Cell Biology 3

Diffusion

movement of atoms or small molecules from high to low concentration until equilibrium; no energy required

osmosis

diffusion of water across semipermeable membrane

what happens when cell is placed in pure water

water is hypotonic relative to cell; cell is hypertonic and swells (lyses)

what happens when cell is placed in very salty solution?

solution is hypertonic relative to cell; cell is hypotonic and shrinks (crenates)

define facilitated diffusion

passive transport via protein channels for molecules that can’t freely pass membrane (Na+); no energy required

define saturated process

transport rate is limited by the number of available channels

what is so special about insulting in regards to saturation process?

insulin signals cells to insert more glucose transporters into membrane via vesicle fusion; this prevents saturation and enables rapid glucose uptake

what is active transport?

moves molecules against concentration gradient using ATP

describe sodium-potassium pump process

1. Na+ bind to receptor inside cell membrane; ATP binds

2. ATP is converted to ADP and phosphate, releasing energy that changes pumps shape and expels Na+

3. K+ bind on extracellular side

4. pump reverts to go shape moving K+ inside

what is exocytosis?

vesicles fuse with plasma membrane to expel molecules; no carrier proteins involved; contents don’t cross membrane individually

2 types of endocytosis:

phagocytosis and pinocytosis

define phagocytosis

membrane engulfs a particle; lysosome cause enzymatic breakdown

define pinocytosis

membrane pinches off extracellular fluid with solutes; bulk uptake, less efficient and less tightly regulated than channel mediated transport

channels v carriers

channels don’t change shape while open and is always passive; carriers change shape during transport and can be passive or active

intracellular receptor

steroid hormones cross membrane and bind to receptors inside cell

membrane bound receptors

peptide/protein hormones can’t cross hydrophobic membrane; bind externally and get receptors to transmit signals inside

Schwann cells purpose

form myelin sheath to insulate axon

what is action potential?

rapid depolarisation and depolarisation of the cell membrane

what are the steps of action potential?

1. depolarisation

2. repolarisation

3. refractory period

describe depolarisation

its the sudden increase in membrane potential; this opens sodium channels and allows them to enter cell and make it more positive; once it reaches -55mV voltage gated sodium channels open getting cell to become 40mV

describe repolarisation

sodium channels close while potassium channels open releasing them out of cell; this makes cell more negative than outside; potassium channels are open longer than necessary making the membrane hyper polarise.

describe the refractory period

once sodium channels have opened they become inactivated (unresponsive to stimuli); so for a brief period of time the membrane doesn’t undergo action potential; lasts from when membrane reaches threshold til it returns to resting membrane potential