Module 2: Section 5 - Biological Membranes

Question 1

Name three functions of membranes at the surface of cells (PLASMA membranes)

Answer 1

1) they are a barrier between the cell and its environment, controlling which substances enter and leave the cell. They’re partially permeable - they let some molecules through but not others. Substances can move across the plasma membrane by diffusion, osmosis or active transport
2) they allow recognition by other cells, e.g. the cells of the immune system
3) they allow cell communication (sometimes called cell signalling)

Question 2

Name five functions of membranes within cells

Answer 2

1) the membranes around organelles divide the cell into different compartments - they act as a barrier between the organelle and the cytoplasm. This makes different functions more efficient, e.g. the substances needed for respiration (like enzymes) are kept together inside mitochondria
2) they can form vesicles to transport substances between different areas of the cell
3) they control which substances enter and leave the organelle, e.g. RNA leaves the nucleus via the nuclear membrane. They are also partially permeable
4) you can get membranes within organelles - these act as barriers between the membrane contents and the rest of the organelle, e.g. thylakoid membranes in chloroplasts
5) membranes within cells can be the site of chemical reactions, e.g. the inner membrane of a mitochondrion contains enzymes needed for respiration

Question 3

The structure of all membranes is basically the same. They’re composed of lipids (mainly phospholipids), proteins and carbohydrates (usually attached to proteins or lipids). In 7 points, briefly explain the fluid mosaic model.

Answer 3

1) in 1972, the fluid mosaic model was suggested to describe the arrangement of molecules in the membrane
2) in the model, phospholipid molecules form a continuous, double layer (bilayer)
3) this bilayer is ‘fluid’ because the phospholipids are constantly moving
4) cholesterol molecules are present within the bilayer
5) protein molecules are scattered through the bilayer, like tiles in a mosaic
6) some proteins have a polysaccharide (carbohydrate) chain attached - these are called glycoproteins
7) some lipids also have a polysaccharide chain attached - these are called glycolipids

Question 4

Phospholipids form a barrier to dissolved substances. What is the role of phospholipids and how does their structure help them to function?

Answer 4

1) phospholipid molecules have a ‘head’ and a ‘tail.’
2) the head is hydrophillic - it attracts water
3) the tail is hydrophobic - it repels water
4) the molecules automatically arrange themselves into a bilayer - the heads face out towards the water on either side of the membrane
5) the centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances (like ions) through it - it acts as a barrier to these dissolved substances. (but fat-soluble substances, e.g. fat-soluble vitamins, can dissolve in the bilayer and pass directly through the membrane).

Question 5

How does cholesterol give the membrane stability?

Answer 5

1) cholesterol is a type of lipid
2) it’s present in all cell membranes (except bacterial cell membranes)
3) cholesterol molecules fit between the phospholipids. 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

Question 6

How do proteins control what enters and leaves the cell?

Answer 6

1) some proteins form channels in the membrane - these allow small or charged particles through
2) other proteins (called carrier proteins) transport molecules and ions across the membrane by active transport and facilitated diffusion
3) proteins also act as receptors for molecules (e.g. hormones) in cell signalling. When a molecule binds to the protein, a chemical reaction is triggered inside the cell

Question 7

How do glycolipids and glycoproteins act as receptors for messenger molecules?

Answer 7

1) glycolipids and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecules
2) they’re also sites where drugs, hormones and antibodies bind
3) they act as receptors for cell signalling
4) they’re also antigens - cell surface molecules involved in the immune response

Question 8

Why do cells need to communicate with each other?

Answer 8

Cells need to communicate with each other to control processes inside the body and to respond to changes in the environment.

Question 9

How do cells communicate with each other?

Answer 9

Cells communicate with each other using messenger molecules:

1) one cell releases a messenger molecule (e.g. a hormone)
2) this molecule travels (e.g. in the blood) to another cell
3) this messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

Question 10

Cell membrane receptors play an important role in cell signalling. In four steps, tell me how receptors work please

Answer 10

1) proteins in the cell membrane act as receptors for messenger molecules
2) receptor proteins have specific shapes - only messenger molecules with a complementary shape can bind to them
3) different cells have different types of receptors - they respond to different messenger molecules
4) a cell that responds to a particular messenger molecule is called a target cell

Question 11

Draw a diagram to show how messenger molecules bind to target cells

Answer 11

see page 52 :)

Question 12

When is glucagon released and how does it bind to receptors?

Answer 12

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 13

Drugs also bind to cell membrane receptors - how? Also, please use the example of antihistamines

Answer 13

1) many drugs work by binding to receptors in cell membranes
2) they either trigger a response in the cell, or block the receptor and prevent it from working

EXAMPLE: antihistamines - cell damage causes the release of histamine. Histamine binds to receptors on the surface of other cells and causes inflammation. Antihistamines work by blocking histamine receptors on cell surfaces. This prevents histamine from binding to the cell and stops inflammation.

