Structure And Function Of Membranes ✅

Question 1

What functions do membranes have

Answer 1

Physical Barriers (separate intracellular environments from extracellular environments)

Regulate the exchange of substances in and out of cell (partially permeable)

Compartmentalisation (membranes enclose and isolate organelles, enabling them to maintain specific environments for chemical reactions)

Support for the cytoskeleton

Sites of chemical reactions

Sites of cell communication (cell signaling)

Formation of spheres called vesicles (used in bulk transport)

Question 2

When are channel proteins used

Answer 2

Facilitated diffusion

Question 3

When are carrier proteins used

Answer 3

Facilitated diffusion and active transport

Question 4

What is the role of glycoproteins

Answer 4

Receptors (neurotransmitters, peptide hormones and drugs)

Question 5

What is the role of glycolipids

Answer 5

Cell recognition (act as antigens)

Question 6

Why can membrane structure be visualized as a fluid mosaic model

Answer 6

Fluid because phospholipids are able to move within the bilayer
Mosaic because proteins are scattered throughout the bilayer, like tiles in mosaic

Question 7

What is a phospholipid tail and head in relation to water called

Answer 7

Hydrophobic core
Hydrophilic head

Question 8

Where is Cholesterol what’s it’s purpose

Answer 8

In the bilayer (looks like a darker phospholipid)
Adds strength

Question 9

Where is a extrinsic protein

Answer 9

Protein molecule lying on the surface

Question 10

Where is a intrinsic protein what does it contain

Answer 10

Protein molecule spanning the phospholipid layer
Pore

Question 11

Which way do hydrophobic tails point

Answer 11

Point inwards

Question 12

Which way do hydrophilic head point

Answer 12

Point outwards

Question 13

What does an extrinsic protein look like that’s partly embedded

Answer 13

An extrinsic protein that is partly in bilayer and partly outside

Question 14

What is a glycoprotein what does it look like

Answer 14

Branching carbohydrate portion of a protein which acts as a recognition site for chemicals eg hormones
Like a branch off of a protein

Question 15

What is a glycolipid what does it look like

Answer 15

Acts as a recognition site eg for cholera toxins
A branch off phospholipid

Question 16

What factors affect membrane structure

Answer 16

Temperature and solvents

Question 17

What effect does temperature have on membrane structure

Answer 17

Decreased temp reduces fluidity (phospholipids move less due to lower kinetic energy)

Increased temp causes greater fluidity and therefore increases permeability, but a membrane will lose its structure and break apart if temp continues to rise

Question 18

What effect does solvents have on membrane structure

Answer 18

Organic, weakly polar (eg ethanol) or non polar (eg benzene) solvents disrupt or dissolve membranes, making them more permeable

Question 19

Practical investigating membrane permeability, what is the procedure

Answer 19

5 test tubes of alcohol solution (0%=control,10%,20%,30%,40%) are set up. Beetroot cylinders added to each tube, after 5-10mins the colour of each alcohol solution is measured using a colorimeter

A red pigment (betacyanin) leaks from cells into solution when membranes are damaged. The intensity of the colour released is proportional to level of cellular damage

Question 20

Practical: permeability
3 control variables
Why is procedure repeated 3 times

Answer 20

Control: temperature, volume of alcohol solution, SA:V ratio of beet cylinders, pH

Repeated 3 times: reproducibility is increased, spread of results can be assessed, greater confidence in accuracy of results

Question 21

What is diffusion

Answer 21

Net (overall) movement of particles from a region of higher concentration to a region of lower concentration. It is a passive process (ie not requiring ATP)

Question 22

What Diffuses through the bilayer

Answer 22

Some particles can diffuse between phospholipid molecules. Large lipid-soluble molecules (eg steroid hormones), non polar molecules (eg oxygen), very small polar molecules (eg water) are able to pass directly through the bilayer

Question 23

What is facilitated diffusion

Answer 23

Process of particles passing through transmembrane proteins
Ions and large polar molecules (eg glucose and amino acids) pass through proteins rather then between phospholipids

Question 24

What are the 2 types of proteins used for facilitated diffusion

Answer 24

Channel proteins: pores, which can be gated (ie open and closed) allowing the diffusion of ions

Carrier proteins: they have shapes that allow only the passage of specific molecules or ions

Question 25

What factors affect diffusion rates

Answer 25

temp (higher temp provides particles with more kinetic energy)

Steeper concentration gradient- greater the difference in concentration either side of membrane, greater the diffusion rate

Shorter diffusion pathway- thin membranes reduce the distance particles have to move

Greater SA

Higher concentration of carrier proteins increase the rate of facilitated diffusion

Question 26

What is active transport

Answer 26

Energy is required because particles are being moved against concentration gradient (region of lower conc to a region of higher conc). Energy in form of ATP

Question 27

How are carrier proteins used in active transport

Answer 27

Use them as pumps to take the form of bulk transport

Carrier proteins change shape so allows particles to pass through then, due to ATP hydrolysis (which produces phosphate ions)

Question 28

Describe the mechanism by which carrier proteins function

Answer 28

Molecules bind to carrier protein and ATP attaches to the membrane protein in the inside of the cell

Binding of phosphate ion to protein causes the protein to change shape so that access for the molecules is open to the inside of the membrane but closed to the outside

Question 29

What is bulk transport

Answer 29

Movement of large molecules (eg enzymes and hormones) in and out of cells, of particles within the vesicles

