Lesson 52-53 - Gas exchange in insects and plants

Question 1

Why are specialised exchange surfaces often located inside the organism?

Answer 1

They must be thin (for a large SA:V ratio and short diffusion pathway)

Question 2

Why do insects not have a surface that’s good for exchange?

Answer 2

They have a hard exoskeleton made of the polysaccharide chitin (minimises water loss but impermeable to gases). Therefore the organism’s surface is no good for exchange.

Question 3

Describe the process of gas exchange in insects

Answer 3

• Terrestrial insects have microscopic air-filled pipes (tracheae) they use for gas exchange.
• Air moves into tracheae through pores on the surface (spiracles).
• O2 travels down the conc. gradient towards the cells
• The tracheae branch off into smaller tracheoles, which have thin, permeable walls and go to individual cells, so O2 diffuses directly into respiring cells - the insect’s circulatory system doesn’t transport O2.
• CO2 from cells moves down its own conc. gradient towards the spiracles to be released into atmosphere.

Question 4

What is the movement of gases through the tracheal system in insects caused by?

Answer 4

1. Steep conc./diffusion gradients
2. Contraction of muscles around the tracheal system, causing rhythmic abdominal movements
3. Ends of tracheoles are filled with water

Question 5

What happen in insects when there’s a conflict between water loss and gas exchange?

Answer 5

Spiracles close to minimise water loss.
It is an adaptation to manage the conflict between gas exchange and water loss

Question 6

Explain why abdominal pumping increases the efficiency of gas exchange between the tracheoles and muscle tissue of the insect (2 marks)

Answer 6

1. More air/O2 enters or air/O2 enters quickly;
2. (So) maintains / greater diffusion or concentration gradient;

Question 7

Where are chloroplasts found in a plant and what are they used for?

Answer 7

Found in the leaf and used for photosynthesis

Question 8

Aerobic respiration equation

Answer 8

glucose + oxygen → carbon dioxide + water + ATP
C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP

Question 9

Anaerobic respiration equation

Answer 9

glucose → lactic acid + ATP
C6H12O6 → C3H6O3 + ATP

Question 10

What is absorbed and released from a plant during the day and night?

Answer 10

Day: O2 released and CO2 absorbed
Night: CO2 released and O2 absorbed

Question 11

What are the key structures of a leaf, from top to bottom?

Answer 11

Waxy cuticle
Upper epidermis
Palisade mesophyll
Spongy mesophyll with vascular bundle
Lower epidermis
Guard cells with chloroplasts and stoma
Waxy cuticle

Question 12

What are the adaptations of the leaves of dicotyledonous plants for gas exchange?

Answer 12

• The leaf is thin and flat: just a few cells thick, therefore has a large SA:V ratio.
• Stomata: mostly on the lower surface, ensure air inside the leaf exchanges with the air outside.
• Diffusion distances are short - no cells are far from a source of air as there are many interconnecting air spaces between the stomata and mesophyll cells.
• Diffusion occurs through these air spaces - this is more rapid than through water.

Question 13

What happens to the guard cell to open and close the stoma?

Answer 13

Cells turgid - so stoma open
Cells flaccid - so stoma closed

Question 14

What are the similarities and differences between gas exchange in plants and insects?

Answer 14

Similarities:
1. Obtain the gases they need from the air by diffusion down concentration gradients
2. Movement of gases is controlled by pore like structures (spiracles/stomata)

Differences:
1. Insects deliver air to cells via a system of tubes that are not present in the leaf
2. Insect muscle contraction can assist with movement of the air (especially larger insects).

Question 15

How do plants and insects reduce water loss?

Answer 15

Insects: They close their spiracles using muscles. They also have a waterproof, waxy cuticle all over their body, and tiny hairs around their spiracles, both of which reduce evaporation.

Plants: Stomata are kept open during the day (to allow gas exchange) water enters the guard cells, making them turgid, opening the stomatal pore. If the plant starts to get dehydrated, guard cells lose water and become flaccid, closing the pore.

Question 16

What are xerophytes?

Answer 16

Plants specially adapted for life in warm, dry or windy habitats.

Question 17

What are some examples of xerophytic adaptations?

Answer 17

• Stomata sunk in pits to trap water vapour, reducing the conc. gradient of water between the leaf and the air, reducing evaporation from leaf.
• A layer of ‘hairs’ on the epidermis to trap water vapour round the stomata.
• Curled leaves with the stomata inside, protecting them from the wind (wind increases rate of diffusion and evaporation)
• Less stomata, so fewer places for water to escape.
• Thicker waxy, waterproof cuticles on leaves and stems to reduce evaporation