Cellular Respiration

\Definition and Overview

Cellular respiration is a fundamental metabolic process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It is essential for providing energy to support cellular activities.

• Definition: Cellular respiration is the process by which cells break down glucose and other organic molecules to produce ATP, the cell’s main energy currency.

• Ultimate Source of Energy: The sun is the ultimate source of energy for most life on Earth, as it drives photosynthesis, which in turn provides the organic molecules used in cellular respiration.

• Relationship to Photosynthesis: Photosynthesis captures energy from sunlight to produce glucose, while cellular respiration releases that energy for cellular work.

• General Formula:

• Reactants and Products: Reactants: glucose and oxygen; Products: carbon dioxide, water, and ATP.

• Breathing and Cellular Respiration: Breathing supplies oxygen for cellular respiration and removes carbon dioxide produced by it.

Key Concepts

• Exergonic Reactions: Cellular respiration is exergonic, meaning it releases energy.

• ATP: The main energy carrier molecule produced during cellular respiration.

• Oxidation and Reduction:

  • Oxidation: Loss of electrons from a molecule.

  • Reduction: Gain of electrons by a molecule.

  • Redox Reactions: Chemical reactions involving the transfer of electrons.

• Electron Acceptors: NAD+ and FAD are intermediate electron acceptors, reduced to NADH and FADH2 during respiration.

Stages of Cellular Respiration

Cellular respiration occurs in several stages, each with specific locations, requirements, and products.

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into two pyruvate molecules; does not require oxygen; produces ATP and NADH.

• Pyruvate Oxidation: Occurs in the mitochondrial matrix; converts pyruvate to acetyl-CoA, producing NADH and CO2.

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes the breakdown of glucose; produces ATP, NADH, FADH2, and CO2.

• Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; occurs in the inner mitochondrial membrane; uses NADH and FADH2 to generate a proton gradient, driving ATP synthesis via ATP synthase.

Summary Table: Stages of Cellular Respiration

Glycolysis Details

• Steps: 10 steps, divided into two phases: energy investment and energy payoff.

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP to form ATP.

• Regulation: Glycolysis is regulated by feedback inhibition, especially at the enzyme phosphofructokinase.

• Spontaneity: Glycolysis is overall exergonic but requires initial energy input.

Citric Acid Cycle (Krebs Cycle)

• Occurs: Twice per glucose molecule (once per pyruvate).

• Key Intermediates: Citrate, oxaloacetate, acetyl-CoA.

• ATP Synthase: Enzyme complex that synthesizes ATP using the proton gradient generated by the electron transport chain.

Oxidative Phosphorylation

• Parts: Electron transport chain (ETC) and chemiosmosis.

• Oxygen's Role: Final electron acceptor in the ETC; absence of O2 halts the process.

• ATP Production: Most ATP is produced here via chemiosmosis.

Fermentation

• Definition: Anaerobic process that allows glycolysis to continue by regenerating NAD+.

• Types: Lactic acid fermentation and alcoholic fermentation.

Biomolecules for ATP Production

• Cells can use carbohydrates, fats, and proteins to make ATP.

Photosynthesis

Definition and Overview

Photosynthesis is the process by which photoautotrophs convert light energy into chemical energy, producing organic molecules and oxygen from carbon dioxide and water.

• General Equation:

• Producers: Photoautotrophs (e.g., plants, algae, cyanobacteria) perform photosynthesis.

• Heterotrophs: Organisms that obtain energy by consuming other organisms.

Chloroplast Structure

• Thylakoids: Membranous sacs where light reactions occur.

• Grana: Stacks of thylakoids.

• Stroma: Fluid surrounding thylakoids; site of the Calvin cycle.

• Chlorophyll: Main pigment that absorbs light energy.

Photosynthesis: Reactants and Products

• Source of Atoms: Carbon from CO2, hydrogen from H2O, oxygen from both CO2 and H2O.

• Fate of O Atoms: Oxygen released comes from water, not CO2.

• Oxidation/Reduction: Water is oxidized (loses electrons), CO2 is reduced (gains electrons).

Stages of Photosynthesis

• Light Reactions: Occur in thylakoid membranes; require light; produce ATP, NADPH, and O2.

• Calvin Cycle (Dark Reactions): Occur in the stroma; use ATP and NADPH to fix CO2 into sugars; do not require light directly.

Summary Table: Stages of Photosynthesis

Photosystems and Pigments

• Photosystem II and I: Protein complexes that absorb light and drive electron transport; require photons to excite electrons.

• Carotenoids: Accessory pigments that protect chlorophyll and broaden the spectrum of light absorption.

• Color Least Effective: Green light is least effective because it is reflected, not absorbed, by chlorophyll.

• Leaf Color Change: In fall, chlorophyll degrades, revealing carotenoids (yellow/orange pigments).

Photophosphorylation

• Definition: Production of ATP using the energy of sunlight during the light reactions.

• Gradient: Proton gradient across the thylakoid membrane drives ATP synthesis via ATP synthase.

Calvin Cycle Details

• Steps: Three main phases: carbon fixation, reduction, and regeneration of RuBP.

• Key Molecules: RuBP (ribulose bisphosphate), G3P (glyceraldehyde-3-phosphate).

• G3P: Produced in the Calvin cycle; can be converted into glucose and other organic molecules.

• Light Requirement: Does not require light directly but depends on ATP and NADPH from light reactions.

Photorespiration

• Definition: A process where oxygen is fixed instead of CO2, reducing photosynthetic efficiency.

Types of Photosynthesis

• C3 Plants: Use the Calvin cycle directly; most common; less efficient in hot, dry environments.

• C4 Plants: Partition carbon fixation and the Calvin cycle into different cell types to minimize photorespiration.

• CAM Plants: Open stomata at night to reduce water loss; fix CO2 at night and run the Calvin cycle during the day.

Comparison Table: C3, C4, and CAM Photosynthesis

Ecological and Global Importance

• Base of Food Chain: Photosynthesis produces glucose, which is the primary energy source for most life forms.

• Importance: Provides oxygen and organic molecules necessary for life.

• Impact of Rising CO2: Can increase photosynthetic rates but may also contribute to climate change.

• Greenhouse Effect: Accumulation of CO2 and other gases traps heat, leading to global warming.

• Deforestation: Reduces the planet’s capacity to absorb CO2, exacerbating climate change.

Additional info:

• Some details, such as the exact number of ATP produced, may vary depending on cell type and conditions.

• Tables and comparisons are inferred from standard biology curricula.