Question 30

What is endocytosis

Answer 30

Bulk transport into cells. Vesicles are formed by the plasma membrane being pinched off and 2 forms exist

Question 31

What are the 2 forms from endocytosis

Answer 31

Pinocytosis: a cell engulfs liquid and small dissolved particles

Phagocytosis: a cell engulfs large solid material (eg a white blood cell engulfing a bacterium)

Question 32

How does exocytosis occur

Answer 32

Vesicles are formed by Golgi apparatus and fuse with cell surface membrane, releasing their contents (eg hormones)

Question 33

What is osmosis

Answer 33

Diffusion of water through a partially permeable membrane down a water potential gradient

Question 34

What is the effect of high water potential on animal and plant cells

Answer 34

Net movement of water: enters cell
Animal: swells and bursts (lysis)
Plant: swells and becomes turgid

Question 35

What is the effect of an equal (same solute concentration in cell and surrounding solution) on animal and plant cells

Answer 35

Net movement: water leaves and enters cell at equal rates
Animal: no change
Plant: no change

Question 36

What is the effect of a low water potential on animal and plant cells

Answer 36

Net movement of water: leaves cell
Animal: shrinks (cremation)
Plant: plasmolysis (membrane pulls away from cell wall)

Question 37

Outline 3 roles of membranes within cells (3 marks)

Answer 37

Compartmentalization of organelles [1]
Anchors/ supports cytoskeleton [1]
A site for chemical reactions (eg thylakoid membranes in chloroplasts) [1]
Vesicle formation [1]

Question 38

State and explain which type of protein within membranes would be used in following cases:
A: uptake of glucose into a cell
B: bonding of adrenaline to a liver cell (4marks)

Answer 38

A: Carrier proteins [1], because they enable facilitated diffusion of large molecules [1]
B: glycoproteins [1], because the carbohydrate attached to protein acts as a receptor [1]

Question 39

Some cells, such as proximal convoluted tubules of kidneys, require different proteins to be present on either side of cell. Suggest why an increase in membrane fluidity would be a problem for these cells (2 marks)

Answer 39

Proteins could move within membrane [1]
And end up in wrong positions within the membrane [1]

Question 40

Describe how temperature affects the permeability of membranes (3 marks)

Answer 40

Increases in temperature raise membrane fluidity and therefore permeability [1]
By increasing the kinetic energy of phospholipids [1]
Which widens the spaces between the phospholipids molecules [1]

Question 41

Other than the effect of phospholipids, why else might membrane function be damaged at high temperatures (2 marks)

Answer 41

Proteins in the membrane are denatured [1]
Preventing cell signaling/ transmembrane transport [1]

Question 42

Explain how excessive ethanol consumption might disrupt membrane function in cells (5marks)

Answer 42

Ethanol is lipid soluble [1]
And can insert itself between phospholipids [1]
Increasing membrane permeability [1]
Extra particles can leave and enter cells [1]
Thereby altering particle concentrations and cell content [1]

Question 43

Glucose molecules are reabsorbed from
Proximal convoluted tubules in the kidney. Explain why the cell membranes of the proximal convoluted tubule
A: are folded into microvilli;
B: contain a high concentration of carrier proteins (4 marks)

Answer 43

A: Microvilli increase SA [1]
Which increases diffusion rate [1]

B: for facilitated diffusion of glucose [1]
Which is large and polar and therefore cannot diffuse through the phospholipid bilayer [1]

Question 44

State how following particles are likely to diffuse through a membrane. Explain your answers
A: co2
B: potassium ions (4 marks)

Answer 44

A: diffusion through the bilayer [1] because CO2 is small and non polar [1]

B: facilitated diffusion [1]
Because potassium ions are charged and therefore not lipid soluble [1]

Question 45

Outline the characteristics of particles that are transported using expcytosis (4 marks)

Answer 45

Molecules being transported out of cells [1]
Which are larger/ polar [1]
Often proteins [1]
Examples: hormones/ enzymes/ neurotransmitters [1]

Question 46

Suggest how vesicles formed during phagocytosis would differ from those formed during pinocytosis. Explain your answer (3 marks)

Answer 46

Larger vesicles for phagocytosis [1]
Phagocytosis transports large solid material into cells [1]
Pinocytosis transports small, dissolved particles and small amounts of water into cells [1]

Question 47

State whether the following sentences are true or false
A: diluting a solution will lower water potential
B: no solutions can have a water potential of 0
C: a liver cell is in danger of plasmolysing if placed in a solution with a high water potential (3 marks)

Answer 47

A: false [1]
B: true [1]
C: false [1]

Question 48

An erythrocyte with a water potential of -500kPa was placed in solutions with a range of water potentials. The water potentials are listed below. Describe what would happen to the erythrocyte in each case
A: -470kPa
B: -500kPa
C: -1200kPa (3marks)

Answer 48

A: increase in cell size (possible lysis/ bursting but unlikely because the difference in water potential is small) [1]
B: no change [1]
C: cell shrinks/ experiences crenation [1]

Question 49

Suggest why some plants wilt when the soil they are growing in becomes too salty (4 marks)

Answer 49

Water potential in the soil water is reduced [1]
Water leaves plant (root hair) cells via osmosis [1]
Plant cells plasmolyse [1]
And lose turgor [1]