Module 2: Biological membranes

Question 1

What are the functions of Plasma membranes on the outer surface of a cell?

Answer 1

1) Acts as a barrier between the cell and the environment.

2) Controls what goes in and out of the cell. It is partially permeable therefore lets some substances through but not others via diffusion/osmosis/active transport.

3) Cell recognition: glycoproteins on the plasma membrane act as antigens, telling the immune system that the cell is not a foreign pathogen.

4) Cell signalling: receptors on the plasma membrane bind to hormones or other chemicals released by other cells.

Question 2

What are the functions of Plasma membranes within a cell?

Answer 2

1) Compartmentalises the cells: internal membranes act as a barrier between organelles and the cytoplasm, allowing them to function more efficiently by keeping enzymes and molecules in one place.

2) Vesicles can form: membranes can form vesicles to transport substances between different areas of the cell.

3) Intracellular membranes are partially permeable. The membranes of some organelles are folded, increasing their surface area and making chemical reactions more efficient.

Question 3

What is the name + year of the model regarding the Plasma membrane?

Answer 3

The fluid mosaic model (1972)

Question 4

Describe the Fluid Mosaic model 1972.

Answer 4

They are all composed of lipids, (phospholipids), proteins and carbohydrates..

The plasma membrane is made up of a BILAYER of phospholipids with proteins and cholesterol dispersed throughout the structure.

Hydrophilic heads on the outer surface and has hydrophobic tails on the inner surface.

Question 5

What are the 6 main components in the bilayer?

Answer 5

Glycoproteins

Channel proteins: extrinsic/intrinsic

Glycoproteins

Glycolipids

Cholesterol

Phospholipids

Question 6

Describe Phospholipids.

Answer 6

Consists of a hydrophilic head and a 2 hydrophobic tails which point towards each other, away from water.

They are the main component of the plasma membrane and forms a barrier to anything which is not a lipid - soluble such as ions and glucose.

The centre of the bilayer is hydrophobic, so the membrane doesn’t allow water-soluble substances, such as ions and polar molecules, to diffuse through it- it acts as a barrier to theses dissolved substances.

Question 7

Describe the role of Cholesterol in the plasma membrane.

Answer 7

It gives the membrane stability. (It is a lipid)

At higher temperatures, they bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This makes the membrane less fluid and more rigid.

At lower temperatures, cholesterol prevents the phospholipids from packing too closely together, so increases membrane fluidity.

It also has hydrophobic regions, so its able to create a further barrier to polar substances moving through the membrane.

Question 8

Describe the role of intrinsic/integral proteins in the plasma membrane.

Answer 8

Intrinsic proteins are fully embedded in the membrane from one side to the other.

Because they pass through the lipid bilayer, the proteins have hydrophobic amino acids on the outer surface. These hydrophobic amino acids can interact with the hydrophobic fatty acid tails in the phospholipid bilayer.

Protein channels are intrinsic proteins as they consist of a channel running through the centre of the bilayer. The channel is lined with hydrophilic amino acids and is filled with water molecules. Protein channels allow water/soluble molecules and ions to diffuse through.

Carrier proteins are intrinsic proteins and they can change their shape or position to transfer molecules or ions from one side of the membrane to the other.

Question 9

Describe the role of extrinsic proteins in the plasma membrane.

Answer 9

They are found on the outer surface of one side of the membrane or the other.

Glycoproteins have a carbohydrate attached to a protein. They act as recognition sites (where drugs, hormones and antibodies bind to) and antigens.

Glycolipids are phospholipids that have a carbohydrate attached to them. They also act as recognition sites (where drugs, hormones and antibodies bind to) and antigens as well. They increase membrane stability by forming hydrogen bonds with surrounding water molecules.

Question 10

What are the 2 factors that affect membrane permeability?

Answer 10

Solvents.

Temperature.

Question 11

What effect do solvents have on membrane permeability?

Answer 11

The permeability of cell membranes depends on the solvent surrounding them.

Some solvents, such as ethanol, dissolve the lipids in a cell membrane leaving it to lose its structure. Ethanol increases membrane permeability more than methanol.

