3 Organisms exchange substances in their environment

Question 1

Explain the advantage for larger animals of having a specialised system that facilitates oxygen uptake. (2)

Answer 1

Larger organisms have a smaller surface area to volume ratio. This enables faster diffusion.

Question 2

Describe the relationship between size and surface area to volume ratio of organisms. (1)

Answer 2

As the size increases, the surface area to volume ratio decreases.

Question 3

Explain why oxygen uptake is a measure of metabolic rate in organisms. (1)

Answer 3

Oxygen is used in respiration to provide ATP.

Question 4

Explain how the counter-current principle allows efficient oxygen uptake in the fish gas exchange system. (2)

Answer 4

The blood and water flow in opposite directions.
The concentration gradient is maintained along the length of the filament.

Question 5

Describe and explain the mechanism that causes the lungs to fill with air. (3)

Answer 5

The diaphragm muscles contract and the external intercostal muscles contract.
This causes a volume increase and pressure decrease.
The air moves down a pressure gradient, into the lungs.

Question 6

Describe and explain the advantage of the counter-current principle in gas exchange across a fish gill. (3)

Answer 6

The water and blood flow in opposite directions.
This maintains a concentration gradient of oxygen.
Diffusion occurs across the length of the filament.

Question 7

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

Answer 7

The oxygen moves across the alveolar epithelium to the epithelium of the blood capillary.

Question 8

Explain how one feature of an alveolus allows efficient gas exchange to occur. (2)

Answer 8

The alveolar epithelium is one cell thick, which creates a short diffusion pathway, so diffusion happens faster.

Question 9

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

Answer 9

The human gas exchange system consists of the trachea, bronchi, bronchioles and alveoli.
As we breathe in, the diaphragm and the external intercostal muscles contract.
This causes an increase in volume and decrease in pressure in thoracic cavity, which causes air to move in.
As we breathe out, the diaphragm relaxes and the internal intercostal muscles contract.
This causes a decrease in volume and an increase in pressure in the thoracic cavity which causes air to be moved out.

Question 10

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

Answer 10

1. Tracheoles have thin walls so there is a short diffusion distance to cells.
2. It is highly branched so there is a large surface area for diffusion to occur.
3. The tracheae provides tubes full of air so there is fast diffusion into insect tissues.

Question 11

Explain two ways in which the structure of fish gills are adapted for efficient gas exchange. (2)

Answer 11

1. Many lamellae increase the surface area,
2. Thin surface so there is a short diffusion pathway.

Question 12

Explain the function of ATP hydrolase. (2)

Answer 12

ATP hydrolase converts ATP to ADP and Pi, which releases energy. This energy allows ions to be moved against a concentration gradient.

Question 13

The movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum. Explain how. (2)

Answer 13

This movement of Na+ maintains a concentration gradient for Na+ from the ileum into the cell. The Na+ moves in by co-transport, bringing glucose with it.

Question 14

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

Answer 14

The micelles have bile salts and fatty acids, which make the fatty acids more soluble in water.
This releases fatty acids to the cell lining the ileum, maintaining a higher concentration of fatty acids to the cell lining the ileum. The fatty acids are absorbed by diffusion.

Question 15

Describe the role of enzymes in the digestion of proteins in a mammal. (4)

Answer 15

Enzymes hydrolyse the polypeptides. Endopeptidases produce shorter polypeptides and exopeptidases produce dipeptides. Dipeptidase then acts on the dipeptide to produce single amino acids.

Question 16

Explain the advantages of lipid droplet and micelle formation. (3)

Answer 16

The droplets increase the surface area for enzyme action so there is faster digestion of lipids.
Micelles carry fatty acids and glycerol through the membrane to the intestinal epithelial cell.

Question 17

Cells lining the ileum of mammals absorb the monosaccharide glucose by co-transport with sodium ions. Explain how. (3)

Answer 17

Sodium ions are actively transported from the ileum cell into the blood. This maintains a diffusion gradient for sodium to enter the cells from the gut, and with it, glucose is brought with it. Glucose enters via facilitated diffusion with sodium ions.

Question 18

Give the pathway a red blood cell takes when travelling in the human circulatory system from a kidney to the lungs. Do NOT include descriptions of pressure changes in the heart or the role of heart valves in your answer. (3)

Answer 18

The red blood cell travels from the renal vein through to the vena cava. From there, it travels to the right atrium, passing through to the right ventricle and eventually to the pulmonary artery, where the red blood cell will be passed to the lungs where it will be reoxygenated.

Question 19

Tissue fluid is formed from the blood at the arteriole end of a capillary bed.

Explain how water from tissue fluid is returned to the circulatory system. (4)

Answer 19

The plasma proteins remain in the arteriole end of the capillary bed. This creates a water potential gradient. Water moves to blood by osmosis and returns to the blood via the lymphatic system.

Question 20

Describe two precautions a student should when clearing away after a dissection. (2)

Answer 20

Disinfect instruments.

Put organs in a separate bin to dispose.

Question 21

Explain how an arteriole can reduce the blood flow into capillaries. (2)

Answer 21

The muscle contracts which constricts the arteriole which reduces blood flow into capillaries.

Question 22

Describe the advantage of the Bohr effect during intense exercise (2)

Answer 22

The Bohr effect increases the dissociation of oxygen for aerobic respiration at the muscle cells.

Question 23

Describe and explain the effect of increasing carbon dioxide concentration on the dissociation of oxyhaemoglobin. (2)

Answer 23

Increasing the carbon dioxide concentration will decrease haemoglobin’s affinity for oxygen, by decreasing the pH of the blood.

Question 24

Binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind. Explain why. (2)

Answer 24

The binding of the first oxygen molecule changes the tertiary structure of the haemoglobin. This uncovers a second binding site for the oxygen molecule to bind.

Question 25

Explain the role of the heart in the formation of tissue fluid. (2)

Answer 25

The contraction of the ventricles produces high blood pressure, which forces water and other substances out of the blood capillaries.

Question 26

Describe the cohesion-tension theory of water transport in the xylem. (5)

Answer 26

Water is lost from the leaf due to transpiration. This lowers the water potential of the mesophyll cells. Water is pulled up by the xylem, creating tension. Water molecules cohere via hydrogen bonds, forming a continuous water column. This causes the adhesion of water to the walls of the xylem, transporting the water.

Question 27

Describe the mass flow hypothesis for the mechanism of translocation in plants. (4)

Answer 27

Leaf sugars are actively transported into the phloem by companion cells. This lowers the water potential of the sieve cells and water enters by osmosis. This increase in pressure causes mass movement towards the root. The sugars are converted in the root for use in respiration or as storage.

Question 28

Describe the transport of carbohydrate in plants. (5)

Answer 28

Sucrose is actively transported into the phloem cells by the companion cells. This lowers the water potential in the phloem, leading to water moving in from the xylem via osmosis. This produces high hydrostatic pressure resulting in mass flow to respiring cells. Sucrose is unloaded into storage cell.

Question 29

The mass flow hypothesis is used to explain the movement of substances through the phloem. Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube. (3)

Answer 29

Sucrose is actively transported into the phloem, which lowers the water potential. Water moves into the phloem by osmosis from the xylem, causing the pressure to increase.