Cellular Respiration: Overview and Pathways

Introduction to Cellular Respiration

Cellular respiration is a series of metabolic processes that convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It is central to both catabolism (breaking down molecules for energy) and anabolism (using energy to build molecules).

• Catabolism: Breakdown of complex molecules to release energy.

• Anabolism: Synthesis of complex molecules using energy.

Four Interconnected Processes of Cellular Respiration

1. Glycolysis

2. Pyruvate Processing

3. The Citric Acid Cycle (Krebs Cycle)

4. Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis)

Inputs and Outputs of Each Process

Fate of Atoms Consumed in Foods

• Carbon atoms from glucose are released as CO2.

• Hydrogen atoms are transferred to NAD+ and FAD, forming NADH and FADH2.

• Oxygen atoms are used as the final electron acceptor, forming H2O.

Energy Transfer and Transformation

• Energy from glucose is gradually transferred to ATP via substrate-level and oxidative phosphorylation.

• NADH and FADH2 carry high-energy electrons to the electron transport chain.

• Proton gradients drive ATP synthesis via ATP synthase.

Glycolysis: Steps and Regulation

Glycolysis is the process by which glucose is broken down into pyruvate, producing ATP and NADH.

• Two Phases:

  • Energy Investment Phase: 2 ATP are used to phosphorylate glucose and its intermediates.

  • Energy Payoff Phase: 4 ATP and 2 NADH are produced per glucose molecule.

• Regulation: Key regulatory step is catalyzed by phosphofructokinase, which is inhibited by high levels of ATP (feedback inhibition).

• Consequences of Pathway Alteration: Disruption can lead to accumulation or depletion of intermediates, affecting energy production and metabolic balance.

Fermentation

Fermentation allows cells to generate ATP from glucose in the absence of oxygen by regenerating NAD+ from NADH.

• Inputs: Glucose, 2 ADP, 2 Pi

• Outputs: 2 ATP, 2 NAD+, and end products such as lactate (in animals) or ethanol and CO2 (in yeast).

• ATP Yield: Much lower than aerobic respiration (2 ATP per glucose).

Pyruvate Processing, Citric Acid Cycle, and Electron Transport Chain

Pyruvate Processing

Pyruvate is transported into the mitochondria and converted into acetyl CoA, producing NADH and CO2.

• Regulation: Controlled by feedback inhibition; high levels of ATP, NADH, or acetyl CoA inhibit the process.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of glucose by breaking down acetyl CoA into CO2 and transferring electrons to NAD+ and FAD.

• Energy Harvest: Each turn produces 3 NADH, 1 FADH2, and 1 ATP (or GTP) per acetyl CoA.

• Regulation: Controlled by substrate availability and feedback inhibition by ATP and NADH.

• Pathway Alteration: Disruption can reduce ATP production and cause accumulation of intermediates.

Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) uses electrons from NADH and FADH2 to pump protons across the mitochondrial membrane, creating a proton gradient that drives ATP synthesis via ATP synthase.

• Relationship to ATP Synthase: The proton gradient powers ATP synthase to convert ADP and Pi into ATP.

• Effects of Alteration: Inhibiting ETC components or ATP synthase reduces or stops ATP production.

• Substrate-Level vs. Oxidative Phosphorylation:

  • Substrate-Level Phosphorylation: Direct transfer of phosphate to ADP (in glycolysis and citric acid cycle).

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

Photosynthesis: Light Reactions and Carbon Fixation

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.

• Overall Equation:

Light-Capturing Reactions

• Inputs: H2O, light, NADP+, ADP, Pi

• Outputs: O2, NADPH, ATP

• Pigments: Chlorophylls absorb light energy, initiating electron transport.

Calvin Cycle (Carbon Fixation Reactions)

• Inputs: CO2, ATP, NADPH

• Outputs: Glucose (or G3P), ADP, NADP+

• Key Enzyme: Rubisco catalyzes the fixation of CO2.

Redox Changes in Photosynthesis

• CO2 is reduced to form glucose.

• H2O is oxidized to form O2.

• Energy is stored in the chemical bonds of glucose.

Electron Flow in Chloroplasts

• Role of Rubisco: Catalyzes the first step of carbon fixation in the Calvin cycle.

• Energy Transformations: Light energy is converted to chemical energy (ATP, NADPH), then used to reduce CO2 to sugars.

• Pathway Alterations: Disruption can decrease ATP, NADPH, or O2 production.

Carbon Fixation Pathways

• C3 Pathway: Most common; CO2 fixed directly by rubisco.

• C4 and CAM Pathways: Adaptations to minimize photorespiration and water loss in hot, dry environments.

Reduction of CO2 to Sugars

• Stimuli: Light intensity, CO2 concentration, and temperature affect photosynthesis rate.

• Fate of Sugar: Used for energy, stored as starch, or used to build other organic molecules.

• Source of Plant Mass: The carbon in CO2 is incorporated into sugars, which accumulate as plant biomass.

Cell Communication: Mechanisms and Pathways

Extracellular Material and Cell Walls

• Cell Wall (Plants): Provides structure and protection; composed mainly of cellulose.

• Extracellular Matrix (ECM, Animals): Network of proteins and carbohydrates; supports cells and mediates signaling.

• Integrins: Transmembrane proteins that connect the ECM to the cytoskeleton, facilitating communication and adhesion.

Interactions Between Adjacent Cells

• Plant Cells: Connected by plasmodesmata, allowing direct cytoplasmic exchange.

• Animal Cells: Connected by gap junctions, tight junctions, and desmosomes.

• Cadherins: Calcium-dependent adhesion proteins important for cell-cell adhesion in animals.

• Notch Signaling: Direct cell-to-cell communication pathway; Notch receptor activation influences cell fate decisions.

Long-Distance Cell Communication

• Lipid-Soluble Signals: (e.g., steroid hormones) diffuse through membranes and bind intracellular receptors.

• Lipid-Insoluble Signals: (e.g., peptide hormones) bind to cell surface receptors (G-protein coupled or enzyme-linked).

Signal Reception and Processing

• Intracellular Receptors: Bind lipid-soluble signals; receptor-signal complex acts as a transcription factor.

• G-Protein Coupled Receptors (GPCRs): Activate intracellular G-proteins, triggering signaling cascades.

• Enzyme-Linked Receptors: Often have intrinsic kinase activity; initiate phosphorylation cascades.

• Notch Receptors: Mediate direct cell-cell signaling without diffusible intermediates.

Deactivation of Signaling

• Removal of signal molecule, receptor internalization, or dephosphorylation of signaling proteins can deactivate pathways.

Comparison of Receptor Types

Effects of Pathway Alterations

• Changes in receptors or signaling proteins can amplify, reduce, or block signal transduction, affecting cell behavior and function.

Example: A mutation in a GPCR may prevent hormone response, while overactive kinase signaling can lead to uncontrolled cell growth (cancer).

Additional info: These notes synthesize core concepts from cellular respiration, photosynthesis, and cell communication, providing a foundation for understanding energy flow and information exchange in cells.