Chapter 4: Cell Membranes And Transport

Question 1

What is a micelle?

Answer 1

A micelle is a phospholipid molecule that arranges itself in a spherical form in aqueous solutions.

Question 2

How wide is the phospholipid bilayer?

Answer 2

7nm

Question 3

What is the phospholipid bilayer?

Answer 3

The phospholipid bilayer makes up the cell surface membrane that is made up of hydrophilic phosphate heads facing outwards and hydrophobic tails facing inwards. It is partially permeable.

Question 4

Why is the membrane structure described as a fluid mosaic model?

Answer 4

It is described as fluid because the phospholipids and proteins can move about by diffusion. The word ‘mosaic’ describes the pattern produced by the scattered protein molecules when the cell membrane is viewed from above.

Question 5

Explain the factors that affect the fluidity of a membrane. (3)

Answer 5

1. The more unsaturated they are, the more fluid the membrane. This is because the unsaturated fatty acid tails are bent(branched) and therefore fit together more loosely.
2. The longer the tail, the less fluid the membrane.
3. As temperature decreases, membranes become less fluid.

Question 6

What are intrinsic proteins? What are its features?

Answer 6

Proteins that are found embedded within the membrane are called intrinsic proteins. They may be found in the inner layer, outer layer or most commonly spanning the whole membrane, in whihc case they are known as transmembrane proteins.
Intrinsic proteins have hydrophobic and hydrophilic regions. They stay in the membrane because the hydrophobic regions, made from hydrophobic amino acids are next to the hydrophobic fatty acid tails are repelled by the watery environment on either side of the membrane. The hydrophilic regions, made of hydrophilic amino acids are repelled by the hydrophobic interior of the membrane and therefore face into the aqueous environment on either side of the membrane.

Question 7

What are extrinsic proteins?

Answer 7

Extrinsic proteins are found on the inner and outer surface of the membrane. Many are bound to intrinsic proteins or to phospholipids.

Question 8

What is one of the functions of phospholipids in cell membranes?

Answer 8

Because the tails of phospholipids are hydrophobic/non-polar, they act as a barrier to most water soluble molecules.

Question 9

Describe the features of a cholesterol molecule.

Answer 9

They are relatively small molecules that, like phospholipids, have hydrophobic tails and hydrophilic heads, so they fit neatly between phospholipids with their heads at the membrane surface.

Question 10

In which organisms can cholesterol be found?

Answer 10

Cell membranes in animal cells contain almost as much cholesterol as phospholipid.
Cholesterol is much less common in plant cell membranes.
It is absent from prokaryotes.

Question 11

Explain the functions of cholesterol in cell membranes.

Answer 11

At low temperatures, cholesterol increases the fluidity of the membranes by preventing the close packing of phospholipid tails, allowing the cell to survive at lower temperatures. The interaction between the cholesterol molecules and phospholipid tails helps to stabilise cells at higher temperatures when the membrane would otherwise become too fluid.
Cholesterol also helps with preventing water soluble molecules from entering the cell due to its hydrophobic tail.
Helps with mechanical stability.

Question 12

What is the glycocalyx? What are its features?

Answer 12

Carbohydrate chains attached to membrane proteins (glycoproteins) and lipids (glycolipids) form hydrogen bonds with the water molecules to stabilise the membrane structure, forming a sugary coating to the cell known as the glycocalyx.

Question 13

What does the glycocalyx mainly comprise of in animal and plant cells?

Answer 13

In animal cells, the glycocalyx is formed mainly from glycoproteins; in plant cells, the glycocalyx is formed mainly from glycolipids.

Question 14

What are the functions of the carbohydrate chains of the glycocalyx? (4)

Answer 14

They act as receptors:

1. Some can act as ‘signalling receptors’, because they form a part of the signalling system that coordinates the activities of the cell. The receptors recognise messages like hormones and neurotransmitters, binding with them and in turn, triggering a series of chemical reactions inside the cell.
2. Some are involved in endocytosis wherein the receptors bind to the molecules that are to be engulfed by the cell surface membrane.
3. A third group of receptors are involved in binding cells to other cells(cell adhesion) in tissues and organs of animals.

Some glycolipids and glycoproteins act as antigens or cell-markers, allowing cell-to-cell recognition.

Question 15

What are some of the functions of proteins in the cell surface membrane?

Answer 15

1. Many proteins act as transport proteins, providing hydrophilic channels for ions and polar molecules to pass through the membrane. There are two types of transport protein: carrier and channel.
2. Other membrane proteins may be enzymes e.g. Can catalyse the hydrolysis of molecules.
3. Some proteins on the inside of the cell are attached to a system of protein filaments that make up the cytoskeleton. This helps to maintain the shape of the cell.

Question 16

How does cell signalling work with hydrophobic signalling molecules?

Answer 16

The hydrophobic signalling molecules can diffuse directly across the cell surface membrane and bind to receptors in the cytoplasm and nucleus.

Question 17

Describe the process of cell signalling with a hydrophilic signalling molecule?

Answer 17

The signalling pathway starts with the signal arriving at a protein receptor on the surface of the cell surface membrane. The receptor is a specific shape which recognises the signal and only cells with this receptor can recognise the signal. The signal brings about a change in the shape of the receptor, and since this spans the membrane, this change in shape allows the message to be transmitted to the inside of the cell is known as signal tranduction. The change in shape allows it to interact with the next component of the pathway known as the ‘G protein’ which in turn transmits the message. The G protein acts as a switch to bring about the release of the second messenger, a small molecule which diffuses through the cell relaying the message. Many second messenger molecules can be made in response to one receptor molecule being stimulated causing an amplification of the original signal. The second messenger typically activates an enzyme, which in turn activates further enzymes, until an enzyme is produced which brings about the required change. The sequence of events triggered by the G protein is known as a signalling cascade.

