Gas exchange in plants

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Lesson notes

Overview of Gas Exchange in Plants

Plants exchange gases with the environment to survive.
**Photosynthesis**: takes in **carbon dioxide** and releases **oxygen**.
**Respiration**: takes in **oxygen** and releases **carbon dioxide**.
The leaf is the main organ for gas exchange.

Stomata

**Stomata** are small openings mainly on the **underside** of leaves.
Each stoma is surrounded by two **guard cells** that control its opening and closing.
When open, **carbon dioxide enters** and **oxygen and water vapour exit**.
Stomata allow gas exchange while regulating water loss.

Guard Cells

**Guard cells** swell and open stomata when the plant has plenty of water.
When water is scarce, guard cells shrink and close stomata to prevent water loss.
They control the balance between gas exchange and water conservation.

Spongy Mesophyll

The **spongy mesophyll** layer has loosely arranged cells with **air spaces**.
Air spaces facilitate the movement of gases through the leaf.
Carbon dioxide diffuses through air spaces to reach palisade cells for photosynthesis.

Palisade Mesophyll

The **palisade mesophyll** layer consists of **tightly packed cells**.
These cells are rich in **chloroplasts**, the site of photosynthesis.
The arrangement maximises light absorption for photosynthesis.

Waxy Cuticle

The **waxy cuticle** is a thin, protective layer covering the leaf surface.
It prevents excessive water loss by reducing evaporation.
It allows light to penetrate for photosynthesis.

Cross-section of a leaf showing cuticle, epidermis, palisade and spongy mesophyll, air spaces, stoma and guard cells.

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Practice questions

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1.Where are stomata mainly located on a leaf?
Easy
- AUnderside of the leaf
- BUpper side of the leaf
- CBoth sides equally
- DOnly on the leaf edges
2.Which cells control the opening and closing of stomata?
Easy
- AGuard cells
- BPalisade cells
- CSpongy mesophyll cells
- DEpidermal cells
3.During photosynthesis, plants take in which gas?
Easy
- ACarbon dioxide
- BOxygen
- CNitrogen
- DWater vapour
4.Which layer of the leaf contains loosely arranged cells with air spaces?
Easy
- ASpongy mesophyll
- BPalisade mesophyll
- CWaxy cuticle
- DEpidermis
5.What is the main function of the waxy cuticle on a leaf?
Easy
- APrevent excessive water loss
- BAbsorb carbon dioxide
- CCarry out photosynthesis
- DOpen and close stomata
6.When water is scarce, what happens to the stomata?
Medium
- AThey close to prevent water loss
- BThey open wider to absorb more carbon dioxide
- CThey remain unchanged
- DThey move to the upper leaf surface
7.Which statement about gas exchange in plants is correct?
Medium
- APlants take in oxygen and release carbon dioxide only during respiration
- BPlants take in carbon dioxide and release oxygen only during photosynthesis
- CPlants take in oxygen and release carbon dioxide only during photosynthesis
- DPlants take in carbon dioxide and release oxygen only during respiration
8.Why are palisade mesophyll cells well suited for photosynthesis?
Medium
- AThey are tightly packed and rich in chloroplasts
- BThey contain large air spaces
- CThey are located on the leaf underside
- DThey have a waxy coating

