Chapter 5: Membrane Structure and Function

Fluid-Mosaic Model of Membranes

The fluid-mosaic model describes the dynamic and complex structure of biological membranes, which are essential for cellular function and compartmentalization.

• Phospholipid bilayer: Hydrophilic heads face outward toward aqueous environments; hydrophobic tails face inward, away from water.

• Lateral movement: Lipids and proteins can move sideways within the layer, contributing to membrane fluidity.

• Proteins: Embedded (integral) or attached (peripheral) in the bilayer, serving various functions.

• Cholesterol: Modulates membrane fluidity, preventing rigidity in cold and excessive permeability in heat.

• Selectively permeable: Allows certain substances to cross while restricting others.

Phospholipid Behavior and Chemical Properties

Phospholipids self-assemble into bilayers due to their amphipathic nature, influencing membrane permeability.

• Hydrophilic heads: Interact with water, facing outward.

• Hydrophobic tails: Face inward, allowing hydrophobic substances to pass more easily.

• Permeability: Hydrophobic molecules cross easily; hydrophilic molecules require transport proteins.

Membrane Fluidity: Temperature, Fatty Acids, and Cholesterol

Membrane fluidity is affected by environmental and molecular factors.

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Saturated fatty acids: Straight tails pack tightly, reducing fluidity.

• Unsaturated fatty acids: Kinked tails (double bonds) prevent tight packing, increasing fluidity.

• Cholesterol: Maintains optimal fluidity across temperature ranges.

Roles of Membrane Proteins

Membrane proteins perform diverse functions critical to cell survival.

• Transport: Channel and carrier proteins facilitate movement of substances.

• Receptors: Receive and transmit signals.

• Enzymatic activity: Catalyze reactions at the membrane.

• Cell-cell recognition: Enable immune response and tissue organization.

• Intercellular joining: Form junctions between cells.

• Attachment: Connect to extracellular matrix (ECM) and cytoskeleton for structural support.

Diffusion and Molecular Movement

Diffusion is the passive movement of molecules from high to low concentration.

• Direction: Molecules move down their concentration gradient.

• Permeability: More permeable solutions allow easier movement.

Substances Crossing the Membrane

• Readily crossing: Small, hydrophobic, nonpolar molecules.

• Require transport proteins: Large, hydrophilic, polar molecules.

Types of Passive Transport

Passive transport does not require energy and moves substances down their concentration gradient.

• Simple diffusion: Direct movement through the bilayer (small, nonpolar molecules).

• Facilitated diffusion: Movement via transport proteins (large, polar molecules, ions).

Diffusion vs. Osmosis

Osmosis is a specific type of diffusion involving water.

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

• Osmosis: Movement of water across a selectively permeable membrane, often facilitated by aquaporins.

• Conditions: Occurs when solute concentrations differ across a membrane.

Tonicity: Isotonic, Hypertonic, Hypotonic

Tonicity describes the effect of a solution on cell volume.

• Isotonic: No net water movement.

• Hypertonic: Cell loses water; plant cells become flaccid (plasmolysis).

• Hypotonic: Cell gains water; plant cells become turgid.

Active vs. Passive Transport

Active transport requires energy to move substances against their gradient; passive transport does not.

• Active transport: Uses ATP; includes primary (e.g., sodium-potassium pump), secondary (co-transport), and bulk transport (exocytosis, endocytosis).

• Passive transport: Includes simple diffusion, facilitated diffusion, and osmosis.

Co-Transport

Co-transport couples the movement of one substance down its gradient with another against its gradient (secondary active transport).

• Factors: ATP availability, membrane potential, concentration gradients, temperature, pH, substrate availability.

Bulk Transport

Cells uptake large materials via endocytosis and release them via exocytosis.

• Endocytosis: Includes phagocytosis (eating), pinocytosis (drinking), and receptor-mediated endocytosis.

• Exocytosis: Release of substances via vesicles.

Chapter 6: An Introduction to Metabolism

Catabolic vs. Anabolic Reactions

Metabolic pathways are classified as catabolic (breaking down) or anabolic (building up).

• Catabolic: Release energy by breaking down complex molecules (e.g., cellular respiration).

• Anabolic: Use energy to build complex molecules (e.g., photosynthesis).

Types of Energy

Energy exists in various forms and can be transformed within biological systems.

• Potential energy: Stored due to position or structure.

• Chemical energy: Potential energy in chemical bonds.

• Kinetic energy: Energy of motion.

• Thermal energy: Total kinetic energy of molecules.

• Heat: Transfer of thermal energy between objects.

