Chapter 7: Membranes – Structure and Function (General Biology Study Notes)

Chapter 7: Membranes

Overview

This chapter explores the structure and function of biological membranes, focusing on their roles in cellular organization, transport, signaling, and cell-to-cell interactions. The plasma membrane is a fundamental feature of all cells, providing a selectively permeable barrier between the cell and its environment.

Roles of Biological Membranes

Boundary and Compartmentalization

• Plasma membrane separates the living cell from its surroundings, maintaining distinct internal conditions.

• Membrane-bound organelles within eukaryotic cells have their own internal environments, separated from the cytoplasm.

Selective Permeability and Regulation

• Membranes regulate the passage of substances, allowing some molecules to cross more easily than others.

• This regulation helps maintain homeostasis, even as external conditions change.

Surface for Chemical Reactions

• Many enzymes are embedded in membranes, facilitating controlled biochemical reactions.

• Membranes help bring reactants and catalysts together, and can separate reactants and products to avoid equilibrium.

Cell Recognition and Signaling

• Proteins and glycoproteins in membranes function in chemical recognition and cell signaling.

• Membrane carbohydrates act as markers to distinguish cell types and individuals.

The Lipid Bilayer and the Fluid Mosaic Model

Structure of the Lipid Bilayer

• Biological membranes are primarily composed of a phospholipid bilayer with embedded proteins and carbohydrates.

• Phospholipids are amphipathic molecules, having hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Phospholipids spontaneously form bilayers in aqueous environments due to their cylindrical shape and amphipathic nature.

Fluid Mosaic Model

• Proposed by Singer and Nicolson in 1972, this model describes membranes as a mosaic of proteins floating in or on a fluid lipid bilayer.

• Membranes are held together by weak hydrophobic interactions, allowing lateral movement of lipids and proteins.

• Proteins are not randomly distributed; they are often grouped for specific functions.

• Other lipids, such as cholesterol and glycolipids, are present and contribute to membrane properties.

Membrane Fluidity

• Fluidity depends on temperature and lipid composition.

• Membranes rich in unsaturated fatty acids are more fluid than those with saturated fatty acids.

• Cholesterol acts as a "fluidity buffer," stabilizing membrane fluidity across temperature changes.

• Organisms can adjust membrane fluidity by changing fatty acid profiles or adding stabilizers.

Membrane Proteins and Their Functions

Types of Membrane Proteins

• Integral proteins: Penetrate the hydrophobic core; often span the membrane (transmembrane proteins).

• Peripheral proteins: Bound to the membrane surface, usually via ionic or hydrogen bonds to integral proteins.

• Transmembrane proteins typically have hydrophobic alpha-helices spanning the bilayer.

Functions of Membrane Proteins

• Transport: Move specific substances across the membrane.

• Enzymatic activity: Catalyze reactions at the membrane surface.

• Signal transduction: Receive and transmit signals from outside the cell.

• Cell-cell recognition: Identify and interact with other cells.

• Intercellular joining: Connect cells together.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix (ECM).

Transport and Transfer Across Cell Membranes

Selective Permeability

• Cell membranes allow some substances to pass freely (e.g., small nonpolar molecules, water), while others require transport proteins.

• Examples of freely transported molecules: O2, CO2, H2O.

• Examples of molecules requiring transport: glucose, amino acids, ions.

Diffusion and Osmosis

• Diffusion: Net movement of particles from high to low concentration, driven by random motion.

• Concentration gradient: Difference in concentration across a membrane; provides energy for diffusion.

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

• Osmotic pressure: Tendency of water to move into a solution due to solute concentration.

Solution Tonicity and Water Balance

• Isotonic: Equal osmotic pressure; no net water movement.

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

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

• Turgor pressure: Hydrostatic pressure in cells with cell walls, maintaining structure.

Types of Membrane Transport

• Passive transport: Movement down a concentration gradient; includes simple diffusion and facilitated diffusion (requires transport proteins, but no energy).

• Active transport: Movement against a concentration gradient; requires energy (usually ATP) and carrier proteins.

• Facilitated diffusion: Passive transport assisted by proteins (e.g., aquaporins for water, glucose transporters).

• Carrier-mediated active transport: Uses ATP to power protein pumps (e.g., sodium-potassium pump).

• Linked cotransport: Uses the gradient of one ion (e.g., Na+, K+, H+) to drive the transport of another substance against its gradient.

Key Equations

• Rate of diffusion: $\text{Rate} \propto \text{Temperature} \times \text{(1/Size)}$

• Sodium-potassium pump: Moves 3 Na+ out, 2 K+ in per ATP hydrolyzed.

Bulk Transport: Exocytosis and Endocytosis

• Exocytosis: Fusion of vesicles with the plasma membrane to secrete materials outside the cell.

• Endocytosis: Uptake of materials via vesicle formation; includes phagocytosis ("cell eating"), pinocytosis ("cell drinking"), and receptor-mediated endocytosis.

Specialized Contacts (Junctions) Between Cells

Types of Cell Junctions

• Anchoring junctions (Desmosomes): Hold cells tightly together; merge cytoskeletons for strength; not involved in transport.

• Tight junctions: Seal adjacent cells to prevent passage of materials between them.

• Gap junctions: Selective pores between animal cells; allow small molecules and ions to pass directly between cells.

• Plasmodesmata: Channels between plant cells; connect plasma membranes and allow exchange of materials.

Comparison Table: Cell Junctions

Junction Type	Structure	Function	Organism
Desmosome (Anchoring)	Protein plaques, cytoskeletal filaments	Strong adhesion, structural support	Animals
Tight Junction	Protein strands sealing membranes	Barrier to fluid movement	Animals
Gap Junction	Cylinders of 6 protein subunits	Direct passage of small molecules	Animals
Plasmodesmata	Channels through cell wall	Exchange of materials between cells	Plants

Key Terms and Definitions

• Selectively permeable: Allows some substances to cross more easily than others.

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

• Concentration gradient: Difference in concentration across a space.

• Osmosis: Diffusion of water across a membrane.

• Tonicity: Ability of a solution to cause a cell to gain or lose water.

• Isotonic: No net water movement.

• Hypertonic: Cell loses water.

• Hypotonic: Cell gains water.

• Turgor pressure: Pressure exerted by water inside the cell against the cell wall.

Summary

Biological membranes are dynamic structures essential for compartmentalization, regulation, communication, and transport in cells. Their unique composition and organization enable a wide range of cellular functions, from maintaining homeostasis to facilitating complex signaling and interactions between cells.