Question 14

The permeability of cell membranes is affected by different conditions, e.g. temperature, solvent type and solvent concentration. You can investigate how these things affect permeability by doing an experiment using beetroot. Beetroot cells contain a coloured pigment that leaks out - the higher the permeability of the membrane, the more pigment leaks out of the cell. How could you investigate how temperature affects beetroot membrane permeability (in 5 steps)?

Answer 14

1) cut five equal sized pieces of beetroot and rinse them to remove any pigment released during cutting.
2) place the five pieces in five different test tubes, each with 5 cm cubed of water
3) place each test tube in a water bath at a different temperature, e.g. 10 degrees C, 20 degrees C, 30 degrees C, for the same length of time
4) remove the pieces of beetroot from the tubes, leaving just the coloured liquid
5) now you need to use a colorimeter - a machine that passes light through the liquid and measures how much of that light is absorbed. The higher the permeability of the membrane, the more pigment is released, so the higher the absorbance of the liquid

Question 15

What happens to the permeability of membranes when the temp is below 0 degrees C?

Answer 15

The phospholipids don’t have much energy, so they can’t move very much. They’re packed closely together and the membrane is rigid. But channel proteins and carrier proteins in the membrane deform, increasing the permeability of the membrane. Ice crystals may form and pierce the membrane making it highly permeable when it thaws

Question 16

What happens to the permeability of membranes when the temp is between 0 degrees C and 45 degrees C?

Answer 16

The phospholipids can move around and aren’t packed as tightly together - the membrane is partially permeable. As the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane

Question 17

What happens to the permeability of membranes when the temp is above 45 degrees C?

Answer 17

The phospholipid bilayer starts to melt and the membranes become more permeable. Water inside the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins deform so they can’t control what enters or leaves the cell - this increases the permeability of the membrane

Question 18

Explain how different solvents and their concentration can affect the permeability of cell membranes

how could you investigate the effects of different solvents on the permeability of cell membranes

what does increasing the concentration of the solvent lead to

Answer 18

1) surrounding cells in a solvent (such as ethanol) increases the permeability of their cell membranes
2) this is because solvents dissolve the lipids in a cell membrane, so the membrane loses its structure
3) some solvents increase cell permeability more than others, e.g. ethanol increases cell permeability more than methanol
4) you could investigate the effects of different solvents by doing an experiment using beetroot like the one on a previous flashcard
5) increasing the concentration of the solvent will also increase membrane permeability

Question 19

Define diffusion

Answer 19

1) diffusion is the net movement of particles from an area of a higher concentration to an area of a lower concentration
2) molecules will diffuse both ways, but the net movement will be to the area of lower concentration. This continues until particles are evenly distributed throughout the liquid or gas

Question 20

Define concentration gradient

Answer 20

The concentration gradient is the path from an area of higher concentration to an area of lower concentration. Particles diffuse down a concentration gradient.

Question 21

Diffusion is a passive process - what does this mean?

Answer 21

diffusion is a passive process - no energy is needed to make it happen

Question 22

What four factors does the rate of diffusion depend on?

Answer 22

1) the concentration gradient - the higher it is, the faster the rate of diffusion
2) the thickness of the exchange surface - the thinner the exchange surface, the faster the rate of diffusion
3) the surface area - the larger the surface area, the faster the rate of diffusion
4) the temperature - the warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster

Question 23

Phenolphthalein is a pH indicator - it’s pink in alkaline solutions and colourless in acidic solutions. How can you use it to investigate diffusion in agar jelly?

Answer 23

1) first, make up some agar jelly with phenolphthalein and dilute sodium hydroxide. This will make the jelly a lovely shade of pink
2) then fill a beaker with some dilute hydrochloric acid. Using a scalpel, cut out a few cubes from the jelly and put them in the beaker of acid
3) if you leave the cubes for a while they’ll eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide

Using this experiment, you can investigate the different factors that affect diffusion, e.g. surface area, concentration gradient and temperature

Question 24

Why is facilitated diffusion necessary and how does it work?

Answer 24

1) some larger molecules (e.g. amino acids, glucose) ions and polar molecules don’t diffuse directly through the phospholipid bilayer of the cell membrane
2) instead they diffuse through carrier proteins or channel proteins in the cell membrane - this is called facilitated diffusion
3) like diffusion, facilitated diffusion moves particles down a concentration gradient, from a higher to a lower concentration
4) it’s also a passive process - it doesn’t use energy

Question 25

How do carrier proteins move large molecules into or out of the cell, down their concentration gradient?