Entropy and the Second Law of Thermodynamics

Entropy measures disorder; energy transfers increase entropy.

• Entropy (S): Higher disorder means higher entropy.

• Second law: Energy transfers increase the entropy of the universe.

• Example: Dye dispersing in water increases entropy.

Maintaining Order in Living Systems

Life maintains order by being an open system, exchanging energy and matter with the environment.

• Coupling: Endergonic (energy-requiring) processes are coupled to exergonic (energy-releasing) ones.

Free Energy and Reaction Direction

Free energy determines whether a reaction is spontaneous.

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

• Equation:

• Spontaneous: (exergonic)

• Nonspontaneous: (endergonic)

• Equilibrium:

Exergonic vs. Endergonic Reactions

• Exergonic: Net release of free energy; spontaneous ( negative).

• Endergonic: Absorbs free energy; nonspontaneous ( positive).

Spontaneity vs. Reaction Rate

Spontaneity indicates energetic favorability, not speed.

• Spontaneous: Occurs without energy input.

• Rate: How fast a reaction proceeds.

Examples of Reaction Types

• Sodium-potassium pump: Endergonic, nonspontaneous.

• Polymer synthesis: Endergonic.

• ATP hydrolysis: Exergonic ().

• Diffusion: Exergonic, spontaneous.

Reaction Coupling in Cells

Cells couple exergonic and endergonic reactions to drive unfavorable processes.

• ATP hydrolysis: Provides energy for endergonic reactions.

Role of ATP

ATP powers cellular work.

• Chemical work: Drives endergonic reactions (e.g., polymer synthesis).

• Transport work: Pumps substances across membranes.

• Mechanical work: Powers movement (e.g., cilia, muscle contraction).

Enzymes and Reaction Rate

Enzymes lower activation energy, speeding up reactions without altering equilibrium.

• Active site: Substrate binds via weak interactions.

• Induced fit: Enzyme changes shape for snug substrate binding.

• Reusable: Enzymes can catalyze multiple reactions.

Factors Affecting Reaction Rate

• Activation energy: Lower activation energy increases rate.

• Enzyme concentration: More enzymes increase rate.

• Inhibitors: Decrease rate (competitive or noncompetitive).

Regulation of Metabolic Pathways

Enzymes regulate metabolic pathways via activation energy and inhibition.

• Allosteric regulation: Regulatory molecules bind at sites other than the active site.

• Feedback inhibition: End product inhibits pathway enzyme.

Chapter 7: Cellular Respiration and Fermentation

Relationship Between Photosynthesis and Cellular Respiration

Photosynthesis and cellular respiration are interconnected processes in the cycling of matter and flow of energy.

• Photosynthesis:

• Cellular respiration:

• Reactants and products: Products of one are reactants of the other.

• Energy: Stored in glucose by photosynthesis, released as ATP by respiration.

Chemical Equation and Energy Change in Cellular Respiration

• Equation:

• Steps: Glycolysis, citric acid cycle, oxidative phosphorylation.

Redox Reactions: Oxidation and Reduction

Redox reactions involve electron transfer.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Oxidizing agent: Electron acceptor.

• Reducing agent: Electron donor.

• Example:

Redox in Combustion and Cellular Respiration

• Combustion: Fuel is oxidized; O2 is reduced.

• Cellular respiration: Glucose is oxidized to CO2; O2 is reduced to H2O.

NAD+ Function in Cellular Respiration

NAD+ acts as an electron carrier, accepting electrons and becoming NADH.

• Oxidizing agent: Accepts electrons in glycolysis, pyruvate oxidation, citric acid cycle.

• Electron transport chain: NADH donates electrons, helping build H+ gradient.

Major Steps of Cellular Respiration

ATP Generation: Substrate-Level vs. Oxidative Phosphorylation

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP (glycolysis, citric acid cycle).

• Oxidative phosphorylation: ATP produced via electron transport chain and chemiosmosis.

Events and Energy in Each Step

• Glycolysis: Splits glucose; exergonic overall; ATP via substrate-level phosphorylation.

• Pyruvate oxidation: Exergonic; no ATP produced.

• Citric acid cycle: Exergonic; ATP via substrate-level phosphorylation.

• Oxidative phosphorylation: Exergonic; ATP via chemiosmosis.

Anaerobic Conditions and Fermentation

When O2 is limited, cells switch to fermentation to regenerate NAD+.

• Fermentation: Pyruvate reduced to lactate, oxidizing NADH to NAD+.

• ATP yield: Lower than aerobic respiration; lactate accumulates.

