9: Nutrition and Gas Exchange in Plants

Question 1

Write the word equation showing the overall process of photosynthesis.

Answer 1

Carbon dioxide + water — light energy / chlorophyll —> carbohydrates + oxygen

Question 2

State whether plants can make proteins and lipids.

Answer 2

Yes, they can do so with the intermediates of photosynthetic reactions.

Question 3

Explain why plants are described as autotrophs.

Answer 3

They can make organic substances (eg. carbohydrates) from inorganic substances (eg. carbon dioxide and water).

Question 4

Name the mode of nutrition for plants.

Answer 4

Autotrophic nutrition

Question 5

State the importance of plants as autotrophs.

Answer 5

Plants, as autotrophs, are producers in the ecosystem. Many organisms depend on them directly or indirectly for food, and they are the basic food source for other organisms.

Question 6

Name the two groups of elements provided by minerals to plants.

Answer 6

Major elements (macronutrients) and trace elements (micronutrients)

Question 7

List 6 examples of major elements needed by plants.

Answer 7

Nitrogen, phosphorus, potassium, magnesium, sulphur, calcium

Question 8

List 3 examples of trace elements needed by plants.

Answer 8

Boron, copper, zinc

Question 9

State the forms of nitrogen in soil which is absorbed by plants.

Answer 9

Nitrate ions and ammonium ions

Question 10

State the main functions of nitrogen in plants.

Answer 10

It is used for the synthesis of amino acids, proteins, nucleic acids, and chlorophyll.

Question 11

State the deficiency diseases when a plant is provided with insufficient nitrogen.

Answer 11

Its leaves turn yellow (chlorosis) and it suffers from poor growth.

Question 12

State the forms of phosphorus in soil which is absorbed by plants.

Answer 12

Phosphate ions

Question 13

State the main functions of phosphorus in plants.

Answer 13

It is used for the synthesis of cell membranes, nucleic acids and ATP. It is also required in some enzymatic reactions.

Question 14

State the deficiency diseases when a plant is provided with insufficient phosphorus.

Answer 14

Its leaves turn purple and it suffers from poor growth with poorly developed roots.

Question 15

State the forms of potassium in soil which is absorbed by plants.

Answer 15

Potassium ions

Question 16

State the main functions of potassium in plants.

Answer 16

It promotes photosynthesis and transport in plants. It is also required in some enzymatic reactions.

Question 17

State the deficiency diseases when a plant is provided with insufficient potassium.

Answer 17

The edges of the leaves are blackened and it suffers from poor growth.

Question 18

State the forms of magnesium in soil which is absorbed by plants.

Answer 18

Magnesium ions

Question 19

State the main functions of magnesium in plants.

Answer 19

It is used for the synthesis of chlorophyll and it forms part of the chlorophyll molecules.

Question 20

State the deficiency diseases when a plant is provided with insufficient magnesium.

Answer 20

Its leaves turn yellow (chlorosis) and it suffers from poor growth.

Question 21

State an example of plant growth promoted by nitrogen.

Answer 21

Leaf development

Question 22

State an example of plant growth promoted by phosphorus.

Answer 22

Flowering

Question 23

What is the main site of gas exchange in terrestrial plants?

Answer 23

Leaves

Question 24

State the properties and functions of the epidermis of terrestrial dicotyledonous plants.

Answer 24

It is the outermost layer of cells covering the upper and lower surfaces of the leaf. It protects the inner layers of cells.
There are tiny pores called stomata in the epidermis.
Epidermal cells have no chloroplasts except the guard cells.

Question 25

State the properties and functions of the stomata of terrestrial dicotyledonous plants.

Answer 25

Each stomata is surrounded by **two guard cells**, which control the **opening and closing** of the stomata. The stomata allow gases to **diffuse into and out of** the leaf.

Question 26

Compare the number of stomata on the upper and lower epidermis of terrestrial dicotyledonous plants.

Answer 26

There are usually **more stomata** in the **lower epidermis** than in the upper epidermis.

Question 27

State the properties and functions of the cuticle of terrestrial dicotyledonous plants.

Answer 27

It is a **thin waxy layer** covering the upper and lower epidermis. It **prevents excessive water loss** by evaporation from the leaf.

Question 28

State the properties and functions of the palisade mesophyll of terrestrial dicotyledonous plants.

Answer 28

It is the internal tissue of the leaf. The cells are cylindrical, **closely packed**, and contain **many chloroplasts**. The air spaces among the cells are relatively **narrow**.

Question 29

State the properties and functions of the spongy mesophyll of terrestrial dicotyledonous plants.

