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2.7Structures and Functions in Living Organisms

Gas Exchange — Edexcel Linear - OL Revision Notes & Topical Questions

15 min read 22 parts Lesson 22 of 25 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

Gas Exchange in Plants — EDEXCEL LINEAR - OL notes: Explain how gases are exchanged by diffusion in plants, how leaf structure and stomata are adapted, and how gas exchange differs by day and night.Particles in liquids and gases are in constant motion.

  • guard cells
  • compensation point
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4

Part 1 of 5Diffusion in gas exchange

Particles in liquids and gases are in constant motion.

Diffusion in gas exchange
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.
Single-celled organisms such as amoeba exchange gases directly by diffusion across the cell membrane, because their SA:V ratio is large.
Multicellular organisms such as plants and animals have a smaller SA:V ratio and so have specialised exchange surfaces: leaves with stomata in plants,
  • and lungs with alveoli in animals.

Part 2 of 5Leaf structure and the pathway of gases

The leaf is adapted both for photosynthesis and for gas exchange: it is flat and thin, giving a large surface area and short diffusion distance; the spongy mesophyll has air spaces for gases to move; and the lower epidermis contains stomata.

Pathway of carbon dioxide in the day: atmosphere → through the open stomata → air spaces around the spongy mesophyll → dissolves in the film of water on the mesophyll cell walls → into mesophyll cells → chloroplasts, where it is used in photosynthesis.

Leaf structure and the pathway of gases
Oxygen follows the reverse path, diffusing from the cells to the air spaces and out through the stomata,
  • because its concentration is higher inside the leaf than outside.
Some of the oxygen made is used in respiration.

Stomata are pores between two guard cells, mainly on the lower epidermis.

Leaf structure and the pathway of gases
The guard cells control the opening and closing of the stomata.
When water enters the guard cells, they become turgid and bend apart, opening the pore; when they lose water they become flaccid and close it.
Stomata open in daylight and close at night, reducing water loss when no photosynthesis is happening.

Part 3 of 5Gas exchange: night versus day

Plants photosynthesise only with sufficient light, but their cells respire all the time.

Day (bright light)Night
PhotosynthesisYes, faster than respirationNo
RespirationYesYes
Gas taken inCarbon dioxide (net)Oxygen
Gas given outOxygen (net)Carbon dioxide

At the compensation point, at dawn and dusk, photosynthesis and respiration rates are equal so there is no net gas exchange. During the day plants photosynthesise and respire, but the rate of photosynthesis is much greater, so there is a net uptake of CO2 and release of O2. At night only respiration occurs.

Part 4 of 5Why plants need gas exchange and how water loss is limited

Plants need carbon dioxide for photosynthesis and oxygen for respiration, and both must diffuse in and out through the same pores, the stomata.

1
This creates a problem: when stomata are open, water vapour also diffuses out by transpiration.
2
Plants solve it by having the guard cells close the stomata when water is short or in the dark, by having a waxy cuticle that is waterproof,
  • and by placing most stomata on the lower surface where it is cooler and less exposed to wind and sunlight.
3
Plants living in dry places have extra adaptations such as sunken stomata and rolled leaves.
4
Gas exchange in the root cells also occurs by diffusion from air spaces in the soil,
  • which is why waterlogged soil, with little oxygen, can kill roots.

Exam tip

  • when asked how a plant is adapted for gas exchange, give the structure and its function, such as stomata allowing carbon dioxide in, air spaces giving a short diffusion path through the leaf, and thin flat leaves giving a large surface area.
  • For night versus day, state which process is happening and the net gas movement.

Part 5 of 5Worked examples

  1. Trace the path of carbon dioxide from the air to a chloroplast. [4]4 marks

  2. Explain how guard cells open the stomata. [3]3 marks

  3. State the net gas exchange at night. [2]2 marks

  4. Why are most stomata on the lower surface? [1]1 mark

  5. What is the compensation point? [1]1 mark

Key terms

TermMeaning
StomaA pore in the leaf epidermis for gas exchange (plural stomata).
Guard cellOne of two cells that control the opening of a stoma.
Spongy mesophyllLeaf layer with air spaces for gas diffusion.
Compensation pointLight level at which photosynthesis equals respiration.
Gas exchangeMovement of gases across an exchange surface.

Avoid these mistakes

1
Saying plants respire only at night.
2
Saying stomata open at night in most plants.
3
Saying carbon dioxide diffuses out of the leaf all the time.
4
Saying oxygen is only a waste product, ignoring its use in respiration.

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