Cell Membranes and Transport

Question 1

What are the main components of cell surface and organelle membranes?

Answer 1

• phospholipids
• proteins
• glycoproteins
• glycolipids
• cholesterol
• channel proteins
• carrier proteins
• cotransporters

Question 2

Describe the fluid mosaic model of membrane structure.

Answer 2

The fluid mosaic model states that membranes are made of a phospholipid bilayer. This means that two layers of phospholipids, each with the hydrophobic fatty acid tails pointing inwards and the hydrophilic phosphate-containing heads pointing outwards, form the main part of the membrane. Scattered throughout the membrane are proteins, some of which are structural, others are involved in transport (like channel and carrier proteins). Glycoproteins and glycolipids (carbohydrates are attached) are found, as is cholesterol.

Question 3

What is the role of glycoproteins and glycolipids in membranes?

Answer 3

Glycoproteins and glycolipids have carbohydrates attached to their surface, and act as receptors for information-carrying molecules, such as hormones. They aid in communication of the cell/organelle with its outside environment.

Question 4

What is the role of phospholipids in membranes?

Answer 4

Phospholipids make up the phospholipid bilayer in membranes, and are what make the membranes partially permeable. Because the fatty acid tails are hydrophobic, the inside of the membrane will not allow water or other polar substances through the membrane without using special transport methods. However, small, non-polar (lipid-soluble) substances are able to diffuse through the membrane easily, so it is partially permeable. The phospholipids are fluid (can move), which is why the model of membrane structure is known as the fluid-mosaic model.

Question 5

What is the role of channel proteins in membranes?

Answer 5

Channel proteins are proteins which provide a pathway through which polar substances can enter a cell/organelle. Channel proteins have hydrophilic amino acids lining the inside of the channel, which allows polar substances, which cannot dissolve straight through the phospholipid bilayer, a way to diffuse into the cell (facilitated diffusion).

Question 6

What is the role of carrier proteins in membranes?

Answer 6

Carrier proteins are also involved in facilitated diffusion. Substances can bind to the binding sites of the carrier protein, which changes its shape, allowing the substance to pass through the membrane. This enables the transport of larger/polar molecules which are kept out by the phospholipid bilayer.

Question 7

What is the role of cotransporters in cell membranes?

Answer 7

Cotransporters enable substances to be transported alongside other substances. The other substance is moving down its concentration gradient, so the process does not require energy directly, even though the original substance is moving against its own concentration gradient (though ATP is required to create an electrochemical gradient).

Question 8

What is the role of cholesterol on membranes?

Answer 8

Cholesterol stabilises membranes by restricting the movement of phospholipids. It does this by binding to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This gives rigidity to the membrane, and helps animal cells maintain the correct shape. Cholesterol also has hydrophobic regions, so acts as a further barrier to polar substances attempting to move through the membrane.

Question 9

How does temperature affect membranes?

Answer 9

At low temperatures (below 0 degrees), the phospholipids don’t have much energy, so can’t move very fast. This makes the membrane more rigid. The very low temperature may also cause some proteins in the membrane to denature, which increases the rigidity. At temperatures in between 0 and 45 degrees, the membrane is partially permeable as the phospholipids can move and aren’t packed as closely together. At high temperatures (above 45 degrees), the phospholipid bilayer starts to break down and the membrane becomes more permeable. Channel proteins and carrier proteins denature, which increases the permeability of the membrane.

Question 10

What is the method for the membrane permeability practical?

Answer 10

1. Use a scalpel to cut 5 equally sized pieces of beetroot then rinse each one to remove pigment released during cutting
2. Add each one to a different test tube containing equal volumes of water
3. Place each one in a water bath at a different temperature for the same length of time
4. Remove the beetroot pieces
5. Set up a colorimeter with a blue filter (it measures the amount of light absorbed by a sample)
6. Add distilled water to a cuvette so it is 3/4 full, put it into the colorimeter with the non-frosted sides facing forward and calibrate the machine to zero.
7. Use a pipette to transfer a sample of liquid from the first test tube to a clean cuvette (3/4 full)
8. Put the cuvette in the colorimeter (the correct way round) and record the absorbance of the solution
9. Repeat this for each of the solutions (using a clean pipette and cuvette each time)
10. Draw a graph of results

Question 11

What does a higher absorbance of the beetroot solution indicate?

Answer 11

This indicates that the membrane is more damaged, as the most pigment has been leaked from the cells (as the pigment is what is absorbing the light).

Question 12

Why is beetroot used for the membrane permeability practical?