Answer 29

It is the internal tissue of the leaf. The cells are irregular in shape, relatively **loosely packed**, and contain **fewer chloroplasts** when compared to palisade mesophyll cells. The air spaces among the cells are relatively **larger**.

Question 30

State two types of tissue in the vascular bundle.

Answer 30

Xylem and phloem.

Question 31

Describe the process of gases diffusing into the leaf of a terrestrial dicotyledonous plant.

Answer 31

1. Gases from the environment diffuse **into the air space** through the **stoma**. 2. Gases **dissolve** in the water film on the surfaces of the mesophyll cells and then **diffuse** into the cells. 3. Gases in the cells closer to the water film **diffuse to the neighbouring cells**.

Question 32

Describe the process of gases diffusing out from the leaf of a terrestrial dicotyledonous plant.

Answer 32

1. Gases produced by the cells further away from the water film **diffuse to the neighbouring cells** towards the air space. 2. Gases **diffuse** to the water film on the surfaces of the mesophyll cells and then **dissolve** in the water film. 3. Gases diffuse into the air space and then diffuse out of the leaf through the **stoma**.

Question 33

State 4 structural adaptations of leaves for gas exchange about the overall leaf structure.

Answer 33

Leaves are broad, flat, thin, and present in **large numbers**.

Question 34

Explain the significance of leaves being broad, flat, and large in number in a plant.

Answer 34

These provide a **large surface area** for the **diffusion of gases** from/to the surrounding air to/from the inside of the leaf through the stomata.

Question 35

Explain the significance of leaves being thin.

Answer 35

This **shortens the diffusion distance** of gases between the plant body and the atmosphere.

Question 36

State 5 structural adaptations of leaves for gas exchange about the internal leaf structure.

Answer 36

1. The spongy mesophyll cells are **loosely packed**. 2. There are numerous air spaces among spongy mesophyll cells. 3. There is a **water film** on the surfaces of the mesophyll cells. 4. There are **stomata** in the epidermis. 5. **Guard cells** are present in the epidermis to control the opening and closing of the stomata.

Question 37

Explain the significance of leaves having loosely packed spongy mesophyll cells.

Answer 37

This provides a **large surface area** for the diffusion of gases **within the leaf**.

Question 38

Explain the significance of leaves having numerous air spaces among spongy mesophyll cells.

Answer 38

This allows the gases to **diffuse freely** within the leaf.

Question 39

Explain the significance of leaves having a water film on the surface of mesophyll cells.

Answer 39

This allows gases to **dissolve** and then diffuse into or out of the cells.

Question 40

Explain the significance of leaves having stomata in the epidermis.

Answer 40

This allows gases to **diffuse** from/to the outside atmosphere to/from the inside of the leaves freely.

Question 41

Explain the significance of leaves having guard cells in the epidermis.

Answer 41

Guard cells are present to control the opening and closing of stomata. This ensures that the **rate of gas exchange** can be regulated.

Question 42

State the differences in leaf structures of submerged plants and terrestrial dicotyledonous plants.

Answer 42

The leaves of submerged plants **do not have cuticle**. They are usually very thin and have **few or no stomata** in the epidermis of both sides.

Question 43

State the main site of gas exchange in submerged plants.

Answer 43

All over the leaf surface

Question 44

State the differences in leaf structures of floating plants and terrestrial dicotyledonous plants.

Answer 44

Leaves of floating plants have stomata in the **upper epidermis only**. The lower epidermis is not covered by cuticle.

Question 45

State the main site of gas exchange in floating plants.

Answer 45

Gas exchange takes place mainly through the **stomata** in the **upper epidermis**.

Question 46

State how gas exchange takes place in the stems of herbaceous plants.

Answer 46

The stems of herbaceous plants have **stomata** for gas exchange.

Question 47

State how gas exchange takes place in the stems of woody plants.

Answer 47

The stems of woody plants are covered by a layer of **cork**, which is **impermeable** to gases. Gas exchange takes places through **lenticels**, which are special openings in the cork layer that **cannot be closed or opened**.

Question 48

State the main site of gas exchange in plant roots.

Answer 48

Since the surfaces of roots are **not covered by cuticle**, gas exchange can take place **all over the surfaces** of the roots.

Question 49

How do plants obtain energy for body activities?

Answer 49

The energy is obtained from the **breakdown of food** in cells by **respiration**.

Question 50

Compare the required conditions for photosynthesis and respiration.

Answer 50

Photosynthesis takes place **only in the presence of light** while respiration takes place **all the time**.

Question 51

Explain the net uptake and release of gases of a leaf in the daytime.

Answer 51

In the daytime, the rate of photosynthesis is **higher than that of respiration**. More carbon dioxide is uptaken for photosynthesis than it is released by respiration, and more oxygen is released by photosynthesis than it is uptaken for respiration. As a result, there is a **net uptake of carbon dioxide** and **net release of oxygen**.

