Exchange of Substances

Question 1

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

Answer 1

Large animals have lower SA:V ratio
Overcomes long diffusion pathway

Question 2

Use your knowledge of surface area to volume ratio to explain the higher
metabolic rate of a mouse compared to a horse

Answer 2

Mice are smaller so higher SA:V ratio
So lose heat faster
Need a higher metabolic rate to release heat from respiration to maintain body temp

Question 3

Tidal volume is the volume of air inhaled and exhaled during a single breath when a person is resting. The tidal volume in a person with emphysema is reduced compared with the tidal volume in a healthy
person.
Suggest and explain how a reduced tidal volume affects the exchange of
carbon dioxide between the blood and the alveoli.

Answer 3

A reduced tidal volume means that less air leaves the alveoli which means there is a greater concentration of CO2 in alveoli
So the concentration gradient is decreased
So CO2 diffuses more slowly out of the blood

Question 4

Explain how the counter-current principle allows efficient oxygen uptake in
the fish gas exchange system.

Answer 4

Water and blood flow in opposite directions
This means concentration of Oxygen is higher in water than blood so concentration gradient maintained along the whole lamella

Question 5

Describe and explain the mechanism that causes lungs to fill with air

Answer 5

Diaphragm contracts and flattens and external intercostal muscles contract and move up and out
This causes volume of lungs to increase so pressure decreases
Air moves down pressure gradient into lungs

Question 6

If alveolar epithelium cells die inside the human body they are replaced by non-specialised, thickened tissue.
Explain why death of alveolar epithelium cells reduces gas exchange in
human lungs

Answer 6

Reduced surface area
Causes an increase in diffusion distance
Reduces rate of diffusion between alveoli and capillaries

Question 7

Use your knowledge of gas exchange in leaves to explain why plants grown in soil with very little water grow only slowly

Answer 7

Stomata close
Reduces intake of CO2 which reduces rate of photosynthesis so less glucose production

Question 8

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

Answer 8

Walls one cell thick: reduces diffusion distance and increases rate of diffusion

Question 9

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

Answer 9

Lots of branched tracheoles: high SA for diffusion
Tracheal fluid: moves out into tissues during exercise to increase rate of diffusion
Tracheoles have thin walls to decrease diffusion distance

Question 10

Explain two ways in which the structure of fish gills is adapted for efficient
gas exchange

Answer 10

Many lamellae so large surface area
Thin surface so short diffusion pathway

Question 11

Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels

Answer 11

Micelles are made of bile salts and monoglycerides
Carry monoglycerides to lining of the ileum
Absorbed by diffusion
Triglycerides are reformed in cells
Vesicles move to cell membrane

Question 12

Describe the role of micelles in the absorption of fats into the cells lining
the ileum

Answer 12

Micelles are made up of bile salts and fatty acids
They make fatty acids more soluble in water
They carry the fatty acids to the lining of the ileum
Maintain higher concentration of fatty acids at lining of the ileum
The fatty acids can then diffuse through the cell membrane

Question 13

Describe the role of enzymes in the digestion of proteins in a mammal

Answer 13

Exopeptidases act from the ends of the protein molecule and hydrolyse peptide bonds at the ends to leave dipeptides
Endopeptidases work from within the molecule and hydrolyse peptide bonds to provide a greater surface area for exopeptidases to work on
Dipeptidases hydrolyse peptide bonds between dipeptides to leave two amino acids

Question 14

Explain the advantages of lipid droplet and micelle formation

Answer 14

Emulsifies lipids to increase surface area
So faster rate of digestion
Micelles carry fatty acids and monoglycerides to membrane

Question 15

Cells lining the ileum of mammals absorb the monosaccharide glucose by co-transport with sodium ions. Explain how

Answer 15

Na/K pump actively transports Na+ out of the cell
Creates a concentration gradient for Na+ to move back in
Na+ moves in by facilitated diffusion with a glucose attached

Question 16

Give the pathway a red blood cell takes when travelling in the human
circulatory system from a kidney to the lungs

Answer 16

Renal vein
Vena Cava to right atrium
Right atrium to pulmonary artery to lungs

Question 17

Tissue fluid is formed from blood at the arteriole end of a capillary bed.
Explain how water from tissue fluid is returned to the circulatory system

Answer 17

Plasma proteins remain in blood
so water potential in blood is lower
so it moves in by osmosis.
lymphatic system and the water moves around as lymph and re-enters circulatory system in the vena cava

Question 18

Explain how an arteriole can reduce the blood flow into capillaries

Answer 18

Arterioles have muscle walls that can contract
Narrows lumen

Question 19

Describe the advantage of the Bohr effect during intense exercise

Answer 19

Increase dissociation of oxygen
For aerobic respiration at the tissues

Question 20

Describe and explain the effect of increasing carbon dioxide concentration
on the dissociation of oxyhaemoglobin

Answer 20

Increased dissociation of oxygen
By decreasing blood pH

Question 21

Binding of one molecule of oxygen to haemoglobin makes it easier for a
second oxygen molecule to bind.
Explain why.

Answer 21

Cooperative bonding
When one molecule of oxygen binds it changes the tertiary structure of haemoglobin
Uncovers another binding site/Haem group for oxygen to bind to

Question 22

Explain the role of the heart in the formation of tissue fluid

Answer 22

The heart creates high hydrostatic pressure which forces water to move out of capillaries into tissues

Question 23

Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube.

Answer 23

Sucrose is actively transported into the phloem
Causes a decrease in water potential in the phloem
Causes water to diffuse in by osmosis from the xylem which generates a positive hydrostatic pressure at the source

Question 24

Phloem pressure is reduced during the hottest part of the day. Use information in the graph above along with your understanding of transpiration and mass flow to explain why

Answer 24

Highest rate of transpiration
Lots of water leaves through stomata
Less movement of water from the xylem to the phloem

Question 25

Explain why coloured water moved up the stalks.

Answer 25

Water leaves the leaves by transpiration through the stomata
Causes water to move through leaf cells to replace it and pull water up from the xylem, generating tension
Due to cohesion of water molecules, water column is maintained and all water molecules are pulled up

Question 26

Describe the cohesion-tension theory of water transport in the xylem

Answer 26

Water evaporates in leaves and leaves through stomata by transpiration
Creates a water potential gradient so water moves across leaf cells
Creates tension and pulls water out of xylem
Water in xylem has cohesion due to hydrogen bonding and this means it maintains a water column and all water is therefore pulled
Water also overcomes gravity by adhesion to xylem walls

Question 27

The student wanted to determine the rate of water loss per mm2 of surface area of the leaves of the shoot in Figure 1.
Outline a method she could have used to find this rate. You should assume that all water loss from the shoot is from the leaves.

Answer 27

Measure surface area of the leaves by drawing around them and counting squares to estimate the surface area.
Repeat this for both sides
Divide the rate by the total surface area of the leaves

Question 28

Describe the mass flow hypothesis for the mechanism of translocation in
plants.

Answer 28

Sucrose in source is actively transported into companion cells
This lowers the water potential in the sieve element
This causes water to move from the xylem to the phloem by osmosis
This generates a positive hydrostatic pressure at the source which causes the sucrose to move towards the sink
Sugars used/converted in sink

Question 29

How is the xylem adapted to its function?

Answer 29

No end walls, forms a continuous hollow tube for water transport
Cells are dead, no organelles so reduce friction
Walls are strengthened by lignin to provide structure
There are pits in the cell wall with no lignin so water and ions can enter and exit the vessel