Cell Membrane Structure & Function

Cell Membrane Structure & Function

• Phospholipids are the most abundant lipids in the plasma membrane.
• Phospholipids are amphipathic molecules, meaning they have both hydrophobic and hydrophilic regions.
• The fluid mosaic model describes the plasma membrane as a fluid structure with various proteins embedded in it.

Fluidity of Membranes

• Phospholipids in the plasma membrane can move within the bilayer.
• Most lipids and some proteins drift laterally or rotate within the membrane.
• Flip-flop of molecules across the membrane is rare.

Temperature and Membrane Fluidity

• As temperature increases, membranes transition from a solid (gel) state to a more fluid state.
• Membranes with shorter fatty acid chains are more fluid than those with longer chains. Shorter chains result in less surface area and fewer van der Waals/hydrophobic interactions between neighboring phospholipid molecules.
• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids. Unsaturated fatty acid kinks prevent tight packing.

Cholesterol and Membrane Fluidity

• In animal cells, cholesterol affects membrane fluidity differently at different temperatures.
• At warm temperatures, cholesterol restrains phospholipid movement, decreasing membrane fluidity.
• At cool temperatures, cholesterol prevents tight packing and increases membrane fluidity.

Membrane Proteins and Their Functions

• A membrane is a collection of different proteins embedded in the fluid matrix of the lipid bilayer.
• Proteins determine most of the membrane's specific functions.
• Peripheral proteins are bound to the membrane surface.
• Integral proteins penetrate the hydrophobic core.
• Transmembrane proteins span the membrane.
• The hydrophobic regions of integral proteins consist of nonpolar amino acids, often coiled into α-helices.

Six Major Functions of Membrane Proteins

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

Carbohydrates in Cell-Cell Recognition

• Cells recognize each other by binding to carbohydrates on the plasma membrane.
• Membrane carbohydrates may be bonded to lipids (glycolipids) or commonly to proteins (glycoproteins).
• Carbohydrates vary among species, individuals, and cell types.

Selective Permeability

• Cells must exchange materials with their surroundings.
• Plasma membranes are selectively permeable, regulating the cell's molecular traffic.
• Hydrophobic molecules can dissolve in the lipid bilayer and pass through rapidly (e.g., water).
• Polar molecules (e.g., sugars) do not cross easily.
• Charged substances (ions) cannot pass through the lipid bilayer.

Transport Proteins

• Transport proteins allow passage of hydrophilic substances across the membrane.
• Channel proteins have hydrophilic channels, acting as tunnels for molecules/ions.
• Aquaporins facilitate the passage of water.
• Carrier proteins bind to molecules and change shape to transport them across the membrane.

Passive Transport

• Diffusion is the tendency for molecules to spread out evenly in an available space.
• Molecules diffuse down their concentration gradient, from high to low concentrations.
• Diffusion across a biological membrane is passive transport because no energy is required.

Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane.
• Water diffuses from a region of lower solute concentration to a region of higher solute concentration.

Water Balance of Cells Without Cell Walls

• Tonicity is a solution's ability to cause a cell to gain or lose water.
• Isotonic solution: Solute concentration is the same as in the cell.
• Hypertonic solution: Solute concentration is higher than in the cell; cell loses water.
• Hypotonic solution: Solute concentration is lower than in the cell; cell gains water (and may burst).

Water Balance of Cells with Cell Walls

• Cell walls help maintain water balance.
• A plant cell in a hypotonic solution will swell until the wall prevents further uptake, becoming turgid.
• In an isotonic solution, there is no net water movement, and the cell becomes flaccid.
• In a hypertonic solution, the plasma membrane pulls away from the cell wall, a process called plasmolysis.

Facilitated Diffusion

• In facilitated diffusion, transport proteins speed passive movement of molecules across the membrane.
• Channel proteins provide corridors for specific molecules/ions.
• Aquaporins facilitate water diffusion.
• Ion channels open/close in response to stimuli (gated channels).
• Carrier proteins change shape to translocate solute-binding sites across the membrane.

Active Transport

• Active transport moves substances against their concentration gradient (low to high).
• Active transport requires energy, usually in the form of ATP.
• The sodium-potassium pump is an example of active transport.

Bulk Transport

• Exocytosis is the process where transport vesicles fuse with the membrane to release their contents.
• Endocytosis is the process where the cell takes in macromolecules by forming new vesicles from the plasma membrane.
• Phagocytosis ("cellular eating"): Engulfing a solid particle.
• Pinocytosis ("cellular drinking"): Taking up fluid.
• Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation.

Co-transport

• Symport: Two molecules transported in the same direction.
• Antiport: Two molecules transported in opposite directions.