Question 52

Explain the net uptake and release of gases of a leaf at night.

Answer 52

At night, photosynthesis stops and **only respiration occurs**. Oxygen is uptaken and carbon dioxide is released in respiration. As a result, there is a **net uptake of oxygen** and a **net release of carbon dioxide**.

Question 53

Describe the net exchange of carbon dioxide between a plant and the atmosphere in complete darkness.

Answer 53

In complete darkness, photosynthesis does not occur and only respiration takes place. The plant has a **net release of carbon dioxide**.

Question 54

Explain the net exchange of carbon dioxide between a plant and the atmosphere as light intensity increases from complete darkness to that of the compensation point.

Answer 54

As light intensity increases, photosynthesis starts to take place. However, its rate is still lower than that of respiration. The rate of carbon dioxide uptake for photosynthesis is **lower than** the rate of carbon dioxide release by respiration. So, there is still a **net release of carbon dioxide** from the plant.

Question 55

Explain the net exchange of carbon dioxide between a plant and the atmosphere at the compensation point.

Answer 55

At the compensation point, **the rate of photosynthesis equals the rate of respiration**. The rate of carbon dioxide uptake for photosynthesis is **equal to** the rate of carbon dioxide release by respiration. **No net exchange of carbon dioxide** occurs.

Question 56

Explain the net exchange of carbon dioxide between a plant and the atmosphere when light intensity increases from that of the compensation point to a higher intensity.

Answer 56

As light intensity keeps increasing, the rate of photosynthesis becomes higher than that of respiration. The rate of carbon dioxide uptake for photosynthesis is **higher than** the rate of carbon dioxide release by respiration. Therefore, there is a **net uptake of carbon dioxide** by the plant.

Question 57

Explain why the net uptake of carbon dioxide between a plant and the atmosphere reaches a maximum after reaching a certain light intensity.

Answer 57

There is another factor which **limits** the rate of photosynthesis. The rate of photosynthesis **reaches a maximum** and does not increase with increasing light intensity, thus the net uptake of carbon dioxide for photosynthesis also reaches a maximum.

Question 58

Describe the net exchange of oxygen between a plant and the atmosphere in complete darkness.

Answer 58

In complete darkness, photosynthesis does not occur and only respiration takes place. The plant has a **net uptake of oxygen**.

Question 59

Explain the net exchange of oxygen between a plant and the atmosphere as light intensity increases from complete darkness to that of the compensation point.

Answer 59

As light intensity increases, photosynthesis starts to take place. However, its rate is still lower than that of respiration. The rate of oxygen release by photosynthesis is **lower than** the rate of oxygen uptaken for respiration. So, there is still a **net uptake of oxygen** from the plant.

Question 60

Explain the net exchange of oxygen between a plant and the atmosphere at the compensation point.

Answer 60

At the compensation point, **the rate of photosynthesis equals the rate of respiration**. The rate of oxygen release by photosynthesis is **equal to** the rate of oxygen uptaken for respiration. **No net exchange of oxygen** occurs.

Question 61

Explain the net exchange of oxygen between a plant and the atmosphere when light intensity increases from that of the compensation point to a higher intensity.

Answer 61

As light intensity keeps increasing, the rate of photosynthesis becomes higher than that of respiration. The rate of oxygen release by photosynthesis is **higher than** the rate of oxygen uptake for respiration. Therefore, there is a **net release of oxygen** by the plant.

Question 62

Explain why the net release of oxygen reaches a maximum after reaching a certain light intensity.

Answer 62

There is another factor which **limits** the rate of photosynthesis. The rate of photosynthesis **reaches a maximum** and does not increase with increasing light intensity, thus the net release of oxygen by photosynthesis also reaches a maximum.

Question 63

The maximum and minimum rate of carbon dioxide uptake of a plant per day is -2 mg h^-1 and 6 mg h^-1. Calculate the maximum rate of photosynthesis of the plant in terms of the rate of carbon dioxide uptake.

Answer 63

6 + 2 = 8 mg h^-1

Question 64

The maximum rate of photosynthesis of a plant in a day can be calculated by its maximum and minimum rate of carbon dioxide uptake. State one assumption in this calculation.

Answer 64

It is assumed that the rate of respiration of the plant is **constant throughout the day**.

Question 65

The maximum and minimum rate of oxygen uptake of a plant per day is 4 mg h^-1 and -4 mg h^-1. Calculate the maximum rate of photosynthesis of the plant in terms of the rate of oxygen uptake.

Answer 65

4 + 4 = 8 mg h^-1