How Cells Harvest Chemical Energy: Cellular Respiration and Fermentation

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How Cells Harvest Chemical Energy

Introduction to Cellular Respiration

Cells require energy to perform essential functions, and this energy is primarily harvested from organic molecules through the process of cellular respiration. This chapter explores how cells convert chemical energy from food into usable energy in the form of ATP, focusing on the interconnected processes of photosynthesis and cellular respiration.

Photosynthesis and Cellular Respiration: Energy Flow in Ecosystems

Overview of Energy Conversion

Photosynthesis captures energy from sunlight in chloroplasts, converting carbon dioxide and water into glucose and oxygen.
Cellular respiration occurs in mitochondria, breaking down glucose into carbon dioxide and water, releasing energy to form ATP.
Some energy is lost as heat during these conversions.

Key Equations:

Photosynthesis:
Cellular Respiration:

Example: Plants use photosynthesis to produce glucose, which animals then use in cellular respiration to generate ATP for cellular work.

Breathing and Cellular Respiration: Distinctions

Definitions and Relationships

Breathing (Respiration) refers to the physical exchange of gases (O2 in, CO2 out) between an organism and its environment.
Cellular respiration is the metabolic process by which cells harvest energy from organic molecules, requiring oxygen and producing carbon dioxide as a waste product.

Example: Oxygen inhaled into the lungs is transported via the bloodstream to muscle cells, where it is used in cellular respiration to produce ATP.

Cellular Respiration: Energy Transfer and ATP Production

Exergonic Nature and Efficiency

Cellular respiration is an exergonic (energy-releasing) process that transfers energy from glucose bonds to ATP.
Up to 32 ATP molecules can be produced per glucose molecule, with about 34% efficiency; the rest is lost as heat.

Equation:

Example: The energy conversion efficiency of cellular respiration is higher than that of most man-made engines.

Redox Reactions in Cellular Respiration

Electron Transfer and Energy Release

Cellular respiration involves oxidation-reduction (redox) reactions, where electrons are transferred from glucose to oxygen.
Oxidation: Loss of electrons from a molecule (e.g., glucose).
Reduction: Gain of electrons by a molecule (e.g., oxygen).
Glucose is oxidized to CO2; oxygen is reduced to H2O.

Key Coenzyme: NAD+ accepts electrons and hydrogen to become NADH, which delivers electrons to the electron transport chain.

Example: NADH acts as an electron shuttle, transferring high-energy electrons to the electron transport chain, ultimately reducing oxygen to water.

Stages of Cellular Respiration

Three Main Stages

Glycolysis: Occurs in the cytosol; breaks down glucose into two pyruvate molecules.
Pyruvate Oxidation and Citric Acid Cycle: Occur in mitochondria; oxidize pyruvate to CO2 and generate NADH and FADH2.
Oxidative Phosphorylation: Electron transport chain and chemiosmosis produce most ATP.

Glycolysis

Occurs in the cytoplasm.
Converts one glucose molecule into two pyruvate molecules.
Net gain: 2 ATP (by substrate-level phosphorylation) and 2 NADH per glucose.
Two phases: Energy investment (uses 2 ATP) and energy payoff (produces 4 ATP and 2 NADH).

Pyruvate Oxidation

Pyruvate is transported into mitochondria.
Each pyruvate is converted to acetyl CoA, producing 1 NADH and releasing 1 CO2 per pyruvate.

Citric Acid Cycle (Krebs Cycle)

Completes the oxidation of organic molecules.
Each acetyl CoA enters the cycle, joining with oxaloacetate to form citrate.
For each acetyl CoA: 2 CO2, 3 NADH, 1 FADH2, and 1 ATP are produced.
Per glucose (2 turns): 4 CO2, 6 NADH, 2 FADH2, 2 ATP.

Oxidative Phosphorylation

Occurs in the inner mitochondrial membrane.
NADH and FADH2 donate electrons to the electron transport chain.
Electron flow drives H+ pumping, creating a gradient used by ATP synthase to generate ATP (chemiosmosis).
Oxygen is the final electron acceptor, forming water.
Produces about 28 ATP per glucose.

Summary Table: ATP Yield per Glucose

Stage	ATP Produced
Glycolysis	2
Citric Acid Cycle	2
Oxidative Phosphorylation	~28
Total	~32

Fermentation: Anaerobic Energy Harvesting

Fermentation Pathways

Occurs when oxygen is absent.
Relies on glycolysis for ATP production (2 ATP per glucose).
Regenerates NAD+ from NADH to allow glycolysis to continue.

Lactic Acid Fermentation

Pyruvate is reduced to lactate, regenerating NAD+.
Occurs in muscle cells and some bacteria.
Used in dairy industry (cheese, yogurt).

Alcohol Fermentation

Pyruvate is converted to ethanol and CO2, regenerating NAD+.
Occurs in yeast and some bacteria.
Used in baking and alcohol production.

Metabolic Versatility: Other Fuels for Cellular Respiration

Alternative Substrates

Cells can use carbohydrates, fats, and proteins as fuel for cellular respiration.
Fats yield more than twice as much ATP per gram as carbohydrates.
Proteins are used as a last resort, after sugars and fats.

Biosynthesis

Cells use intermediates from respiration for biosynthesis of macromolecules (e.g., amino acids, nucleotides, lipids).
ATP generated is used for building cellular structures and performing functions.