Chapter 5 Plasma Membrane

Question 1

What is the plasma membrane of animal cells made of?

Answer 1

It is a phospholipid bilayer. A phospholipid is made of a glycerol bonded to 2 fatty acids and a phosphate group. The phosphate head is hydrophilic and fatty acids are hydrophobic (an amphipathic molecule). The bilayer is made of two rows of phospholipids with the fatty acid tails facing inwards towards each other.

Question 2

Why is the phospholipid bilayer described as the fluid mosaic model?

Answer 2

‘Fluid’- components like carrier proteins can move freely within the membrane.
‘Mosaic’- Components of different size and shapes fit together.

Question 3

What is the role of membranes?

Answer 3

Compartmentalisation- Membranes separate different cells and organelles so they can carry out their individual function.
Selectively permeable- This allows the membrane to control what enters and leaves the cell.
Communication- Used in cell signalling via receptors on the surface of the plasma membrane.
Chemical reactions- The plasma membrane is also a site where chemical reactions take place.

Question 4

What are intrinsic proteins?

Answer 4

Intrinsic proteins are found embedded in the membrane.

Question 5

An example of an intrinsic protein is a carrier protein. How does a carrier protein work?

Answer 5

Carrier proteins transports substances across a membrane by changing shape. Carrier proteins can actively transport substances against a concentration which would require ATP. Carrier proteins can also transport molecules along the concentration gradient, which would not require ATP and be known as facilitated diffusion.

Question 6

Other than carrier proteins, what is another example of an intrinsic protein?

Answer 6

Channel proteins.
Channel proteins helps with transporting substances via facilitated diffusion. The substances transported through these channels are polar or charged so cannot simply diffuse through the membrane. Channel proteins have specific function; different channel proteins transports specific molecules. For example, aquaporins are channel proteins that only transport water molecules. There are also sodium and potassium ion channels.

Question 7

Why are proteins able to lie in the phospholipid bilayer?

Answer 7

A proteins tertiary structure is composed of polar, charged and non charged regions. The polar and charged regions of a protein is unable to interact with the fatty acid tails of the phospholipid bilayer. Hence, the protein arranges itself in a certain way so that the polar and charged region is found on the interior and the non-charged regions are found on the exterior (these proteins are globular).

Question 8

What are extrinsic proteins? Give 2 examples of extrinsic proteins.

Answer 8

Extrinsic proteins are found partially embedded in the plasma membrane. Two examples of extrinsic proteins are glycoproteins and glycolipids. Glycoproteins or glycolipids is where a carbohydrate chain is attached to either a phospholipid or protein in the membrane.

Question 9

What are some functions of glycoproteins and glycolipids?

Answer 9

The carbohydrate chain on glycoprotein or glycolipid can be used as an antigen. The shape of the carbohydrate chain is specific to different cells; the carbohydrate chain on a bacteria cell would be different from the one on a body cell. This is what white blood cells use to differentiate pathogens from body cells in an immune response.
Receptors for cell signalling. The carbohydrate chains can also be receptors, where molecules like hormones can bind on to. The carbohydrate chain is typically from a glycoprotein, because the protein attached tends to be an enzyme, which initiates a reaction as a result of hormone signalling.

Question 10

What is cholesterol and its function?

Answer 10

Cholesterol is a lipid molecule found in the plasma membrane that regulates the fluidity of the membrane. The cholesterol molecules contains an -OH group that cannot interact wit the fatty acid tails. This -OH group is found facing out of the membrane.

Question 11

What forces are found in the phospholipid bilayer? Where are they found?

Answer 11

Weak intermolecular forces are found between fatty acid tails on different phospholipids. They are also found between proteins and phospholipids. These intermolecular forces stabilises the structure of the membrane, but, phospholipids and other components are still able to move around a bit.

Question 12

How does increasing temperature affect the phospholipids in the membrane?

Answer 12

Increasing temperature means giving thermal energy.
Thermal energy transferred to kinetic energy.
Kinetic energy causes phospholipids to vibrate.
When they start to violently vibrate, intermolecular forces between the phospholipids start to break.
This reduces stability of the membrane and opens up gaps in the membrane.
More molecules can simply diffuse through, hence, permeability increases.

Question 13

How does increasing temperature affect the proteins in the membrane?

Answer 13

Increasing temperature causes the proteins to denature as bonds are broken in the tertiary and quaternary structure of the protein.
A denatured protein changes shape of the protein, so the protein may no longer be able to transport molecules across the membrane. Hence, the permeability of the membrane is altered.

Question 14

How does non-polar solvents affect the fluidity of the membrane?

Answer 14

Non-polar solvents are able to interact with the fatty acid tails in the phospholipid bilayer. For this reason, non-polar solvents are able to slip between fatty acid tails and stay there. This in turn disrupts the intermolecular forces between the fatty acid tails and causes them to break. The phospholipids move further apart, leaving gaps in the membrane. Now molecules are able to simply diffuse through the membrane more easily and hence, permeability as well as fluidity increases.

Question 15

How does ethanol as a non-polar solvent bring about the side affects of alcohol?

