Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a series of metabolic pathways that extract energy from organic molecules, primarily glucose, to produce ATP. This process is essential for both plant and animal cells and involves redox reactions and energy transformations.

• ATP (Adenosine Triphosphate): The main energy currency of the cell, used to power most cellular work.

• Energy Flow: Energy enters ecosystems as sunlight, is stored in organic molecules via photosynthesis, and is released as heat during cellular respiration.

• Cellular Respiration: Occurs mainly in mitochondria, breaking down organic molecules to generate ATP.

• Key Waste Products: Carbon dioxide (CO2) and water (H2O).

• Catabolic Pathways: Pathways that break down molecules and release energy.

Catabolic Pathways and ATP Production

• Aerobic Respiration: Uses oxygen as the final electron acceptor, yielding the most ATP.

• Anaerobic Respiration: Uses other molecules as final electron acceptors in the absence of oxygen.

• Fermentation: Partial degradation of sugars without oxygen, producing less ATP.

Redox Reactions: Oxidation and Reduction

Redox reactions are central to cellular respiration, involving electron transfer between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Reducing Agent: Electron donor.

• Oxidizing Agent: Electron acceptor.

• Energy Release: Electrons moving toward more electronegative atoms (like oxygen) release energy.

Oxidation of Organic Fuel Molecules

• Glucose Oxidation: Electrons are transferred from glucose to oxygen, releasing energy for ATP synthesis.

• Organic Molecules: Rich in hydrogen, they are excellent sources of high-energy electrons.

The Stages of Cellular Respiration

Cellular respiration consists of three main stages:

1. Glycolysis: Occurs in the cytoplasm; breaks down glucose into two pyruvate molecules.

2. Pyruvate Oxidation and Citric Acid Cycle: Occurs in mitochondria; completes the breakdown of glucose.

3. Oxidative Phosphorylation: Electron transport chain and chemiosmosis produce most ATP.

Glycolysis

• Location: Cytoplasm.

• Phases: Energy investment (uses 2 ATP) and energy payoff (produces 4 ATP and 2 NADH).

• Net Yield: 2 ATP and 2 NADH per glucose.

• Oxygen Requirement: Can occur with or without oxygen.

Pyruvate Oxidation

• Conversion: Pyruvate is converted to acetyl CoA by pyruvate dehydrogenase.

• Reactions: Releases CO2, reduces NAD+ to NADH, and forms acetyl CoA.

The Citric Acid Cycle (Krebs Cycle)

• Steps: Eight steps, each catalyzed by a specific enzyme.

• First Step: Acetyl group joins oxaloacetate to form citrate.

• Cycle: Citrate is decomposed back to oxaloacetate.

• Electron Carriers: NADH and FADH2 carry electrons to the electron transport chain.

• Turns: Runs twice per glucose molecule.

Oxidative Phosphorylation

• Electron Transport Chain: Embedded in the inner mitochondrial membrane; passes electrons through carrier molecules.

• Chemiosmosis: Energy from electron transport pumps protons, creating a gradient that drives ATP synthesis via ATP synthase.

• Proton-Motive Force: The gradient of protons across the membrane, used to do cellular work.

ATP Yield from Cellular Respiration

• Efficiency: About 34% of energy from glucose is transferred to ATP; the rest is lost as heat.

• Variability: Exact ATP yield varies due to coupling efficiency, electron shuttle mechanisms, and use of proton-motive force for other work.

Fermentation

• Purpose: Extends glycolysis by regenerating NAD+ using organic molecules as electron acceptors.

• Types: Alcohol fermentation and lactic acid fermentation.

Connections to Other Metabolic Pathways

• Intersections: Glycolysis and the citric acid cycle are central to various catabolic and anabolic pathways.

• Catabolic Pathways: Funnel electrons from carbohydrates, proteins, and fats into cellular respiration.

Regulation of Cellular Respiration

• Feedback Mechanisms: Primarily feedback inhibition regulates enzyme activity at strategic points to prevent wasteful production.

