5- membranes

Question 1

Membrane proteins

various roles

Answer 1

Most form alpha helicies crossing the membrane
Membrane inserted part is HYDROPHOBIC
VARIOUS roles: transporters and channels, anchors, receptors, enzymes

Question 2

Fluid mosaic model of membranes

Answer 2

Phospholipids are amipathic and form bilayers with hydrophobic tails pointing inwards and polar heads pointing out

Membranes are lucid - lipids and proteins can diffuse laterally

Lipid molecules can diffuse laterally, and rotate around itself and can bend/ flex the lipid tail

Flip flop - when one individual molecule flips from one leaflet of membrane to another, UNFAVORABLE as one hydrophobic region would go through water. Only happens with help from flippase enzymes

Question 3

Membrane permeability

Answer 3

Molecules diffuse from high conc to low conc
Not all molecules can diffuse across membranes
Small non polar molecuels can cross easily = simple diffusion
Small uncharged polar molecules can cross but less easily e.g. h2o
Larger uncharged polar molecules are less likely
IOns are not able to cross via diffusion

Question 4

Passive and active transport across membranes

Answer 4

PASSIVE = transport DOWN a chemical gradient, no energy source needed

ACTIVE = transport UP a chemical gradient, only occurs with energy source

Question 5

Transporters

Answer 5

Bind a solute from one side of the membrane into a binding site in the middle of the membrane, protein undergoes change and flips so that molecule can be released to the other side

Can allow passive or active transport (active = atp)

Sometimes called PUMPS

Relatively slow

Question 6

Channel proteins

Answer 6

Continous channel across moleule
Only allows passive transport ( facilitated diffusion )
Can be ligand gated or voltage gated
Relatively fast

Question 7

How can ions move across the membrane

Answer 7

Transporters and channel proteins

Question 8

2 types of active transport

Answer 8

Primary active transport - driven by ATP

Secondary active transport - driven by cotransport of Na+

Question 9

Passive transport - simple diffusion

Answer 9

Molecules that can diffuse across the membrane flow DOWN their conc gradient
The rate of transport is directly proportional to the conc gradient across the membrane ( triangle c )
e.g. o2, co2, steroid hormones

Question 10

Passive transport - facilitated diffusion

Answer 10

Rate of transport has a hyperbolic dependence on triangle c ( conc difference), maximum rate WILL be reached

Lower kM means faster transportation

Important example is glucose via GLUT transporters

Question 11

2 types of passive transport

Answer 11

Facilitated

Simple

Question 12

Primary active transport

example

Answer 12

Na+/K+ ATPase is an example

Pumps 3 sodiums out of cell and 2 potassium into cell for every ATP used

Question 13

Secondary active transport

Example

Answer 13

Na/K+ atpase generates a Na+ gradient ( and a membrane potential )
Gradient can be used as a ENERGY SOURCE to drive other transport reactions - either in the same direction ( symport ) or opposite direction ( antiport)

Question 14

Transport processes in gut epithelial cell

Answer 14

Uptake of sugars

Glucose absorbed from lumen to blood via epithelial cells

Question 15

Receptors

Answer 15

Proteins that bind to signalling molecules ( ligands ) producing a cellular response
LOCK AND KEY
Non covalnet

Question 16

Ligands

Answer 16

Signalling molecules
Bind to receptor on cell
e.g. hormones, cytokines, neurotransmitters

Question 17

2 key types of receptors in terms of placement

Answer 17

INTRACELLULAR

CELL SURFACE

Question 18

Intracellular receptors

Answer 18

Protein found INSIDE the ell, so in order to bind, the ligand must diffuse across the membrane

These receptors are only for ligands small and hydrophobic and therefore can cross the membrane e.g. receptors for steroid hormones, thyroid hormones and vitamin D

Question 19

Cell surface receptors

Answer 19

Membrane proteins with a ligand binding region on the OUTSIDE of the cell, causes a confirmational change which is transmitted across the membrane to the inside of the cell and response stimulated by intracellular signalling molecules

Question 20

3 types of cell surface receptor

Answer 20

Ligand gated ion channels
G protein coupled receptors ( GPCR’s)
Enzyme coupled receptors

Question 21

Ligand gated ion channels

Answer 21

e.g. nitotine acetylcholine receptor found in neuromuscular junctions
Channels usually shut, but when ligand binds a change occurs and the channel OPENS
Ion moves down concentration gradient
Hydrophobic molecules

Question 22

GPCRs

Answer 22

Ligand binds to receptor changing the shape of the receptor, which activates associated G protein, which binds to GTP
ACtivated G protein interacts with an associated enzyme, activating the enzyme which catalysts a reaction which transmits a signal
Stimulate enzymes that produce secondary messengers e.g. cyclic AMP ( cAMP ) causing signal amplification

Question 23

Enzyme coupled receptors

Answer 23

Binding of an extracellular ligand causes ENZYMATIC activity on the intracellular side
Ligand binds to the receptor, enzymes are activated in the cell, GTP is transformed into cAMP causing downstream signalling of other secondary molecules in the cell
Ligand binding often causes dimerization and activation ( 2 molecules come together forming a reaction)
Include receptors for insulin and growth factors

Question 24

RTK

Answer 24

Receptor tyrosine kinases phosphorylate proteins on tyrosine
Enzyme coupled receptors
Disregulation of RTKS linked to cancer
( ligand binds , enzyme works )