Cellular Respiration: How Cells Harvest Chemical Energy

Overview of Energy Harvesting in Cells

Cells require energy to perform essential life processes. This energy is primarily obtained through two mechanisms: cellular respiration and photosynthesis. Cellular respiration is the process by which cells extract energy from organic molecules, while photosynthesis is the process by which plants and some other organisms convert solar energy into chemical energy.

• Cellular respiration occurs in both aerobic (with oxygen) and anaerobic (without oxygen) conditions.

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

Photosynthesis and Cellular Respiration: The Energy Cycle

Photosynthesis and cellular respiration are interconnected processes that sustain life on Earth. Photosynthesis captures energy from sunlight to produce organic molecules and oxygen, while cellular respiration breaks down these molecules to release energy for cellular work.

• Photosynthesis occurs in chloroplasts and produces glucose and oxygen as a byproduct.

• Cellular respiration occurs in mitochondria and produces ATP, carbon dioxide, and water.

• ATP (adenosine triphosphate) is the main energy currency of the cell.

Example: Plants use photosynthesis to make glucose, which is then used by both plants and animals in cellular respiration to generate ATP.

Cellular Respiration: The Overall Chemical Equation

Cellular respiration is an exergonic (energy-releasing) process that transfers energy from glucose to ATP. The overall chemical equation for the aerobic respiration of glucose is:

• Oxygen is required for the reaction.

• Carbon dioxide is released as a waste product.

• Water is produced and used by the cell.

• ATP is generated to power cellular functions.

Breathing and Cellular Respiration

Breathing (ventilation) is the physical process of exchanging gases with the environment. It supplies oxygen for cellular respiration and removes carbon dioxide produced as a waste product.

• Oxygen inhaled becomes part of water in the body.

• Carbon dioxide exhaled is produced from the breakdown of organic molecules.

Redox Reactions in Cellular Respiration

Cellular respiration involves a series of redox (reduction-oxidation) reactions in which electrons are transferred from fuel molecules (such as glucose) to oxygen.

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

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

• NAD+ is an electron carrier that becomes reduced to NADH during these reactions.

• Electrons are passed through the electron transport chain, ultimately reducing oxygen to water.

Example: In the reaction , glucose is oxidized and oxygen is reduced.

Stages of Cellular Respiration

Cellular respiration consists of three main stages, each with distinct locations and functions:

1. Glycolysis (in the cytoplasm)

2. Pyruvate oxidation and the Citric Acid Cycle (in the mitochondrial matrix)

3. Oxidative phosphorylation (in the inner mitochondrial membrane)

Stage 1: Glycolysis

Glycolysis is the metabolic pathway that breaks down glucose (6 carbons) into two molecules of pyruvate (3 carbons each). It occurs in the cytosol and does not require oxygen.

• Net gain: 2 ATP (4 produced, 2 used), 2 NADH, and 2 pyruvate per glucose molecule.

• Consists of an energy investment phase (uses ATP) and an energy payoff phase (produces ATP and NADH).

• Intermediates are formed between the initial reactant and the final product.

Stage 2: Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate oxidation converts pyruvate into acetyl CoA, releasing CO2 and generating NADH. The citric acid cycle (Krebs cycle) completes the breakdown of glucose, producing CO2, NADH, FADH2, and a small amount of ATP.

• For each glucose: 2 acetyl CoA, 2 CO2, and 2 NADH from pyruvate oxidation.

• Each turn of the citric acid cycle yields: 2 CO2, 3 NADH, 1 FADH2, and 1 ATP (per acetyl CoA).

• For both turns (per glucose): 4 CO2, 6 NADH, 2 FADH2, and 2 ATP.

Stage 3: Oxidative Phosphorylation

Oxidative phosphorylation includes the electron transport chain and chemiosmosis. It occurs in the inner mitochondrial membrane and produces most of the ATP generated by cellular respiration.

• NADH and FADH2 donate electrons to the electron transport chain.

• Energy from electrons is used to pump H+ ions into the intermembrane space, creating a proton gradient.

• Oxygen is the final electron acceptor, forming water.

• ATP synthase uses the proton gradient to synthesize ATP from ADP and inorganic phosphate.

ATP Yield from Cellular Respiration

Additional info: The exact ATP yield can vary depending on cell type and conditions.

Fermentation: Anaerobic Harvesting of Energy

Fermentation allows cells to produce ATP without oxygen. It relies on glycolysis and regenerates NAD+ by reducing pyruvate to lactic acid (lactic acid fermentation) or to ethanol and CO2 (alcohol fermentation).

• Produces only 2 ATP per glucose (from glycolysis).

• Used by muscle cells under anaerobic conditions, yeasts, and some bacteria.

• Fermentation is less efficient than aerobic respiration.

Example: Yeast cells perform alcohol fermentation to produce ethanol and CO2 in the absence of oxygen.

Connections Between Metabolic Pathways

Cells can use carbohydrates, fats, and proteins as fuel for cellular respiration. These molecules enter the pathway at different points after being converted to intermediates of glycolysis, pyruvate oxidation, or the citric acid cycle.

• Carbohydrates are broken down to glucose.

• Fats are hydrolyzed to fatty acids and glycerol, which enter glycolysis or the citric acid cycle.

• Proteins are deaminated and their carbon skeletons enter as intermediates.

Summary Table: Comparison of Aerobic Respiration and Fermentation

Evolutionary Significance of Glycolysis

Glycolysis is a universal pathway found in nearly all organisms, indicating its ancient evolutionary origin. It occurs in the cytosol and does not require oxygen, suggesting it evolved before the atmosphere became oxygen-rich.

• Highly conserved across prokaryotes and eukaryotes.

• Functions as the foundation for both aerobic and anaerobic metabolism.