Photosynthesis

Introduction to Photosynthesis

Photosynthesis is the process by which plants, algae, and certain prokaryotes convert solar energy into chemical energy, producing organic molecules from carbon dioxide and water. It is fundamental to life on Earth.

• Autotrophs: Organisms that produce their own food from inorganic substances (e.g., plants).

• Heterotrophs: Obtain organic molecules by consuming other organisms.

Sites and Structures of Photosynthesis

• Chloroplasts: Organelles in leaf mesophyll cells where photosynthesis occurs.

• Stroma: Dense fluid inside the chloroplast.

• Thylakoids: Membranous sacs within the stroma, often stacked into grana, containing chlorophyll.

• Stomata: Microscopic pores for gas exchange (CO2 in, O2 out).

• Veins: Transport water and export sugars.

The Photosynthetic Equation and Redox Nature

• Overall Equation:

• Redox Process: CO2 is reduced to sugar; H2O is oxidized to O2.

• Endergonic: Requires energy input from sunlight.

Stages of Photosynthesis

• Light Reactions (in thylakoids): Convert solar energy to chemical energy (ATP and NADPH), split water, and release O2.

• Calvin Cycle (in stroma): Uses ATP and NADPH to convert CO2 into sugars through carbon fixation.

The Nature of Sunlight and Pigments

• Light: Electromagnetic energy; visible spectrum (380–740 nm) is used in photosynthesis.

• Pigments: Absorb specific wavelengths; chlorophyll a is the main pigment, with chlorophyll b and carotenoids as accessory pigments.

• Photons: Discrete packets of light energy; shorter wavelengths have higher energy.

• Leaves appear green: Chlorophyll absorbs violet-blue and red light, reflecting green.

Measuring Pigment Absorption

• Spectrophotometer: Measures light absorption at different wavelengths, producing an absorption spectrum.

• Action Spectrum: Shows effectiveness of different wavelengths in driving photosynthesis.

• Accessory Pigments: Broaden the spectrum and provide photoprotection.

Excitation of Chlorophyll

• Excitation: Absorption of light excites electrons to a higher energy state.

• Energy Release: In isolation, energy is released as heat or fluorescence.

Photosystems and Electron Flow

• Photosystem: Reaction-center complex surrounded by light-harvesting complexes.

• Types: Photosystem II (P680) and Photosystem I (P700).

• Light-harvesting complexes: Transfer energy to the reaction center, where an excited electron is transferred to a primary electron acceptor.

Linear Electron Flow

• Main Pathway: Involves both photosystems, producing ATP and NADPH.

• Water Splitting: Provides electrons and releases O2.

• Electron Transport Chain: Generates a proton gradient for ATP synthesis (photophosphorylation).

• NADP+ Reduction: Forms NADPH.

Cyclic Electron Flow

• Process: Electrons cycle back to Photosystem I, producing ATP but not NADPH or O2.

• Function: Provides additional ATP for the Calvin cycle and may protect cells from light-induced damage.

Chemiosmosis and ATP Synthesis

• Mechanism: Both chloroplasts and mitochondria use chemiosmosis to generate ATP, but differ in energy sources and spatial organization.

The Calvin Cycle

• Purpose: Uses ATP and NADPH to convert CO2 into the three-carbon sugar glyceraldehyde 3-phosphate (G3P).

• Phases: Carbon fixation, reduction, and regeneration of the CO2 acceptor (RuBP).

• Key Enzyme: Rubisco catalyzes the attachment of CO2 to RuBP.

• Energy Requirement: For each G3P produced, the cycle consumes 9 ATP and 6 NADPH.

Adaptations in Carbon Fixation

• Photorespiration: Occurs when rubisco binds O2 instead of CO2, reducing photosynthetic efficiency.

• C3 Plants: Convert CO2 into a 3-carbon sugar; use photorespiration.

• C4 Plants: Fix CO2 into a four-carbon compound in mesophyll cells, which is transported to bundle-sheath cells for the Calvin cycle, minimizing photorespiration (e.g., corn, sugarcane).

