chapter 5 part 1

Question 1

What are membranes:

Answer 1

Membranes are the structures that separate the contents of cells from their environment.
They also separate the different areas within cells (organelles) from each other and the cytosol.
Some organelles are divided further by internal membranes.

Question 2

What is compartmentalisation:

Answer 2

The formation of separate membrane-bound areas in a cell
Compartmentalisation is vital to a cell as metabolism includes many different and often incompatible reactions.
Containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions, such as chemical gradients, to be maintained, and protects vital cell components.

Question 3

Membrane structure:

Answer 3

• The cell surface membrane which separates the cell from its external environment is known as the plasma membrane.
• Membranes are formed from a phospholipid bilayer.
• The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane.
• Cells normally exist in aqueous environments.
• The inside of cells and organelles are also usually aqueous environments.
• Phospholipid bilayers are perfectly suited as membranes because the outer surfaces of the hydrophilic phosphate heads can interact with water.

Question 4

Cell membrane theory:

Answer 4

• Membranes were seen for the first time following the invention of electron microscopy, which allowed images to be taken with higher magnification and resolution.
• Images taken in the 1950s showed the membrane as two black parallel lines - supporting an earlier theory that membranes were composed of a lipid bilayer.
• In 1972 American scientists Singer and Nicolson proposed a model, building upon an earlier lipid-bilayer model, in which proteins occupy various positions in the membrane.
• The model is known as the fluid-mosaic model

Question 5

The model is known as the fluid-mosaic model because…

Answer 5

because the phospholipids are free to move within the layer relative to each other (they are fluid), giving the membrane flexibility, and because the proteins embedded in the bilayer vary in shape, size, and position (in the same way as the tiles of a mosaic).
- This model forms the basis of our understanding of membranes today.

Question 6

diagram of the fluid mosaic model

Answer 6

Question 7

Membrane proteins:

Answer 7

Membrane proteins have important roles in the various functions of membranes.
There are two types of proteins in the cell-surface membrane - intrinsic and extrinsic proteins.

Question 8

Intrinsic proteins:

Answer 8

• Intrinsic proteins, or integral proteins, are transmembrane proteins that are embedded through both layers of a membrane.
• They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place.
• Channel and carrier proteins are intrinsic proteins.
• They are both involved in transport across the membrane.

Question 9

Channel proteins

Answer 9

provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes.
They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins.

Question 10

Carrier proteins

Answer 10

have an important role in both passive transport (down a concentration gradient) and active transport (against a concentration gradient) into cells.
This often involves the shape of the protein changing.

Question 11

Glycoproteins:

Answer 11

• intrinsic proteins.
• embedded in the cell-surface membrane with attached carbohydrate (sugar) chains of varying lengths and shapes.
• play a role in cell adhesion (when cells join together to form tight junctions in certain tissues) and as receptors for chemical signals.
• When the chemical binds to the receptor, it elicits a response from the cell.
• This may cause a direct response or set off a cascade of events inside the cell. This process is known as cell communication or cell signalling.

Question 12

Examples of cell communication or cell signalling include:

Answer 12

The binding of the neurotransmitters triggers or prevents an impulse in the next neurone

receptors for peptide hormones, including insulin and glucagon, which affect the uptake and storage of glucose by cells.
Some drugs act by binding to cell receptors. For example, 3 blockers are used to reduce the response of the heart to stress.

Question 13

Glycolipids:

Answer 13

Glycolipids are similar to glycoproteins.
They are lipids with attached carbohydrate (sugar) chains.
These molecules are called cell markers or antigens and can be recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism).

Question 14

Extrinsic proteins:

Answer 14

Extrinsic proteins or peripheral proteins are present in one side of the bilayer.
They normally have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins.
They can be present in either layer and some move between layers.

Question 15

Cholesterol:

Answer 15

• a lipid with a hydrophilic end and a hydrophobic end, like a phospholipid.
• It regulates the fluidity of membranes.
• positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together.
• so adds stability to membranes without making them too rigid.
• prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.

Question 16

Sites of chemical reactions:

Answer 16

• Like enzymes, proteins in the membranes forming organelles, or present within organelles, have to be in particular positions for chemical reactions to take place.
• For example, the electron carriers and the enzyme ATP synthase have to be in the correct positions within the cristae (inner membrane of mitochondrion) for the production of ATP in respiration.
• The enzymes of photosynthesis are found on the membrane stacks within the chloroplasts.

Question 17

Unlike proteins, membranes are not…

Answer 17

denatured by high temperatures - when they lose their structure they should be described as disrupted or destroyed.
Membranes control the passage of different substances into and out of cells (and organelles).
If membranes lose their structure, they lose control of this and cell processes will be disrupted.
A number of factors affect membrane structure including temperature and the presence of solvents.

Question 18

Temperature:

Answer 18

• Phospholipids in a cell membrane are constantly moving.
• When temperature is increased the phospholipids will have more kinetic energy and will move more.
• This makes a membrane more fluid and it begins to lose its structure.
• If temperature continues to increase the cell will eventually break down completely.
• This loss of structure increases the permeability of the membrane, making it easier for particles to cross it.
• Carrier and channel proteins in the membrane will be denatured at higher temperatures.
• These proteins are involved in transport across the membrane so as they denature,
  membrane permeability will be affected.

