University:
Felician University
Course:
BIO-310 | Cell Biology
Academic year:

2020
Views:

386

Pages:

1
Author:

Clint W.

membrAne TrAnsport Properties of Membranes Types of Membrane Transport Cell membranes have two key properties • Semi-permeable (only certain things can cross) • Selective (membranes can regulate material passage) Membrane transport can either be: • Passive (along concentration gradient, no ATP expenditure) • Active (against concentration gradient, ATP is required) Passive Transport Simple Diffusion The net movement of particles from a region of higher concentration to a region of lower concentration (i.e. along the gradient) until equilibrium is reached Facilitated Diffusion The passive movement of molecules across a cell membrane via the aid of a membrane protein (carrier / channel protein) • Involves large / charged molecules (e.g. ions, glucose, etc.) • Involves small / lipophilic molecules (e.g. O2, CO2, steroids) • E.g. Voltage-gated channels control the flow of ions in neurons High [ ] Low [ ] Protein Channel Carrier Protein Osmosis Osmolarity The net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of higher solute concentration (diffusion of free water molecules) Osmolarity is a measure of solute concentration Low solute concentration High solute concentration 12 H2O total ; 12 H20 free 12 H2O total ; 0 H2O free Solutions can be measured as: • Hypertonic: High solute concentration (gains water) • Hypotonic: Low solute concentration (loses water) • Isotonic: Same solute concentration (no net flow) net Hypertonic Isotonic Hypotonic Active Transport Vesicular Transport Active transport uses energy (ATP) to move molecules against a concentration gradient (i.e. from low to high) • Molecule binds to a transmembrane protein pump • Hydrolysis of ATP causes a conformational change, translocating the molecule across the membrane • E.g. Sodium-potassium pumps move ions in neuron The fluidity of the plasma membrane allows it to break and reform around certain materials (this process requires ATP) • Exocytosis: Materials released from a cell via vesicles • Endocytosis: Materials internalised within a vesicle Sometimes molecules are passively coupled to an actively transported molecule (co-transport) • Symport: Both molecules move the same direction • Antiport: Molecules move in opposite directions Intracellular vesicles can move materials between cell organelles • E.g. rough ER → Golgi complex → plasma membrane