic and endergonic reactions.

Example: In cellular respiration, glucose is oxidized and oxygen is reduced, producing ATP.

Glycolysis

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. It occurs in the cytoplasm and does not require oxygen.

• Location: Cytosol (cytoplasm)

• Process: One glucose (6C) is split into two pyruvate (3C) molecules.

• Products: 2 ATP (net), 2 NADH, 2 pyruvate

Example: Glycolysis provides ATP for short bursts of energy in muscle cells.

Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle)

Pyruvate from glycolysis is transported into the mitochondria, where it is converted to acetyl CoA, which enters the citric acid cycle.

• Pyruvate Oxidation: Pyruvate is converted to acetyl CoA, releasing CO2.

• Citric Acid Cycle: Acetyl CoA enters the cycle, producing CO2, NADH, FADH2, and ATP.

Equation: $\text{Acetyl CoA} + 3\ NAD^+ + FAD + GDP + P_i + 2\ H_2O \rightarrow 2\ CO_2 + 3\ NADH + 3\ H^+ + FADH_2 + GTP + CoA$

Electron Transport Chain (ETC) and Oxidative Phosphorylation

The ETC is a series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, generating a proton gradient used to produce ATP.

• Location: Inner mitochondrial membrane

• Electron Flow: NADH/FADH2 → ETC → O2 (final electron acceptor)

• ATP Synthesis: Proton gradient drives ATP synthase to produce ATP from ADP and Pi.

Equation: $\text{NADH} + H^+ + 1/2\ O_2 \rightarrow \text{NAD}^+ + H_2O$

Anaerobic Respiration and Fermentation

When oxygen is not available, cells can generate ATP through anaerobic pathways such as fermentation.

• Fermentation: Converts pyruvate to lactate (in animals) or ethanol and CO2 (in yeast), regenerating NAD+ for glycolysis.

• ATP Yield: Much lower than aerobic respiration (2 ATP per glucose).

Example: Muscle cells use lactic acid fermentation during intense exercise.

Photosynthesis Overview

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

• Light Reactions: Occur in the thylakoid membranes; convert light energy to ATP and NADPH, releasing O2.

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix CO2 into glucose.

Equation: $6\ CO_2 + 6\ H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6\ O_2$

Key Comparison Table: Aerobic vs. Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration/Fermentation
Oxygen Required?	Yes	No
ATP Yield (per glucose)	~30-32 ATP	2 ATP
End Products	CO2, H2O	Lactate or Ethanol + CO2

Review Tips and Practice

• Know the locations and main products of glycolysis, Krebs cycle, and ETC.

• Understand the role of electron carriers (NADH, FADH2).

• Be able to compare aerobic and anaerobic pathways.

• Practice tracing the flow of carbon and electrons through cellular respiration and photosynthesis.