Charlene Sci 7.4 Gas Exchange in Plants PDF

## 7.4 Gas exchange in plants - Green plants carry out photosynthesis to make their own food. - During photosynthesis, they use carbon dioxide and produce oxygen. - They also carry out respiration to release energy in food. - During respiration, they use oxygen and produce carbon dioxide. - Green...

## 7.4 Gas exchange in plants - Green plants carry out photosynthesis to make their own food. - During photosynthesis, they use carbon dioxide and produce oxygen. - They also carry out respiration to release energy in food. - During respiration, they use oxygen and produce carbon dioxide. - Green plants take in carbon dioxide during photosynthesis and release oxygen. - Green plants take in oxygen and release carbon dioxide during respiration. ### Practical 7.9 Studying uptake or release of carbon dioxide in plants **Aim:** To find out the changes in carbon dioxide content in boiling tubes with green leaves under light and dark conditions. **Apparatus and materials per group:** - 4 boiling tubes - 4 stoppers - 1 test tube rack - 1 measuring cylinder (10 cm³) - 1 hydrogencarbonate indicator - 1 table lamp - 2 pieces of cotton thread - 4 fresh green leaves - aluminium foil **Hydrogencarbonate indicator shows different colours at different levels of carbon dioxide:** | Hydrogencarbonate Indicator | Carbon dioxide level | |---|---| | Purple | Lower than normal level | | Red | Normal level in air (about 0.04%) | | Yellow | Higher than normal level | **Procedure:** 1. Add 3 cm³ of hydrogencarbonate indicator to each of four boiling tubes (A, B, C, and D). Record the colour of the indicator in each tube. 2. Put two leaves into tubes A and C. Make sure the leaves do not touch the indicator. 3. Stopper the four tubes. Make sure there is no air leakage. 4. Wrap tubes C and D with aluminium foil. 5. Leave the four tubes in bright light for five hours. Record the colour of the indicator in each tube. **Results** | Tube | Color of hydrogencarbonate indicator at the beginning | Color of hydrogencarbonate indicator after five hours | |---|---|---| | A | Red | Purple | | B | Red | Red | | C | Red | Yellow | | D | Red | Red | **Discussion** 1. What is the purpose of setting up tubes B and D? - Tubes B and D are control set-ups. - They make sure the results in tubes A and C are caused by the presence of the green leaves. 2. Compare the results of the following tubes. What can you tell about the gas exchange of leaves? * **Tube A:** The indicator in tube A **turns purple** while the indicator in tube B **remains red**. This shows that under light conditions, the green leaves **take in** carbon dioxide. * **Tube C:** The indicator in tube C **turns yellow** while the indicator in tube D **remains red**. This shows that under dark conditions, the green leaves **give out** carbon dioxide. ### The net gas exchange in plants depends on the relative rates of photosynthesis and respiration. **Under light conditions:** - Plants carry out both photosynthesis and respiration. - The rate of photosynthesis is higher than the rate of respiration. - The plants use more carbon dioxide in photosynthesis than they produce in respiration. - As a result, there is a net uptake of carbon dioxide. - The plants produce more oxygen in photosynthesis than they use in respiration. - As a result, there is a net release of oxygen. **Under dark conditions:** - Plants carry out respiration but not photosynthesis. - There is a net release of carbon dioxide and a net uptake of oxygen. ### Gas exchange through stomata - On the leaves of most plants, there are pores called **stomata**. - They allow gases like carbon dioxide and oxygen to move into or out of the leaves. ### Practical 7.11 Observing stomata **Aim:** To observe stomata on the leaf epidermis under a microscope **Apparatus and materials per group:** - 1 Light microscope - 1 Prepared slide of leaf epidermis **Procedure:** 1. Observe a prepared slide of the leaf epidermis under a microscope at a low magnification (e.g. 100X). - After seeing a clear image, observe the slide again at a high magnification (e.g. 400X). 2. Draw a labelled diagram to show a stoma and the cells surrounding it. **Skill tips:** - When making a biological drawing of the specimen: - Use a sharp HB pencil. - Draw smooth, continuous lines. Do not sketch or shade your drawing. - Draw different parts in proportion. - Put labels on the sides using straight labelling lines. These lines must not cross one another. - Write a title for your drawing. - This should include the name of the specimen and the power of magnification. **Key point:** - Gas exchange in plants is mainly carried out through the **stomata**. ### Section summary 7.4 1. The net gas exchange in plants depends on the relative rates of photosynthesis and respiration. 2. Gas exchange in plants under light and dark conditions: | Conditions | Net gas exchange | |---|---| | Light conditions: Rate of photosynthesis > rate of respiration | - Net uptake of carbon dioxide <br> - Net release of oxygen| | Dark conditions: Respiration only | - Net uptake of oxygen <br> - Net release of carbon dioxide | 3. Gas exchange in plants is mainly carried out through the stomata. ### Section exercise 7.4 **Level 1** Questions 1 and 2: Write ‘T’ for a true statement and ‘F’ for a false statement in the boxes provided. 1. Plants do not need to take in oxygen from the atmosphere. [ ] 2. When the rate of respiration of a plant is higher than that of photosynthesis, there is a net release of carbon dioxide from the plant. [ ] **Level 2** 3. [Image of a potted plant near a window] a. State the net uptake and release of gases of the leaves of the plant at noon in a warm sunny day. Explain your answer. (3 marks) b. In plants, gas exchange mainly occurs through a structure on the leaf epidermis. Name the structure. (1 mark)