Membrane Structure and Function

Fluid-Mosaic Model of Membranes

The fluid-mosaic model describes the structure of biological membranes as a dynamic combination of lipids and proteins. This model explains how membranes maintain flexibility and allow for various cellular processes.

• Phospholipids form a bilayer, with hydrophobic tails facing inward and hydrophilic heads facing outward.

• Membrane proteins are embedded within or attached to the bilayer, serving diverse functions.

• The membrane is "fluid" because lipids and proteins can move laterally within the layer.

Example: The plasma membrane of animal cells is a classic example of the fluid-mosaic model.

Phospholipid Behavior and Chemical Properties

Phospholipids are amphipathic molecules, meaning they have both hydrophilic and hydrophobic regions. Their chemical properties dictate their arrangement and movement within membranes.

• Hydrophobic tails avoid water, causing them to face inward.

• Hydrophilic heads interact with aqueous environments inside and outside the cell.

• Phospholipids can move laterally, but rarely flip-flop across the bilayer.

Factors Affecting Membrane Fluidity

Membrane fluidity is influenced by temperature, fatty acid composition, and cholesterol content.

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

• Fatty acid composition: Unsaturated fatty acids (with double bonds) increase fluidity; saturated fatty acids decrease fluidity.

• Cholesterol: Acts as a "fluidity buffer," preventing membranes from becoming too rigid or too fluid.

Roles of Membrane Proteins

Membrane proteins perform a variety of functions essential for cell survival.

• Transport: Move substances across the membrane.

• Enzymatic activity: Catalyze reactions at the membrane surface.

• Signal transduction: Relay signals from outside to inside the cell.

• Cell-cell recognition: Allow cells to identify each other.

• Intercellular joining: Connect adjacent cells.

• Attachment: Anchor the membrane to the cytoskeleton or extracellular matrix.

Diffusion and Movement Across Membranes

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

• Simple diffusion: Small, nonpolar molecules (e.g., O2, CO2) move directly through the membrane.

• Facilitated diffusion: Larger or polar molecules (e.g., glucose, ions) require transport proteins.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Example: Water moves into a cell placed in a hypotonic solution by osmosis.

Tonicity and Water Movement

Tonicity describes the relative concentration of solutes in solutions separated by a membrane.

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

• Hypertonic: Higher solute concentration outside; water moves out of the cell.

• Hypotonic: Lower solute concentration outside; water moves into the cell.

Active vs. Passive Transport

Transport across membranes can be passive (no energy required) or active (energy required).

• Passive transport: Includes simple and facilitated diffusion; moves substances down their concentration gradient.

• Active transport: Moves substances against their concentration gradient; requires ATP.

Co-Transport and Bulk Transport

Co-transport involves the simultaneous movement of two substances across a membrane, often using the energy from one to drive the other. Bulk transport allows cells to uptake large materials.

• Co-transport: Example: Sodium-glucose co-transporter.

• Bulk transport: Includes endocytosis (uptake) and exocytosis (release).

Metabolism and Energy

Catabolic vs. Anabolic Reactions

Metabolic reactions are classified as catabolic (breaking down molecules) or anabolic (building molecules).

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

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

Types of Energy

Energy exists in various forms within biological systems.

• Potential energy: Stored energy due to position or structure.

• Chemical energy: Energy stored in chemical bonds.

• Kinetic energy: Energy of motion.

• Thermal energy: Energy due to random molecular motion.

• Heat: Transfer of thermal energy.

Entropy and the Second Law of Thermodynamics

Entropy is a measure of disorder in a system. The second law states that entropy tends to increase in spontaneous processes.

• Living systems maintain order by using energy from their environment.

• Example: Cells use energy from food to build ordered structures.

Free Energy and Reaction Direction

Free energy () determines whether a reaction is spontaneous.

• Exergonic reactions: Release energy; ; spontaneous.

• Endergonic reactions: Require energy; ; non-spontaneous.

