Cellular Respiration

Overview and Importance

Cellular respiration is a fundamental catabolic process by which cells convert glucose into ATP, the primary energy currency used to power cellular activities. This process occurs in all prokaryotic and eukaryotic cells, though the mechanisms differ slightly between these groups. - ATP is used for various cellular functions, including movement, muscle contraction, cell division, and transport processes. - Passive processes such as diffusion and osmosis do not require ATP. - Cellular respiration is a universal process, but eukaryotes utilize mitochondria, while prokaryotes rely on their cytoplasm and cell membrane.

Summary Reaction

The overall reaction for cellular respiration is: - A single molecule of glucose yields approximately 30-32 ATP molecules. - Direct combustion of glucose releases too much energy; cellular respiration captures it in manageable ATP packets.

Relationship to Photosynthesis

Cellular respiration and photosynthesis are interconnected metabolic pathways: - Photosynthesis is anabolic (builds glucose), while cellular respiration is catabolic (breaks down glucose). - Both processes involve organelles with complex internal membranes (chloroplasts and mitochondria). - Plants produce glucose via photosynthesis, then use it in mitochondria for cellular respiration. - Animals obtain glucose by consuming plants or other animals.

Three Sequential Steps of Cellular Respiration

Step 1: Glycolysis

Glycolysis is the first step, occurring in the cytoplasm and does not require oxygen. It is considered a primitive process, present in both prokaryotes and eukaryotes. - Location: Cytoplasm - Input: Glucose (C6H12O6) - Output: 2 Pyruvate, 2 ATP (net), 2 NADH - Energy budget: 2 ATP invested, 4 ATP produced, net yield 2 ATP - Isomers: Glucose and fructose are hexose sugars with the same formula but different spatial arrangements.

Step 2: Citric Acid (Krebs) Cycle

The citric acid cycle occurs in the mitochondrion and is limited to eukaryotic cells. Each glucose molecule produces two pyruvates, so the cycle runs twice per glucose. - Location: Mitochondrion - Input: 2 Pyruvate (from glycolysis) converted to Acetyl CoA - Output: 2 ATP (1 per cycle), NADH, FADH2, CO2 - Key intermediates: Citrate (citric acid), malate (malic acid), fumarate, succinate, oxaloacetate, α-ketoglutarate - Cycle produces: 1 ATP, 3 NADH, 1 FADH2 per turn

Step 3: Electron Transport Chain (ETC)

The ETC is located on the inner mitochondrial membrane (cristae) and is the main site of ATP production. - Location: Mitochondrion (cristae) - Input: NADH, FADH2, O2 - Output: H2O, ~26-28 ATP - Oxygen: Required as the final electron acceptor; plants derive O2 from photolysis, animals inhale it. - ATP synthase: Enzyme responsible for ATP production, recognized by the 1997 Nobel Prize in Chemistry (Paul Boyer & John Walker).

Summary Table: Cellular Respiration Steps

Anaerobic Respiration and Fermentation

Overview

Anaerobic respiration and fermentation occur in organisms living in oxygen-poor environments. These processes are less efficient, producing fewer ATP molecules. - Examples: Mud flats, hot sulfur springs, deep-sea vents, cake batter (yeasts). - Yeasts: Unicellular fungi capable of both aerobic and anaerobic metabolism.

Fermentation Pathways

Fermentation produces economically important by-products: - Baking yeast: Glucose → ATP + CO2 (CO2 causes dough to rise) - Alcoholic yeast: Glucose → ATP + Ethanol + CO2 (used in alcoholic beverage production)

Key Terms and Concepts

• ATP (Adenosine Triphosphate): The energy currency of the cell.

• Glycolysis: The breakdown of glucose into pyruvate, yielding ATP and NADH.

• Citric Acid Cycle (Krebs Cycle): A series of reactions that generate ATP, NADH, and FADH2 from Acetyl CoA.

• Electron Transport Chain: A sequence of proteins that transfer electrons and produce ATP.

• Fermentation: Anaerobic process producing ATP and by-products like ethanol or CO2.

• Catabolic Pathway: Breaks down complex molecules, releasing energy.

• Anabolic Pathway: Builds complex molecules, requiring energy.

Additional info:

• Paul Boyer and John Walker were awarded the Nobel Prize for their work on ATP synthase, a key enzyme in cellular respiration.

• Malic acid and citric acid, familiar from fruits, are intermediates in the citric acid cycle present in all eukaryotic cells.