Unit 3 - Exchanging substances

Question 1

The relationship between surface

area to volume ratio and metabolic

rate for a smaller organism.

Answer 1

1. (Smaller so) larger surface area to volume ratio;
2. More heat loss (per gram)
3. Faster rate of respiration, releases more heat

Question 2

Explain the advantage for larger animals of having a specialised system that facilitates oxygen uptake.

Answer 2

1.      Large(r) organisms have a small(er) surface area:volume (ratio);

OR

Small(er) organisms have a large(r) surface area:volume (ratio);

2.      Overcomes long diffusion pathway

OR

Faster diffusion

Question 3

Plants – explain why stomata open due to increase in light intensity (1)

Answer 3

allowing carbon dioxide to enter for photosynthesis;

Or

for gas exchange allowing photosynthesis

Question 4

Plants -Describe how carbon dioxide in the

air outside a leaf reaches

mesophyll cells inside the leaf (4)

Answer 4

1. (Carbon dioxide enters) via stomata; Reject stroma
2. (Stomata opened by) guard cells;
3. Diffuses through air spaces;
4. Down diffusion gradient;

Question 5

Plants – describe & explain an advantage and disadvantage to having a higher stomatal density

Answer 5

Advantage

1.      More carbon dioxide uptake;

2.      More photosynthesis so faster/more growth;

Disadvantage

3.      More water loss/transpiration

Accept plant wilts for ‘more water loss’

4.      Less photosynthesis so slower/less growth;

Question 6

Plants - Adaptations to

desert plants (6)

Answer 6

1. Hairs so ‘trap’ water vapour and water potential gradient decreased;
2. Stomata in pits/grooves so ‘trap’ water vapour and water potential gradient decreased;
3. Thick (cuticle/waxy) layer so increases diffusion distance;
4. Waxy layer/cuticle so reduces evaporation/transpiration;
5. Rolled/folded/curled leaves so ‘trap’ water vapour and water potential gradient decreased;
6. Spines/needles so reduces surface area to volume ratio;

Question 7

fish - counter-current

mechanism (3)

Answer 7

1. Water and blood flow in opposite directions;
2. Blood always passing water with a higher oxygen concentration;
3. Diffusion/concentration gradient (maintained) along (length of) lamella/filament;

Question 8

Insects - Describe & explain how the structure of the insect gas exchange system:

*   provides cells with sufficient oxygen

Answer 8

1.      Spiracles (lead) to tracheae (that lead) to tracheoles;

2.      Open spiracles allow diffusion of oxygen from air

OR

Oxygen diffusion through tracheae/tracheoles;

3.      Tracheoles are highly branched so large surface area (for exchange);

4.      Tracheole (walls) thin so short diffusion distance (to cells)

OR

Highly branched tracheoles so short diffusion distance (to cells)

OR

Tracheoles push into cells so short diffusion distance;

5.      Tracheole walls are permeable to oxygen;

Question 9

Insects - Describe & explain how the structure of the insect gas exchange system:

limits water loss.(2)

Answer 9

1. Cuticle/chitin in tracheae impermeable so reduce water loss;
2.       Spiracles close (eg.during inactivity) preventing water loss;

Question 10

Insects - Abdominal Pumping (3)

Answer 10

1. Abdominal pumping/pressure in tubes linked to carbon dioxide release;
2. (Abdominal) pumping raises pressure in body;
3. Air/carbon dioxide pushed out of body /air/carbon dioxide moves down pressure gradient (to atmosphere)

Question 11

Insects - Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange.

Answer 11

1. Tracheoles have thin walls so short diffusion distance to cells;
2. Highly branched / large number of tracheoles so short diffusion distance to cells;
3. Highly branched / large number of tracheoles so large surface area (for gas exchange);
4. Tracheae provide tubes full of air so fast diffusion (into insect tissues);
5. Fluid in the end of the tracheoles that moves out (into tissues) during exercise so faster diffusion through the air to the gas exchange surface;
   OR
   Fluid in the end of the tracheoles that moves out (into tissues) during exercise so larger surface area (for gas exchange);
6. Body can be moved (by muscles) to move air so maintains diffusion / concentration gradient for oxygen / carbon dioxide;

Question 12

Lungs - Describe and explain one feature of the alveolar epithelium that makes the epithelium well adapted as a surface for gas exchange.

Answer 12

Mark in pairs
1. Flattened cells
OR
Single layer of cells;
Reject thin cell wall/membrane
Accept thin cells
Accept ‘one cell thick’
2. Reduces diffusion distance/pathway;
3. Permeable;
4. Allows diffusion of oxygen/carbon dioxide;
5. moist
6. Increase rate of diffusion

Question 13

Lungs – describe and explain inhaling (4)

Answer 13

1. Diaphragm (muscle) contracts and external intercostal muscles contract;
   Ignore ribs move up and out
2. (Causes volume increase and) pressure decrease;
3. Air moves down a pressure gradient
   Ignore along OR
   Air enters from higher atmospheric pressure;

Question 14

Lungs - Describe the pathway taken by an oxygen molecule from an alveolus to the blood. (2)

Answer 14

1. (Across) alveolar epithelium;
2. Endothelium of capillary;

Question 15

Lungs - Explain how one feature of an alveolus allows efficient gas exchange to occur.

