Membranes and transport

The components of the plasma membrane

• Phospholipids: main fabric
• Cholesterol: tucked between the hydrophobic tails(core of the membrane)
• integral proteins: embedded the bilayer
• peripheral proteins: on the inner or outer surface, not embedded
• carbohydrates: (attached to proteins or lipids)on the extracellular side

Diffusion and osmosis

• osmosis: the net movement of water across a semipermeable membrane from lower concentration to higher concentration
• diffusion: molecules move from higher concentration to lower concentration as a result of probabilities

the solute molecules physically knocking the water molecules backwards, making them less likely to cross

• tonicity: the ability of an extracellular solution to make water into or out of a cell by osmosis
• osmolarity: the total concentration of all solutes in the solution
• *only solutes that cannot cross the membrane
• hypotonic: extracelluar's osmolarity lower, inside the cell's higher(water out to in)
• hypertonic: extracelluar's osmolarity higher, inside the cell's lower(water in to out)
• isotonic: both's osmolarity same(no net movement of water)

Passive transport

• does not require to expend energy bc a substance diffusing down its concentration gradient
• concentration gradient: a region where the concentration of a substance changes(from higher to lower)
• non polar and small molecules readily diffuse across(carbon dioxide, oxygen..)
• polar and charged molecules have trouble crossing the core membrane
• facilitated diffusion: channels and carrier proteins

Aquaporins: channel for water molecules

carrier proteins: change shape in response to binding of their target molecule

Active transport

• the cell expends energy(ATP) to move substance against its concentration gradient
• membrane potential: an electrical potential difference(voltage) across cell membrane
• inside of the cell having extra negative charges(generally -40 to -80 millivolts)
• electrochemical gradient: the combination of concentration gradient and voltage(that affects an ion's movement)
• Active transport-primary active transport: sodium-potassium pump

sodium-potassium pump

• move Na+ out, K+ into cells 
• uses ATP as an energy source
• generating the voltage across the membrane(electrogenic pumps)

The sodium-potassium pump cycle

1. the pump is open to the inside of the cell, take up 3*Na+
2. trigger the pump to hydrolyze ATP(phosphorylation, phosphate group attach to the pump)
3. phosphorylation make the pump change shape, release 3*Na+
4. the pump take up 2*K+, triggers removal of the phosphate group attached to the pump
5. with the phosphate group gone, the pump change back to its original shape
6. the pump release 2*k+, back to step 1

membrane potential generated by the pump

1. too many K+ inside the cell(since the pump), the gradient is very steep
2. K+ move out of the cell(via channels), negative charge inside is growing
3. (negative charge inside big enough)K+ stops being imported, the voltage(difference, not flow)disappear

Secondary active transport

• the electrochemical gradients store energy
• secondary active transport uses the energy to move other substances against their own gradients
• when Na+ return to the interior of the cell via channel or carrier protein, glucose share the carrier(as a cotransporter)
• cotransporter: symporter(both move same direction) and antiporter(move in opposite direction)

Bulk transport

• Endocytosis: move particles into a cell by enclosing them in vesicle
• Phagocytosis: cell-eating, large particles are transported into the cell
• Pinocytosis: cell-drinking, cell takes in small amounts of extracellular fluid
• Receptor-mediated endocytosis: receptor proteins(on the cell surface) capture a specific target molecule *they cluster in coated pits(ex. Clathrin)
• Exocytosis: materials are transported from the inside to the outside of the cell(like from the Golgi apparatus)