Section 3 - Unit 6: Exchange

Question 1

Describe how proteins are digested in the human gut (4 marks)

Answer 1

• Hydrolysis of peptide bonds
• Endopeptidases break polypeptides into smaller peptide chains
• Exopeptidases remove terminal amino acids
• Dipeptidases hydrolyse / break down dipeptides into amino acids

Question 2

Suggest and explain why the combined actions of endopeptidases and exopeptidases are more efficient than exopeptidases on their own (2 marks)

Answer 2

• Endopeptidases hydrolyse the internal peptide bonds

- Exopeptidases hydrolyse the bonds at the end, increasing surface area

Question 3

Suggest the advantage of producing protease in an inactive form inside cells in the pancreas (2 marks)

Answer 3

• The protease will not digest the proteins inside cells

- So the cell is not damaged

Question 4

Explain how the epithelial cells that line the small intestine are adapted for the absorption of glucose (6 marks)

Answer 4

• Microvilli provide a large surface area;
• Many mitochondria to produce ATP for active transport
• Carrier proteins for active transport
• And for facilitated diffusion;
• Co-transport of sodium (ions) and glucose
• Membrane-bound enzymes digest disaccharides

Question 5

Describe the role of the enzymes of the digestive system in the complete breakdown of starch (5 marks)

Answer 5

• Amylase
• Breaks down starch into maltose
• Maltase
• Breaks down maltose into glucose
• Overall the enzymes hydrolyse
• The glycosidic bonds

Question 6

Describe the processes involved in the absorption of the products of starch digestion (5 marks)

Answer 6

• Glucose moves in with sodium into the epithelial cell
• Via channel protein
• Sodium removed by active transport
• Into the blood
• This maintains a low concentration of sodium, causing a concentration gradient
• Glucose moves into blood
• By facilitated diffusion

Question 7

Describe the role played by RER in the formation of chylomicrons (2 marks)

Answer 7

• Proteins are synthesised by RER

- With the involvement of ribosomes

Question 8

Suggest and explain how the chylomicrons leave the epithelial cell (2 marks)

Answer 8

• Via exocytosis

- Since it’s too large for it to leave in any other way

Question 9

State and explain which feature is present in an epithelial cell for rapid diffusion of substances from the lumen of the gut into the cytoplasm (2 marks)

Answer 9

• Microvilli

- Increase surface area

Question 10

Describe how oxygen in the air reaches capillaries surrounding alveoli in the lungs (4 marks)

Answer 10

• Trachea, bronchi and bronchioles
• Down pressure gradient
• Down diffusion gradient
• Across alveolar epithelium
• Across capillary endothelium/epithelium

Question 11

Describe and explain how the counter-current system leads to efficient gas exchange across the gills of a fish (3 marks)

Answer 11

• Water and blood flow in opposite directions
• This maintains a concentration gradient
• Along the whole length of the gill

Question 12

Describe two adaptations of the structure of alveoli for efficient gas exchange (2 marks)

Answer 12

• Thin walls

- Large surface area

Question 13

Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf (3 marks)

Answer 13

• CO2 enters via stomata
• Stomata opened by guard cells
• Diffuses through air spaces
• Down a diffusion gradient

Question 14

Describe and explain how the lungs are adapted to allow rapid exchange of oxygen between air in the alveoli and blood in the capillaries around them (5 marks)

Answer 14

Any five from:

• Alveoli 𝘄𝗮𝗹𝗹𝘀 are folded to provide a large surface area
• Many capillaries provide a large surface area;
• So fast 𝗱𝗶𝗳𝗳𝘂𝘀𝗶𝗼𝗻
• Alveoli/capillary walls are thin so short distance between alveoli and blood
• Flattened epithelium
• So short 𝗱𝗶𝗳𝗳𝘂𝘀𝗶𝗼𝗻 pathway;
• Thus fast 𝗱𝗶𝗳𝗳𝘂𝘀𝗶𝗼𝗻
• Circulation of blood
• Maintains a concentration gradient
• So fast 𝗱𝗶𝗳𝗳𝘂𝘀𝗶𝗼𝗻

