Cell Membranes: Structure, Function, and Transport Mechanisms

Phospholipid bilayer

Double layer of phospholipids forming cell membranes.

Fluid Mosaic model

Describes membrane structure with proteins in lipids.

Hydrophobic interior

Nonpolar region of the membrane repels water.

Polar phospholipid heads

Hydrophilic parts facing water inside and outside.

Nonpolar phospholipid tails

Hydrophobic parts facing inward, away from water.

Transport proteins

Facilitate movement of substances across membranes.

Glycolipids

Carbohydrates attached to lipids for cell recognition.

Glycoproteins

Carbohydrates attached to proteins for cell adhesion.

Semi-permeable barrier

Allows selective passage of substances through membrane.

Non-polar molecules

Easily cross membranes without assistance.

Polar molecules

Cannot diffuse across membranes without help.

Ions

Cannot cross membranes unaided, e.g., Na+.

Membrane composition

Varies among different cells and organelles.

Saturated fatty acids

Less fluidity in membranes, more stable.

Unsaturated fatty acids

Increase membrane fluidity, especially in cold.

Peripheral membrane proteins

Not embedded, interact with membrane surface.

Integral membrane proteins

Embedded in membrane, have hydrophilic and hydrophobic parts.

Transmembrane proteins

Span the membrane, protruding on both sides.

Freeze-fracture method

Technique to visualize membrane proteins.

Cell junctions

Structures that connect adjacent cells.

Tight junctions

Prevent leakage between cells, seal intercellular spaces.

Desmosomes

Provide mechanical stability, hold cells together.

Gap junctions

Channels for communication between adjacent cells.

Extracellular Matrix (ECM)

Protein mix surrounding cells, aiding communication.

Integrin

Protein mediating cell attachment to ECM.

Passive transport

Movement of substances without energy input.

Diffusion

Movement from high to low concentration.

Simple diffusion

Nonpolar molecules cross membranes unaided.

Facilitated diffusion

Polar molecules use proteins to cross membranes.

Carrier proteins

Transport molecules across membranes, limited speed.

Active transport

Movement against concentration gradient, requires ATP.

Sodium-potassium pump

Moves Na+ out and K+ into cells.

Primary active transport

Direct use of ATP for transport.

Secondary active transport

Uses established gradients for energy.

Endocytosis

Process of bringing substances into the cell.

Exocytosis

Process of expelling substances from the cell.

Phagocytosis

Cell engulfs large particles or cells.

Pinocytosis

Cell takes in fluids and dissolved substances.

Receptor-mediated endocytosis

Specific uptake of substances via receptors.

Clathrin

Protein aiding in vesicle formation during endocytosis.

Osmosis

Diffusion of water across a semipermeable barrier.

Tonicity

Comparison of solute concentrations in solutions.

Hypertonic solution

Higher solute concentration, water moves toward it.

Hypotonic solution

Lower solute concentration, water moves away from it.

Cell-cell recognition

Cells identify and bind to each other.

Cell adhesion

Cells stick together to maintain tissue structure.

Cell junction types

Include tight junctions, desmosomes, and gap junctions.

Membrane fluidity

Affected by lipid composition and temperature.

Cold-blooded animals adaptation

Change fatty acids to maintain membrane fluidity.