membranes

Question 1

membrane definition

Answer 1

semi permeable barriers that surround the cytoplasm of a cell and control the movement of substances in and out

Question 2

components of a cell surface membrane according to fluid mosaic model

Answer 2

phospholipid
glycolipid
glycoprotein
extrinsic protein
intrinsic protein
cholesterol
protein channel

Question 3

function of phospholipids in a membrane

Answer 3

has a hydrophobic fatty acid tail and a hydrophilic polar phosphate head, enabling these molecules to form bilayers in aq solution - majority of biological membranes

fatty acid tails are unsaturated so have some kinks, this prevents molecules form stacking too closely therefore improving flexibility of membrane

phospholipids aren’t stationary, they constantly move around each other which gives the membrane flexibility

Question 4

function of glycolipids in a membrane

Answer 4

a phospholipid with a carbohydrate attached

acts as antigens/markers that allow the cell to be recognised

Question 5

function of glycoprotein in a membrane

Answer 5

a protein with a carbohydrate attached to it

this carbohydrate acts as a receptor, e.g. for chemicals/hormones

Question 6

what is an extrinsic protein in a membrane

Answer 6

a protein that goes all the way through the membrane

Question 7

what is an intrinsic protein in a membrane

Answer 7

a protein that is only partially submerged in the membrane

Question 8

function of cholesterol in a membrane

Answer 8

hydrophilic end can interact with phosphate heads and hydrophobic tail can interact with fatty acid tails

pulls phospholipid molecules closer together

increases stability of the membrane as have kinks like fatty acids which takes up more space and ensures the other molecules don’t get too close together so membrane cannot crystallise

Question 9

function of transport proteins in a membrane

Answer 9

water is polar but also very small so can squeeze through bilayer (passive diffusion via osmosis)

larger molecules, e.g. glucose - which may be non-polar and able to passively diffuse across, are too big to fit through the bilayer so need specific transport proteins

Question 10

6 functions of membranes

Answer 10

compartmentalisation - making organelles/cells with specific environments, isolation of metabolic pathways

creation of concentration gradients

selective permeability - allows for active + passive transport of molecules

cellular communication - glycoprotein receptors and glycolipids that act as cell markers allow for identification and communication via chemical messengers/hormones

cell adhesion

fluidity - if phospholipids crystallise cell no longer works

Question 11

2 factors affecting membrane permeability

Answer 11

temperature and solvent concentration

Question 12

how does temperature increasing affect membrane permeability

Answer 12

as temp increases, lipids gain kinetic energy = become more fluid = effectiveness as a barrier decreases = diffusion rate increases - REVERSIBLE

as temp increases, proteins begin to denature = no longer specific = substances can freely pass through - IRREVERSIBLE

Question 13

how does temperature decreasing affect membrane permeability

Answer 13

as temp decreases towards freezing channel proteins become deformed and ice crystals form which pierce membrane = increased permeability when cell is thawed again

Question 14

how does solvent concentration increasing affect membrane permeability

Answer 14

increasing concentration of organic solvents increases membrane permeability as they dissolve lipids = membrane loses structure

Question 15

partially permeable definition

Answer 15

a membrane that allows some substances to cross but not others

Question 16

why is it called the fluid mosaic model

Answer 16

phospholipids can move around each other freely = fluid
proteins embedded vary in size, shape and posotion = mosaic

Question 17

how can a colorimeter be used to show the permeability of membranes

Answer 17

a more permeable membrane means more dye/pigment leaks out
this means less light is absorbed by the detector

Question 18

outline an experiment to investigate membrane permeability

Answer 18

• beetroot cells contain betalain
• when membranes are disrupted the pigment leaks out
• cut pieces of equal size and place in solvent (water at diff temps or organic solvents at diff concs)
• leave all pieces for same amount of time
• solutions should then be placed in a colorimeter and absorbance should be measured (must be recalibrated each time to 100% absorbance)

Question 19

suggest 3 improvements to colorimeter experiment

Answer 19

• cut pieces from same place on beetroot
• blot surface of beetroot before experiment
• use same volume of solvent

Question 20

how does a channel protein work

Answer 20

these provide a hydrophilic channel that allows the passive movement of polar molecules down a concentration gradient

water filled channel is lined with hydrophilic amino acids on the inside

channel proteins are specific

Question 21

how does a carrier protein work

Answer 21

molecule binds to protein on outside of membrane, protein changes shape and releases the molecule on the other side

this conformational change requires energy - active transport

carrier proteins are specific

Question 22

what is facilitated diffusion

Answer 22

passive diffusion across a membrane down a concentration gradient through protein channels - necessary for most polar molecules

Question 23

what is endocytosis

Answer 23

the bulk transport of material into cells - a form of active transport

Question 24

what are the 2 types of endocytosis

Answer 24

phagocytosis = solids
pinocytosis = liquids

Question 25

outline the process of endocytosis

Answer 25

• cell surface membrane comes into contact with the material to be transported, causing it to bend inwards
• the membrane enfolds the material and fuses, forming a vesicle
• the vesicle pinches off and moves into the cytoplasm to transfer material into cell

Question 26

what is exocytosis

Answer 26

the bulk transport of material out of a cell - a form of active transport

Question 27

outline the process of exocytosis

Answer 27

• vesicles (e.g. the ones formed by the golgi apparatus) moves towards cell surface membrane
• the vesicle fuses to the membrane and the contents are released out of the cell

Question 28

is endocytosis/exocytosis passive or active transport and why

Answer 28

both are active

energy is needed for the movement of vesicles along cytoskeleton + cell changing shape + fusion of cell membranes

Question 29

2 examples of where endocytosis/exocytosis occurs in living organisms

Answer 29

• secretion of proteins/hormones e.g. insulin in the pancreas
• phagocytosis of pathogens as part of the immune response

Question 30

1 example of where facilitated diffusion occurs in living organisms

Answer 30

• movement of glucose and amino acids through cell surface membrane, both in plant and animal cells

Question 31

what is diffusion

Answer 31

the net passive movement of particles down the concentration gradient

can be across a membrane/barrier or without any barrier

Question 32

when will diffusion stop

Answer 32

when a concentration equilibrium is reached

Question 33

what is active transport

Answer 33

the movement of particles across a membrane against the concentration gradient, requiring both energy and carrier proteins

Question 34

2 examples of active transport in organisms

Answer 34

• proton pumps to allow facilitated diffusion of sucrose into companion cells in plants
• movement of Ca2+ ions out of cardiac muscle cells

Question 35

2 examples of diffusion in living organisms

Answer 35

• diffusion of oxygen through alveoli in the lungs
• diffusion of CO2 into leaf through stomata

Question 36

what is osmosis

Answer 36

the net passive movement of water molecules down the concentration gradient across a semi permeable membrane

Question 37

1 example of osmosis in living organisms

Answer 37

• movement of water into root cells from the soil

Question 38

what is the highest water potential

Answer 38

0
this is pure water, with no solute dissolved

Question 39

why do membranes exist as bilayers

Answer 39

both the inside and outside of cells are aq, so a single layer would repel on one side

bilayers mean that hydrophilic heads can face the cytoplasm and tissue fluid with hydrophobic tails facing away from both environments, within the bilayer