Question 18

What is diffusion?

Answer 18

Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down the concentration gradient, as a result of the random movement of particles.

Question 19

Explain the factors that affect the rate of diffusion across a membrane. (5)

Answer 19

1. The steepness of the concentration gradient: The greater the difference in gradient, the greater number of particles moving in two directions, the faster the rate of diffusion.
2. Temperature: At high temperatures, molecules have more kinetic energy than at lower temperatures and hence move around faster, increasing the rate.
3. Surface area: The greater the surface area, the more molecules can cross it at any one moment.
4. Distance: The time it takes a molecule to reach a certain destination by diffusion increases rapidly with distance travelled. The rate falls in proportion to the square of the distance.
5. The nature of the molecules or ions: Large molecules require more energy to get them moving than small molecules. Non-polar molecules including oxygen, can cross the phospholipid bilayer easily. Water molecules, despite being polar, can diffuse rapidly across the cell surface membrane because they are small enough.

Question 20

What is facilitated diffusion?

Answer 20

Facilitated diffusion is the diffusion of a substance through transport proteins in a cell membrane. The proteins provide hydrophilic areas that allow the molecules or ions to pass through them from a region of higher concentration to lower concentration, down the concentration gradient.

Question 21

What are the two types of proteins involved in facilitated diffusion?

Answer 21

Channel and carrier

Question 22

What are channel proteins?

Answer 22

They are water-filled pores which act as a gate to allow charged substances to diffuse through the membrane. Each is highly specific, allowing certain substances to pass. They have a fixed shape and can be gated to control ion exchange. This process does not use ATP and is in facilitated diffusion.

Question 23

What are carrier proteins?

Answer 23

Carrier proteins, unlike channel proteins, do not have a fixed shape and instead flip between two shapes. It is involved in active transport where it uses ATP to move substances up a concentration gradient. It is also used in facilitated diffusion down the concentration gradient without the use of energy. The binding site of a carrier protein is alternately open to one side of the membrane, then the other.

Question 24

What is osmosis?

Answer 24

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, as a result of their random movement.

Question 25

What is water potential?

Answer 25

Water potential is the tendency of water to move out of a solution.

Question 26

What is the water potential of pure water at atmospheric pressure 0?

Answer 26

Water potential is 0

Question 27

What is solute potential?

Answer 27

Solute potential is the contribution of the concentration of a solution to water potential.

Question 28

What is pressure potential?

Answer 28

The contribution of pressure to the water potential of a solution.

Question 29

What happens to animal cells in varying concentrated solutions?

Answer 29

In pure water/ dilute solution, the animal cell swells and bursts. In a concentrated solution, the animal cell shrinks.

Question 30

A solution with solutes mixed in it will have what water potential?

Answer 30

It will have a water potential less than 0.

Question 31

How does solute potential change with the concentration of the solute?

Answer 31

The higher the concentration of solute, the more negative the value of the solute potential.

Question 32

What does the rate of facilitated diffusion depend on?

Answer 32

It depends on how many carrier and channel proteins there are in a membrane, and in the case of channel proteins, whether they are open or not.

Question 33

What happens to water potential when pressure is increased?

Answer 33

When pressure is increased, water potential increases.

Question 34

What is the protoplast?

Answer 34

The living part of the cell inside the cell wall.

Question 35

What is the formula for water potential?

Answer 35

Water potential= solute potential+pressure potential

Question 36

Describe how a plant cell becomes ‘turgid’.

Answer 36

When a plant cell is surrounded by pure water/dilute solution, water will enter the cell by osmosis. As the cell begins to swell, the cell wall pushes against the expanding protoplast and pressure potential increases, increasing the water potential of the cell, until the water potential of the cell equals the water potential outside the cell. When a plant cell is fully inflated with water, it is described as fully turgid.

Question 37

How does plasmolysis occur?

Answer 37

When a plant cell is surrounded by a concentrated solution, water will leave the cell by osmosis. The protoplast gradually shrinks until it is exerting no pressure at all on the cell wall. At this point pressure potential is 0, so the water potential=solute potential. Both the solute molecules and water molecules of the external solution can pass through the freely permeable cell wall, allowing the protoplast to remain in contact with the external solution. As the protoplast continues to shrink, it begins to pull away from the cell wall in a process known as plasmolysis. A cell in which it has happened is said to be plasmolysed.
Eventually, as with an animal cell, an equilibrium is reached when the water potential has decreased until it equals that of the external solution.

Question 38

What is active transport?

Answer 38

Active transport is the movement of molecules or ions through carrier proteins across a cell membrane, against the concentration gradient, using energy from ATP.

Question 39

How does active transport work? Example?

Answer 39

Active transport is achieved by carrier proteins, each which have a specific shape complementary to a particular molecule, moving against the concentration gradient, using the ATP made in the cell as energy to make the carrier protein change shape. In the sodium-potassium pump (Na+-K+), the role is to pump 3 sodium ions out of the cell at the same time allowing 2 potassium ions into the cell. As both the ions are positively charged, this creates a net difference and hence a potential difference with the inside of the cell being more negative than the outside.

Question 40

What is endocytosis?

Answer 40

Endocytosis is the bulk movement of liquids(pinocytosis) or solids(phagocytosis) into a cell, by the infolding of the cell membrane to form vesicles containing the substance. Endocytosis is an active process that requires ATP.

Question 41

What is exocytosis?

Answer 41

It is the bulk movement of liquids or solids out of a cell, by the fusion of vesicles containing the substances with the membrane. Exocytosis is an active process requiring ATP.