transport across cell membranes

Question 1

Describe the fluid mosaic model of membranes

Answer 1

Fluid: phospholipid bilayer in which individual phospholipids can move so is flexible

Mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded

Question 2

Explain the role of cholesterol and glycolipids in membranes

Answer 2

• Cholesterol: connects phospholipids and reduces fluidity to make bilayer more stable
• Glycolipids: cell signalling and cell recognition

Question 3

Explain the function of extrinsic proteins in membranes

Answer 3

1. Binding sites/receptors
2. Antigens
3. Bind cells together
4. Involved in cell signalling

Question 4

Explain the function of intrinsic proteins in membranes

Answer 4

1. Electron carriers
2. Channel proteins
3. Carrier proteins

Question 5

Explain the functions of membranes within cells

Answer 5

• Provide an internal transport system
• Selectively permeable to regulate the passage of molecules into/out of organelles
• Provide reaction surface
• Isolate organelles from cytoplasm for specific metabolic reactions

Question 6

Explain the functions of the cell surface membrane

Answer 6

• Isolates cytoplasm from the extracellular environment
• Selectively permeable to regulate transport of substances
• Involved in cell signalling/recognition

Question 7

Name and explain 3 factors that affect membrane permeability

Answer 7

1. Temperature: high temperature denatures membrane proteins, phospholipid molecules have more kinetic energy and move further apart
2. pH changes tertiary structure of membrane proteins
3. Solvent may dissolve membrane

Question 8

Describe when membranes become less fluid

Answer 8

1. An increased proportion of saturated fatty acid chains as the chains pack together tightly and therefore there is a high number of intermolecular forces between the chains
2. A lower temperature as the molecules have less energy and therefore are not moving as freely which causes the structure to be more closely packed

Question 9

Describe when membranes become more fluid

Answer 9

1. An increased proportion of unsaturated fatty acid chains as these chains are bent, which means the chains are less tightly packed together and there are fewer intermolecular forces
2. At higher temperatures, the molecules have more energy and therefore move more freely, which increases membrane fluidity

Question 10

Define osmosis

Answer 10

Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established

Question 11

Explain water potential in a dilute solution

Answer 11

• Low concentration of solute
• High concentration of free water
• High water potential

Question 12

Describe water potential in a concentrated solution

Answer 12

• High concentration of solute
• Low concentration of free water
• Low water potential

Question 13

What is the water potential of pure water?

Answer 13

0 kPa

Question 14

Explain how osmosis can negatively affect plant and animal cells

Answer 14

Osmosis into the cell:
- Plant = swells, cell turns turgid
- Animal = cell lysis (bursts)

Osmosis out of the cell:
- Plant = shrinks, flaccid
- Animal = shrinks, dies

Question 15

Define simple diffusion

Answer 15

The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.

Question 16

What type of process is simple diffusion and why?

Answer 16

Diffusion is a passive process because it doesn’t require ATP from energy.

Question 17

Define facilitated diffusion

Answer 17

The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration via specific proteins in the phospholipid bilayer

Question 18

Explain how to channel protein work

Answer 18

• Hydrophilic channels bind to specific ions
• One side of the protein closes and the other opens

Question 19

Explain how carrier protein work

Answer 19

• Binds to complementary molecule
• Conformational change releases molecules on other side of membrane

Question 20

Name 5 factors that affect the rate of diffusion

Answer 20

1. Temperature
2. Diffusion distance
3. Surface area
4. Size of molecule
5. Concentration gradient

Question 21

State Fick’s Law

Answer 21

Rate of diffusion = (surface area x concentration difference) / diffusion distance

Question 22

How are cells adapted to maximise the rate of transport across their membranes?

Answer 22

• Many carrier/channel protein
• Folded membrane increases the surface area

Question 23

Explain how the concentration gradient affects the rate of diffusion

Answer 23

A greater difference in concentration means a greater difference in the number of molecules passing in the two directions and therefore a faster rate of diffusion.

Question 24

Explain how temperature affects the rate of diffusion

Answer 24

Molecules and ions have more kinetic energy at higher temperatures therefore they will move faster, resulting in a higher rate of diffusion.

Question 25

Explain how surface area affects the rate of diffusion

Answer 25

The greater the surface area across which diffusion is taking place, the greater the number of molecules or ions that can cross it at any moment and therefore the faster diffusion occurs.

Question 26

Describe how the properties of molecules or ions affect the rate of diffusion

Answer 26

• Large molecules diffuse more slowly than smaller ones as they require more energy to move
• Uncharged and non-polar molecules diffuse directly through the phospholipid bilayer
• Non-polar molecules diffuse more quickly than polar ones as they are soluble in the non-polar phospholipid bilayer

Question 27

Define active transport

Answer 27

The movement of molecules/ions into or out of a cell from a region of lower concentration to higher concentration using ATP and carrier proteins

Question 28

Compare and contrast active transport and facilitated diffusion

Answer 28

1. Both involve carrier protein
2. Active transport requires energy from ATP hydrolysis, facilitated diffusion is a passive process
3. Facilitated diffusion may also involve channel protein

Question 29

Define co-transport

Answer 29

The movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient

Question 30

Explain how co-transport is involved in the absorption of glucose AND/OR amino acids in the small intestine

Answer 30

(glucose diffuses from the lumen of the gut into the ileum, then into a capillary)

1. Na+ is actively transported out of epithelial cells and into the bloodstream
2. Na+ concentration is lower in epithelial cells than in the lumen of the gut, creating diffusion gradient of Na+
3. Transport of glucose/amino acids from the lumen to epithelial cells is coupled to the facilitated diffusion of Na+ down the concentration gradient

Question 31

How does the cell get energy for active transport?

Answer 31

The energy required is provided by ATP (adenosine triphosphate) produced during respiration. The ATP is hydrolysed to release energy

Question 32

Why is energy required in active transport?

Answer 32

The energy is required to make the carrier proteinchange shape, allowing it to transfer the molecules or ions across the cell membrane

Question 33

Define endocytosis

Answer 33

Type of active transport that moves particles into a cell by enclosing them in a vesicle made out of plasma membrane

Question 34

Define exocytosis

Answer 34

Cells transporting signalling chemicals or waste products from the inside of the cell to the outside of the cell in vesicles that fuse with the plasma membrane

Question 35

Why is active transport a selective process

Answer 35

It’s a selective process because molecules have the right shape and size to fit into the tertiary structure of a carrier protein