Chapter 7

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Membrane Structure and Function

Membrane Properties

The plasma membrane is a fundamental structure in all cells, separating the internal environment from the external surroundings and compartmentalizing organelles. Its unique composition allows for selective transport and cellular communication.

Plasma Membrane: A boundary common to all cells, composed primarily of a phospholipid bilayer.
Phospholipid Bilayer: Consists of two layers of phospholipids, each with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.
Amphipathic: Phospholipids possess both hydrophilic and hydrophobic regions, enabling spontaneous bilayer formation in water.
Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins, cholesterol, glycolipids, and glycoproteins embedded within the lipid bilayer. Components are mobile, not fixed.
Cholesterol: Acts as a "fluidity buffer," restraining movement at high temperatures and preventing tight packing at low temperatures.
Peripheral Proteins: Attached to the membrane surface.
Integral Proteins: Penetrate the hydrophobic core; those spanning the membrane are called transmembrane proteins.
Glycolipids & Glycoproteins: Carbohydrates covalently bonded to lipids or proteins, respectively, important for cell recognition.

Membrane Fluidity:

Fluidity is essential for membrane function.
Temperature affects fluidity: cooling causes membranes to solidify.
Unsaturated fatty acids increase fluidity; saturated fatty acids decrease it.
Cholesterol moderates fluidity changes due to temperature.

Functions of Membrane Proteins:

Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to cytoskeleton and extracellular matrix (ECM)

Cell "Fingerprint": Glycolipids and glycoproteins on the membrane surface provide unique identifiers for cell recognition, varying among species and cell types.

Membrane Selectivity

The plasma membrane is selectively permeable, allowing certain substances to cross while restricting others. This selectivity is based on the chemical nature of the phospholipid bilayer and the presence of transport proteins.

Selectively Permeable: Only specific molecules can pass freely.
Freely Diffusing Substances: Small, non-charged molecules (e.g., CO2, O2, glycerol, alcohol, H2O).
Restricted Substances: Ions and large polar molecules require transport proteins.
Transport Proteins: Facilitate movement of hydrophilic substances. Channel proteins (e.g., aquaporins for water) provide tunnels; carrier proteins change shape to shuttle molecules.

Passive Transport

Passive transport involves the movement of substances across the membrane without energy input, driven by concentration gradients.

Concentration Gradient: Difference in concentration across a space.
Diffusion: Movement of molecules from high to low concentration until dynamic equilibrium is reached.
Dynamic Equilibrium: Net movement stops; concentrations are uniform.
Facilitated Diffusion: Passive movement of molecules via transport proteins; no energy required.
Osmosis: Diffusion of water across a selectively permeable membrane.

Tonicity and Water Balance:

Tonicity: Ability of a solution to cause a cell to gain or lose water.
Isotonic Solution: Equal solute concentration; cell volume remains constant.
Hypotonic Solution: Lower solute concentration outside; cell gains water, swells (animal cell may burst, plant cell becomes turgid).
Hypertonic Solution: Higher solute concentration outside; cell loses water, shrivels (plant cell undergoes plasmolysis).
Plasmolysis: Plant cell membrane pulls away from cell wall due to water loss.
Osmoregulation: Regulation of water and solute balance; contractile vacuole in freshwater protists pumps out excess water.

Table: Effects of Solution Types on Animal and Plant Cells

Solution Type	Animal Cell	Plant Cell
Isotonic	Volume constant	Flaccid (wilts)
Hypotonic	Swells, may burst	Turgid (firm)
Hypertonic	Shrivels	Plasmolysis (membrane pulls away)

Active Transport

Active transport moves substances against their concentration gradients, requiring energy (usually ATP).

Active Transport: Movement from low to high concentration; requires ATP.
Membrane Potential: Voltage difference across the membrane due to unequal ion distribution.
Electrochemical Gradient: Combined effect of concentration gradient and membrane potential.
Electrogenic Pump: Transport protein generating voltage across membrane (e.g., sodium-potassium pump in animals, proton pump in plants).
Cotransport: Active transport of one solute drives passive transport of another.

Example: Sodium-Potassium Pump

Maintains electrochemical gradient by pumping 3 Na+ out and 2 K+ in.
Creates voltage across membrane.

Equation for Sodium-Potassium Pump:

Bulk Transport

Bulk transport moves large molecules or quantities across the membrane via vesicles, requiring energy.

Exocytosis: Vesicles fuse with membrane to release contents outside the cell.
Endocytosis: Cell takes in macromolecules by forming vesicles from the membrane.
Types of Endocytosis:
- Phagocytosis: "Cellular eating"; intake of large particles.
- Pinocytosis: "Cellular drinking"; intake of fluids and small particles.
- Receptor-mediated Endocytosis: Specific molecules (ligands) bind to receptors, triggering vesicle formation.
Bulk Transport Requires Energy: All forms require ATP.

Table: Types of Bulk Transport

Type	Description	Energy Required?
Exocytosis	Release of substances outside cell via vesicles	Yes
Phagocytosis	Intake of large particles	Yes
Pinocytosis	Intake of fluids/small particles	Yes
Receptor-mediated Endocytosis	Intake of specific molecules via receptor binding	Yes

Key Terms and Definitions

Plasma membrane: The cell's boundary, regulating entry and exit of substances.
Phospholipid bilayer: Double layer of phospholipids forming the membrane.
Amphipathic: Molecule with both hydrophilic and hydrophobic regions.
Fluid mosaic model: Describes the dynamic, protein-studded nature of the membrane.
Cholesterol: Steroid affecting membrane fluidity.
Peripheral proteins: Proteins attached to membrane surface.
Integral proteins: Proteins embedded within the membrane.
Transmembrane proteins: Integral proteins spanning the membrane.
Glycolipids: Lipids with attached carbohydrates.
Glycoproteins: Proteins with attached carbohydrates.
Selectively permeable: Allows some substances to pass, restricts others.
Transport proteins: Facilitate movement of substances across membrane.
Aquaporins: Channel proteins for water transport.
Concentration gradient: Difference in concentration across space.
Diffusion: Movement from high to low concentration.
Passive transport: Movement without energy input.
Facilitated diffusion: Passive transport via proteins.
Osmosis: Diffusion of water.
Tonicity: Effect of solution on cell water balance.
Isotonic solution: Equal solute concentration.
Hypertonic solution: Higher solute concentration outside cell.
Hypotonic solution: Lower solute concentration outside cell.
Plasmolysis: Plant cell membrane pulls away from wall.
Osmoregulation: Regulation of water/solute balance.
Contractile vacuole: Organelle pumping excess water out (in protists).
Active transport: Movement against gradient, requires energy.
Membrane potential: Voltage across membrane.
Electrochemical gradient: Combined chemical and electrical gradient.
Electrogenic pump: Protein generating voltage gradient.
Proton pump: Moves protons out, major in plants.
Cotransport: Coupled transport of substances.
Exocytosis: Export of substances via vesicles.
Endocytosis: Import of substances via vesicles.
Phagocytosis: Intake of large particles.
Pinocytosis: Intake of fluids/small particles.
Receptor-mediated endocytosis: Specific intake via receptor binding.

Example: The protist Paramecium uses a contractile vacuole to maintain water balance in a hypotonic environment.

Additional info: Membrane structure and function are central to cell physiology, affecting transport, signaling, and energy conversion. Understanding these concepts is foundational for advanced topics in biology.