Question 15

Describe and explain the mechanism that causes forced expiration (4 marks)

Answer 15

• Contraction of internal intercostal muscles
• Relaxation of diaphragm muscles/of external intercostal muscles
• Causes decrease in volume of chest / thoracic cavity
• Air pushed down a pressure gradient

Question 16

Explain the role of the diaphragm in breathing out (3 marks)

Answer 16

• Diaphragm moves up
• Reduces volume of thorax
• So the pressure in the thorax is higher than outside so air moves out

Question 17

Name the structure through which gases enter and leave the body of an insect (1 mark)

Answer 17

• Spiracle

Question 18

Name the small tubes that carry gases directly to and from the cells of an insect (1 mark)

Answer 18

• Tracheole

Question 19

Explain the movement of oxygen into the gas exchange system of an insect when it is at rest (3 marks)

Answer 19

• Oxygen used in 𝗮𝗲𝗿𝗼𝗯𝗶𝗰 respiration
• So oxygen concentration gradient established
• And oxygen 𝗱𝗶𝗳𝗳𝘂𝘀𝗲𝘀 in

Question 20

Describe a method you could use to find the surface area of a leaf (3 marks)

Answer 20

• Draw around leaf on graph paper
• Count squares
• Multiply by two for upper and lower leaf surface

Question 21

Suggest an advantage of an insect opening its spiracles at a lower frequency in very dry conditions (1 mark)

Answer 21

• No water is lost

Question 22

Removal of water from the tracheoles increases the rate of diffusion of oxygen between the tracheoles and muscle tissue. Suggest one reason why (1 mark)

Answer 22

• The volume is increased

Question 23

Explain how downward movement of the diaphragm leads to air entering the lungs (2 marks)

Answer 23

• Increases volume in thorax
• Lowers pressure in lungs
• So air pushed in by pressure gradient

Question 24

Explain how reduced elasticity of the lungs can cause breathing difficulties (2 marks)

Answer 24

• Alveoli will not inflate

- Breathing out particularly affected

Question 25

A fish uses its gills to absorb oxygen from water. Explain how the gills of a fish are adapted for efficient gas exchange (6 marks)

Answer 25

• Large surface area provided by lamellae
• Thin epithelium/distance between water and blood;
• Water and blood flow in opposite directions (countercurrent)
• This maintains concentration gradient (along gill)
• Circulation replaces blood saturated with oxygen
• Ventilation replaces water (as oxygen is removed)

Question 26

Describe the difference in the composition of gases in inhaled and exhaled air. Explain how these differences are caused (6 marks)

Answer 26

• Inhaled air contains more oxygen than exhaled air
• Inhaled air contains less carbon dioxide than exhaled air
• Inhaled air contains less water vapour
• Percentage of nitrogen also changes
• Respiration results in lower blood oxygen
• Oxygen enters blood
• By diffusion;
• Water vapour diffuses from moist surface

Question 27

In normal breathing, describe the part played by the intercostal muscles during inspiration (3 marks)

Answer 27

• Muscles contract
• Ribs move upwards and out
• Increasing volume in lungs

Question 28

Describe and explain how fish maintain a flow of water over their gills (4 marks)

Answer 28

• Mouth opens
• Floor of mouth lowered;
• Water enters due to decreased pressure/increased volume
• Mouth closes
• Floor raised results in increased pressure/decreased volume
• Increased pressure forces water over gills

Question 29

Describe and explain how the structure of the mammalian breathing system enables efficient uptake of oxygen into the blood (6 marks)

Answer 29

• Alveoli provide a large surface area;
• Walls of alveoli thin to provide a short diffusion pathway
• Walls of capillary thin for a short diffusion pathway
• Walls of alveoli have flattened cells
• Cell membrane permeable to gases
• Many blood capillaries provide a large surface area
• Intercostal muscles present to maintain a concentration gradient
• Branching of bronchi for efficient flow of air
• Cartilage rings keep airways open