Answer 15

1. (The alveolar epithelium) is one cell thick;
   Reject thin membrane
2. Creating a short diffusion pathway / reduces the diffusion distance;

Question 16

Lungs - Describe the gross structure of the human gas exchange system (1)

Answer 16

1. Named structures – trachea, bronchi, bronchioles, alveoli;

Question 17

Lungs – Describe how we breathe in and out.(4)

Answer 17

1. Breathing in – diaphragm contracts and external intercostal muscles contract;
2. (Causes) volume increase and pressure decrease in thoracic cavity (to below atmospheric, resulting in air moving in);
   For thoracic cavity accept ‘lungs’ or ‘thorax’.
   Reference to ‘thoracic cavity’ only required once.
3. Breathing out - Diaphragm relaxes and internal intercostal muscles contract;
   Accept diaphragm relaxes and (external) intercostal muscles relax and lung tissue elastic (so recoils).
4. (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric, resulting in air moving out);

Question 18

Digestion – How are proteins digested? (4)

Answer 18

1. Hydrolysis of peptide bonds;
2. Endopeptidases break polypeptides into smaller peptide chains;
3. Exopeptidases remove terminal amino acids;
4. Dipeptidases hydrolyse/break down dipeptides into amino acids;

Question 19

Digestion – Compare endopeptidase and exopeptidase (3)

Answer 19

1. Endopeptidases hydrolyse internal (peptide bonds);
2. Exopeptidases remove amino acids/hydrolyse (bonds) at end(s);
3. More ends or increase in surface area (for exopeptidases);

Question 20

Digestion - Describe the action of membrane-bound dipeptidases and explain their importance.(2)

Answer 20

1. Hydrolyse (peptide bonds) to release amino acids;
2. Amino acids can cross (cell) membrane by facilitated diffusion;
   OR
   Maintain concentration gradient of amino acids for absorption;

Question 21

Digestion – Describe the complete digestion of starch by a mammal. (5)

Answer 21

1. Hydrolysis;
2. (Of) glycosidic bonds;
3. (Starch) to maltose by amylase;
4. (Maltose) to glucose by disaccharidase/maltase;
5. Disaccharidase/maltase membrane-bound;

Question 22

Digestion - Function of bile salts and micelles (3)

Answer 22

1. (Bile salts emulsify lipids forming) droplets which increase surface areas (for lipase / enzyme action);
2. (So) faster hydrolysis / digestion (of triglycerides / lipids);
3. Micelles carry fatty acids and glycerol / monoglycerides to / through membrane / to (intestinal epithelial) cell;

Question 23

Digestion – describe lipid digestion (3)

Answer 23

1. lipase hydrolyses triglycerides
2. ester bonds
3. Form monoglycerides and fatty acids

Question 24

Digestion – Explain the advantages of emulsification and micelle formation. (2)

Answer 24

1. Droplets increase surface areas (for lipase / enzyme action);
2. (So) faster hydrolysis / digestion (of triglycerides / lipids);
3. Micelles carry fatty acids and glycerol / monoglycerides to / through membrane / to (intestinal epithelial) cell;

Question 25

Absorption - Describe and explain two features you would expect to find in a cell specialised for absorption. (4)

Answer 25

1. Folded membrane/microvilli so large surface area (for absorption); Reject references to ‘villi’.
   Accept ‘brush border’ for ‘microvilli’.
2. Large number of co-transport/carrier/channel proteins so fast rate (of absorption)
   OR Large number of co-transport/carrier proteins for active transport
   OR Large number of co-transport/carrier/channel proteins for facilitated diffusion;
3. Large number of mitochondria so make (more) ATP (by respiration)
   OR Large number of mitochondria for aerobic respiration
   OR Large number of mitochondria to release energy for active transport;
4. Membrane-bound (digestive) enzymes so maintains concentration gradient (for fast absorption);

Question 26

Absorption - Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels. (4)

Answer 26

1. Micelles contain bile salts and fatty acids/monoglycerides;
2. Make fatty acids/monoglycerides (more) soluble (in water)
   OR Bring/release/carry fatty acids/monoglycerides to cell/lining (of the iluem)
   OR Maintain high(er) concentration of fatty acids/monoglycerides to cell/lining (of the ileum);
3. Fatty acids/monoglycerides absorbed by simple diffusion;
4. Triglycerides (re)formed (in cells);
   Accept chylomicrons form
5. Vesicles move to cell membrane;

Question 27

Absorption - Describe the role of micelles in the absorption of fats into the cells lining the ileum (2)

Answer 27

1. Micelles include bile salts and fatty acids;
   Ignore other correct components of micelles.
2. Make the fatty acids (more) soluble in water;
   For ‘fatty acids’ accept fats / lipids.
3. Bring/release/carry fatty acids to cell/lining (of the ileum);
   For ‘fatty acids’ accept fats/lipids.
4. Maintain high(er) concentration of fatty acids to cell/lining (of the ileum);
5. Fatty acids (absorbed) by diffusion;