Unit 1 Chapter 2A Membranes and Transport

Question 1

With reference of the nature of water molecules and the properties of cellular structures, describe how water molecules enter a plant cell.

Answer 1

• Cell wall is fully permeable to water
• Water can pass through cell wall freely by simple diffusion
• Water is repelled by the hydrophobic core of the phospholipid bilayer of the cell membrane
• Channel proteins in the cell membrane provide a hydrophilic pathway for water to pass through the cell membrane to enter the cell (facilitated diffusion)

Question 2

Explain how the movement of chloride ions into the mucus facilitates the movement of water molecules.

Answer 2

• The movement of chloride ions causes the concentration of chloride ions in the mucus to be higher than that in the tissue fluid
• Leads to a NET movement of water molecules into the mucus by osmosis
• From a region of higher water potential in the tissue fluid to a region of lower water potential in the mucus

Question 3

Fluid mosaic model

Answer 3

• Phospholipid + some protein molecules can move laterally within each phospholipid bilayer
• Different type of proteins scatter among the phospholipid molecules in cell membrane (e.g. peripheral proteins, integral proteins…)
• Contains cholesterol

Question 4

What is the function of cholersterol in membrane fluidity

Answer 4

-> Moderate temperature: bind to fatty acid tail of phospholipid molecules to reduce movement (reduce fluidity)

-> Low temperature: hinder closer packing of fatty acid tails of phospholipid molecules (hinder solidification of membrane; increase fluidity)

Question 5

What is the function of cholesterol in membrane stability

Answer 5

• rigid ring structure of cholersterol
• confers rigidity

Question 6

Define passive transport

Answer 6

No energy required: diffusion + facilitated diffusion

Question 7

Define diffusion

Answer 7

net movement of molecules from a region of higher concentration to region of lower concentration down the concentration gradient across a partially permeable membrane

Question 8

What are the different kinds of diffusion

Answer 8

• Simple diffusion: small, uncharged, non-polar and hydrophobic molecules
• Facilitated diffusion: polar, charged, hydrophilic molecules (requires carrier proteins / channel proteins)

Question 9

Functions of carrier proteins

Answer 9

• change shape of specific proteins that binds
• release molecules on the other side of the membrane

Question 10

Define osmosis

Answer 10

Diffusion / net movement of water molecules down a water potential gradient through a partially permeable membrane

Question 11

Rate of diffusion formula

Answer 11

Surface area x concentration gradient / thickness of exchange membrane

Question 12

Arrangement of phospholipids in cell membrne

Answer 12

1. Phospholipids are arranged “tail-to-tail” in a bilayer in cell membrane with hydrophilic phosphate heads associate with water in aqueous environment inside a cell and water in aqueous environment outside a cell
2. The hydrophobic fatty acid tails orientate away from water in aqueous environment inside and outside a cell, thus forming the hydrophobic core of the phospholipid bilayer in cell membrane

Question 13

Process of endocytosis

Answer 13

1. Cell membrane extends to form foot-like pseudopodia to surround the particles
2. Part of the cell membrane then pinches off to form a vesicle to enclose the particles
3. Cell membrane folds inward to form a pit to surround the particles
4. The pit deepens and part of the cell membrane pinches off to form a pit to enclose the particles

Question 14

(past paper: spec) Compare and contrast exocytosis and endocytosis (4)

Answer 14

Similarities
- both used to transport large particles
- both involves phosopholipid membrane vescicles

Differences
- exocytosis: export of materials (out of cell)
- endocytosis: import of materials (into the cell)

Question 15

(past paper: spec) Explain why oxygen molecules can pass directly through the cell membrane. (3)

Answer 15

• oxygen molecule is small
• oxygen molecule is non-polar
• pass between gaps in the cell membrane

Question 16

(past paper: spec) The enzyme OMP decarboxylas increases the rate of carbon dioxide removal from orotidine monophosphate by 10^17 times. State how OMP decarboxylas increases the rate of this reaction. (2)

Answer 16

• acts as a biological catalyst (1)
• forms an enzyme-substrate complex (1)

Question 17

(past paper: spec) The enzyme OMP decarboxylase increases the rate of carbon dioxide removal from orotidine monophosphate by 10^17 times. Explain why OMP decarboxylase catlyses this reaction only. (3)