• CAM Plants: Open stomata at night to fix CO2 into organic acids; during the day, CO2 is released for the Calvin cycle, reducing water loss (e.g., succulents, pineapples).

Cell Communication: Mechanisms and Pathways

Introduction to Cell Communication

Cell communication is essential for coordinating activities in multicellular organisms, allowing cells to detect and respond to environmental signals. It is crucial for processes such as growth, immune responses, and development.

Evolution of Cell Signaling

• Origins: Likely evolved in prokaryotes and single-celled eukaryotes, later adapted by multicellular organisms.

• Quorum Sensing: Enables bacteria to sense population density via signaling molecules, influencing behaviors like biofilm formation and toxin secretion.

• Yeast Signaling: Saccharomyces cerevisiae uses secreted factors to locate cells of the opposite mating type, initiating a signal transduction pathway for mating.

• Molecular Mechanisms: Signal transduction is conserved across yeasts and mammals.

Types of Cell Signaling

• Local Signaling: Includes direct contact (via cell junctions or surface molecules) and local regulators (e.g., growth factors).

• Paracrine Signaling: Secreted molecules affect nearby cells.

• Synaptic Signaling: In the nervous system, neurotransmitters cross synapses to target cells.

• Long-distance Signaling: Hormones travel through the circulatory system to reach distant target cells.

The Three Stages of Cell Signaling

1. Reception: Detection of a signaling molecule by a receptor protein.

2. Transduction: Conversion of the signal to a form that can bring about a specific cellular response, often through a series of steps (signal transduction pathway).

3. Response: The specific cellular activity triggered by the transduced signal.

1. Signal Reception

• Ligand: A signaling molecule that binds specifically to a receptor.

• Binding: Highly specific; usually causes a shape change in the receptor, initiating signal transduction.

• Receptors: Most are in the plasma membrane; some are intracellular.

Types of Membrane Receptors

• G protein-coupled receptors (GPCRs): Largest family of cell-surface receptors; involved in many physiological processes.

• Receptor tyrosine kinases (RTKs): Enzyme-linked receptors that can activate multiple pathways; often implicated in cancer when dysfunctional.

• Ion channel receptors: Ligand-gated channels that open or close in response to signaling molecules, allowing ions to flow across the membrane.

Intracellular Receptors

• Location: Cytoplasm or nucleus.

• Ligands: Bind small or hydrophobic molecules (e.g., steroid hormones).

• Hormone-receptor complex: Often acts as a transcription factor, regulating gene expression.

2. Signal Transduction

• Phosphorylation Cascade: Protein kinases sequentially activate each other by adding phosphate groups.

• Protein Phosphatases: Remove phosphates, turning off the signal.

• Amplification: Cascades amplify the signal and provide opportunities for regulation.

Second Messengers

• Second Messengers: Small, nonprotein molecules or ions (e.g., cyclic AMP, Ca2+) that relay signals inside the cell.

• Cyclic AMP (cAMP): Produced from ATP by adenylyl cyclase; activates protein kinase A.

• Calcium ions (Ca2+): Widely used as a second messenger; concentration is tightly regulated by cellular compartments.

• Inositol trisphosphate (IP3) and diacylglycerol (DAG): Produced by cleavage of membrane phospholipids; help release Ca2+ from the endoplasmic reticulum.

3. Cellular Responses to Signals

• Nuclear Responses: Activation of transcription factors in the nucleus, leading to changes in gene expression.

• Cytoplasmic Responses: Changes in enzyme activity or ion channel function.

Regulation of the Response

• Amplification: Enzyme cascades can greatly increase the magnitude of the response.

• Specificity: Different cell types have different proteins, allowing for specific responses to the same signal.

• Scaffolding Proteins: Organize and increase the efficiency of signal transduction pathways.

• Termination: Inactivation mechanisms ensure that signals are not perpetuated longer than necessary.

Programmed Cell Death (Apoptosis)

• Apoptosis: A form of programmed cell death that prevents damage to neighboring cells by packaging cellular components into vesicles for digestion.