2.1.5 biological membranes

Question 1

What is compartmentalisation?

Answer 1

Compartmentalisation is the formation of separate membrane-bound areas in a cell.

Question 2

Why is compartmentalisation vital to a cell?

Answer 2

It allows incompatible reactions with different reaction conditions to be contained in different sections=> allows specific conditions for cellular reactions.

Question 3

What is the role of the plasma membrane?

Answer 3

-Keep all cellular components in the cell
-Allow selected molecules to move in and out the cell
-Site for biochemical reactions
-Allow a cell to change shape
-Isolate organelles from the rest of the cytoplasm (compartmentalisation)

Question 4

What is the plasma membrane comprised of?

Answer 4

A phospholipid bilayer:
Hydrophobic heads are pointed outwards and hydrophilic tails are towards each other.

Question 5

What is the fluid mosaic model?

Answer 5

Phospholipids are free to move within the layer relative to each other=>giving the membrane flexibility.
Proteins embedded in the bilayer vary in shape, size, and position.

Question 6

What components make up the fluid mosaic model?

Answer 6

-Glycoprotein
-Glycolipids
-Hydrophobic heads and hydrophilic tails (phospholipids)
-Extrinsic proteins
-Intrinsic proteins
-Cholestrol

Question 7

What are intrinsic proteins?

Answer 7

Transmembrane proteins that are embedded through both layers of a membrane.
They have amino acids with hydrophobic R-groups on external surface that interact with hydrophobic core.

Question 8

What are two examples of intrinsic proteins?

Answer 8

-Carrier proteins
-Channel proteins

Question 9

What are channel proteins?

Answer 9

Channel proteins are intrinsic proteins which provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes.

Question 10

What are carrier proteins?

Answer 10

Carrier proteins are intrinsic proteins which have a role in both passive transport and active transport into cells.

Question 11

What are glycoproteins?

Answer 11

Glycoproteins are intrinsic proteins embedded in the cell-surface membrane with attached carbohydrate chains.

Question 12

What is the role of glycoproteins?

Answer 12

-Cell adhesion
-Receptors for chemical signals

Question 13

What are some examples of cell signalling/communication?

Answer 13

-receptors for neurotransmitters e.g. acetylcholine.
-receptors for peptide hormones, insulin and glucagon.

Question 14

What are glycolipids?

Answer 14

Glycolipids are intrinsic proteins, lipids with attached carbohydrate chains.
These molecules are called cell markers/antigens and can be recognised by the immune system and non-self.

Question 15

What are extrinsic proteins?

Answer 15

Present in one side of the bilayer.
Normally have hydrophilic R-groups on their outer surfaces and interact with polar heads of phospholipids or intrinsic proteins.

Question 16

What is cholesterol?

Answer 16

Cholesterol is a lipid with a hydrophilic end and a hydrophobic end (C27H46O)

Question 17

What does cholesterol do for plasma membranes?

Answer 17

-Adds stability to membranes without making them too rigid.
-Prevent molecules from being too solid by stopping phospholipid molecules from grouping too closely and crystallising.

Question 18

How does temperature affect membrane structure?

Answer 18

-Increasing temperature->increases kinetic energy of phospholipids=>membrane becomes more fluid and begins to lose structure.
If temperature keeps rising, the cell will eventually completely break down.
Loss of structure increase permeability of membrane.
-Carrier and channel proteins will denature at higher temperature=>membrane permeability affected.

Question 19

How do solvents affect membrane structure?

Answer 19

Non-polar or less polar solvents than water will dissolve membrane=>the alcohol used in antiseptic wipes will dissolve the membrane of bacteria.
Less concentrated alcohols will not dissolve the membrane but will enter the cell membrane and disrupt the membrane=>membrane becomes more fluid and permeable.

Question 20

What is the procedure for investigating membrane permeability?

Answer 20

1) cut five beetroot pieces equally (using cork borer).
2) was beetroot under water and place in 100ml of distilled water.
3) add to water baths with increasing 10 degree intervals
4) samples of water were taken five minutes after the temperature was reached.
5) use a colorimeter to measure absorbance of samples.

Question 21

What is diffusion?

Answer 21

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
A passive process.

Question 22

What factors affect rate of diffusion?

Answer 22

-temperature-> higher the temperature the higher the rate of diffusion because the particles have more kinetic energy.
-concentration difference-> the greater the difference in concentration the faster the rate of diffusion because the higher concentration to lower concentration will be larger.

Question 23

What is simple diffusion?