Answer 17

• OMP decarboxylase ==> specific (1)
• active site of the enzyme has particular shape (1)
• binds only with orotidine monophosphate (1)

Question 18

Gas exchange surface adaptations

Answer 18

1. thin alveoli wall - reduce diffusion distance
2. many alveoli - increase SA for diffusion
3. capillary network - maintain conc gradient

Question 19

(past paper: Jan19) Explain the arrangmenet of phospholipids in liposomes. (somd type of double layer membrane)

Answer 19

• hydrophobic tails repelled by the aqueous environment
• hydrophilic heads interact with / associate in the aqueous environment

do not just state the directions, must explain

Question 20

Factors affecting rate of gas exchange

Answer 20

1. surface area
2. temperature
3. diffusion distance
4. concentration gradient
5. permeability

Question 21

Relationship between mass and surface area of alveoli

Answer 21

1. As mass increases, demand for oxygen increases
2. The surface area of alveoli increases to allow for increased gas exchange to increase oxygen for respiration to provide energy for metabolic need

Question 22

Suggest how the heart enables organs to carry out effective gas exchange

Answer 22

1. Heart generates pumping force to maintain continuous flow of oxygenated blood to and
   deoxygenated blood away from alveoli
2. Hence maintaining steep concentration gradient

Question 23

Suggest how double circulation enables mammals to carry out effective gas exchange

Answer 23

1. Double circulatory system separates oxygenated and deoxygenated blood
2. Ensures deoxygenated blood delivers to alveoli for gas exchange
3. Oxygenated blood is delivered to body cells
4. Maintaining the steep concentration gradient for effective gas exchange
5. Ensures different pressures in the heart. Blood pumps at a lower pressure to the lungs to prevent
   the alveoli from being damaged.

Question 24

Tuberculosis and gas exchange

Answer 24

1. Alveoli are destroyed by the tubercle
2. Hence the tubercle has reduced the surface area for gas exchange by diffusion
3. Gas exchange is reduced leading to severe breathing problems
4. The tubercle has damaged the lung blood vessels causing coughing up of blood
5. Severe coughing is due to the patient trying to cough up dead tissue

Question 25

Emphysema and gas exchange

Answer 25

1. The alveoli has a smaller surface area 2. Therefore the rate of diffusion of oxygen into the bloodstream will be slower

Question 26

(past paper: May19) The red blood cells of giraffe are about one third of the size of human red blood cells, so that they can pass through the very narrow capillaries. The small size of the red blood cells ensures that the legs of the giraffe have a good supply of oxygen. Explain why smaller red blood cells increase supply of oxygen to the legs. (2)

Answer 26

- because (the smaller red blood cells will have) a greater surface area (1) - and therefore oxygen will be able to diffuse (into / out of the RBCs) faster (1) Ignore easier to diffuse / more can diffuse

Question 27

Explain the differences in the uptake of substance K and substance L (2)

Answer 27

- substance K is taken up by active transport because active transport {works against / not affected by} the concentration gradient (1) - substance L is taken up by diffusion until {the concentration on the inside of the cell is equal to the concentration on the outside of the cell / no net movement / equilibrium reached} (1)

Question 28

Pseudopodia are formed by cytoplasm flowing towards the membrane and changing its shape. Explain why the membrane is able to change its shape when cytoplasm flows towards it. (2)

Answer 28

- the membrane is fluid (1) - DO NOT ACCEPT flexible / strong IGNORE unqualified - references to fluid mosaic structure / model phospholipids (and proteins) can move (within the membrane) (1) - ACCEPT more phospholipids added to the membrane (presence of) cholesterol contributes to fluidity (1)

Question 29

Explain how the uptake of substances would be affected if the amoeba increased its number of pseudopodia. (2)

Answer 29

- (more pseudopodia would) increase the surface area (of the pseudopodia / amoeba / cell / membrane) (1) - therefore {uptake / diffusion} would be faster (1)

Question 30

The absorption of glucose into the bloodstream equires membrane transport proteins. Explain why glucose cannot diffuse into the bloodstream.

Answer 30

- Polar / hydrophilic molecule - Cannot pass through the phospholipids / fatty acids / tails which are non-polar