Membranes and Transport PDF

The membrane is selectively permeable  Getting across the barrier Membranes define a barrier between an inside and an outside in a cell the inside must interact with the outside! Examples of the selective permeability of membranes Include: 1. Bulk transfer by endo and exocyto...

The membrane is selectively permeable  Getting across the barrier Membranes define a barrier between an inside and an outside in a cell the inside must interact with the outside! Examples of the selective permeability of membranes Include: 1. Bulk transfer by endo and exocytosis 2. Secreting and importing proteins 3. Solute molecules such as ions e.g. Na+, K+ Cl- 4. Solute molecules such as METABOLITES e.g. sugars, amino acids  Ways of getting across the barrier SIMPLE DIFFUSION e.g. oxygen Diffusion can only work down a concentration gradient towards equilibrium: HIGH LOW Small uncharged polar molecules can diffuse across the membrane best Osmosis: a special case of simple diffusion Diffusion of water is known as OSMOSIS The plasma membrane is highly permeable to water In relation to the cell interior solutions can be: 1. HYPOTONIC 2. ISOTONIC 3. HYPERTONIC  Facilitated diffusion Still diffuses down the gradient but larger and/or polar molecules are helped FACILITATED DIFFUSION can be sub-divided into two types, requiring two different types of integral membrane protein: 1. Carrier proteins: binding of solute causes a conformational change in the membrane molecule which transports the solute across the membrane 2. Channel proteins: form a hydrophilic passage through the membrane  ACTIVE TRANSPORT Active transport can move molecules against a concentration gradient Active transport can be sub-divided into classes 1. Direct active transport: The energy required to move against the gradient comes from ATP hydrolysis 2. Indirect transport: The energy required to move against the gradient comes from co-transport of a solute favorably down its gradient Bulk transfer: exo and endocytosis  Differences in cytosolic ion concentrations The cytosolic pH is maintained at around 7.2 by regulating hydrogen ion concentration In general the K+ is higher intracellular than extracellular In general the Na+ is lower intracellular than extracellular Direct ATP powered pumps maintain an ionic gradient across the membrane The ionic concentration of inside cells is different to that outside the cell The differences in ion concentration between the inside and outside is maintained by ATP powered ion pumps and ion channels The differences in ion concentration means there is electrical potential of about 70mV across the membrane  Synthesis of the membrane Cells synthesise new membranes by the expansion of existing membranes Lipid precursers are synthesised in the cytosol Synthesis continues in the ER Many membrane lipids are distributed to their target membranes by vesicles Some membranes grow receive membrane lipids by a vesicle independent mechanism a c b