Membrane Transport and Cell Signaling

Plasma Membrane: Structure and Function

The plasma membrane (PM) is a selectively permeable barrier found in all cells, separating the internal environment from the external surroundings. Its major functions include protection, communication, and regulation of transport.

• Composition: Primarily composed of a phospholipid bilayer, proteins, and carbohydrates.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins and lipids moving laterally within the bilayer.

• Amphipathic Molecule: Phospholipids have both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Membrane Fluidity: Influenced by temperature and cholesterol. Higher temperatures increase fluidity; cholesterol stabilizes membrane at both high and low temperatures.

Proteins in Membranes:

• Integral proteins: Span the membrane; involved in transport and signaling.

• Peripheral proteins: Attached to membrane surface; involved in cell communication and structure.

• Transport proteins: Facilitate movement of substances across the membrane.

• Receptor proteins: Receive and transmit signals.

Carbohydrates: Glycoproteins and glycolipids are involved in cell recognition and signaling.

Selectively Permeable: The membrane allows certain molecules (e.g., small, nonpolar) to cross easily, while others require transport proteins.

• Easy passage: Small, nonpolar molecules (O2, CO2).

• Restricted passage: Large, polar, or charged molecules.

Transport Mechanisms

• Passive Transport: Movement without energy input; includes diffusion, osmosis, and facilitated diffusion.

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a membrane.

• Facilitated Diffusion: Passive transport aided by proteins.

• Aquaporin: Channel protein for water transport.

• Active Transport: Movement against concentration gradient, requires energy (ATP).

• Endocytosis: Bulk transport into the cell.

• Exocytosis: Bulk transport out of the cell.

Osmosis and Tonicity

• Hypertonic: Higher solute concentration outside; water leaves cell.

• Hypotonic: Lower solute concentration outside; water enters cell.

• Isotonic: Equal solute concentration; no net water movement.

• Effects on Cells:

  • Animal cells: Lyse (burst) in hypotonic, shrivel in hypertonic, normal in isotonic.

  • Plant cells: Turgid (firm) in hypotonic, flaccid in isotonic, plasmolyzed in hypertonic.

• Turgid: Firm, swollen plant cell.

• Flaccid: Limp plant cell.

• Lyse: Animal cell bursting.

Cell Signaling

• Signal Transduction Pathway: Series of steps converting a signal to a cellular response.

• Types of Signaling:

  • Paracrine: Local signaling to nearby cells.

  • Endocrine: Long-distance signaling via hormones.

  • Synaptic: Signaling between neurons.

  • Direct Contact: Cell-to-cell communication via membrane proteins.

• Stages of Cell Signaling:

  1. Reception: Signal detected by receptor.

  2. Transduction: Signal relayed and amplified.

  3. Response: Cellular activity triggered.

• Ligand: Molecule that binds to a receptor to initiate signaling.

• Responses: Changes in gene expression or cellular activity.

An Introduction to Metabolism

Metabolism and Metabolic Pathways

Metabolism is the sum of all chemical reactions in an organism. Metabolic pathways are sequences of reactions, each catalyzed by a specific enzyme.

• Catabolic pathways: Break down molecules, releasing energy.

• Anabolic pathways: Build molecules, requiring energy.

Energy and Thermodynamics

• Energy: Capacity to do work.

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy.

• Chemical Energy: Energy stored in chemical bonds.

• First Law of Thermodynamics: Energy cannot be created or destroyed.

• Second Law of Thermodynamics: Entropy (disorder) increases in spontaneous processes.

• Entropy: Measure of disorder.

Free Energy and Reactions

• Free Energy (G): Energy available to do work.

• Spontaneous processes: Occur without energy input; increase entropy.

• Exergonic Reaction: Releases free energy; .

• Endergonic Reaction: Requires energy input; .

• Energy Coupling: Using exergonic reactions to drive endergonic ones.

ATP: Structure and Function

• ATP (Adenosine Triphosphate): Main energy currency of cells.

• Hydrolysis of ATP: Releases energy by breaking phosphate bond.

• Phosphorylated: Molecule with added phosphate group.

• ATP Regeneration:

Enzymes and Catalysis

• Enzyme: Protein catalyst speeding up reactions.

• Activation Energy: Energy needed to start a reaction.

• Substrate: Reactant acted on by enzyme.