Increasing the concentration of the solvent will also increase membrane permeability.

Question 12

What effect does temperature have on membrane permeability?

Answer 12

Temperatures below 0’C:
The phospholipids don’t have much energy, so they can’t move very much. They are packed closely together and the membrane is rigid.
But channel/carrier proteins will denature, which increases membrane permeability. Ice crystals may form and pierce the membrane, making it highly permeable when it defrosts.

Temperatures between 0-45’C:
The phospholipids aren’t closely packed together and can move around, allowing the membrane to be partially permeable. This is because the phospholipids have more energy.

Temperatures above 45’C:
The phospholipid bilayer starts to melt and the membrane becomes more permeable. Water inside the cell expands, putting pressure on the membrane. Channel/carrier proteins in the membrane denature so they cant control what enters or leaves the cell. This increases the permeability of the membrane.

Question 13

Describe the practical investigating membrane permeability.

Answer 13

Prepare eight cylinders of beetroot of equal size.

Rinse each piece to remove any pigment released during cutting.

If you are investigating the effect of temperature, prepare eight water baths of varying temperatures ranging from 0-70’C.

Prepare a series of test tubes containing the same volume of water (e.g. 10 cm3).

Place the tubes in different water for five minutes.

Place a single sample of beetroot into each of the eight test tubes. Leave for 15 minutes.

Use forceps to remove the pieces of beetroot from each tube. Keep the coloured liquid and transfer to a cuvette.

Use a colorimeter to measure how much light is absorbed by each liquid. The darker the solution (i.e. the more permeable the membrane), the more light is absorbed.

Draw a graph plotting absorbance against temperature.

Question 14

What is cell signalling?

Answer 14

Cell signalling starts when one cell releases a messenger molecule (e.g. hormone).

This molecule travels to another cell (e.g. in the blood).

The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane.

The binding then triggers a change in the cell, e.g. a series of chemical signals is set off.

Question 15

What are the role of membrane receptors?

Answer 15

Proteins in the cell membrane act as receptors for messenger molecules. These receptor proteins are called ‘membrane-bound receptors’

They have specific shapes where only messenger molecules with a complementary shape can bind to them.

Different cells have different types of receptors that respond to different messenger molecules.

A cell that responds to a particular messenger is called a target cell.

Question 16

Describe hormones as messenger molecules.

Answer 16

They work by binding to receptors in cell membranes and triggering a response in the cell.

For example, Glucagon is a hormone that’s released when there isn’t enough glucose in the blood. it binds to receptors on liver cells, causing the liver cells to break down stores of glycogen to glucose.

Question 17

Describe the role of drugs as messenger molecules.

Answer 17

They bind to receptors in cell membranes and either trigger a response in the cell, or block the receptor and prevent it from working.

For example, the chemical endorphin, binds to opioid receptors in the brain and reduce the transmission of pain signals

Morphine works by binding to the same opioid receptors as endorphins, also triggering a reduction in pain signals.

Antihistamines work by blocking histamine receptors on cell surfaces. This prevents histamine from binding to the cell and stops inflammation.

Question 18

What is Diffusion?

Answer 18

It is the net movement of particles from an area of higher concentration to an area of lower concentration.

It is a passive process- no energy is needed for it to happen.

Question 19

What are the 4 main factors that affect the rate of diffusion?

Answer 19

The concentration gradient- the higher it is, the faster the rate of diffusion.

The thickness of the exchange surface- the thinner the exchange surface, the shorter the distance the particles have to travel allowing a faster rate.

The surface area- the larger the surface area, the faster the rate of diffusion.

The temperature- the warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster.

Question 20

Describe the practical investigating the rate of diffusion.

Answer 20

You need to make up cubes of agar jelly containing phenolphthalein and dilute sodium hydroxide (an alkali).

Place in a beaker containing dilute hydrochloric acid. The cubes will turn pink to colourless as the acid diffuses into the cubes and neutralises the sodium hydroxide.

Temperature – repeat the experiment with several beakers, placing each in a water bath set to different temperatures. The agar cubes should be cut to exactly the same size. You’d expect to find that higher temperatures cause the cubes to lose their colour more quickly.

