L1

Question 1

cell membrane permeability depends on

Answer 1

molecular size, lipid solubility, and charge

Question 2

what is the cell membrane composed of?

Answer 2

a phospholipid bilayer

Question 3

permeability of the membrane to:
- lipid-soluble molecules and gases
- water soluble molecules
- organic anions (proteins)

Answer 3

• lipid-soluble molecules and gases diffuse through readily
• water soluble molecules cannot cross without help
• impermeable to organic anions (proteins)

Question 4

define simple diffusion

Answer 4

passive movement of a substance down its concentration gradient

Question 5

what molecules pass the membrane through simple diffusion?

Answer 5

small, lipid-soluble molecules and gases (O2, CO2, ethanol, urea etc) pass either directly through the phospholipid bilayer or through pores

Question 6

the relative rate of diffusion is roughly proportional to

Answer 6

the concentration gradient across the membrane

Question 7

define facilitated diffusion

Answer 7

a process of diffusion, where molecules diffuse across membranes with the assistance of carrier proteins

Question 8

what molecules require facilitated diffusion?

Answer 8

polar molecules like sugars and amino acids

Question 9

describe the energy requirement of facilitated diffusion

Answer 9

• movement of substance is down its concentration
• the energy comes from the concentration gradient of the solute
• passive

Question 10

steps of facilitated diffusion

Answer 10

the molecule to be transported binds to the transporter on 1 side and causes a conformational change, causing transporter to close and open on opposite side of the membrane

Question 11

define active transport

Answer 11

a mechanism to move selected molecules across cell membranes, against their concentration gradient

Question 12

describe the process of active transport

Answer 12

• substance binds to protein carrier that changes conformation to move substance across the membrane
• requires energy from ATP hydrolysis

Question 13

define secondary active transport

Answer 13

when a substance is carried up its concentration gradient without ATP catabolism;

Question 14

how does secondary active transport work?

Answer 14

sequential binding of a substance and ions to specific sites in the transporter protein induces a conformational change in the protein

Question 15

how is secondary active transport powered?

Answer 15

kinetic energy of movement of one substance down its concentration gradient powers the simultaneous transport of another up its concentration gradient

Question 16

describe the structure of membrane channels

Answer 16

• membrane spanning protein forms a pore right through the membrane
• 4-5 protein subunits fit together such that a central pore is created through membrane, through which specific ions can diffuse through
• these pore loops of the protein molecule dangle inside the channel

Question 17

how is the selectivity of membrane channels established?

Answer 17

• physical properties of the pore loops create a selectivity filter
• only specific molecules can diffuse through by means of size and electric charge

Question 18

gated channels

Answer 18

• channels can be closed off by a branch of the protein structure which functions as a gate
• under certain conditions, the gate is closed and no diffusion takes place; under other conditions the gate is open and selective diffusion is allowed
• the protein components switch between 2 shapes; one creates an open pore, the other blocks the hole

Question 19

factors determining channel protein shape:

Answer 19

• ligand gated channels: binding of a chemical agent
• voltage gated channels: voltage across the membrane

Question 20

ligand gated channels

Answer 20

• part of the body’s chemical signalling system
• the binding of a receptor with its ligand usually triggers events at the membrane, such as activation of an enzyme

Question 21

voltage gated channels

Answer 21

sensitive to the potential difference across the membrane (eg depolarisation), which changes the conformation of the channel subunits causing a diffusion pore to be created

Question 22

voltage sensing mechanism of voltage gated channels

Answer 22

in the 4th transmembrane domain of the protein, the S4 segment:
- the natural position of the S4 wing is up towards the outer surface of the cell membrane. but when the membrane is polarised, the positively charged wing is attracted downwards to the negatively charged inner surface of the membrane
- depolarisation of the membrane to about -50mV no longer provides sufficient electrical attraction to hold the S4 wing downwards, so it migrates back up
- in the up position, S4 removes a structural occlusion from the pore such that ions can now diffuse through it

Question 23

define endocytosis

Answer 23

inward pinching of membrane to create a vesicle; usually receptor-mediated to capture proteins, from outside to inside

Question 24

define exocytosis

Answer 24

partial or complete fusion of vesicles with cell membrane for bulk trans-membrane transport of specific molecules, from inside to outside

Question 25

state the 2 different types. of exocytosis with different docking mechanisms

Answer 25

- exocytosis 1: the more rapid mechanism has been dubbed the 'Kiss and Run' - exocytosis 2: full exocytosis

Question 26

exocytosis 1

Answer 26

- secretory vesicles dock and fuse with the plasma membrane at specific locations called 'fusion pores' - vesicle can connect and disconnect several times before the contents are emptied - used for low rate of signalling

Question 27

exocytosis 2

Answer 27

- involves complete fusion of the vesicle with the membrane, leading to total release of vesicle contents at once - necessary for delivery of membrane proteins and high levels of signalling - must be counterbalanced by endocytosis to stabilise membrane surface area

Question 28

what body cells generate membrane potential?

Answer 28

all

Question 29

to generate membrane potential we need 2 conditions:

Answer 29

1. create a concentration gradient 2. semi-permeable membrane that allows one ion species to diffuse across the membrane more than others

Question 30

how can we create a concentration gradient to generate membrane potential?

Answer 30

an enzyme ion pump (functions as an ATPase) must actively transport certain ion species across the membrane to create a concentration gradient

Question 31

Na+/K+ pumps

Answer 31

- all cell membranes are loaded with these - Na+/K+ dependent ATPase is an enzyme - for each ATP molecule broken down, 3 Na+ ions are pumped out and 2K+ pumped inn (creates a concentration gradient)

Question 32

if the Na/K inequality leads to a potential difference of -10mV, why is the actual resting MP in neurons closer to -70mV?

Answer 32

- the membrane is most permeable to K+ ion - K+ diffuses out of cell, down concentration gradient, via K+ channels - cations accumulate on the outside of the membrane, leaving a net negativity inside membrane

Question 33

-ve membrane potential means

Answer 33

inside more negative than outside

Question 34

until when does the efflux of K+ ions occur?

Answer 34

- until electrochemical equilibrium is reached - build up of + charge on the outside of the membrane so further diffusion is repelled by the electromagnetic force

Question 35

what two things are equal at equilibrium?

Answer 35

electrical work to repel outward cation diffusion = chemical work of diffusion down concentration gradient

Question 36

membrane potential at equilibrium is determined by

Answer 36

concentration gradient

Question 37

how can we calculate equilibrium potential?

Answer 37

using the Nernst equation

Question 38

Nernst equation

Answer 38

- describes the balance between the chemical work of diffusion with electrical work of repulsion - the equation gives the potential difference across the membrane, inside with respect to outside, at equilibrium

Question 39

if you calculate equilibrium potential using the Nernst equation, you get

Answer 39

-90mV

Question 40

result of the Nernst equation is only valid if

Answer 40

one ion species (K+ in this case) is diffusing across the membrane