Chapter 4 - Transport Across Membranes

Question 1

Describe the structure of the phospholipid bilayer and the 2 common features of it?
Refer to the head and the tail

Answer 1

Phospholipid bilayer is made up of a phosphate group, a molecule of glycerol and 2 fatty acids

• has a HYDROPHILIC HEAD = attracted to water
• has a HYDROPHOBIC TAIL = repels water

Question 2

What is meant by the fluid mosaic model, referring to the phospholipid bilayer?

Answer 2

Fluid: molecules (phospholipids and proteins) can move horizontally in membrane

Mosaic: proteins (channel and carrier) are embedded in phospholipid bilayer.

Question 3

What is meant by diffusion? Is it passive or active?

Answer 3

The net movement of molecules or ions from a region of high concentration to a region of lower concentration

• it is a passive movement

Question 4

1. SIMPLE DIFFUSION

What can pass through, and why can they do this?

Answer 4

• small, lipid soluble, non-polar molecules e.g. oxygen/ carbon dioxide

-these molecules are hydrophobic so dissolve well in lipids, but not in water. Therefore they can pass straight through the phospholipid bilayer.

Question 5

2. FACILITATED DIFFUSION

What sort of molecules can pass through and how does facilitated diffusion work?

Answer 5

-important for moving substances too large or charged to get through phospholipid bilayer directly.

• uses CHANNEL and CARRIER proteins that are specific to particular substances e.g. Na+ ion channels, glucose carriers
• specificity depends on shape and charge of transport protein

Question 6

How do channel and carrier proteins work?

Answer 6

CHANNEL:
- substances pass straight through a water-filled pore.

CARRIER:
- Substance binds to carrier protein, then carrier protein changes shape to transport substance to other side of membrane

Question 7

How would you speed up diffusion? [5]

Answer 7

1. Increase concentration gradient
2. Increase SA over which diffusion is occurring
3. Reduce thickness of exchange surface
4. Increase temperature = molecules have more KE
5. Increase number of channel/ carrier proteins

Question 8

3. OSMOSIS

What is meant by osmosis?

Answer 8

A special type of diffusion using a passive process

• it is the net diffusion of water from a region of high WATER POTENTIAL to a region of lower WATER POTENTIAL through a partially permeable membrane

Question 9

How does osmosis work?

Answer 9

• despite being polar, water molecules are very small so pass through bilayer via simple diffusion
  HOWEVER
• most osmosis occurs via facilitated diffusion through special channel proteins called aquaporins.

Question 10

How is the water potential gradient affected by solute concentration?

Answer 10

• water molecules stick to sodium ions which are too big to pass through the membrane therefore…
• a higher solute concentration will decrease the number of FREE water molecules so water moves from a low water potential (0kPa) to an even lower water potential (-40kPa)

Question 11

4. ACTIVE TRANSPORT

What is active transport, is it an active or passive process?

Answer 11

Active transport is where substances are transported from a region of low concentration to a region of high concentration, AGAINST their concentration gradient

• it is an active process; energy is provided by hydrolysis of ATP

Question 12

How does active transport work?

Answer 12

1. ATP binds to a special type of Carrier protein, which is then hydrolysed to form ADP and Pi, releasing energy in order to change the shape of the carrier protein
2. Molecules bind to this carrier protein and Pi attaches to the membrane protein on the inside of the cell
3. The binding of the phosphate ion to the protein cause the protein to change shape so that access for the molecules is open to the inside but closed to outside.

Question 13

5. CO-TRANSPORT

Describe what occurs during the co-transport of glucose from the ileum lumen to the blood.

Answer 13

1. Sodium ions pass from lumen to the epithelial cell by facilitated diffusion through a CARRIER PROTEIN
2. As sodium ions pass through, the protein changes shape so that glucose is SIMULTANEOUSLY brought in AGAINST its concentration gradient.
3. Glucose leaves to the blood by facilitated diffusion through base of cell
4. Sodium ions actively transported out of base of epithelial cell using ATP
5. This removal of sodium ions maintains the sodium ion concentration gradient needed to maintain uptake of glucose

Question 14

What are the adaptations of the cells to maximise co-transport? [7]

Answer 14

1. Many channel/ carrier proteins so fast rate of absorption
2. Many carrier proteins for active transport
3. Many carrier/ channel proteins for facilitated diffusion
4. Many mitochondria so make more ATP by respiration
5. Many mitochondria for aerobic respiration
6. Many mitochondria to release energy for active transport

Question 15

6. ENDOCYTOSIS AND EXOCYTOSIS

How does endo and exocytosis work?

Answer 15

ENDOCYTOSIS: cell surface membrane engulfs particles to form a vesicle

EXOCYTOSIS: vesicle fuses with cell surface membrane to release substances OUT OF THE CELL

Question 16

What is meant by passive and active when referring to movement of molecules?

Answer 16

Passive: it uses energy already present in solution rather than from ATP

Active: it uses energy from hydrolysis of ATP