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Cellular Respiration, Photosynthesis, and ATP Generation Overview

Overview of Cellular Respiration

  • Cellular respiration is a series of processes that generate ATP from the stepwise catabolism of glucose.
    • Simplified equation: glucose + O₂ → CO₂ + H₂O + energy.

Components of Cellular Respiration

  • Comprises several metabolic processes:
    1. Glycolysis
    2. Pyruvate oxidation
    3. Citric acid cycle
    4. Oxidative phosphorylation

Cellular Locations

  • Each process takes place in specific locations within the cell:
    • Glycolysis: Cytoplasm
    • Pyruvate oxidation: Mitochondrial matrix
    • Citric acid cycle: Mitochondrial matrix
    • Oxidative phosphorylation: Inner mitochondrial membrane

Glycolysis

Changes in the 6C Molecule through Glycolysis

  • Starts with glucose (6C), which undergoes various transformations to produce:
    • Glyceraldehyde 3-phosphate (G3P) (3C) and then pyruvate (3C).

Investment and Harvesting Phases

  • Investment Phase:
    • Initial energy investment where ATP is utilized to modify glucose and breakdown into intermediates.
  • Energy Harvesting Phase:
    • ATP and NADH are produced, generating a net gain of ATP.

Roles of Enzymes

  • Kinases: Enzymes that add phosphate groups to substrates.
  • Isomerases: Enzymes that rearrange the structure of a molecule.
  • Dehydrogenases: Enzymes that remove hydrogen atoms and transfer them to electron carriers.

Accounting from One Molecule of Glucose

  • ATP Generated: 2 ATP (net gain)
  • Reduced Electron Carriers (NADH) Produced: 2 NADH

Key Molecules

  • Glucose: Starting molecule (6C).
  • Glyceraldehyde 3-phosphate (G3P): Intermediate (3C).
  • Pyruvate: End product (3C).

Pyruvate Oxidation

Changes in the 3C Molecule through Pyruvate Oxidation

  • Converts pyruvate into acetyl CoA (2C), releasing CO₂.

Roles of Enzymes

  • Dehydrogenases: Facilitate the conversion by removing electrons from pyruvate.

Accounting for Products from One Molecule of Glucose

  • ATP Generated: 0 ATP (no net gain in this step).
  • Reduced Electron Carriers (NADH) Produced: 2 NADH.

Key Molecules

  • Pyruvate: Converted into acetyl CoA.
  • Acetyl CoA: Enters the citric acid cycle (2C).
  • Coenzyme A: Assists in the transfer of acetate.

Citric Acid Cycle

Changes in Molecules (2C, 4C, 6C)

  • Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).
  • Progresses through transformations to regenerate oxaloacetate.

Roles of Enzymes

  • Kinases: Participate in phosphorylation reactions.
  • Dehydrogenases: Remove electrons producing NADH and FADH2.

Accounting for Products from One Molecule of Glucose

  • ATP Generated: 2 ATP (1 per cycle; 2 cycles from 1 glucose).
  • Reduced Electron Carriers:
    • NADH Produced: 6 NADH.
    • FADH₂ Produced: 2 FADH₂.

Key Molecules

  • Acetyl: Derived from Acetyl-CoA entering the cycle.
  • Oxaloacetate: Reactant and product, necessary for cycle continuity.
  • Citrate: Intermediate molecule produced in the cycle.

Oxidative Phosphorylation

Processes Involved

  • Involves the electron transport chain and ATP synthase.
    • Electron Transport Chain Role: Transfers electrons through multiple complexes; generates a proton gradient.
    • ATP Synthase Role: Uses the proton gradient to synthesize ATP from ADP.

Electron Journey in the Chain

  • Source of Electrons: Nadh and FADH2 donate electrons to the chain.
  • Electron Destination: Electrons are eventually transferred to oxygen forming water.

Role of Chemiosmosis

  • Powers proton pumps that create a proton gradient across the inner mitochondrial membrane.
  • Protons are concentrated in the intermembrane space.

Revisiting Overall Process of Cellular Respiration

  • In the equation: glucose + O₂ → CO₂ + H₂O + energy:
    • What happens to glucose? It is oxidized to CO₂.
    • What happens to oxygen? It is reduced to form H₂O.
    • What happens to CO₂? It is released as a waste product.

Fermentation as an Alternative Mechanism for ATP Generation

Anaerobic Conditions and Oxidative Phosphorylation

  • Oxidative phosphorylation cannot occur under anaerobic conditions as there is no final electron acceptor (oxygen).

Benefits of Fermentation

  • Enables ATP production in the absence of oxygen, allowing survival under anaerobic conditions.

Disadvantages of Fermentation

  • Less efficient than cellular respiration; yields only 2 ATP per glucose.

Lactic Acid Fermentation

  • Organisms: Bacteria and animal muscle cells.
  • Pyruvate: Converted into lactic acid.
  • Significance of Dehydrogenase Involvement: Involved in redox reactions, regenerating NAD+ for glycolysis.

