Cellular Respiration Study Guide

Unit 4: Cellular Energy

Overview

This unit covers the fundamental processes by which cells obtain, store, and utilize energy: cellular respiration and photosynthesis. These processes are essential for life, providing the energy required for cellular activities and the synthesis of organic molecules.

Photosynthesis and Cellular Respiration: Energy for Life

Photosynthesis

• Definition: The process by which chloroplasts in plant cells capture sunlight energy and convert it into chemical energy stored in sugars.

• Key Reactants: Carbon dioxide (CO2) and water (H2O).

• Key Products: Glucose (C6H12O6) and oxygen (O2).

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

Cellular Respiration

• Definition: The process by which cells break down glucose to release energy, producing ATP, carbon dioxide, and water.

• Key Reactants: Glucose and oxygen.

• Key Products: Carbon dioxide, water, and ATP.

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

Connection: Photosynthesis and cellular respiration are complementary processes in the ecosystem, cycling energy and matter.

Cellular Respiration: Aerobic Harvesting of Energy

Breathing and Cellular Respiration

• Breathing: The physical exchange of gases (O2 in, CO2 out) between an organism and its environment.

• Cellular Respiration: The metabolic process inside cells that uses O2 to break down food molecules and release energy.

• Relationship: Oxygen from breathing is used in cellular respiration; carbon dioxide produced is expelled during exhalation.

Stages of Cellular Respiration

• Stage 1: Glycolysis (in cytosol)

  • Breaks down glucose into two molecules of pyruvate.

  • Produces 2 ATP (net) and 2 NADH.

  • Equation: $\text{Glucose} \rightarrow 2 \text{Pyruvate} + 2 \text{ATP} + 2 \text{NADH}$

• Stage 2: Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle) (in mitochondria)

  • Pyruvate is converted to Acetyl CoA, which enters the Krebs cycle.

  • Produces CO2, ATP, NADH, and FADH2.

  • Equation (per glucose): $2 \text{Pyruvate} \rightarrow 6CO_2 + 2 \text{ATP} + 8 \text{NADH} + 2 \text{FADH}_2$

• Stage 3: Oxidative Phosphorylation (in mitochondria)

  • Includes electron transport chain and chemiosmosis.

  • Uses NADH and FADH2 to generate most ATP (up to 32 per glucose).

  • Oxygen is the final electron acceptor, forming water.

Overall Chemical Equation

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

Exergonic Reaction: Cellular respiration releases energy, with about 34% captured as ATP and the rest lost as heat.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

• Oxidation: Loss of electrons or hydrogen atoms.

• Reduction: Gain of electrons or hydrogen atoms.

• Mnemonic: "LEO the lion says GER" (Lose Electrons = Oxidation, Gain Electrons = Reduction).

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

Electron Carriers: NADH and FADH2

• Definition: Coenzymes that transport electrons during cellular respiration.

• Function: NAD+ and FAD accept electrons and hydrogen, becoming NADH and FADH2.

• Role: Carry electrons to the electron transport chain for ATP production.

Fermentation: Anaerobic Harvesting of Energy

Overview

Fermentation allows cells to produce ATP without oxygen, using glycolysis followed by alternative pathways to regenerate NAD+.

Lactic Acid Fermentation

• Occurs in: Humans (muscle cells) and bacteria.

• Products: Lactic acid (toxic by-product).

• Purpose: Regenerates NAD+ for glycolysis to continue.

• Applications: Used in yogurt production; causes muscle fatigue.

Alcoholic Fermentation

• Occurs in: Plants and fungi (yeast).

• Products: Ethanol and CO2.

• Purpose: Regenerates NAD+ for glycolysis to continue.

• Applications: Used in bread and wine production.

Evolutionary Significance of Glycolysis

Ancient Origins

• Glycolysis is found in all domains of life, indicating it evolved early.

• Does not require oxygen or mitochondria, suitable for early Earth conditions.

Checkpoint Questions and Key Concepts

• Compare the processes and locations of cellular respiration.

• Explain how breathing and cellular respiration are related.

• Provide the overall chemical equation for cellular respiration.

• Of the three main stages of cellular respiration, which one does not take place in the mitochondria? Answer: Glycolysis (occurs in cytosol).

• Compare the reactants, products, and energy yield of alcohol and lactic acid fermentation.

• Explain how carbohydrates, fats, and proteins are used as fuel for cellular respiration. Additional info: Fats store more energy per gram than polysaccharides, providing efficient energy reserves for animals.

Table: Comparison of Aerobic Respiration and Fermentation

Process	Oxygen Required?	Main Products	ATP Yield (per glucose)	Location
Aerobic Respiration	Yes	CO2, H2O, ATP	~32-36	Cytosol & Mitochondria
Lactic Acid Fermentation	No	Lactic Acid, ATP	2	Cytosol
Alcoholic Fermentation	No	Ethanol, CO2, ATP	2	Cytosol

Diagram: Mitochondrion Structure

• Outer membrane: Encloses the organelle.

• Inner membrane: Contains folds (cristae) for electron transport chain.

• Matrix: Site of Krebs cycle.

• Intermembrane space: Accumulates protons for chemiosmosis.

• ATP synthase: Enzyme that synthesizes ATP using the proton gradient.

Summary

• Cellular respiration and photosynthesis are central to energy flow in living systems.

• Cellular respiration consists of glycolysis, pyruvate oxidation/Krebs cycle, and oxidative phosphorylation.

• Fermentation provides an alternative pathway for ATP production in the absence of oxygen.

• Redox reactions and electron carriers are essential for energy transfer.

• Glycolysis is an ancient, universal metabolic pathway.