Surface area – repeat using cubes cut to different sizes and calculate their surface area to volume ratio. Smaller cubes, with a larger surface area to volume ratio, will turn colourless quicker.

Concentration – repeat the experiment with beakers containing different acid concentrations. Cut your agar cubes to exactly the same size and place in the separate beakers. The cubes in the highest concentration of hydrochloric acid will go colourless the fastest.

Question 21

What is facilitated diffusion?

Answer 21

Large/charged particles diffuse through carrier or channel proteins in the cell membrane. These protein molecules allow hydrophilic substances to cross the membrane without interacting with the hydrophilic centre of the bilayer.

It is a passive process- doesn’t require energy.

Question 22

Describe active transport?

Answer 22

It uses energy to move molecules/ions across plasma membranes, against a concentration gradient.

A molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side.

ATP is used to move the solute against its concentration gradient.

Question 23

What is Endocytosis?

Answer 23

If substances are too large to cross the membrane, they enter the cell by endocytosis. The cell surrounds the substance and folds its membrane around it. The membrane then pinches off to engulf the substance, which causes a vesicle to form inside the cell containing the ingested substance.

This is an active process so will require energy in the form of ATP.

An example of endocytosis is when phagocytes carry out phagocytosis, in which the phagocyte engulfs a whole bacterium in order to destroy it.

Question 24

What is Exocytosis?

Answer 24

When large substances need to leave the cell, such as hormones and digestive enzymes, they do so by exocytosis. These substances will be contained inside vesicles which move towards the plasma membrane and fuse with it.

This causes the substances to either be released outside of the cell or they will be inserted straight into the membrane (for example, if the substance is a membrane protein).

Exocytosis is an active process which requires ATP.

Question 25

What is Osmosis?

Answer 25

Osmosis is the movement of water molecules down its concentration gradient across a partially permeable membrane. This means water molecules move from an area of higher water potential to an area of lower water potential.

It is a passive process so does not require energy in the form of ATP.

Question 26

Define water potential.

Answer 26

Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution.

Question 27

What is the water potential of pure water.

Answer 27

It is zero.

Adding solutes to pure water lowers its water potential- so the water potential of any solution is always negative. The more negative the water potential, the stronger the concentration of solutes in the solution.

Question 28

Describe isotonic solutions.

Answer 28

If two solutions have the same water potential, they are said to be isotonic. Cells in an isotonic solution wont lose or gain any water- there’s no net movement of water molecules because there’s no difference in water potential between the cell and the surrounding solution. Both plant and animal cells will stay the same when placed in an isotonic solution.

Question 29

Describe hypotonic solutions.

Answer 29

When a plant cell is placed in hypotonic (dilute, higher water potential) solution, water moves into the cell by osmosis. The vacuole swells and the cytoplasm pushes against the cell wall. The cell becomes turgid and the cell wall protects it from bursting.

Question 30

Describe Hypertonic solutions.

Answer 30

When plant cells are placed in a hypertonic (concentrated, lower water potential) solution, water moves out of the cell by osmosis. The cell becomes flaccid/shrinks and the cell membrane pulls away from the cell wall (plasmolysis)

Question 31

Describe the practical investigating the effect of water potential on plant cells.

Answer 31

Prepare cylinders of potato using a cork borer and cut into three.
Measure the mass of the potato pieces using a mass balance.

Prepare serial dilutions of sucrose solution. First, make a 2M solution with a volume of 10 cm3. Transfer half (5 cm3) to another test tube, with an equal volume of water. Repeat three more times so that you end up with solutions of 2M, 1M, 0.5M, 0.25M and 0.125M.

Place the potato pieces in each solution and leave for 30 minutes.
Remove from solution and dry with a paper towel.

Record the mass of each potato piece and calculate the percentage change in mass.

Draw a graph with percentage change in mass on the y-axis and concentration of sucrose solution on the x-axis.

Read off the sucrose concentration where the change in mass = 0. Look up the water potential for this sucrose concentration – it’s the same as the water potential of the potato cells.