More questions

9.What happens to guard cells when the plant has plenty of water?
Medium
- AThey swell and open the stomata
- BThey shrink and close the stomata
- CThey become rigid and close the stomata
- DThey disintegrate
10.Which of the following statements about the waxy cuticle is false?
Hard
- AIt prevents excessive water loss
- BIt allows light to penetrate
- CIt is a thin, protective layer
- DIt is the main site of photosynthesis
11.In the leaf cross-section, which structure is labelled with the number 1?
Hard
- ACuticle
- BUpper epidermis
- CPalisade mesophyll
- DSpongy mesophyll
12.The air spaces in the spongy mesophyll are important because they:
Hard
- AAllow carbon dioxide to diffuse to palisade cells
- BStore water for the plant
- CPrevent water loss
- DProduce chloroplasts
13.Which gas is taken in by a plant during photosynthesis?
Easy
- ACarbon dioxide
- BOxygen
- CNitrogen
- DWater vapour
14.Stomata are mainly located on which part of the leaf?
Easy
- AUpper surface
- BLower surface
- CBoth surfaces equally
- DLeaf edge
15.A plant is placed in a sealed container with a sensor that measures carbon dioxide concentration. The container is kept in bright light for 2 hours. Which graph shows the expected change in carbon dioxide concentration?
Medium
- AGradual increase
- BGradual decrease
- CNo change
- DIncrease then decrease
16.A student counts the number of stomata per mm² on the upper and lower surfaces of a leaf. The results show 120 stomata per mm² on the lower surface and 20 per mm² on the upper surface. What is the ratio of stomata on the lower surface to the upper surface?
Hard
- A6:1
- B1:6
- C120:20
- D20:120
17.In an experiment, a leaf is placed in a solution and air is bubbled through it. The bubbles are collected and tested. The gas collected turns a glowing splint brighter. Which process is occurring?
Medium
- APhotosynthesis
- BRespiration
- CTranspiration
- DEvaporation
18.Which of the following leaf structures is primarily responsible for preventing water loss?
Medium
- AWaxy cuticle
- BStomata
- CSpongy mesophyll
- DPalisade mesophyll
19.Guard cells control the opening and closing of which structure?
Easy
- AStomata
- BCuticle
- CAir spaces
- DChloroplasts
20.A plant is kept in a dark room for 24 hours and then placed in bright light. Which gas will the plant release into the air after 1 hour in bright light?
Hard
- AOxygen
- BCarbon dioxide
- CNitrogen
- DWater vapour
21.The diagram shows a cross-section of a leaf. Which layer contains air spaces that help gases move?
Medium
- ASpongy mesophyll
- BPalisade mesophyll
- CUpper epidermis
- DLower epidermis
22.In the leaf cross-section diagram, which labelled part is responsible for photosynthesis?
Medium
- APalisade mesophyll
- BSpongy mesophyll
- CCuticle
- DStomata
23.A student measures the rate of oxygen production from a plant at different light intensities. The results show that oxygen production increases with light intensity up to a point, then levels off. What is the most likely reason for the levelling off?
Hard
- AAnother factor (e.g. carbon dioxide) becomes limiting
- BThe plant becomes saturated with oxygen
- CThe stomata close completely
- DRespiration stops
24.Which of the following is NOT a gas exchanged by plants?
Medium
- ANitrogen
- BOxygen
- CCarbon dioxide
- DWater vapour
25.In an experiment to measure the rate of photosynthesis, a student places a pondweed in a beaker of water and counts the number of bubbles produced per minute. Which variable should be kept constant to ensure a fair test?
Medium
- ADistance of the lamp from the pondweed
- BNumber of bubbles per minute
- CTemperature of the water
- DSize of the beaker
26.A student uses a potometer to measure the rate of water uptake by a leafy shoot. The shoot is placed in bright light. After 30 minutes, the water level in the capillary tube has moved 5 mm. The student then covers the leaves with petroleum jelly and repeats the measurement. What would be the most likely result?
Hard
- AThe water level moves faster because the jelly prevents water loss.
- BThe water level moves slower because the stomata are blocked.
- CThe water level stays the same because the plant stops respiring.