Spontaneity vs. Reaction Rate

Spontaneity refers to whether a reaction can occur without input of energy, while rate refers to how fast it occurs.

• Spontaneous reactions may still be slow if activation energy is high.

Reaction Coupling and ATP

Cells couple exergonic and endergonic reactions to drive unfavorable processes. ATP is the primary energy currency.

• ATP hydrolysis releases energy to drive endergonic reactions.

• Example: Sodium-potassium pump uses ATP to transport ions.

Enzymes and Activation Energy

Enzymes are biological catalysts that lower activation energy, increasing reaction rates.

• Substrates bind to the enzyme's active site.

• Enzymes are reusable and specific to their substrates.

• Inhibitors can decrease enzyme activity (competitive or noncompetitive).

Cellular Respiration

Relationship Between Cellular Respiration and Photosynthesis

Cellular respiration and photosynthesis are complementary processes in the flow of energy and matter.

• Photosynthesis stores energy in glucose; cellular respiration releases it.

Redox Reactions in Cellular Respiration

Redox reactions involve the transfer of electrons.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Oxidizing agent: Accepts electrons.

• Reducing agent: Donates electrons.

Example: NAD+ is reduced to NADH during glycolysis.

Major Steps of Cellular Respiration

Cellular respiration consists of several stages, each with specific reactants, products, and locations.

• Glycolysis: Occurs in cytosol; glucose is split into pyruvate.

• Pyruvate oxidation: Occurs in mitochondrial matrix; pyruvate is converted to acetyl-CoA.

• Citric acid cycle: Occurs in mitochondrial matrix; acetyl-CoA is oxidized, producing NADH and FADH2.

• Oxidative phosphorylation: Occurs at inner mitochondrial membrane; ATP is generated via electron transport chain and chemiosmosis.

ATP Generation Mechanisms

ATP is produced by substrate-level phosphorylation and oxidative phosphorylation.

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

• Oxidative phosphorylation: Uses energy from electron transport chain.

Anaerobic Conditions and Fermentation

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

• Lactic acid fermentation: Occurs in muscle cells.

• Alcohol fermentation: Occurs in yeast.

Photosynthesis

Autotrophs vs. Heterotrophs

Photosynthesis is performed by autotrophs, while heterotrophs rely on consuming organic molecules.

• Autotrophs: Produce their own food (e.g., plants).

• Heterotrophs: Obtain food by consuming other organisms.

Leaf and Chloroplast Structure

Leaves and chloroplasts are structured to maximize photosynthesis.

• Leaves: Broad surface area, thin structure.

• Chloroplasts: Contain thylakoids, stroma, and grana.

Photosynthesis Net Reaction

Stages of Photosynthesis

Photosynthesis consists of light reactions and the Calvin cycle.

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

• Calvin cycle: Occurs in stroma; uses ATP and NADPH to fix carbon dioxide.

Light Absorption and Pigments

Different pigments absorb different wavelengths of light, affecting photosynthetic efficiency.

• Chlorophyll: Main pigment; absorbs blue and red light.

• Absorption spectrum: Graph showing pigment absorption at various wavelengths.

Photosystems and Electron Transport

Photosystems I and II capture light energy and transfer electrons through an electron transport chain.

• PSII: Initiates water-splitting; generates O2, electrons, and H+.

• PSI: Produces NADPH.

• ATP synthase: Uses H+ gradient to produce ATP.

Calvin Cycle

The Calvin cycle fixes carbon dioxide into organic molecules using ATP and NADPH.

• Three phases: Carbon fixation, reduction, regeneration of RuBP.

• Major reactants: CO2, ATP, NADPH.

• Major products: G3P (a sugar), ADP, NADP+.

Comparison of Photosynthesis and Cellular Respiration

Both processes involve electron carriers, electron transport chains, H+ gradients, and ATP synthase, but differ in direction and purpose.

• Photosynthesis: Stores energy; produces glucose.

• Cellular respiration: Releases energy; breaks down glucose.

Comparison Table: Citric Acid Cycle vs. Calvin Cycle