Answer 25

Carrier proteins move large molecules into or out of the cell, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.

1) first, a large molecule attaches to a carrier protein in the membrane
2) then, the protein changes shape
3) this releases the molecule on the opposite side of the membrane

Question 26

How do channel proteins move large molecules into or out of the cell, down their concentration gradient?

Answer 26

Channel proteins form pores in the membrane for charged particles to diffuse through (down their concentration gradient). Different channel proteins facilitate the diffusion of different charged particles

Question 27

Active transport uses energy to move molecules and ions across plasma membranes, against a concentration gradient. This process involves carrier proteins. How does it work? Oi, draw a diagram too of the active transport of calcium ions. The answer is on page 56

Answer 27

1) the process is pretty similar to facilitated diffusion - 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
2) the only difference is that energy is used (from ATP - a common source of energy used in the cell), to move the solute against its concentration gradient
3) the diagram you’ve drawn should show the active transport of calcium ions (Ca2+)

Question 28

Cells can take in substances by endocytosis. Explain how in 5 steps please

Answer 28

1) some molecules are way too large to be taken into a cell by carrier proteins, e.g. proteins, lipids and some carbs
2) instead a cell can surround a substance with a section of its plasma membrane
3) the membrane then pinches off to form a vesicle inside the cell containing the ingested substance - this is endocytosis
4) some cells also take in much larger objects by endocytosis - for example, some white blood cells (mainly phagocytes) use endocytosis to take in things like microorganisms and dead cells so that they can destroy them
5) like active transport, this process also uses ATP for energy

Question 29

Cells can secrete substances by exocytosis, please explain how

Answer 29

1) some substances produced by the cell (e.g. digestive enzymes, hormones, lipids) need to be released from the cell - this is done by exocytosis
2) vesicles containing these substances pinch off from the sacs of the Golgi apparatus and move towards the plasma membrane
3) the vesicles fuse with the plasma membrane and release their contents outside the cell
4) some substances (like membrane proteins) aren’t released outside the cell - instead they are inserted straight into the plasma membrane
5) exocytosis uses ATP as an energy source

Question 30

Explain osmosis

Answer 30

Osmosis is the diffusion of water molecules across a partially permeable membrane down a water potential gradient. This means water molecules move from an area of higher water potential (i.e. higher concentration of water molecules) to an area of lower water potential (i.e. lower concentration of water molecules).

Question 31

Define water potential

Answer 31

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

Question 32

What has the highest water potential?

Answer 32

Pure water has the highest water potential. All solutions have a lower water potential than pure water

Question 33

How does an animal cell behave in a hypotonic solution, an isotonic solution and a hypertonic solution?

Answer 33

In a hypotonic solution (solution with a higher water potential than the cell): net movement of water molecules is into the cell. Cell bursts

In a isotonic solution (solution with the same water potential as the cell): water molecules pass into and out of the cell in equal amounts. The cell stays the same

In a hypertonic solution (solution with a lower water potential than the cell): net movement of water molecules is out of the cell. The cell shrinks

Question 34

How does a plant cell behave in a hypotonic solution, an isotonic solution and a hypertonic solution?

Answer 34

In a hypotonic solution (solution with a higher water potential than the cell): net movement of water is into cell. The vacuole swells. The vacuole and cytoplasm push against the cell wall. The cell becomes turgid (swollen)

In a isotonic solution (solution with the same water potential as the cell): water molecules move into and out of the cell in equal amounts. The cell stays the same.

In a hypertonic solution (solution with a lower water potential than the cell): net movement of water is out of the cell. The cell becomes flaccid. The cytoplasm and the membrane pull away from the cell wall. This is called plasmolysis

Question 35

You can do an experiment, using potato cylinders, to find out the water potential of plant tissue. This experiment involves putting potato cylinders into different concentrations of sucrose solution - remember, the higher the sucrose solution, the lower the water potential. How would you carry out this experiment?

Answer 35

1) prepare sucrose solutions of the following concentrations: 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M
2) use a cork borer or chip maker to cut potatoes into the same sized pieces (they need to be about 1cm in diameter)
3) divide the chips into groups of three and use a mass balance to measure the mass of each group
4) place one group in each solution
5) leave the chips in the solution for as long as possible (making sure they all get the same amount of time). Try to leave them for at least 20 minutes
6) remove the chips and pat dry gently with a paper towel
7) weigh each group again and record your results
8) calculate the percentage change in mass for each group
9) plot your results in a graph (like the one on pg 59)

NB: you can do a similar experiment using chickey eggs that have had their shells dissolved. The remaining membrane is partially permeable, so it’s a good model for showing the effects of osmosis in animal tissue