- DThe water level moves faster because the leaves are protected.
27.A student sets up an experiment with two sealed glass jars, each containing a plant. Jar A is placed in bright light. Jar B is placed in darkness. After 24 hours, the air in each jar is tested for carbon dioxide concentration. Which statement correctly predicts the results?
Hard
- AJar A has more carbon dioxide than Jar B because photosynthesis produces CO2.
- BJar B has more carbon dioxide than Jar A because respiration releases CO2 and no photosynthesis removes it.
- CBoth jars have the same carbon dioxide concentration because plants respire at the same rate in light and dark.
- DJar A has more carbon dioxide than Jar B because respiration is faster in light.
28.Which of the following is the correct order of structures that carbon dioxide passes through when entering a leaf?
Medium
- AStoma → spongy mesophyll air space → palisade mesophyll cell
- BPalisade mesophyll cell → spongy mesophyll air space → stoma
- CStoma → palisade mesophyll cell → spongy mesophyll air space
- DSpongy mesophyll air space → stoma → palisade mesophyll cell
29.A leaf is placed in a solution and exposed to light. Bubbles of gas are produced. When a glowing splint is inserted into the gas, it relights. What is the gas?
Medium
- ACarbon dioxide
- BOxygen
- CNitrogen
- DWater vapour
30.The diagram shows a cross-section of a leaf. Which labelled part is the site of the majority of photosynthesis?
Hard
- AA (upper epidermis)
- BB (palisade mesophyll)
- CC (spongy mesophyll)
- DD (lower epidermis)
31.Which of the following is a function of the waxy cuticle on a leaf?
Easy
- ATo absorb carbon dioxide
- BTo prevent water loss
- CTo carry out photosynthesis
- DTo open and close stomata
32.In an experiment, a student places a leaf in a sealed flask with a carbon dioxide sensor. The flask is kept in bright light. Which graph shows the expected change in carbon dioxide concentration over time?
Medium
- AA line increasing steadily
- BA line decreasing steadily
- CA line staying constant
- DA line increasing then decreasing
33.A student investigates the effect of light intensity on the rate of photosynthesis using an aquatic plant. They count the number of bubbles released per minute at different distances from a lamp. The results are shown in the table. What conclusion can be drawn?
Hard
- AThe rate of photosynthesis is highest at 50 cm.
- BThe rate of photosynthesis increases as light intensity increases.
- CThe rate of photosynthesis is not affected by light intensity.
- DThe rate of photosynthesis decreases as light intensity increases.
34.Which gas is released by plants during respiration?
Easy
- AOxygen
- BCarbon dioxide
- CNitrogen
- DHydrogen
35.Guard cells control the opening and closing of stomata. When do guard cells cause stomata to close?
Medium
- AWhen the plant has plenty of water
- BWhen the plant is in bright light
- CWhen the plant is short of water
- DWhen the plant is photosynthesising rapidly
36.A student wants to compare the number of stomata on the upper and lower surfaces of a leaf. Which piece of apparatus is most suitable for obtaining a clear view of the stomata?
Hard
- AHand lens
- BLight microscope
- CRuler
- DBalance
37.Which of the following statements about gas exchange in plants is correct?
Easy
- ADuring photosynthesis, plants take in oxygen and release carbon dioxide.
- BDuring respiration, plants take in carbon dioxide and release oxygen.
- CDuring photosynthesis, plants take in carbon dioxide and release oxygen.
- DPlants do not respire; only animals respire.
38.Stomata are small openings mainly located on which part of the leaf?
Easy
- AUpper surface only
- BLower surface mainly
- CBoth surfaces equally
- DOnly on the leaf edges
39.Which layer of the leaf contains loosely arranged cells with air spaces that help gases move?
Medium
- APalisade mesophyll
- BSpongy mesophyll
- CWaxy cuticle
- DEpidermis
40.Which of the following statements is false?
Hard
- APalisade mesophyll cells contain many chloroplasts for photosynthesis.
- BSpongy mesophyll cells are tightly packed with no air spaces.
- CGuard cells are responsible for opening and closing stomata.
- DThe waxy cuticle helps reduce water loss from the leaf.