Question 30

Explain how young fish get enough oxygen to their cells without having gills (2 marks)

Answer 30

• Diffusion of oxygen across skin

- Since there’s a large SA:V ratio

Question 31

Explain how xerophytic plants are adapted to dry habitats (6 marks)

Answer 31

Any 3 from:

• Fewer Stomata to avoid dehydration
• Sunken stomata to trap moist air and decrease the water potential gradient between the inside of leaves and outside (this reduces the rate of diffusion)
• Hairs to trap water vapour
• Waxy Cuticle to reduce evaporation
• Curled leaves as a shelter from the wind to reduce rate of water loss

Question 32

Explain two ways in which insects are adapted for efficient gas exchange (4 marks)

Answer 32

• Tracheae have rings for strength to prevent them from collapsing
• Contraction of muscles allows the mass movement of air so speeds up gas exchange

Question 33

Explain why lactic acid is produced inside the muscles of insects and how this increases the rate of gas exchange (4 marks)

Answer 33

• Anaerobic respiration takes place whilst flying
• This lowers the water potential in muscle cells
• So water moves in from the tracheoles via osmosis (due to large concentration gradient)
• This in turn reduces the volume of tracheoles so air moves in

Question 34

Explain how insects are adapted for both efficient gas exchange and limited water loss (6 marks)

Answer 34

• Small SA:V ratio to minimise the area of which water is lost
• Rigid outer skeleton of chitin and a waterproof cuticle
• Spiracles which can be opened and closed to reduce water loss

Question 35

Explain how triglycerides are absorbed into the blood (7 marks)

Answer 35

• Lipids are digested to form fatty acids (FA) and monoglycerides (MG)
• Micelles are formed however in the epithelial cells, they breakdown and release MG and FA again
• MG and FA are then transported to the ER where they form triglycerides
• Here they associate with cholesterol and lipoproteins to form chylomicrons
• Chylomicrons move out of the cells via exocytosis
• And enter the lacteals (lymphatic capillaries in centre of villi)
• The triglycerides in chylomicrons are then hydrolysed by an enzyme in endothelial cells of blood capillaries from where they diffuse into cells

Question 36

Describe the role of micelles in the absorption of fats into the cells lining the ileum (3 marks)

Answer 36

• Micelles include bile salts and fatty acids
• Make the fatty acids (more) soluble in water
• Brings/carries fatty acids to cell/lining (of the
  ileum)
• Maintain high(er) concentration of fatty acids to
  cell/lining (of the ileum)
• Fatty acids (absorbed) by diffusion

Question 37

The death of alveolar epithelium leads to the formation of unspecialised, thickened tissue. Explain why this reduces gas exchange in human lungs (3 marks)

Answer 37

• Reduced surface area
• Increased distance for diffusion
• Reduced rate of gas exchange

Question 38

Describe the gross structure of the human gas exchange system and how we breathe in and out (6 marks)

Answer 38

• Named structures – trachea, bronchi, bronchioles, alveoli
• Above structures named in correct order
• Breathing in – diaphragm contracts and external intercostal muscles contract
• Causes volume increase and pressure decrease in thoracic cavity (to below atmospheric, resulting in air moving in
• Breathing out - Diaphragm relaxes and internal intercostal muscles contract
• Causes volume decrease and pressure increase in thoracic cavity (to above atmospheric, resulting in air moving out

Question 39

Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange (3 marks)

Answer 39

• Tracheoles have thin walls so short diffusion distance to cells
• Highly branched/large number of tracheoles so short diffusion distance to cells
• Highly branched/large number of tracheoles so large surface area (for gas exchange)
• Tracheae provide tubes full of air so fast diffusion (into insect tissues)
• 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)
• Body can be moved (by muscles) to move air so maintains diffusion/concentration gradient for oxygen/carbon dioxide;