2.1.5 Membranes

Question 1

Cut pieces of agar jelly can be used to investigate the factors affecting diffusion rates in cells.
Four pieces of agar jelly containing universal indicator were soaked in the same concentration of
hydrochloric acid for one minute.
The cubes were then removed and blotted dry.
Which of the following pieces of agar jelly would be the first to turn entirely red?
A) a cube with edges 4cm each
B) a cuboid with edges 2cm, 4cm and 6cm
C) a cuboid with edges 3cm, 3cm and 5cm
D) a sphere with diameter 4cm

Answer 1

B

Question 2

E coli usually grows in conditions where the extracellular concentration of lactose is low.
In such conditions lactose does not easily cross the bacterial cell surface membrane.
Suggest and explain why lactose is unable to cross membranes. (2)

Answer 2

(too) big 
unable to pass between phospholipids 
OR
no / small , concentration gradient 
needs , carrier protein / pump 

Question 3

A common symptom of lactose intolerance in adults is the creation of extra fluid in the large intestine. Suggest why this occurs

Answer 3

Undigested lactose in large intestine lowers water potential
This causes water to move in by osmosis

Question 4

What is the main characteristic of biological membranes?

Answer 4

they are partially permeable

Question 5

What is the role of membranes at the surface of cells?

Answer 5

separates the cell’s components from its external environment (compartmentalisation)
controls what enters/leaves the cell
form pseudopodia in amoeba
may contain enzymes involved in specific metabolic pathways
has antigens for cell recognition
cell-cell signalling
may be site of chemical reactions

Question 6

What is the role of membranes within cells?

Answer 6

form organelles - isolation of contents and reactions from cell cytoplasm (compartmentalisation)
in some organelles metabolic processes occur on membranes ( e.g. mitochondria cristae localise enzymes needed for respiration, chloroplast membranes is where some of photosynthesis occurs)
attachment of proteins like enzymes and ribosomes
- creation of concentration gradients
- cell cell signalling
- Controls what enters and leaves organelles

Question 7

What is the structure of biological membranes?

Answer 7

fluid mosaic model
phospholipid bilayer with hydrophilic heads facing outwards and hydrophobic tails facing inwards
proteins and phospholipids are free to move sideways, making it fluid
proteins spread throughout randomly like a mosaic

Question 8

What is the role of phospholipids?

Answer 8

head orientates outwards towards external aqueous environment
tails orientate away from external environment
helps to form bilayer to separate two aqueous regions
act as a barrier to most water-soluble substances

Question 9

What is the role of cholesterol?

Answer 9

regulates membrane fluidity by being located between phospholipids, binded to the tails
prevents over fluidity at high temps by almost pulling phospholipids closer and making them more packed
prevents losing fluidity at low temps by keeping gaps and pushing neighbouring phospholipids apart slightly

Question 10

What is the role of the glycocalyx?

Answer 10

formed from glycolipids and glycoproteins
contain carbohydrate chains that exit at surface, enabling them to act as receptors
some are involved in cell adhesion and stabilisation

Question 11

What are the two types of transport proteins? How do they work?

Answer 11

integral proteins
carrier proteins: change shape to carry specific complementary molecule across membrane
channel proteins: act as hydrophilic channels to allow ions to pass

Question 12

What are some examples of specialised membranes? HINT: neurones, WBC, Root hair cells, mitochondria)

Answer 12

neurones: protein channels and carriers covering axon allow entry and exit of ions to bring about conduction of electrical impulses, as well as myelin sheath with a protein + (lots of) lipid membrane
white blood cells: special protein receptors that enable them to recognise antigens on foreign cells
root hair cells: carrier proteins to actively transport nitrate ions
mitochondria: inner membranes contain protein electron carriers

Question 13

Define diffusion

Answer 13

net movement, as a result of KE, of a substance from a region of higher conc to a region of lower conc. DOWN GRADIENT

Question 14

What is the diff between simple and facilitated diffusion?

Answer 14

simple: small, non polar lipid soluble molecules. oxygen, carbon dioxide, steroid hormones.
facilitated: channel/carrier proteins help a substance to diffuse. e.g. ions (water-filled protein channels), glucose is too large so transmembrane carrier protein opens.

Question 15

What happens after they have moved down their conc gradient?

Answer 15

still move randomly but remain evenly dispersed = no net diffusion

Question 16

Which molecules are an exception?

Answer 16

water
presence in such high concs that it still often does simple diffusion, even though its polar
in membranes where high rate of movement needed, aquaporins allow this

Question 17

How is conc gradient maintained?

