12.9 Gas Exchange

Question 1

Complete the sentence: The larger an organism is the …………. the SA: volume ratio is.

Answer 1

Smaller

Question 2

Examples of materials exchanged between environment and organisms.

Answer 2

• Oxygen and carbon dioxide (respiratory gases)
• Glucose, amino acids, lipids (nutrients)
• CO2 and Urea (excretory products)

Question 3

Complete the sentence: For exchange to be effective, the surface area of the organism must be ………. Compared to its volume.

Answer 3

Large

Question 4

How do we calculate SA of an organism?

Answer 4

L x H x 6 (faces/sides)

Question 5

How do we calculate the volume of an organism?

Answer 5

L x H x W

Question 6

What is Fick’s Law equation?

Answer 6

Rate of diffusion= (surface area x concentration gradient)/(diffusion distance or pathway)

Question 7

According to Fick’s Law equation, what makes a good exchange surface?

Answer 7

• Large surface area
• Large concentration gradient
• Thin exchange surface (thin walls)
• Shorter diffusion pathway

Question 8

How are single-celled organisms adapted for gas exchange?

Answer 8

Have large enough SA: volume ratios to meet gas exchange needs by diffusion across their surface.

Question 9

How is a concentration gradient of oxygen and carbon dioxide produced in opposite directions?

Answer 9

• Oxygen is required to produce ATP during aerobic respiration.
• Carbon dioxide is produced as a waste product during this process.

Question 10

What is the tracheal system in insects made up of?

Answer 10

Trachea and tracheoles

Question 11

To limit water loss, insects may have…

Answer 11

• Waterproof covering over their body surfaces. Usually, a rigid outer skeleton (exoskeleton) which is covered with a waterproof cuticle.
• Relatively small SA: volume ratio to minimise the area over which water is lost.

Question 12

Explain the movement of oxygen through the insect.

Answer 12

• Oxygen enters the insect by diffusing through spiracles and into the tracheae. Spiracles close.
• Oxygen diffuses through the tracheae into the tracheoles (down a concentration gradient).
• Oxygen is delivered directly to the respiring tissues.

Question 13

What happens to the movement of carbon dioxide through the insect?

Answer 13

Carbon dioxide, which is produced by aerobically respiring tissues, moves in the opposite direction and exits insects when spiracles open.

Question 14

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

Answer 14

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 15

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

Answer 15

1 Large surface area provided by many lamellae over many gill filaments;
2 Increases diffusion/makes diffusion efficient;
3 Thin epithelium/distance between water and blood;
4 Water and blood flow in opposite directions/countercurrent;
5 (Point 4) maintains concentration gradient (along gill)/equilibrium not reached;
6 As water always next to blood with lower concentration of oxygen;
7 Circulation replaces blood saturated with oxygen;
8 Ventilation replaces water (as oxygen removed);

Question 16

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

Answer 16

1. Water and blood flow in opposite directions;
2. Maintains diffusion/concentration gradient of oxygen OR Oxygen concentration always higher (in water);
3. (Diffusion) along length of lamellae/filament/gill/capillary;

Question 17

What are the adaptions of the leaves for gaseous exchange?

Answer 17

• Flat- gives a larger SA: volume ratio
• Many stomata- pores allow air to move in and out of the leaf.
• Air spaces in leaves so a short distance between cells and air (short diffusion distance).

Question 18

Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf.

Answer 18

• Mesophyll cells photosynthesise and this reduces the concentration of CO2 in the cells.
• Carbon dioxide enters via the stomata.
• Stomata open by guard cells.
• Carbon dioxide diffuses through air spaces.
• Down diffusion gradient.

Question 19

Describe how oxygen made by the mesophyll cells leaves the cells and reaches the air outside.

Answer 19

• Mesophyll cells produce O2 as a result of photosynthesis.
• O2 diffuses into the air spaces from the cells.
• This increases the concentration of O2 in the air spaces, causing O2 to move from the air spaces to outside the leaf via the stomata.

Question 20

Other than the distribution of stomata, suggest and explain two xerophytic features the leaves of this plant might have.

Answer 20

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 21

What are the adaptions of dicotyledonous plants to reduce water loss?

Answer 21

• Gases are exchanged through stomata. The guard cells control the opening and closing of stomata. At night, the guard cells close the stomata to prevent water loss by evaporation. (Less CO2 is required at this time of day due to the lack of available sunlight for photosynthesis)
• Small SA: volume ratio.
• Waxy cuticle, which is waterproof and thick, increasing diffusion distance, so less diffusion of substances.
• Hairy leaves trap water vapour. Lower water potential gradient.

Question 22

Explain why plants grown in soil with very little water grow only slowly

Answer 22

1. Stomata close;
2. Less carbon dioxide (uptake) for less photosynthesis/glucose production;