Ch+4+FP module 3

Membrane Structure and Function

• Membrane Composition

  • All cells possess a plasma membrane, whether they are prokaryotic or eukaryotic.

  • The membrane acts as a critical boundary between the cell and the external environment.

  • Membranes regulate the passage of materials into and out of cells.

Membrane Structure

• Phospholipid Bilayer

  • The structure consists of a phospholipid bilayer with embedded proteins (fluid-mosaic model).

  • Polar heads of phospholipids face the exterior and interior of the cell, while non-polar tails face each other.

  • Cholesterol and other steroids are present to regulate membrane fluidity.

• Proteins in Membranes

  • Proteins can be either integral (embedded within the membrane) or peripheral (on the surface).

  • Sugars attach to proteins and lipids, forming glycoproteins and glycolipids.

Membrane Protein Functions

• Channels and Carriers

  • Allow passage of specific target molecules through the membrane.

• Cell Recognition

  • Glycoproteins allow cells to recognize self from invaders.

• Receptors and Enzymes

  • Receptor proteins bind to signal molecules outside the cell, initiating a cellular response (e.g., movement, enzymatic activity, gene expression).

  • Enzymes catalyze metabolic reactions within the membrane.

Membrane Permeability

• Selective Permeability

  • The membrane allows some substances to cross while restricting others.

  • Small, non-charged molecules like oxygen and carbon dioxide can pass freely.

  • Water can cross the membrane, but cells also utilize water channel proteins for better regulation.

  • Large molecules and ions require specific channels or carriers for transport.

  • Transport mechanisms can be passive (e.g., diffusion, osmosis) or active (active transport, exocytosis, endocytosis).

Diffusion

• Defined as the movement of molecules down a concentration gradient.

• Random molecular movement separates molecules, moving them from areas of high concentration to low concentration.

• Diffusion can occur in various media, not solely in cells.

• Solutions consist of solutes (e.g., sugar, protein, salt) dissolved in a solvent (usually water).

• The rate of diffusion is influenced by temperature, pressure, electricity, and molecular size.

Osmosis

• Refers specifically to the diffusion of water across a selectively permeable membrane.

• When solutes cannot cross the membrane, water moves to balance concentration.

• Water will move from areas of high to low concentration of water.

Tonicity

• The solute concentration of a cell's contents relative to its environment determines water movement.

  • Isotonic: Same concentration; no net gain/loss of water.

  • Hypotonic: Lower solute concentration outside the cell; water moves in, causing cell swelling.

  • Hypertonic: Higher solute concentration outside the cell; water exits the cell, resulting in shrinkage.

Transport by Carrier Proteins

• Carrier proteins are specific to target molecules; only specific molecules can pass through.

• Some carrier proteins facilitate transport (passive) while others are involved in active transport requiring ATP.

Active Transport

• Involves moving molecules against their concentration gradient.

• Requires ATP as an energy source.

• Often involves proteins known as "pumps" (e.g., sodium-potassium pump).

Bulk Transport

• Larger molecules and particles use membrane-bound vesicles for transport.

  • Exocytosis: Exports materials from cells using vesicles formed by the Golgi apparatus.

  • Endocytosis: Brings in particles/solutes via vesicles or vacuoles. Types include:

    • Phagocytosis: "Cell eating" — engulfing large particles.

    • Pinocytosis: "Cell drinking" — importing liquids or small particles.

    • Receptor-mediated endocytosis: Specific import where target molecules bind to proteins before vesicle formation.

Phagocytosis

• This is a form of endocytosis that involves the engulfing of large particles or cells into food vacuoles.

• These vacuoles then fuse with lysosomes for digestion.

Pinocytosis and Receptor-Mediated Endocytosis

• Pinocytosis: The process of taking in fluid and small particles into vesicles.

• Receptor-mediated endocytosis: A specific and efficient form of pinocytosis where target molecules bind to receptors before vesicle formation.