Answer 17

molecules entering cells then pass into organelles and are used for metabolic reactions = low conc = keeps more entering
e.g. O2 into cytoplasm into mitochondria for aerobic respiration and CO2 into palisade cells into chloroplasts for photosynthesis

Question 18

What factors affect the rate of diffusion?

Answer 18

Temp: inc = inc KE = inc rate
SA: inc = inc rate
Size of molecules: dec size = inc rate
Distance= inc in thickness = dec rate
Conc gradient = inc steepness= inc rate

Question 19

What is osmosis?

Answer 19

net movement of water molecules from a region of high water potential to a region of lower water potential, across a partially permeable membrane

Question 20

what is water potential? how is it measured?

Answer 20

measure of the tendency of water molecules to diffuse from one region to another

0 to negative numbers

Question 21

What happens if the water potential on both sides of the membrane becomes equal?

Answer 21

• No net osmosis, but molecules will still move randomly
• ISOTONIC (solution has balanced solute conc with another solution

Question 22

Hypotonic

Answer 22

low solute concentration

Question 23

hypertonic

Answer 23

high solute concentration

Question 24

Water potential of cells. What happens if an animal cell has a lower water potential than its surrounding environment?

Answer 24

Water moves IN down the water potential gradient
Often animal cells burst open = Cytolysis
Haemolysis for red blood cells

Question 25

Water potential of cells. What happens if a plant cell has a lower water potential than its surrounding environment?

Answer 25

Water moves IN down the water potential gradient.
Membrane pushes against cell wall = turgid.

Question 26

Water potential of cells. What happens if an animal cell has a higher water potential than its surrounding environment?

Answer 26

Water moves OUT down water potential gradient
animal cells shrivel = Crenated

Question 27

Water potential of cells. What happens if a plant cell has a higher water potential than its surrounding environment?

Answer 27

Plant plasma membrane pulls away from cell wall = Plasmolysed
Plant tissue with plasmolysed cells= flaccid

Question 28

What happens to plasmolysed cells?

Answer 28

Suffer degree of dehydration = metabolism cannot proceed as enzyme-catalysed reactions need to be in solution (need water)

Question 29

What is active transport?

Answer 29

movement of substance against it’s conc gradient, from an area of lower conc to an area of higher conc
needs energy - provided by hydrolysis of ATP

Question 30

What do carrier proteins do?

Answer 30

have specific sites that combine reversibly with only certain solute molecules/ions (complementary shape). Also have a region that binds to and allows hydrolysis of a molecule of ATP.
energy helps it to change shape and carries molecule/ion from one side to the other

Question 31

What are some examples of when carrier proteins are important?

Answer 31

uptake of mineral ions by plant roots
potassium pump helps trigger an electrical impulse
guard cells: K+ions actively transported into guard cells = lowers water potential = water enters = stoma open due to guard cells changing conformation

Question 32

What is bulk transport used for? What does it require?

Answer 32

transport larger molecules , requires ATP

Question 33

What is endocytosis?

Answer 33

movement INTO cell
cell membrane invaginates around substance to form a vesicle and brings it in
phagocytosis: solid matter
pino(endo)cytosis: liquid
e.g. engulfing an invading pathogen

Question 34

What is exocytosis?

Answer 34

movement OUT of cell
vesicle moves towards plasma membrane + fuses with it, releasing molecule. Membrane reforms

Question 35

What happens to phospholipids when temperature drops? What can some organisms e.g fish do?

Answer 35

saturated fatty acids become compressed
many unsaturated fatty acids kinks push adjacent phospholipid molecules away, maintaining fluidity
proportions of unsaturated and saturated fatty acids within a cell membrane determines fluidity
cholesterol prevents phospholipids from packing too closely together
some organisms e.g. fish and microorganisms and plants can change composition of fatty acids in response to lower temps

Question 36

What happens to phospholipids when temperature increases? What might this affect?

Answer 36

Phospholipids move more = increases fluidity = inc permeability
inc in fluidity may affect infolding of membrane and ability of cells to signal to other cells (exocytosis)
cholesterol also reduces increase in membrane fluidity.

Question 37

What is the link between proteins and temperature?

Answer 37

high temps cause atoms to vibrate -> H and ionic bonds broken -> unfold from teritary structure and falls apart. carrier proteins may change shape = inc or dec permeability.
affects enzymes -> alter rate of reactions they catalyse and could become denatured.
cytoskeleton threads: if this and membrane embedded proteins denature, then the membrane will fall apart and inc permeability.

Question 38

What is the effect of solvents?

Answer 38

organic solvents such as acetone will damage membranes as they dissolve lipids
hydrophobic/non polar substances can get between fatty acids and break H bonds = inc permeability.