Answer 23

Simple diffusion is diffusion in the absence of a barrier or membrane.

Question 24

Why are membranes described as partially permeable?

Answer 24

-Hydrophobic interior repels substances with a charge (ions), so they cannot easily pass through.
-Polar molecules e.g. H2O with partial charges can diffuse through membranes very slowly.
-Small polar molecules can pass through more easily than larger ones.

Question 25

What factors affect the rate at which molecules or ions diffuse across membranes?

Answer 25

-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 26

What is facilitated diffusion?

Answer 26

Diffusion across a membrane through protein channels.
Passive process->down a concentration gradient.

Question 27

What are membrane with protein channels called?

Answer 27

Selectively permeable->most channel proteins are specific to one molecule or ion.

Question 28

What is the model cell practical?

Answer 28

Use of dialysis tubing to create ‘model cells’-> dialysis tubing is partially permeable with pores.
1) Tubing is tied on one end and a solution is added and the other side is tied.
2) Cell is then placed into another solution (could be different sizes or concentration of solute molecules).

Question 29

What is active transport?

Answer 29

Active transport is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration.
Requires energy in the form of ATP and carrier proteins.

Question 30

What is the general process of active transport (outside to inside cell)?

Answer 30

1) Molecule or ion binds to receptors in the channel of carrier protein on the outside of the cell.
2) On the inside of the cell ATP binds to carrier protein and is hydrolysed into ADP and phosphate.
3) Binding of the phosphate molecule to carrier protein causes protein to change shape-opening up the inside of the cell.
4) Molecule or ion is released to inside of the cell.
5) Phosphate molecule is released from carrier protein and recombines with ADP to form ATP.
6) Carrier protein returns to original shape.

Question 31

What is bulk transport?

Answer 31

Another form of active transport for large molecules and whole cells which are too large to move through channels or carrier proteins e.g. enzymes, hormones, bacteria.

Question 32

What are some examples of bulk transport?

Answer 32

-Endocytosis: bulk transport of materials into cells.
Two types of endocytosis: phagocytosis for solids and pinocytosis for liquids.
-Exocytosis: bulk transport of materials out of cells.

Question 33

What is osmosis?

Answer 33

Osmosis is a particular type of diffusion, diffusion of water across a partially permeable membrane.
Passive process.

Question 34

What is water potential?

Answer 34

Water potential is the pressure exerted by water molecules as they collide with a membrane or container.

Question 35

What is the water potential of pure water?

Answer 35

0 kPa.

Question 36

What is the highest possible value for water potential?

Answer 36

0 kPa -> the presence of a solute in water lowers the water potential below 0.
The more concentrated the solution, the more negative the water potential.

Question 37

Which way does the net movement of water travel?

Answer 37

Net movement of water from the concentration of higher water potential (less concentrated) to the solution of lower water potential (more concentrated).

Question 38

What is hydrostatic pressure?

Answer 38

Pressure created if the solution is in a closed system e.g. a cell.

Question 39

What happens when there is a higher water potential of external solution compared to cell solution in a red blood cell?

Answer 39

Net movement of water: enters the cell.
The red blood cell swells and bursts.

Question 40

What happens when there is an equal water potential of external solution compared to cell solution in a red blood cell?

Answer 40

Net movement of water: constantly enters and leaves at an equal rate.
There is no change to the red blood cell.

Question 41

What happens when there is a lower water potential of external solution compared to cell solution in a red blood cell?

Answer 41

Net movement of water: leaves cell.
The red blood cell shrinks.

Question 42

What is turgor pressure?

Answer 42

Pressure against the cell wall as a result of increased hydrostatic pressure which pushes the membrane against the cell wall.

Question 43

What happens when there is a higher water potential of external solution compared to cell solution in a plant cell?

Answer 43

Net movement of water: enters cell.
The plant cell swells and becomes turgid.

Question 44

What happens when there is a equal water potential of external solution compared to cell solution in a plant cell?

Answer 44

Net movement of water: water constantly enters and leaves at equal rates.
No change.

Question 45

What happens when there is a lower water potential of external solution compared to cell solution in a plant cell?

Answer 45

Net movement of water: leaves cell.
Plasmolysis and contents shrink.

Question 46

What is the practical for osmosis?

Answer 46

Plant cells:
Equal potato pieces placed into sugar or salt solutions of different concentrations.
The mass is measured before and after the solution.
Animal cells:
Eggs without shell are placed in different concentrations of sugar syrup.