Answer 15

Ethanol is an alcohol that can be consumed. In nervous transmission, the nerve impulses are brought about by the the movement of ions across the membrane. If ethanol as a non-polar solvent is able to disrupt the membrane (make membrane more permeable), there will be more movement of ions across the membrane. This unwanted ion diffusion across the membrane can bring about unwanted nerve impulses.

Question 16

Ethanol at high concentrations can be used as a disinfectant. How does this work?

Answer 16

High concentration of ethanol means there is more alcohol molecules. Hence, the membrane can be disrupted at a higher rate and can even be broken down, due to a lack of intermolecular forces. Using high concentrations of ethanol in alcohols as a disinfectant can kill bacterial cells, by breaking down its membrane.

Question 17

Define diffusion.

Answer 17

Diffusion is the net movement of particles down the concentration gradient.

Question 18

What is facilitated diffusion?

Answer 18

This is diffusion across protein channels in the membrane. The proteins in the membrane is what makes a cell selectively permeable. The permeability of the cell (and plasma proteins present) can be linked to the function of the cell.

Question 19

How does temperature affect diffusion rate?

Answer 19

A higher temperature means more kinetic energy is given to the system. With the increased KE, phospholipids starts to vibrate and move apart slightly. Gaps in the membrane means more molecules can diffuse across. (An increased kinetic energy also means molecules can move faster).

Question 20

How does concentration gradient affect the rate of diffusion?

Answer 20

A steeper concentration gradient means more molecules are likely to move from an area of high concentration to low concentration.

Question 21

Other than temperature and concentration, what other two factors affect the diffusion rate?

Answer 21

Surface area to volume concentration, and thickness of membrane.

Question 22

Define osmosis.

Answer 22

Osmosis is the net movement of water down a water potential gradient across a partially permeable membrane.

Question 23

What units is water potential measured in?

Answer 23

Pa, or kPa

Question 24

What does it mean if a solution has a water potential of 0kPa?

Answer 24

This means the solution is pure water and there is no solutes present.

Question 25

How does values of water potential vary with concentration of solutes in a solution?

Answer 25

Water potential can always ever be 0 or less than 0. A solution with more solutes will have a more negative value.

Question 26

What does it mean if a cell is surrounded by a hypotonic solution?
What is the direction of the net movement of water?
How does this movement of water affect an animal cell?
How does this movement of water affect a plant cell?

Answer 26

If a cell is surrounded by a hypotonic solution, it means the solution has a higher water potential than that of the cell, as it contains less solutes.
Hence, this means that the net movement of water will be going from the solution outside to inside the cell- in this sense, it is moving along the water potential gradient.
This net movement of water into an animal cell will put too much pressure on the cell surface membrane and will cause it to burst or undergo lysis.
The net movement of water into a plant cell will cause it to become turgid. It does not burst because the strong plant cell is able to withstand the pressure of the water.

Question 27

What does it mean if a cell is surrounded by a hypertonic solution?
What is the direction of the net movement of water?
How does this movement of water affect an animal cell?
How does this movement of water affect a plant cell?

Answer 27

If a cell is in a hypertonic solution, the solution has a lower water potential than that of the cell, as the solution contains more solutes.
In this sense, the net movement of water will be from inside the cell to the surrounding solution outside (travelling along the water potential gradient).
The net movement of water out of an animal cell will cause it to shrivel up or undergo crenation.
The net movement of water out of a plant cell will cause it to undergo plasmolysis. The general shape of the plant cell is maintained, but the cell surface membrane detaches itself from the cell wall and goes inwards.

Question 28

What does it mean if a cell is surrounded by a isotonic solution?
What is the direction of the net movement of water?
How does this movement of water affect an animal cell?
How does this movement of water affect a plant cell?

Answer 28

If a cell is in an isotonic solution, the water potential of the solution is the same as the water potential inside the cell.
Hence, this means the rate of movement of water into the cell is equal to the rate of movement of water outside the cell.
If an animal cell is in an isotonic solution, it will exist in its normal state.
If a plant cell is in an isotonic solution, it is flaccid (its normal state), where the cell surface membrane is just touching the cell wall.

Question 29

Define active transport.

Answer 29

This is the movement of particles against a concentration gradient, using ATP and involving protein carriers.

Question 30

ATP is required in active transport, within protein carriers. How is the ATP used?

Answer 30

ATP is hydrolysed to ADP and a phosphate ion. The binding of the phosphate ion to the carrier protein brings about a conformational change, which then allows the carrier proteins to carry molecules across the membrane. When the phosphate ion is released, the carrier protein returns to its original shape.

Question 31

What is bulk transport?

Answer 31

When we refer to active transport, we normally refer to the movement of small molecules and ions. Bulk transport is the movement of large molecules, like glucose or bacteria.

Question 32

Endocytosis is a type of bulk transport. What is endocytosis? Provide examples.

Answer 32

Endocytosis is the bulk transport of molecules into a cell.
An example is phagocytosis, where white blood cells engulf foreign matter.
Another example is pinocytosis, which is the engulfing of liquids.

Question 33

Another type of bulk transport is exocytosis. What is exocytosis?

Answer 33

Exocytosis is the bulk transport of molecules outside of a cell.
An example is if a protein synthesised in a cell is an extracellular enzyme, the enzyme is packaged into vesicles and fuses with the plasma membrane to leave the cell.