Topic 2B: Cell membranes

Question 1

What do cell membranes do?

Answer 1

• Surround cell and individual organelles
• Control exchange of substances
• Help communication with the environment with receptors
• Help organelle function (cristae in mitochondria, thylakoid in chloroplasts)

Question 2

What is the cell membrane bi layer?

Answer 2

• Phospholipid bilayer
• Hydrophilic glycerol heads and hydrophobic fatty acid tails
• Tails face in to create the bi layer and be shielded

Question 3

What substances can easily pass through the bi layer?

Answer 3

• small
• uncharged
• lipid soluble

Question 4

What is the cell membrane model called?

Answer 4

• fluid mosaic

Question 5

What are glycolipids?

Answer 5

lipid with carbohydrate chain attached

Question 6

What are glycoproteins?

Answer 6

proteins with a carbohydrate chain attached

Question 7

What do glycolipids and glycoproteins do?

Answer 7

• Act as receptors (glycoproteins)
• Help to stabilise the membrane

Question 8

What does cholesterol do?

Answer 8

• Type of lipid
• Help regulate fluidity of the membrane
• Bind to hydrophobic tails so they pack more closely together –> restricts the movement of phospholipids and decreases fluidity
• Helps maintain the shape of animal cells (have no cell wall)
• Also have hydrophobic regions to act as a further barrier

Question 9

What do the extrinsic proteins do?

Answer 9

• Channel proteins (facilitated diffusion) and carrier proteins (active transport) help to move substances across the membrane

Question 10

What is membrane permeability like below 0°C?

Answer 10

• Little energy in molecules so little movement (of phospholipids)
  -Closely packed so rigid membrane
• Channel and carrier proteins can denature to increase permeability
• Ice crystals can form and pierce the membrane when it thaws –> inc permeability

Question 11

What is membrane permeability like 0-45°C?

Answer 11

• Less tightly packed, partially permeable
• As temperature inc, phospholipids have more energy so move more and inc permeabiity

Question 12

What is membrane permeability like above 45°C?

Answer 12

• Bi-layer begins to melt –> more permeable
• Water inside expands, inc pressure on the membrane
• Channel and carrier proteins denature, inc permeability

Question 13

Describe the experiment to investigate membrane permeability.

Answer 13

• Solvents disrupt the membrane by inserting themselves into it and increase the permeability
• Beetroot has a red pigment in the vacuole that normally cannot fit through the membrane
• Put beetroot in different solvent concentrations in a water bath and measure the pigment released using a colorimeter and a calibration curve
• Can use different temperature instead of solvent concentrations

Question 14

What is simple diffusion?

Answer 14

• Net movement of particles from high to low concentration
• Continues until molecules are equally distributed
• Passive
• Can occur across a cell membrane (directly)

Question 15

What factors affect the rate of simple diffusion?

Answer 15

• Concentration gradient –> higher gradient = faster diffusion that slows over time as the gradient gets more shallow
• Thickness of the exchange surface –> thinner = shorter diffusion pathway
• Surface area –> higher = faster

Question 16

What is facilitated diffusion?

Answer 16

• Helped diffusion of large, polar or water soluble molecules (glucose, ions etc)
• Move from high to low concentration
• Passive
• Use transport proteins

Question 17

How do carrier proteins work?

Answer 17

• Move large molecules
• Molecule attaches
• Protein changes shape
• Molecule released on the other side of the membrane

Question 18

How do channel proteins work?

Answer 18

• Move charged particles
• Form pores in the membrane
• Molecules diffuse through the pore
• Different channel proteins help different substances move

Question 19

What factors affect the rate of facilitated diffusion?

Answer 19

• Concentration gradient –> higher = faster but slows over time as gradient gets more shallow
• Number of transport proteins –> Once all proteins in use, diffusion cannot happen any faster

Question 20

What is osmosis?

Answer 20

• Diffusion of water from high to low water potential across a partially permeable membrane

Question 21

What is water potential?

Answer 21

• Pure water = 0 water potential
• More negative = higher solute concentration

Question 22

What is isotonic?

Answer 22

• Solutions have equal concentrations

Question 23

What is hypertonic?

Answer 23

• In a higher water concentration (low solute conc)

Question 24

What is hypotonic?

Answer 24

• In a lower water concentration (high solute conc)

Question 25

What factors affect rate of osmosis?

Answer 25

• Water potential gradient –> higher = faster but slows over time as potential gets lower
• Thickness of exchange surface –> thinner = shorter diffusion pathway
• Surface area –> higher = faster

Question 26

What happens to plant cells in different solutions?

Answer 26

• Outside = lower water potential –> water moves out, cytoplasm pulls away from cell wall = plasmolysed
• Outside = higher water potential –> water moves in, cytoplasm pushes against cell wall = turgud

Question 27

What happens to animal cells in different solutions?

Answer 27

• Outside = higher water potential –> water moves in, no cell wall so cell eventually bursts = lysis
• Outside = lower water potential –> water moves out, cytoplasm pulls away from membrane = crenated

Question 28

Describe the experiment to investigate the water potential of potato

Answer 28

• Put potato in different sucrose solutions and in a water bath and leave them
• Measure mass before and after to find change in mass ans then % change in mass
• Plot the results and where the line crosses the x-axis is the sucrose concentration of the potato

Question 29

What is the calculation to make solutions from stock solution?

Answer 29

(Conc required / Conc stock solution) x volume solution required = volume of stock solution to use

Question 30

What is active transport?

Answer 30

• Movement of molecules from low to high concentration (against conc gradient)
• Happens through carrier proteins specific to the substance
• Requires energy

Question 31

How do carrier proteins work in active transport?

Answer 31

• Molecule attaches to the protein
• The protein changes shape
• The molecule is released on the other side of the membrane
• ATP is hydrolysed to ADP + Pi to release energy for this

Question 32

What factors affect the rate of active transport?

Answer 32

Speed of individual carrier proteins –> faster they work = faster rate
- Number of carrier proteins present –> more = faster
- Rate of respiration in the cell and ATP availability

Question 33

What are co-transporters?

Answer 33

• Type of carrier protein that bind to 2 molecules at a time
• The conc gradient of one is used to help move the other one against their conc gradient
• e.g. sodium ions move high –> low conc moving glucose with them (low–>high)

Question 34

How is glucose co-transported and absorbed from the ileum to the blood?

Answer 34

• Sodium ions (Na+) are moved out of epithelium cell and into blood via active transport
• This creates a conc gradient (lower in cell than ileum) so Na+ now diffuse into the cell using the sodium glucose co-transporter from the ileum
• Co-transporter moves glucose with the Na+ against its conc gradient
• Conc of glucose then becomes higher in the cel than the blood so glucose moves out of the cell via facilitated diffusion using a carrier protein into the blood