Revision notes

## Gas Exchange in Plants - Plants exchange gases for **photosynthesis** (CO₂ in, O₂ out) and **respiration** (O₂ in, CO₂ out). ### Key Leaf Structures - **Stomata**: Small openings, mainly on leaf underside. Each stoma is surrounded by two **guard cells** that control opening/closing. Open: CO₂ enters, O₂ and water vapour exit. - **Guard cells**: Swell when water is plentiful → open stomata for gas exchange; shrink when water is scarce → close stomata to prevent water loss. - **Spongy mesophyll**: Loosely arranged cells with air spaces → allow CO₂ to diffuse to palisade cells for photosynthesis. - **Palisade mesophyll**: Tightly packed cells rich in **chloroplasts** (site of photosynthesis). - **Waxy cuticle**: Thin protective layer on leaf surface; prevents excessive water loss by reducing evaporation, while allowing light to penetrate.

Stomata: Small openings mainly on the underside of leaves that allow gas exchange (CO₂ in, O₂ and water vapour out).
Guard cells: Cells surrounding each stoma that control its opening and closing; swell when water is plentiful to open stomata, shrink when water is scarce to close them.
Spongy mesophyll: Layer of loosely arranged cells with air spaces that facilitate gas movement, allowing CO₂ to diffuse to palisade cells.
Palisade mesophyll: Layer of tightly packed cells rich in chloroplasts, where photosynthesis occurs.
Waxy cuticle: Thin protective layer on leaf surface that prevents excessive water loss by reducing evaporation while allowing light to penetrate.

Flashcards

What are the two main gases exchanged in plants during photosynthesis?

Carbon dioxide is taken in and oxygen is released.

What are the two main gases exchanged in plants during respiration?

Oxygen is taken in and carbon dioxide is released.

What are stomata?

Stomata are small openings mainly located on the underside of leaves.

What controls the opening and closing of stomata?

Guard cells control the opening and closing of stomata.

When do guard cells open the stomata?

When the plant has plenty of water, guard cells swell and open the stomata.

When do guard cells close the stomata?

When water is scarce, guard cells shrink and close the stomata to prevent water loss.

What is the function of the spongy mesophyll layer?

It contains loosely arranged cells with air spaces that facilitate gas movement, allowing carbon dioxide to diffuse to palisade cells for photosynthesis.

What is the palisade mesophyll layer?

It is a layer of tightly packed cells rich in chloroplasts, the site of photosynthesis.

What is the waxy cuticle?

It is a thin, protective layer covering the leaf surface that prevents excessive water loss by reducing evaporation while allowing light to penetrate.

True or false: Stomata are mainly located on the upper side of leaves.

False. They are mainly located on the underside of leaves.

True or false: The spongy mesophyll layer is tightly packed with cells rich in chloroplasts.

False. The spongy mesophyll has loosely arranged cells with air spaces; the palisade mesophyll is tightly packed with chloroplasts.

True or false: Guard cells open the stomata when the plant has plenty of water.

True.

True or false: Photosynthesis occurs in the palisade cells.

True.

True or false: During photosynthesis, oxygen is taken in and carbon dioxide is released.

False. During photosynthesis, carbon dioxide is taken in and oxygen is released.

True or false: Only animals undergo respiration; plants do not respire.

False. Plants also respire, taking in oxygen and releasing carbon dioxide.

Describe the role of guard cells in gas exchange.

Guard cells control the opening and closing of stomata. When open, carbon dioxide enters and oxygen and water vapour exit; when closed, water loss is prevented.

Describe the function of the spongy mesophyll layer.

It has loosely arranged cells with air spaces that allow gases to move easily, enabling carbon dioxide to diffuse to palisade cells for photosynthesis.

How is the palisade mesophyll layer suited for photosynthesis?

It consists of tightly packed cells rich in chloroplasts, which are the sites of photosynthesis.

Describe the role of the waxy cuticle.

It is a protective layer that reduces water loss by evaporation while still allowing light to penetrate the leaf.

What exits the leaf through open stomata besides oxygen?

Water vapour also exits through open stomata.

What enters the leaf through open stomata?

Carbon dioxide enters the leaf through open stomata.

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