Effects of Disrupted Cellular Respiration

• Glycolysis and fermentation: Become primary ATP sources.

• ATP yield: Drops; lactate and H+ accumulate.

• Disruption: Less pyruvate, NADH, acetyl-CoA, and ATP produced.

Chapter 8: Photosynthesis

Flow of Matter and Energy: Autotrophs vs. Heterotrophs

Photosynthesis and cellular respiration link plants (autotrophs) and animals (heterotrophs).

• Autotrophs: Produce their own food via photosynthesis.

• Heterotrophs: Consume other organisms; rely on cellular respiration.

• Matter: Cycles between organisms; energy flows and dissipates as heat.

Leaf and Chloroplast Structure for Photosynthesis

Leaf and chloroplast structures maximize photosynthetic efficiency.

• Leaf: Broad, thin, high surface area; upper epidermis is transparent.

• Mesophyll: Packed with chloroplasts near surface.

• Stomata: Regulate gas exchange.

• Veins: Transport water and sugars.

• Thylakoid membranes: Stacked (grana) for increased surface area.

• Chloroplasts: Shift position for optimal light capture.

Net Reaction and Atom Tracing in Photosynthesis

• Equation:

• Carbon in glucose: From CO2.

• O2 released: From H2O.

• Hydrogen in glucose: From water, via NADPH.

Stages of Photosynthesis: Light Reactions and Calvin Cycle

Photosynthesis consists of light reactions and the Calvin cycle, which are interdependent.

• Light reactions: Occur in thylakoid membranes; produce ATP, NADPH, and O2.

• Calvin cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into carbohydrates.

• Interdependence: Calvin cycle needs ATP/NADPH from light reactions; light reactions need ADP, Pi, NADP+ from Calvin cycle.

Location of Photosynthetic Stages

• Light reactions: Thylakoid membranes.

• Calvin cycle: Stroma.

Light Harvesting and Absorption Spectra

Pigments absorb specific wavelengths of light, affecting photosynthetic efficiency.

• Pigments: Absorb visible light; chlorophyll absorbs violet-blue and red best.

• Energy: Shorter wavelengths (violet-blue) have more energy per photon.

• Absorption spectrum: Plots pigment absorption vs. wavelength.

Chlorophyll Excitation and Return to Ground State

• Photon absorption: Excites electron to higher energy state.

• Return: Electron drops to ground state, releasing energy as heat.

Photosystems and Electron Flow

Photosystems capture light energy and transfer electrons through the electron transport chain.

• Photosystem II (PSII): P680 chlorophyll; splits water, releases O2, initiates electron flow.

• Photosystem I (PSI): P700 chlorophyll; reenergizes electrons, reduces NADP+ to NADPH.

• Electron transport chain: Builds H+ gradient for ATP synthesis.

Comparison of PSII and PSI

Water-Splitting Reaction

• Replenishes electrons in PSII.

• Releases O2 as byproduct.

• Contributes H+ for gradient.

ATP and NADPH Production in Photosynthesis

• Electron transport chain: Energy used to pump H+ into thylakoid lumen.

• ATP synthase: H+ flows back to stroma, driving ATP formation.

• NADPH: PSI excites electrons to reduce NADP+.

Calvin Cycle: Phases, Reactants, and Products

The Calvin cycle fixes carbon and synthesizes carbohydrates using ATP and NADPH.

• Carbon fixation: Rubisco attaches CO2 to RuBP; forms 3-carbon molecules.

• Reduction: ATP and NADPH convert 3-PGA to G3P.

• Regeneration: G3P rearranged to regenerate RuBP.

• Reactants: CO2, ATP, NADPH, RuBP.

• Products: G3P (a sugar).

• ATP and NADPH: Provide energy and reducing power.

Electron Carriers, ETC, H+ Gradients, and ATP Synthase: Photosynthesis vs. Respiration

Citric Acid Cycle vs. Calvin Cycle

• Citric acid cycle: Breaks down carbon fuels to CO2; makes NADH/FADH2.

• Calvin cycle: Builds sugars from CO2; uses ATP/NADPH.

Predicting O2, CO2, and Glucose Levels in Closed Systems

• Animals only: O2 decreases, CO2 increases, glucose decreases.

• Plants only (light): CO2 decreases, O2 increases, glucose increases.

• Plants only (dark): O2 decreases, CO2 increases, glucose decreases.

• Both: Net changes depend on balance of photosynthesis and respiration.

Additional info: Academic context and explanations were expanded for clarity and completeness. Tables were recreated for comparison and classification purposes.