• Active Site: Region where substrate binds.

• Specificity: Enzymes act on specific substrates.

• Properties: Lower activation energy, do not change .

• Effect of Heat and pH: Extreme conditions can denature enzymes.

• Cofactors: Non-protein helpers (e.g., vitamins, minerals).

• Competitive Inhibition: Inhibitor binds active site.

• Noncompetitive Inhibition: Inhibitor binds elsewhere, changes enzyme shape.

• Allosteric Regulation: Regulation by binding at a site other than active site.

• Feedback Inhibition: End product inhibits pathway.

Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a catabolic pathway that breaks down glucose to produce ATP. It occurs in three main parts of the mitochondria: matrix, inner membrane, and intermembrane space.

• Fermentation: Anaerobic process producing ATP without oxygen.

• Redox Reaction: Transfer of electrons; oxidation (loss), reduction (gain).

• NADH: Electron carrier.

• Electron Transport Chain: Series of proteins transferring electrons to oxygen.

Stages of Cellular Respiration

• Glycolysis: Occurs in cytosol; splits glucose into pyruvate.

  • Energy investment phase: Uses ATP.

  • Energy payoff phase: Produces ATP and NADH.

• Pyruvate Oxidation: Pyruvate enters mitochondria, converted to Acetyl CoA.

• Citric Acid Cycle: Occurs in matrix; Acetyl CoA combines with oxaloacetate to form citrate. Produces NADH, FADH2, ATP, CO2.

• Oxidative Phosphorylation: Electron transport chain and chemiosmosis; produces most ATP.

ATP Synthesis

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP.

• Oxidative phosphorylation: ATP produced by ATP synthase using energy from electron transport chain.

• ATP Synthase: Enzyme in cristae; uses proton gradient for ATP production.

• Chemiosmosis: Movement of protons across membrane to drive ATP synthesis.

Fermentation

• Occurs when oxygen is absent.

• Major products: Lactic acid or ethanol.

• Aerobic vs Anaerobic: Aerobic uses oxygen; anaerobic does not.

• Pyruvate: Fork in pathway; can enter mitochondria or undergo fermentation.

• Other molecules: Carbohydrates, fats, proteins can enter respiration pathway.

• Role of Oxygen: Final electron acceptor in electron transport chain.

• Regulation: Accumulation of ATP or NADH inhibits process.

ATP Yield Table

Photosynthesis

Autotrophs and Heterotrophs

• Autotroph: Organism that produces its own food.

• Photoautotroph: Uses light energy (plants).

• Chemoautotroph: Uses chemical energy.

• Heterotroph: Consumes other organisms for energy.

Chloroplast Structure

• Chlorophyll: Main pigment for photosynthesis.

• Stomata: Openings for gas exchange.

• Mesophyll cells: Contain chloroplasts.

• Stroma: Fluid inside chloroplast.

• Thylakoid membranes: Site of light reactions.

• Grana: Stacks of thylakoids.

Photosynthesis Equation

Basic chemical equation:

Light Reactions

• Location: Thylakoid membranes.

• Inputs: Light, water.

• Outputs: ATP, NADPH, O2.

• Provides: Energy and reducing power for Calvin cycle.

Color and Pigments

• Chlorophyll: Absorbs blue and red light; reflects green.

• Absorption spectra: Shows wavelengths absorbed.

• Action spectra: Shows effectiveness of wavelengths for photosynthesis.

Photosystems

• Photosystem I and II: Involved in light reactions.

• Deciduous trees: Change color in fall due to breakdown of chlorophyll.

Calvin Cycle

• Location: Stroma.

• Inputs: CO2, ATP, NADPH.

• Outputs: G3P (sugar).

• Phases: Carbon fixation, reduction, regeneration.

• Key enzyme: Rubisco.

Photorespiration and Plant Types

• Photorespiration: Rubisco binds O2 instead of CO2; wastes energy.

• Affinity: Strength of binding.

• C3 plants: Standard Calvin cycle.

• C4 plants: Spatial separation; PEP carboxylase fixes CO2.

• CAM plants: Temporal separation; fix CO2 at night.

• Minimizing photorespiration: C4 and CAM plants use adaptations to reduce O2 binding.

Plant Types Comparison Table

Spatial vs Temporal Separation: C4 plants separate steps in different cells; CAM plants separate steps by time (day/night).