Cellular Respiration

Overview and Importance

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules, primarily glucose, to produce ATP. It occurs in several stages, each with distinct inputs, outputs, and cellular locations. Oxygen is required for aerobic respiration, while fermentation provides an alternative pathway in the absence of oxygen.

• Purpose: To convert biochemical energy from nutrients into ATP, the cell's energy currency.

• Main Steps: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle (Krebs Cycle), Oxidative Phosphorylation.

• Location: Glycolysis in cytosol; other steps in mitochondria.

Detailed Steps of Cellular Respiration

• Glycolysis: Occurs in the cytosol. Glucose (6C) is split into two pyruvate (3C) molecules. Produces 2 ATP (via substrate-level phosphorylation) and 2 NADH.

• Pyruvate Oxidation: Pyruvate is transported into the mitochondria and converted to acetyl-CoA, producing NADH and releasing CO2.

• Citric Acid Cycle (Krebs Cycle): Acetyl-CoA enters the cycle, generating 2 ATP (per glucose), 6 NADH, 2 FADH2, and 4 CO2 (per glucose).

• Oxidative Phosphorylation: Includes the electron transport chain (ETC) and chemiosmosis. NADH and FADH2 donate electrons to the ETC, creating a proton gradient used by ATP synthase to produce ATP (about 26-28 ATP per glucose).

ATP Production Mechanisms

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP during glycolysis and the citric acid cycle.

• Oxidative Phosphorylation: ATP synthesis powered by the proton gradient generated by the ETC.

Key Terms and Concepts

• ATP Synthase: Enzyme that synthesizes ATP using the energy from the proton gradient.

• NADH & FADH2: Electron carriers that transport electrons to the ETC.

• Electron Transport Chain (ETC): Series of protein complexes that transfer electrons and pump protons.

• Final Electron Acceptor: Oxygen in aerobic respiration; forms water.

• Proton Gradient: Drives ATP synthesis via chemiosmosis.

Fermentation

Fermentation is an anaerobic process that allows cells to generate ATP when oxygen is unavailable. It regenerates NAD+ for glycolysis.

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2.

• Purpose: To sustain ATP production in the absence of oxygen.

Redox Reactions

Redox reactions involve the transfer of electrons between molecules, fundamental to energy extraction in cellular respiration.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Example: NAD+ is reduced to NADH during glycolysis and the citric acid cycle.

Aerobic vs. Anaerobic Respiration

• Aerobic: Requires oxygen; produces more ATP.

• Anaerobic: Does not require oxygen; includes fermentation; produces less ATP.

Photosynthesis

Overview and Importance

Photosynthesis is the process by which autotrophic organisms convert light energy into chemical energy, producing glucose and oxygen. It occurs in two main stages: the light-dependent reactions (LDR) and the Calvin cycle (CC).

• Reactants: CO2, H2O, sunlight.

• Products: Glucose, O2.

• Location: Chloroplasts; LDR in thylakoid membranes, CC in stroma.

Light-Dependent Reactions (LDR)

• Linear Electron Flow: Electrons move from water through photosystem II and I, ETC, to NADP+ forming NADPH.

• Key Steps: Splitting of water (releases O2), generation of ATP (via chemiosmosis), production of NADPH.

• Photosystems: Protein complexes with pigments (chlorophyll) that absorb light.

• Electron Carrier: NADPH transports electrons to the Calvin cycle.

• ATP: Produced by ATP synthase using the proton gradient.

Calvin Cycle (CC)

• Purpose: To fix carbon dioxide and synthesize glucose.

• Stages: Carbon fixation, reduction, regeneration of RuBP.

• CO2 Requirement: 3 CO2 molecules are needed to produce one G3P; two G3P combine to form glucose.

Light and Pigments

• Photon: A quantum of light energy.

• Wavelength: Determines energy and color; visible light is used in photosynthesis.

• Electromagnetic Spectrum: Range of all light wavelengths; visible light (400-700 nm) is absorbed by pigments.

• Pigments: Chlorophyll (main pigment), carotenes (accessory pigments).

• Absorption vs. Reflection: Pigments absorb specific wavelengths; green is reflected by chlorophyll.

Photosynthesis Efficiency

• Best Wavelengths: Blue and red light are most effective for photosynthesis.

• Importance: Provides energy for nearly all life, produces oxygen.

C3, C4, and CAM Plants

• C3 Plants: Use the Calvin cycle directly; most common.

• C4 Plants: Adapted to hot, dry environments; fix CO2 in a separate cell layer.

• CAM Plants: Open stomata at night; store CO2 for use during the day.

• Summary: All use photosynthesis but differ in how they fix carbon to minimize water loss and photorespiration.

Autotrophs and Heterotrophs

• Autotrophs: Organisms that produce their own food via photosynthesis (producers).

• Heterotrophs: Organisms that consume other organisms for energy (consumers).

Stomata

• Definition: Pores on leaf surfaces that regulate gas exchange.

• Importance: Allow CO2 in and O2 out; help control water loss.

Summary Table: Cellular Respiration vs. Photosynthesis

Key Equations

• Cellular Respiration:

• Photosynthesis:

Example: During vigorous exercise, muscle cells may switch to lactic acid fermentation when oxygen is scarce, producing lactic acid and sustaining ATP production.

Example: Plants use chlorophyll to absorb blue and red light, driving the light-dependent reactions of photosynthesis.

Additional info: The guide emphasizes the importance of understanding terminology and process details for exam preparation. Students should be familiar with the flow of energy and electrons, the role of electron carriers, and the differences between plant adaptations (C3, C4, CAM).