Answer 12

Beetroot has pigments called anthocyanins stored in the vacuole of its cells, which is released when the membrane is damaged. Therefore, the amount of pigment released indicates the extent to which the membrane is damaged in relation to a named variable (e.g. temperature).

Question 13

How can the effect of solvent concentration of membrane permeability be investigated?

Answer 13

Same method as for temperature, but for this practical, the temperature, and all other factors, must be kept constant. The only factor being varied is the concentration on the solvent you are using (e.g. alcohol, acetone). Increasing the concentration of the solvent should increase the membrane permeability because the solvent dissolves the lipids in the cell membrane, causing it to lose its structure.

Question 14

What is diffusion?

Answer 14

Diffusion is the net movement of particles down a concentration gradient (from an area of high concentration to an area of low concentration).

Question 15

Is diffusion active or passive?

Answer 15

Diffusion is a passive process (it doesn’t require energy).

Question 16

What is simple diffusion and which molecules are transported in this way?

Answer 16

Simple diffusion is the net movement of particles down a concentration gradient, moving directly through a cell membrane by going through the phospholipid bilayer. Because the inside of the phospholipid bilayer is hydrophobic, only small, non-polar (lipid-soluble) molecules can pass through the membrane in this way.

Question 17

How does concentration gradient affect the rate of simple diffusion?

Answer 17

The higher/steeper the concentration gradient, the the faster the rate of diffusion, because diffusion slows over time as the concentrations on each side become closer and eventually reach equilibrium. Therefore, if the concentrations are more different to begin with, the rate will be quicker.

Question 18

How does the thickness of the exchange surface affect the rate of simple diffusion?

Answer 18

The thicker the exchange surface, the slower the rate of diffusion because the particles have to diffuse further.

Question 19

How does surface area affect the rate of simple diffusion?

Answer 19

The larger the surface area, the faster the rate of diffusion, because there is more space for the particles to diffuse, so more can diffuse at once. Some specialised cells, such as those found in the small intestine, have microvilli, which increase the surface area for this reason.

Question 20

What is facilitated diffusion?

Answer 20

Facilitated diffusion is the net movement of particles down a concentration gradient, through carrier or channel proteins. It is mainly large or polar particles which use this method of transport, as they can’t move through the phospholipid bilayer, so can’t undergo simple diffusion.

Question 21

Is facilitated diffusion an active or passive process?

Answer 21

Passive, it doesn’t require energy.

Question 22

How do carrier proteins work in facilitated diffusion?

Answer 22

A large molecule attaches to a carrier protein in the membrane. The protein changes shape and this releases the molecule on the other side of the membrane. The specific carrier protein used depends on the molecule being transported.

Question 23

How do channel proteins work in facilitated diffusion?

Answer 23

Channel proteins form pores in the membrane for polar molecules to diffuse through, down their concentration gradient. Different channel proteins facilitate the diffusion of different polar particles. This works because the protein channel is lined by hydrophilic amino acids on the inside.

Question 24

What factors affect the rate of facilitated diffusion?

Answer 24

Same as for simple diffusion (thickness of membrane, surface area, concentration gradient), but also the number of channel or carrier proteins, as the more there are, the more molecules can be transported at once, so the faster the rate of diffusion.

Question 25

What is osmosis?

Answer 25

Osmosis is the net movement of water molecules from an area of high water potential to an area of low water potential, across a partially permeable membrane.

Question 26

Is osmosis an active or a passive process?

Answer 26

Passive - it doesn’t require energy

Question 27

What are aquaporins?

Answer 27

Aquaporins are special channel proteins that allow the facilitated diffusion (osmosis) of water through cell membranes.

Question 28

What is water potential?

Answer 28

Water potential = solute potential + pressure potential
It is a measure of the potential (or likelihood) of water molecules to diffuse out of or into solution

Question 29

Which has a higher water potential, dilute or concentrated sucrose solution?

Answer 29

Pure water has a water potential of zero. Adding solute makes the water potential negative, and the more solute you add, the more negative the water potential is. Therefore, a dilute sucrose solution would have a higher water potential than a concentrated sucrose solution.

Question 30

What is an isotonic solution?

Answer 30

An isotonic solution has the same water potential as whatever is placed in it, so there will be no net movement of water.

Question 31

What is a hypertonic solution?

Answer 31

A hypertonic solution has a lower water potential than whatever is placed in them. So, if cells are placed in a hypertonic solution, they will lose water by osmosis, as the water potential of the solution inside the cells is greater than that of the solution outside the cells.