Alcohol Fermentation

  • Organisms: Yeasts and some bacteria.
  • Pyruvate: Converted to ethanol and CO₂.
  • Significance of Dehydrogenase Involvement: Converts acetyldehyde back into ethanol; also regenerates NAD+.

Overview of Photosynthesis

  • Photosynthesis generates ATP and NADPH via harvesting light energy, utilized for carbon fixation and sugar generation.
    • Simplified equation: CO₂ + H₂O + energy → sugars + O₂.

Components of Photosynthesis

  • Comprises metabolic processes:
    1. Light reactions
    2. Dark reactions (Calvin cycle)

Cellular Locations of Photosynthesis

  • Light reactions occur in the thylakoid membranes of chloroplasts.
  • Dark reactions occur in the stroma of chloroplasts.

The Nature of Light and Chlorophyll

General Properties of Light

  • Photon: The fundamental unit of light.
  • Wavelengths of visible light range from approximately 400 nm (violet) to 700 nm (red).
  • Greatest Energy Wavelengths: Violet (short wavelengths); Least Energy Wavelengths: Red (long wavelengths).

Chlorophyll Function

  • Chlorophyll acts as a pigment that absorbs light energy, facilitating photosynthesis.
  • Absorbance Spectrum: A graph depicting how much light is absorbed at various wavelengths.
  • Action Spectrum: Shows the rates of photosynthesis at different wavelengths, correlating with absorption spectra of chlorophyll.

Color of Plants

  • Plants appear green because chlorophyll absorbs red and blue wavelengths and reflects green.

Electron Dynamics in Chlorophyll

  • When energized by light, electrons in chlorophyll molecules become excited and move to a higher energy level.
  • Fluorescence: Occurs when excited electrons return to their ground state; energy is released as light.

Organization and Function of Photosystems

Parts of a Photosystem

  • Light Harvesting Complexes: Contain multiple chlorophyll and accessory pigments arranged to capture light efficiently.
  • Reaction Center: Contains “special” chlorophylls that transfer electrons to the primary electron acceptor.

Photon Interaction with Chlorophyll

  • As photons hit chlorophyll molecules in the complexes, they excite electrons, initiating the photosynthetic process.

Types of Photosystems

  • Photosystem II (P680): Absorbs light at a wavelength of 680 nm, initiates linear electron flow.
  • Photosystem I (P700): Absorbs light at a wavelength of 700 nm, involved in both linear and cyclic electron flow.

The Light Reactions

Linear Electron Flow

  • Purpose: Generate ATP and NADPH for the Calvin cycle.
  • Order of Photosystem Utilization: Photosystem II (P680) is first to engage because it can split water to produce O₂.
    • Source of O₂: Generated when water molecules are split during linear electron flow; essential for the electron transport chain.
    • Purpose of Electron Transport Chain: Transfers electrons, establishing a proton gradient to drive ATP synthesis.
    • Final Electron Destination: NADP+ is reduced to NADPH.
    • Protons necessary for ATP synthesis are concentrated within the thylakoid lumen during the light reactions.

Cyclic Electron Flow

  • Purpose: Provides additional ATP when NADPH levels are high and additional ATP is needed for dark reactions.
  • Only Photosystem I (P700) is involved, and during cyclic flow, an electron returns to the same photosystem after passing through the electron transport chain.

The Dark Reactions (Calvin Cycle)

Pathway of Molecules (1C, 5C, 3C)

  • The Calvin cycle uses carbon dioxide (1C) to produce glyceraldehyde 3-phosphate (G3P) (3C) from ribulose 1,5-bisphosphate (RuBP) (5C).

Key Enzyme

  • Rubisco (Ribulose bisphosphate carboxylase/oxygenase): Catalyzes the first step of carbon fixation by attaching CO₂ to RuBP.

Important Phenomena in the Calvin Cycle

  • Carbon Fixation: Incorporation of CO₂ into organic molecules.
  • Reduction: Converts 3-phosphoglycerate to G3P using ATP and NADPH.
  • Regeneration: Reformation of RuBP to continue the cycle.

Redox Reactions in Dark Reactions

  • Involve transferring electrons and storing energy in sugar molecules.

Use of Sugars Beyond Dark Reactions

  • The simple sugars generated are further processed into various macromolecules through different metabolic pathways.

Accounting for Key Molecules

  • Ribulose 1,5-bisphosphate (RuBP): Starting and ending substrate in the cycle.
  • Glyceraldehyde 3-phosphate (G3P): Product of the Calvin cycle, used to form glucose and other carbohydrates.
  • Carbon Dioxide: Essential substrate for the synthesis of organic compounds.

Different Forms of ATP Generation

Three Forms of ATP Generation

  1. Substrate-level Phosphorylation: Directly generates ATP via enzyme-catalyzed reactions.
  2. Oxidative Phosphorylation: ATP synthesis dependent on the electron transport chain and chemiosmosis.
  3. Photophosphorylation: ATP generation through light-driven processes in chloroplasts during photosynthesis.