Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. This process occurs in both producers (such as plants, which store energy as starch) and consumers (such as animals, which store energy as glycogen).

• Reactants: Sugar (glucose) and oxygen gas

• Products: Carbon dioxide, water, and energy (ATP)

• General Equation:

• Producers: Store energy as starch

• Consumers: Store energy as glycogen

• Energy Released: Used for cellular work

Example: Cows (consumers) use cellular respiration to convert glycogen into usable energy for movement and growth.

Photosynthesis and Cellular Respiration in Ecosystems

Photosynthesis and cellular respiration are interconnected processes in ecosystems. Photosynthesis captures sunlight energy to produce glucose and oxygen, while cellular respiration breaks down glucose to release energy, carbon dioxide, and water.

• Photosynthesis: Occurs in chloroplasts of producers (plants, algae)

• Cellular Respiration: Occurs in mitochondria of both producers and consumers

Example: Plants use photosynthesis to make glucose, which is then used in cellular respiration to generate ATP.

Stages of Cellular Respiration

ATP and Electron Carriers

ATP (adenosine triphosphate) is the main energy currency of the cell. During cellular respiration, energy is transferred to ATP and electron carriers such as NADH and FADH2.

• ATP Formation:

• NAD+ and FAD: Electron carriers that become NADH and FADH2 when reduced

Example: NAD+ + 2e- + 2H+ → NADH + H+

Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It involves the splitting of glucose (a six-carbon sugar) into two molecules of pyruvate (three carbons each).

• Location: Cytoplasm

• Inputs: 1 glucose, 2 ATP, 2 NAD+

• Outputs: 2 pyruvate, 4 ATP (net gain: 2 ATP), 2 NADH

Equation:

Transition Reaction (Formation of Acetyl-CoA)

The transition reaction links glycolysis to the citric acid cycle. Pyruvate is transported into the mitochondrion and converted into acetyl-CoA, releasing carbon dioxide and producing NADH.

• Location: Mitochondrial matrix

• Inputs: 2 pyruvate, 2 NAD+, 2 Coenzyme A

• Outputs: 2 acetyl-CoA, 2 NADH, 2 CO2

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle is a series of enzyme-catalyzed reactions in the mitochondrial matrix that completes the breakdown of glucose derivatives, generating electron carriers and ATP.

• Location: Mitochondrial matrix

• Inputs: 2 acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP + 2 Pi

• Outputs (per glucose): 4 CO2, 6 NADH, 2 FADH2, 2 ATP

Example: Each turn of the cycle processes one acetyl-CoA, producing 3 NADH, 1 FADH2, 1 ATP, and 2 CO2.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

The electron transport chain is the final stage of cellular respiration, occurring in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through protein complexes, driving the production of ATP via chemiosmosis.

• Location: Inner mitochondrial membrane

• Inputs: NADH, FADH2, O2

• Outputs: ATP, H2O

• ATP Yield: About 32-34 ATP per glucose molecule

Key Steps:

• Electrons are passed along the chain, releasing energy

• Energy is used to pump protons (H+) across the membrane, creating a gradient

• ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi

• Oxygen acts as the final electron acceptor, forming water

Equation:

Aerobic vs. Anaerobic Respiration

Cellular respiration can proceed with or without oxygen. In the presence of oxygen (aerobic), the process continues through the citric acid cycle and electron transport chain. Without oxygen (anaerobic), cells undergo fermentation.

• Aerobic Respiration: Complete breakdown of glucose, high ATP yield

• Anaerobic Respiration (Fermentation): Incomplete breakdown, low ATP yield, produces lactic acid or ethanol

Example: Muscle cells perform lactic acid fermentation during intense exercise when oxygen is limited.

Summary Table: Major Steps of Cellular Respiration

Additional info: The total ATP yield from one molecule of glucose during aerobic respiration is approximately 36-38 ATP, depending on the cell type and shuttle systems used for transporting electrons into the mitochondria.