Question 32

What is a hypotonic solution?

Answer 32

A hypotonic solution has a higher water potential than whatever is placed in it, so if cells are placed in a hypotonic solution, they will gain water by osmosis as the water potential of the solution inside the cell is lesser than the water potential of the solution outside the cell.

Question 33

What factors affect the rate of osmosis?

Answer 33

• water potential gradient - higher water potential gradient = faster rate of osmosis because the difference in water potential on each side of the membrane decreases over time, so the rate of osmosis levels off
• thickness of exchange surface - thinner = faster osmosis
• surface area - larger surface area = faster rate of osmosis

Question 34

How would you make serial dilutions?

Answer 34

Serial dilutions are a series of solutions, which decrease in concentration by the same factor each time. Set up multiple test tubes, one with a stock solution of known concentration and volume, and the rest with a certain volume of distilled water. If you started with 10cm3 of 2M solution, then you added 5cm3 distilled water, you now have a 1M solution. If 5cm3 is added again, it is now 0.5M, then again gives 0.25M and so on. This can be repeated many times to get increasingly dilute solutions.

Question 35

How do you make up a solution of a particular concentration, without using serial dilutions?

Answer 35

V1=(C2xV2)/C1
C1 - stock concentration
C2 - concentration of working solution (what you are making up)
V1 - volume of stock solution used
V2 - volume of total solution
This formula can be used to work out the volume of stock solution you need (V1), then total volume - V1 is the volume of distilled water you need.

Question 36

What is the method of the osmosis practical?

Answer 36

1. Make up a series of solutions of different concentrations (e.g. 0.1M, 0.2M, 0.3M, 0.4M, 0.5M)
2. Use a cork borer to cut potatoes into identically sized chips. Divide into groups of 3 and measure the mass of the groups using a mass balance
3. Place each group into one of the solutions made up and leave for 20 minutes
4. Remove the chips and pat dry with a tissue, then measure the final mass of each one
5. Draw a graph of results (calibration curve)

Question 37

How can you calculate the water potential of the potato?

Answer 37

The point at which the graph of sucrose concentration against % change in mass crosses the x-axis is the point at which the potato is in isotonic solution. This tells you the concentration of sucrose within the potato. This information can be used to find the water potential (use tables from another source of sucrose concentration against water potential).

Question 38

What is active transport?

Answer 38

Active transport is the net movement of particles across a membrane, against a concentration gradient (from low to high concentration).

Question 39

Is active transport an active or a passive process?

Answer 39

Active transport is an active process - it requires energy.

Question 40

Where does the energy required for active transport come from?

Answer 40

The energy needed for active transport comes from the hydrolysis of ATP to ADP and Pi (the energy released was previously stored in the high energy bond between phosphate groups).

Question 41

How does active transport work?

Answer 41

A molecule attaches to a carrier protein, causing the protein to change shape, which moves the molecule to the other side of the membrane. The protein can only change shape when ATP is hydrolysed, releasing the energy needed to transport the molecule against its concentration gradient. Carrier proteins are the only transport protein used in active transport.

Question 42

What factors affect the rate of active transport?

Answer 42

• speed of individual carrier proteins - the faster the carrier proteins work, the faster the rate of active transport
• the number of carrier proteins - the more carrier proteins there are, the faster the rate of active transport as more molecules can be transported at once
• the rate of respiration in the cell and the availability of ATP (if respiration is fast and lots of ATP is produced, then active transport can occur at a faster rate)
• concentration gradient actually doesn’t really affect the rate of active transport, but thickness of membrane and surface area do

Question 43

What are co-transporters?

Answer 43

Co-transporters are a type of carrier protein which have two binding sites, using the concentration gradient of one molecule to transport another against its concentration gradient.

Question 44

Is co-transport an active or passive process?

Answer 44

Co-transport is an active process because it uses the energy from one molecule’s concentration gradient to move another against its own concentration gradient, however, unlike transitional active transport, it does not directly require ATP.

Question 45

How does the co-transport of glucose work in the mammalian ileum?

Answer 45

Sodium ions are actively transported out of the epithelial cells in the ileum, into the blood by the sodium-potassium pump. This creates a concentration gradient of sodium ions, higher concentration in the lumen of the ileum than inside the cell. Sodium ions bind to co-transporters alongside glucose, and both are released on the other side of the membrane, inside the cell. The concentration of glucose inside the cell increases, and glucose diffuses out of the cell, into the blood, through a channel protein, by facilitated diffusion.