Biology Chapter on Diffusion

Simple Diffusion

• Small, non-polar substances move across membranes without assistance.
• Movement is down a concentration gradient (high to low).
• No energy required.
• Continues until equilibrium is reached.
• Also known as simple transport.
• Very small, non-polar molecules (like O₂ and CO₂) move easily.
• Non-polar steroid hormones and drugs also cross readily.
• Water and glycerol, though polar, can move relatively quickly due to their small size.
• Large molecules and ions have very slow movement across membranes.

Facilitated Diffusion: Channel and Carrier Proteins

• Similar to simple diffusion, it's passive transport across a semi-permeable membrane until equilibrium is reached.
• A key difference: it involves transport proteins.

Channel Proteins

• Act like tunnels.
• Can be ungated (always open) or gated (can open/close).
• Moves small, dissolved charged molecules (like ions).
• Charged particles need help crossing the hydrophobic core.
• Aquaporins are a specific type of channel protein that facilitate water movement.

Carrier Proteins

• Act like revolving doors.
• Undergo conformational changes to move molecules.
• Move a variety of substances from charged particles to large, uncharged ones (like glucose).

Osmosis

• Water movement across a semi-permeable membrane.
• In cells, aquaporins facilitate water movement.
• Moves from high water concentration to low water concentration (or to dilute solutes).
• Water moves in the opposite direction of solute movement.
• Tonicity refers to the osmotic pressure due to solute concentration differences across a membrane, influenced by non-penetrating solutes. It refers to the concentration of solutes in the environment surrounding the cell (external).

Osmosis in Animal Cells

• Isotonic: Equal water movement in and out. Cells are in equilibrium.
• Hypotonic: Lower solute concentration outside the cell. Water moves into the cell, causing it to swell and potentially burst (osmotic lysis).
• Hypertonic: Higher solute concentration outside the cell. Water moves out of the cell, causing it to shrink (crenation).

Osmosis in Plant Cells

• Plant cells have cell walls, affecting osmotic effects.
• Isotonic: Cells are flaccid (lacking firmness). Water movement is in equilibrium but does not provide the pressure necessary for support.
• Hypotonic: Cell wall prevents bursting, resulting in turgor pressure and turgidity.
• Hypertonic: Cell membrane and cytoplasm shrink (plasmolysis) away from the cell wall, but the cell wall retains shape.

Active Transport

• Moves molecules against their concentration gradient (low to high).
• Requires energy in the form of ATP.
• Uses transport proteins called pumps.
• ATP changes the pump's conformation, allowing specific molecules to enter/exit against their gradient.

Types of Pumps

• Often function similarly to carrier proteins but move molecules against the concentration gradient, requiring energy.
• Can be coupled to move two molecules against their concentration gradient.

Sodium-Potassium Pump (Na-K Pump)

• Located in the plasma membrane.
• Simultaneously moves 3 Na+ ions out and 2 K+ ions into the cell.
• The pump oscillates between two conformational states to perform this.

Bulk Membrane Transport

• Transports molecules in large quantities.
• Used for molecules that are too large or polar to pass through the membrane.
• Involves folding of the cell membrane to create vesicles.
• Includes endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) and exocytosis.

Endocytosis

• Phagocytosis: "Cellular eating"; ingestion of cells or large particles. Used for feeding in simpler forms of life and defense in higher organisms.
• Pinocytosis: "Cellular drinking"; ingestion of dissolved materials (unspecific).
• Receptor-mediated endocytosis: Specific intake of molecules that bind to receptors on the cell surface. Ligands bind to receptors, membrane pinches to form a vesicle, ligands detach from receptors, vesicle pinches into two parts – free ligand and empty receptor, ligands fuse with lysosomes, and receptors are returned to the cell surface

Exocytosis

• Movement of materials from inside the cell to the cell surface within membrane-bound vesicles.
• Vesicles form from the Golgi or endocytosis.
• Reverse of endocytosis.
• Functions in secretion, recycling membrane proteins, and restoring the cell membrane.