Cellular Respiration and Energy Harvesting in Cells

How Cells Harvest Energy

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the energy currency of the cell. This process can occur aerobically (with oxygen) or anaerobically (without oxygen, as in fermentation).

• Cellular Respiration: Aerobic process that uses oxygen to harvest energy from glucose, producing carbon dioxide, water, and ATP.

• Fermentation: Anaerobic process that allows energy harvesting in the absence of oxygen, resulting in products like lactic acid or ethanol.

• Metabolic Pathways: Series of chemical reactions in a cell that build and break down molecules for cellular processes.

Learning Objectives

• Describe the three main metabolic pathways involved in the breakdown of glucose.

• Identify the inputs and outputs of each pathway.

• Understand the process of fermentation and its role in energy metabolism.

Global Bioenergetics

Energy Flow in Ecosystems

Energy flows through ecosystems via the conversion of sunlight to chemical energy by photosynthesis and the release of energy by cellular respiration.

• Photosynthesis: Converts sunlight energy into chemical energy stored in glucose.

• Cellular Respiration: Converts chemical energy in glucose into ATP, releasing heat energy as a byproduct.

• Energy Transformations: Chemical energy can be converted to kinetic energy, with some energy lost as heat at each step.

• Cycling of Matter: Matter cycles between photosynthesis and respiration, with CO2 and H2O as key intermediates.

Metabolic Demands

Energy Requirements of Cells and Organisms

Different activities require varying amounts of energy, measured in kilocalories (kcal) per gram of fuel. ATP is the immediate source of energy for cellular work, and is generated from food-derived molecules.

• ATP (Adenosine Triphosphate): The main energy currency of the cell, used to power cellular processes.

• Energy Yield: The amount of ATP produced depends on the type of fuel and the metabolic pathway used.

• Example: Running, swimming, and cycling require more energy (higher kcal) than sitting or walking.

Key Chemical Equation for Cellular Respiration

The overall reaction for aerobic cellular respiration is:

$\mathrm{C_6H_{12}O_6} + 6\ \mathrm{O_2} \rightarrow 6\ \mathrm{CO_2} + 6\ \mathrm{H_2O} + \text{ATP (energy)}$

• Oxidation: Glucose loses hydrogen atoms (is oxidized) to form carbon dioxide.

• Reduction: Oxygen gains hydrogen atoms (is reduced) to form water.

Stages of Cellular Respiration

1. Glycolysis

Glycolysis is the first stage of cellular respiration and occurs in the cytosol. It breaks down one molecule of glucose into two molecules of pyruvate, producing a small amount of ATP and NADH.

• Location: Cytosol

• Oxygen Requirement: Can occur with or without oxygen

• Phases:

  • Energy Investment Phase

  • Cleavage Phase

  • Energy Liberation Phase

• Net Output (per glucose):

  • 2 Pyruvate

  • 2 ATP (net gain)

  • 2 NADH

2. Pyruvate Oxidation

Pyruvate produced in glycolysis is transported into the mitochondrial matrix, where it is converted into Acetyl-CoA, producing NADH and releasing CO2.

• Location: Mitochondrial matrix

• Inputs: 2 Pyruvate

• Outputs (per glucose):

  • 2 Acetyl-CoA

  • 2 NADH

  • 2 CO2

3. Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose by oxidizing Acetyl-CoA to CO2. It generates ATP, NADH, and FADH2 for use in the next stage.

• Location: Mitochondrial matrix

• Inputs (per turn): 1 Acetyl-CoA

• Outputs (per glucose, 2 turns):

  • 4 CO2

  • 2 ATP

  • 6 NADH

  • 2 FADH2

• Cycle: Oxaloacetate is regenerated to start the cycle again.

4. Oxidative Phosphorylation

This stage includes the electron transport chain and chemiosmosis, which together produce the majority of ATP during cellular respiration.

• Location: Inner mitochondrial membrane

• Process: Electrons from NADH and FADH2 are transferred through protein complexes, creating a proton gradient that drives ATP synthesis via ATP synthase.

• ATP Yield: About 28 ATP molecules per glucose (almost 90% of total ATP from respiration)

Summary Table: Stages of Cellular Respiration

Stage	Main Location	Inputs	Outputs	ATP Produced
Glycolysis	Cytosol	Glucose, 2 NAD+, 2 ADP	2 Pyruvate, 2 NADH, 2 ATP	2
Pyruvate Oxidation	Mitochondrial Matrix	2 Pyruvate, 2 NAD+	2 Acetyl-CoA, 2 NADH, 2 CO2	0
Citric Acid Cycle	Mitochondrial Matrix	2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP	2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	10 NADH, 2 FADH2, O2	H2O, ~28 ATP	~28

Fermentation: Anaerobic Energy Harvesting

Overview of Fermentation

Fermentation allows cells to produce ATP without oxygen by regenerating NAD+ for glycolysis. It results in the formation of lactic acid (in animals) or ethanol and CO2 (in yeast).

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid, regenerating NAD+.

• Alcoholic Fermentation: Pyruvate is converted to ethanol and CO2, regenerating NAD+.

• ATP Yield: Only 2 ATP per glucose (from glycolysis).

• Applications: Used in muscle cells during intense exercise and by yeast in brewing and baking.

Connections Between Metabolic Pathways

Integration of Metabolism

Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration. Intermediates from glycolysis and the citric acid cycle can be used for biosynthesis of other molecules.

• Entry Points: Different macromolecules enter respiration at various stages (e.g., fatty acids enter as Acetyl-CoA).

• Anabolism: Some intermediates are diverted for the synthesis of amino acids, nucleotides, and other biomolecules.

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• NADH/FADH2: Electron carriers that store energy used to make ATP.

• Oxidation: Loss of electrons or hydrogen atoms from a molecule.

• Reduction: Gain of electrons or hydrogen atoms by a molecule.

• Substrate-level Phosphorylation: Direct formation of ATP by transfer of a phosphate group to ADP.

• Oxidative Phosphorylation: ATP formation driven by the transfer of electrons through the electron transport chain and chemiosmosis.