topic 2 - cell membrane structure

Question 1

what do cell surface membranes do

Answer 1

surround cells - acting as a barrier between the cell and its environment, controlling which substances enter and leave the cell - they’re partially permeable

Question 2

what are the three ways substances can move across the cell surface membrane

Answer 2

(1) diffusion
(2) osmosis
(3) active transport

Question 3

what does the membranes around organelles do

Answer 3

divides the cell into different compartments - they act as a barrier between the organelle and the cytoplasm

Question 4

what is the basic structure of all cell membranes

Answer 4

composed of lipids, proteins and carbohydrates

Question 5

what is the fluid mosaic model

Answer 5

a model formulated in 1972 which describes describes the structure of the plasma membrane as a mosaic of components - including phospholipids, cholesterol, proteins, and carbohydrates

Question 6

in the fluid mosaic model what do the phospholipid molecules form

Answer 6

a continuous double layer (bilayer) - the bilayer is fluid as the phospholipids are constantly moving

Question 7

why was the fluid mosaic model given this name

Answer 7

because proteins are scattered through the bilayer like tiles in a mosaic

Question 8

what two proteins allow large molecules and ions to pass through the membrane

Answer 8

carrier proteins and channel proteins

Question 9

what do the receptor proteins on the cell surface membrane do

Answer 9

they allow the cell to detect chemicals released from other cells - the chemicals signal to the cell to respond in some way

Question 10

what are the six features of the cell membrane

Answer 10

(1) protein
(2) cholesterol
(3) protein channel
(4) phospholipid
(5) glycolipid
(6) glycoprotein

Question 11

what are glycoproteins

Answer 11

proteins that have a polysaccharide (carbohydrate) chain attached

Question 12

what are glycolipids

Answer 12

lipids with polysaccharide chains called

Question 13

what do phospholipids do

Answer 13

act as a barrier to separate the inside of a cell from its outside environment

Question 14

what is the difference between the head and tail of a phospholipid

Answer 14

the head is hydrophilic - it attracts water
the tail is hydrophobic - it repels water

Question 15

in what way do the phospholipids arrange themselves into a bilayer

Answer 15

the heads face out towards the water on either side of the membrane

Question 16

how is the phospholipid bilayer able to act as a barrier to dissolved substances

Answer 16

since the centre of the bilayer is hydrophobic so the membrane doesn’t allow water soluble substances through it

Question 17

what is cholesterol a type of

Answer 17

lipid

Question 18

what is cholesterol not present in

Answer 18

bacterial cell membranes

Question 19

how does cholesterol make the membrane more rigid

Answer 19

the cholesterol molecules fit between the phospholipids - they bind to the hydrophobic tails of the phospholipids causing them to pack more closely together which restricts the movement of the phospholipids - making the membrane less fluid and more rigid

Question 20

what are two examples of different conditions that could affect the permeability of cell membranes

Answer 20

(1) temperature
(2) solvent concentration

Question 21

why do we use beetroot to investigate how conditions affect permeability

Answer 21

beetroot cells contain a coloured pigment that leaks out - the higher the permeability of the membrane, the more pigment leaks out of the cell

Question 22

what is the first step to investigating how temperature affects beetroot membrane permeability

Answer 22

use a scalpel to carefully cut five equal sized pieces of beetroot - rinse the pieces to remove any pigment released during cutting

Question 23

what is the second step to investigating how temperature affects beetroot membrane permeability

Answer 23

add the five pieces to five different test tubes, each containing 5cm3 of water - use a measuring cylinder or pipette to measure the water

Question 24

what is the third step to investigating how temperature affects beetroot membrane permeability

Answer 24

place each test tube in a water bath at a different temperature for the same length of time (measured using a stopwatch)

Question 25

what is the fourth step to investigating how temperature affects beetroot membrane permeability

Answer 25

remove the pieces of beetroot from the tubes, leaving just the coloured liquid

Question 26

what is the fifth step to investigating how temperature affects beetroot membrane permeability

Answer 26

use a colourimeter - a machines that passes light through the liquid and measures how much of that light is absorbed - the higher the absorbance, the more pigment released, so the higher the permeability of the membrane

Question 27

what is the sixth step to investigating how temperature affects beetroot membrane permeability

Answer 27

connect the colourimeter to a computer and use software to collect the data and draw a graph of the results

Question 28

what happens to the membrane permeability when temperature is below 0C

Answer 28

the phospholipids don’t have much energy, so they can’t move very much - they’re packed closely together and the membrane is rigid, but channel proteins and carrier proteins in the membrane deform, increasing the permeability of the membrane - ice crystals may form and pierce the membrane making it highly permeable when it thaws

Question 29

what happens to the membrane permeability when temperature is between 0 and 45C

Answer 29

the phospholipids can move around and aren’t packed as tightly together - the membrane is partially permeable - as the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane

Question 30

what happens to the membrane permeability when temperature is above 45C

Answer 30

the phospholipid bilayer starts to melt and the membrane becomes more permeable - water inside the cell expands, putting pressure on the membrane - channel proteins and carrier proteins deform so they can’t control what enters or leaves the cell - this increases the permeability of the membrane