Membrane Structure and Function

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a dynamic combination of lipids and proteins. The membrane is flexible, with proteins embedded or attached to a bilayer of phospholipids.

• Fluidity: Lipids and proteins can move laterally within the membrane.

• Mosaic: Diverse proteins are scattered throughout the lipid bilayer.

• Example: Integral proteins span the membrane, while peripheral proteins are attached to its surface.

Selective Permeability

Cell membranes allow certain substances to pass while restricting others, maintaining internal conditions.

• Small, nonpolar molecules (e.g., O2, CO2) pass easily.

• Ions and large polar molecules require transport proteins.

Diffusion and Concentration Gradient

Diffusion is the movement of molecules from high to low concentration, driven by the concentration gradient.

• Passive process: No energy required.

• Example: Oxygen diffuses into cells where its concentration is lower.

Passive Transport

Movement of substances across membranes without energy input.

• Includes: Simple diffusion, facilitated diffusion, and osmosis.

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity describes the effect of a solution on cell volume.

• Isotonic: No net water movement; cell volume remains constant.

• Hypotonic: Water enters cell; cell may swell.

• Hypertonic: Water leaves cell; cell shrinks.

• Osmoregulation: Control of water balance by organisms.

• Example: Paramecium uses contractile vacuole for osmoregulation.

Facilitated Diffusion and Aquaporins

Transport proteins assist movement of substances across membranes.

• Facilitated diffusion: Passive transport via channel or carrier proteins.

• Aquaporins: Channel proteins specifically for water.

Active Transport

Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Example: Sodium-potassium pump maintains ion gradients in animal cells.

Bulk Transport: Exocytosis and Endocytosis

Cells move large molecules or particles via vesicles.

• Exocytosis: Vesicles fuse with membrane to release contents outside.

• Endocytosis: Membrane engulfs material to bring it inside.

• Phagocytosis: Cell engulfs large particles ('cell eating').

• Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

Energy and Metabolism

Energy Types

• Energy: Capacity to do work.

• Kinetic energy: Energy of motion.

• Thermal energy: Kinetic energy of particles; measured as heat.

• Heat: Transfer of thermal energy.

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

• Chemical energy: Potential energy in chemical bonds.

• ATP: Adenosine triphosphate, main energy currency of cells.

Thermodynamics

Study of energy transformations.

• First law: Energy cannot be created or destroyed, only transformed. (where is change in energy, is heat, is work)

• Second law: Every energy transfer increases entropy (disorder). (for spontaneous processes)

• Entropy: Measure of disorder.

Cellular Respiration

Process by which cells extract energy from glucose to produce ATP.

• Exergonic reaction: Releases energy; spontaneous.

• Endergonic reaction: Requires energy input; non-spontaneous.

• Example: Cellular respiration is exergonic; photosynthesis is endergonic.

Metabolism and Metabolic Pathways

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

• Energy coupling: Using energy from exergonic reactions to drive endergonic ones.

• Phosphorylation: Addition of phosphate group to a molecule, often activating it.

Enzymes and Regulation

Enzymes

Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Activation energy: Energy required to start a reaction. (activation energy symbol)

• Substrate: Reactant acted upon by enzyme.

• Active site: Region of enzyme where substrate binds.

• Induced fit: Enzyme changes shape to fit substrate.

Cofactors and Coenzymes

Enzymes may require non-protein helpers.

• Cofactor: Inorganic helper (e.g., metal ions).

• Coenzyme: Organic helper (e.g., vitamins).

Enzyme Inhibition and Regulation

• Competitive inhibitor: Binds to active site, blocking substrate.

• Noncompetitive inhibitor: Binds elsewhere, changing enzyme shape.

• Feedback inhibition: End product inhibits pathway, preventing overproduction.

Summary Table: Types of Membrane Transport

Additional info: These key terms are foundational for understanding cell membrane structure, transport mechanisms, and cellular metabolism, as covered in General Biology chapters on membrane function and energy transformations.