Question 19

Solvents:

Answer 19

• Water, a polar solvent, is essential in the formation of the phospholipid bilayer.
• The non-polar tails of the phospholipids are orientated away from the water, forming a bilayer with a hydrophobic core.
• The charged phosphate heads interact with water, helping to keep the bilayer intact.

Question 20

organic solvents and membranes

Answer 20

• Many organic solvents are less polar than water for example alcohols, or they are non-polar like benzene.
• Organic solvents will dissolve membranes, disrupting cells.
• This is why alcohols are used in antiseptic wipes.
• The alcohols dissolve the membranes of bacteria in a wound, killing them and reducing the risk of infection.

Question 21

alcohols on membrane structure

Answer 21

• Pure or very strong alcohol solutions are toxic as they destroy cells in the body.
• Less concentrated solutions of alcohols, such as alcoholic drinks, will not dissolve membranes but still cause damage.
• The non-polar alcohol molecules can enter the cell membrane and the presence of these molecules between the phospholipids disrupts the membrane.
• When the membrane is disrupted it becomes more fluid and more permeable.
• Some cells need intact cell membranes for specific functions, for example, the transmission of nerve impulses by neurones (nerve cells).

Question 22

what happens when neuronal membranes are disrupted

Answer 22

nerve impulses are no longer transmitted as normal.
- This also happens to neurones in the brain, explaining the changes seen in peoples’ behaviour after consuming alcoholic drinks

Question 23

Investigating membrane permeability:

Answer 23

Beetroot cells contain betalain, a red pigment that gives them their distinctive colour, because of this they are useful for investigating the effects of temperature and organic solvents on membrane permeability.
When beetroot cells membranes are disrupted the red pigment is released and the surrounding solution is coloured.
The amount of pigment released into a solution is related to the disruption of the cell membranes.

Question 24

To investigate the effect of temperature on the permeability of cell membranes a student carried out the following procedure.

Answer 24

Five small pieces of beetroot of equal size were cut using a cork borer.
The beetroot pieces were thoroughly washed in running water, they were then placed in 100 ml of distilled water in a water bath.
The temperature of the water bath was increased in 10°C intervals.
Samples of the water containing the beetroot were taken five minutes after each temperature was reached.
The absorbance of each sample was measured using a colorimeter with a blue filter.

Question 25

Diffusion only occurs between….

Answer 25

different concentrations of the same substance.
The exchange of substances between cells and their environment or between membrane-bound compartments within cells and the cell cytosol is defined as either active (requiring metabolic energy) or passive.
All movement requires energy.
Passive movement, however, utilises energy from the natural motion of particles, rather than energy from an another source.

Question 26

Diffusion:

Answer 26

• Diffusion is the net, or overall, movement of particles (atoms, molecules or ions) from a region of higher concentration to a region of lower concentration.
• It is a passive process and it will continue until there is a concentration equilibrium between the two areas.

Question 27

Equilibrium

Answer 27

Equilibrium means a balance or no difference in concentrations.
- Diffusion happens because the particles in a gas or liquid have kinetic energy (they are moving).
- This movement is random and an unequal distribution of particles will eventually become an equal distribution.
- Equilibrium doesn’t mean the particles stop moving, just that the movements are equal in both directions.

Question 28

A concentration difference is said to be a …

Answer 28

concentration gradient, which goes from high to low concentration.
- Diffusion takes place down a concentration gradient.
- It takes a lot more energy to move substances up a concentration gradient.
- So far diffusion in the absence of a barrier or membrane has been considered - This is simple diffusion.

Question 29

Factors affecting rate of diffusion:

Answer 29

temperature:
the higher the temperature the higher the rate of diffusion.
This is because the particles have more kinetic energy and move at higher speeds.

diffusion distance

concentration difference:
the greater the difference in concentration between two regions the faster the rate of diffusion because the overall movement from the higher concentration to lower concentration will be larger.

Question 30

diffusion distance

Answer 30

• Particles move at high speeds and are constantly colliding, which slows down their overall movement.
• This means that over short distances diffusion is fast, but as diffusion distance increases the rate of diffusion slows down because more collisions have taken place.
• For this reason cells are generally microscopic - the movement of particles within cells depends on diffusion and a large cell would lead to slow rates of diffusion.
• Reactions would not get the substrates they need quickly enough or ATP would be supplied too slowly to energy-requiring processes.

Question 31

The rate of diffusion can be calculated in two ways

Answer 31

• by distance travelled/time and volume filled/time.
  Distance travelled/time is not affected by changes in surface area, whilst volume/time varies depending on the surface area.

Question 32

The rate of diffusion investigation

Answer 32

A student used different sized agar blocks to investigate how the rate of diffusion was affected by surface area.
The agar used to make the blocks contained the indicator phenolphthalein with turns pink in the presence of an alkali.
The agar blocks were immersed in a solution of sodium hydroxide for ten minutes.
The blocks were removed and distance the sodium hydroxide had diffused was measured with a ruler.

Question 33

The rate at which molecules or ions diffuse across membranes is affected by:

Answer 33

surface area - the larger the area of an exchange surface, the higher the rate of diffusion
thickness of membrane - the thinner the exchange surface, the higher the rate of diffusion.

Question 34

passive diffusion of oxygen into the cell of a cornea

Answer 34