Chapter 3 - Cell membranes and transport

Question 1

What is the cell membrane mostly made of?

Answer 1

Proteins and phospholipids.

Question 2

Why are phospholipids important components of membranes?

Answer 2

• They form bi-layers

- This component allows lipid soluble molecules across, but not water soluble molecules.

Question 3

What are the 2 ways proteins can be embedded in the membrane?

Answer 3

• Extrinsic proteins - on either surface of the bi-layer. (Structural support and form recognition sites).
• Intrinsic proteins - extend across both layers. (Carriers or form channels thus allowing active transport of ions).

Question 4

What is the name given to the model of the membrane structure?

Answer 4

Fluid mosaic model.

Question 5

What are the components of the fluid mosaic model?

Answer 5

• Intrinsic proteins (carrier proteins or channel proteins).
• Extrinsic proteins
• Glycoproteins/lipids
• Cholesterol
• Phospholipids (Hydrophilic phosphate head and hydrophobic fatty acid tail).

Question 6

Why is the membrane model called the ‘fluid mosaic’ model?

Answer 6

• The individual phospholipids can move within a layer.

- The proteins vary in shape, size and pattern (mosaic).

Question 7

What does the fluid mosaic model contain cholesterol and glycoproteins/lipids?

Answer 7

Cholesterol (animal cells) makes the membrane more rigid/stable.
Glyco have roles as hormone receptors or in cell-to-cell recognition.

Question 8

How do lipid soluble and water soluble molecules move across the membrane?

Answer 8

Lipid-soluble - dissolve in the phospholipid and diffuse across the membrane.
Water-soluble - Must pass through intrinsic protein molecules which form water filled channels.
The cell surface membrane is selectively permeable to water and some solutes.

Question 9

What is the definition of simple diffusion?

Answer 9

It is the movement of molecules/ions from a region where they’re in high concentration to a region of lower concentration. (Down a concentration gradient).

Question 10

What is the rate of diffusion affected by?

Answer 10

• The concentration gradient
• Thickness of exchange surface/distance of travel over which diffusion takes place.
• The S.A. of the membrane.
• The size of the diffusing molecule.
• Temperature

Question 11

Whats the definition of active transport?

Answer 11

The movement of molecules/ions across a membrane against a concentration gradient, using energy from the hydrolysis of ATP made by the cell in respiration.

Question 12

The rate of active transport is limited by the number of what?

Answer 12

Carrier proteins

Question 13

How does active transport happen?

Answer 13

• Molecule/ion combines with a specific carrier protein.
• ATP transfers a P group to the carrier protein (inside).
• Carrier protein changes shape and carries the molecule across the membrane.
• Molecule is then released into the membrane.

Question 14

What is co-transport and how does it work?

Answer 14

A type of facilitated diffusion that brings molecules/ions into cells together on the same transport protein molecule. E.g. Sodium-glucose transport

• Glucose molecule and 2 Na ions attach to carrier.
• Carrier changes shape and deposits them inside the cell.
• They diffuse through the cell to the other membrane.

Question 15

What is the definition of osmosis?

Answer 15

The net passive diffusion of water molecules across a partially permeable membrane from a region of high water potential to a region of lower water potential.

Question 16

What is water potential?

Answer 16

It is the measure of the free energy of water molecules and is the tendency for water to move. Measured in kPa.

Question 17

What two things you needs to know to calculate the water potential of a plant cell?

Answer 17

Solute potential - A measure of the osmotic strength of a solution.
Pressure potential - The pressure exerted by the cell contents on the cell wall.

Question 18

What is it meant by hypotonic, hypertonic and isotonic?

Answer 18

Hypotonic - Water outside has a higher water potential.
Hypertonic - Water inside has a higher water potential.
Isotonic - Water potentials equal (No net movement).

Question 19

What will decrease the water potential of a solution

?

Answer 19

The addition of a solute.

Question 20

At incipient plasmolysis, what happens to the pressure potential of the cell?

Answer 20

Pressure potential = 0.

Question 21

What is it called when a plant cell can take in m#n#mo more water?

Answer 21

The cell becomes turgid. (Provides support, maintains shape and holds them upright).

Question 22

What happens to animal cells when they’re placed in a hypotonic/hypertonic solution?

Answer 22

They have no cell wall so…
In hypertonic- cell becomes crentated
In hypotonic - lysis occurs and the cell bursts.

Question 23

How does the cell transport materials in bulk?

Answer 23

By Endocytosis - when material is engulfed by extensions of the plasma membrane and cytoplasm making a vesicle.(Phagocytosis -Solid material or Pinocytosis - Liquids)
Or Exocytosis - Substances leave the cell having been transported in a vesicle, which fuses with the cell membrane (digestive enzymes). Both require ATP.

Question 24

What is the definition of facilitated diffusion?

Answer 24

The passive transfer of molecules or ions down a concentration gradient, across a membrane, by protein carrier molecules in the membrane.

Question 25

What are the 2 types of transfer proteins?

Answer 25

Channel proteins - Ions being water soluble can pass through these as the channels are hydrophilic. The open/close according to the needs of the cell.
Carrier proteins - Molecules attach to its binding site, the protein changes shape and releases the molecule on the other side (sugar and amino acids).

Question 26

Why does the facilitated diffusion graph plateau off at the end?

Answer 26

Because all the carriers are occupied and their number has become a limiting factor.

Question 27

How does active transport work?

Answer 27

• Molecules combines with a specific carrier protein.
• ATP transfers a P group to the carrier protein.
• Carrier protein changes shape and carries molecule inside the cell.
• Molecule released and P ion released back to ATP.

Question 28

What effect does cyanide have?

Answer 28

It is a respiratory inhibitor. Therefore, if a process stops happening once cyanide has been added, it indicated that ATP is required for this process.

Question 29

What is Co-transport?

Answer 29

A type of facilitated diffusion that brings molecules/ions into cells together on the same transport protein molecule E.g. Sodium-glucose co-transport.

Question 30

How does Co-transport work (using the sodium-glucose pump as an example)?

Answer 30

• A glucose molecule and 2 sodium ions outside cell attach to a carrier protein.
• Carrier protein changes shape and deposits the glucose and sodium inside the cell.
• The diffuse separately out the cell.

Question 31

What is osmosis?

Answer 31

The net passive diffusion of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential.

Question 32

What does the solute potential measure?

Answer 32

Measures how easily water molecules move out of a solution.

Question 33

What two factors need to be know in order to determine the water potential of a cell?

Answer 33

The solute potential and the pressure potential (caused by the addition of a cell wall in plant cells).

Question 34

What is it called when a plant cell is holding as much water as possible/ when the cell loses water?

Answer 34

Turgid/Flaccid (the process the cell goes through to become flaccid is called plasmolysed).

Question 35

Will water move in/out when the solution is..

• Hypertonic
• Hypotonic
• Isotonic

Answer 35

• Hyper - Water moves out.
• Hypo - Water moves in.
• Iso - No net water movement.

Question 36

What is it called when the external concentration is high enough that the cell has lost just enough water that its membrane begins to be pulled away from the cell wall?

Answer 36

Incipient plasmolysis