Plant Gas Exchange

IGCSE Edexcel Biology
2.39–2.45, 2.70 Gas exchange in leaves, stomata, guard cells and plant excretion
Key Concepts: Plants exchange gases through stomata in their leaves. CO₂ diffuses in for photosynthesis; O₂ diffuses out. At the same time, both gases move in opposite directions during respiration. Guard cells open and close stomata by changing shape. Plants excrete gaseous waste (CO₂ and O₂) through stomata.

Section A — Gas Exchange in Leaves

1. State the gases that enter and leave a leaf during photosynthesis, and the direction of their movement through stomata. [2]
2. Explain why a leaf can exchange gases efficiently, referring to two structural features. [4]
3. On a bright sunny day, a plant is photosynthesising rapidly. Explain whether there is any net gain or net loss of CO₂ from the leaf, and why. [3]
4. State the compensation point and explain what it means in terms of gas exchange. [3]

Section B — Stomata and Guard Cells

5. Describe the structure of a stoma. Include the guard cells in your answer. [2]
6. Explain how stomata open during the day. Include the role of water potential and osmosis. [4]
7. State two conditions under which stomata close, and explain the benefit of closing in each case. [4]
8. Explain the link between stomata being open and transpiration rate. [2]

Section C — Plant Excretion (spec 2.70)

9. State the two main gaseous waste products of plant cells and explain the metabolic reaction that produces each. [4]
10. Explain how plants remove their gaseous waste products. [2]
11. Explain why plants do not require a specialised excretory organ (such as a kidney) for removing gaseous waste. [2]

Total marks: 32

Mark Scheme

1. CO₂ enters the leaf (diffuses in through stomata) [1]; O₂ leaves the leaf (diffuses out through stomata) [1] [2]
2. Any two: large surface area of leaves allows more gas exchange [2]; spongy mesophyll layer has many air spaces for CO₂ and O₂ to diffuse to and from cells [2]; stomata on the leaf surface allow gas exchange between the leaf and the atmosphere [2]; thin leaf = short diffusion distance [2] (max 4) [4]
3. On a bright day, photosynthesis rate > respiration rate [1]; so more CO₂ is used in photosynthesis than is produced by respiration [1]; there is net uptake of CO₂ (CO₂ enters the leaf) [1] [3]
4. The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration [2]; at this point, the gases produced by each process exactly cancel out — no net exchange of CO₂ or O₂ [1] [3]
5. A stoma is a pore in the leaf surface [1]; it is surrounded by two sausage-shaped guard cells whose shape controls whether the pore is open or closed [1] [2]
6. In light, guard cells photosynthesize, producing glucose [1]; glucose increases the solute concentration inside the guard cells, lowering their water potential [1]; water enters by osmosis from surrounding cells, making guard cells turgid [1]; the uneven thickening of guard cell walls causes them to bow outwards, opening the stoma [1] [4]
7. Any two: darkness — stomata close as there is no photosynthesis and no CO₂ needed; closing reduces water loss [2]; drought/water stress — stomata close to prevent wilting and dehydration [2] [4]
8. When stomata are open, water vapour inside the leaf diffuses out through them [1]; the greater the stomatal opening, the steeper the diffusion gradient for water vapour, increasing the rate of transpiration [1] [2]
9. CO₂ is a waste product of aerobic respiration (oxidation of glucose) [2]; O₂ is produced as a by-product of photosynthesis (splitting of water molecules) [2] [4]
10. Both CO₂ and O₂ diffuse out of leaf cells [1]; they pass through the air spaces in the spongy mesophyll and out through stomata by diffusion [1] [2]
11. The gaseous waste products are simply gases that can diffuse out through stomata [1]; no energy is required and no complex organ is needed — the concentration gradient (higher inside cells) drives them out passively [1] [2]