Membrane Structure and Function: Study Notes

Membrane Structure and Function

Overview of Biological Membranes

Biological membranes are essential structures that define the boundaries of cells and organelles, controlling the movement of substances in and out. The plasma membrane is primarily composed of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol, forming a dynamic and complex structure.

Membranes as Fluid Mosaics of Lipid and Protein

Phospholipid Bilayer Structure

The membrane consists of a double layer of phospholipids, with hydrophilic (water-attracting) heads facing outward toward the aqueous environments and hydrophobic (water-repelling) tails facing inward. This arrangement creates a semi-permeable barrier.

• Amphipathic molecules: Phospholipids have both hydrophilic and hydrophobic regions.

• Fluid mosaic model: Proteins are interspersed within the lipid bilayer, creating a mosaic pattern. Both lipids and proteins can move laterally within the layer, contributing to membrane fluidity.

Membrane Fluidity

Membrane fluidity is crucial for proper function, allowing for the movement of proteins and lipids within the bilayer. Factors such as temperature, fatty acid composition, and cholesterol content influence fluidity.

• Unsaturated fatty acids increase fluidity, while saturated fatty acids decrease it.

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

Membrane Proteins and Their Functions

Types of Membrane Proteins

Membrane proteins are classified based on their association with the lipid bilayer:

• Integral proteins: Embedded within the bilayer; most are transmembrane proteins that span the membrane.

• Peripheral proteins: Loosely bound to the membrane surface, not embedded in the lipid bilayer.

Functions of Membrane Proteins

• Recognition: Mark cells for identification (e.g., immune response).

• Anchoring: Attach the membrane to the cytoskeleton for structural support.

• Signal transduction: Act as receptors for signaling molecules.

• Transport: Facilitate movement of substances across the membrane.

• Linkage: Connect adjacent cells.

• Enzymatic activity: Catalyze reactions at the membrane surface.

Role of Membrane Carbohydrates

Glycolipids and Glycoproteins

Carbohydrates attached to lipids (glycolipids) or proteins (glycoproteins) are found on the extracellular surface of the membrane. They play a critical role in cell recognition and communication.

• Self vs. non-self recognition: Important for immune system function.

• Short sugar chains: Serve as identification tags for cells.

Selective Permeability of the Membrane

Transport Across the Membrane

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others. Transport proteins facilitate the movement of specific molecules.

• Channel proteins: Provide corridors for molecules like water (aquaporins).

• Carrier proteins: Bind and transport specific substances across the membrane.

Passive Transport (Diffusion)

Simple Diffusion

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration, driven by the random motion of particles. No energy (ATP) is required.

• Rate of diffusion is affected by temperature, pressure, and electric gradients.

Osmosis

Water Movement Across Membranes

Osmosis is the diffusion of water across a selectively permeable membrane, moving from areas of high water concentration to low water concentration.

• Water balance is critical for cell survival.

Water Balance in Animal and Plant Cells

Tonicity and Its Effects

Tonicity describes the ability of a solution to cause a cell to gain or lose water:

• Isotonic: No net movement of water; cell remains stable.

• Hypertonic: Cell loses water and shrivels.

• Hypotonic: Cell gains water and may burst (animal cells) or become turgid (plant cells).

Facilitated Diffusion

Passive Transport Aided by Proteins

Facilitated diffusion is a type of passive transport where molecules move down their concentration gradient with the help of membrane proteins. No ATP is required.

• Channel proteins: Allow ions and small molecules to pass through.

• Carrier proteins: Undergo conformational changes to transport substances.

Active Transport

Movement Against the Concentration Gradient

Active transport requires energy (usually from ATP) to move substances against their concentration gradient. This process is essential for maintaining cellular homeostasis.

• Sodium-potassium pump (Na+/K+ ATPase): Moves Na+ out of and K+ into animal cells.

• Proton pump: Main pump in plants, fungi, and bacteria.

Transport of Large Particles

Bulk Transport Mechanisms

Large particles and macromolecules are transported via vesicles in processes that require energy:

• Exocytosis: Vesicles fuse with the plasma membrane to secrete substances out of the cell.

• Endocytosis: The cell takes in substances by forming vesicles from the plasma membrane.

Types of Endocytosis

• Phagocytosis: "Cell eating"; the cell engulfs large particles or cells.

• Pinocytosis: "Cell drinking"; the cell takes in extracellular fluid and dissolved solutes.

• Receptor-mediated endocytosis: Specific molecules bind to receptors and are taken into the cell via vesicles.