10: Transpiration, Transport and Support in Plants

Question 1

What is transpiration?

Answer 1

Transpiration is the evaporation of water stored in plants from plant surfaces, which is lost in the form of water vapour.

Question 2

Where does transpiration occur?

Answer 2

Stomata (90-95%), cuticle (5-10%), lenticels in woody plants (0.1%)

Question 3

Name 5 parts of plants containing stomata.

Answer 3

Leaves, sepals, petals, fruits, seeds

Question 4

Describe the process of transpiration through the stomata in leaves of terrestrial plants.

Answer 4

1. Water diffuses out of the mesophyll cells to form a water film on the cell surface.
2. Water of the water film evaporates to form water vapour. The water vapour diffuses into the air space among the mesophyll cells.
3. Water vapour in the air space diffuses from the air space through the stomata to the atmosphere.

Question 5

Describe the process of the creation of transpiration pull.

Answer 5

1. When water diffuses out of the mesophyll cells near the air space to form water vapour in the air space, these mesophyll cells lose water and their water potential decreases to a value which is lower than that of the neighbouring cells.
2. As a result, a water potential gradient is created among these mesophyll cells and their neighbouring cells. There is net water movement from the neighbouring cells into these cells through the differentially permeable cell membrane by osmosis.
3. The process repeats and eventually water moves out of the xylem vessels by osmosis to replace the water loss in mesophyll cells.
4. A water potential gradient is created along a chain of cells across the leaf. This causes water to flow from the xylem vessels to the mesophyll cells near the air space. The water from the xylem vessels is drawn up by transpiration pull from the roots.

Question 6

Name is the layer covering the leaf epidermis of terrestrial plants.

Answer 6

Cuticle

Question 7

State and explain three significances of transpiration.

Answer 7

1. Transpiration creates transpiration pull, which enables water and minerals to be transported from the roots to other parts of the plant.
2. Transpiration pull facilitates the absorption of water by roots.
3. During transpiration, the evaporation of water from the mesophyll cells removes heat from the leaves, providing a cooling effect to prevent plants from being overheated under hot conditions.

Question 8

Explain why plants are adapted to prevent excessive water loss by transpiration.

Answer 8

Excessive transpiration may result in dehydration and death of the plant.

Question 9

Name is the layer covering the leaf epidermis of terrestrial plants.

Answer 9

Cuticle

Question 10

State the difference in stomatal density for upper and lower epidermis of most terrestrial dicotyledonous plants.

Answer 10

The stomatal density of the lower epidermis is higher than that of the upper epidermis.

Question 11

Explain why there are only a few or no stomata on the upper epidermis of most terrestrial dicotyledonous plants.

Answer 11

As the leaves of dicotyledonous plants are usually oriented horizontally, their upper epidermis faces the sun and is hotter than the lower epidermis. The small number of stomata on the upper epidermis reduces water loss by evaporation.

Question 12

State the function of guard cells.

Answer 12

To control the opening and closing of the stomata.

Question 13

State the difference in degree of opening of the stomata in the daytime and at night.

Answer 13

In the daytime, more stomata are open to a greater extent to allow efficient gas exchange. At night, when the need of gas exchange decreases in the absence of photosynthesis, most stomata open to a lesser extent to reduce water loss from the leaves.

Question 14

Explain how guard cells control the opening and closing of the stomata.

Answer 14

Guard cells are bean-shaped with cell wall of uneven thickness. When they take up water from surrounding cells, the inner thicker layer expands to a lesser extent compared to the outer thinner layer. Thus the guard cells bend and the stoma opens. The stoma closes when the guard cells lose water and become flaccid.

Question 15

State and explain three adaptations of different xerophytes to reduce water loss.

Answer 15

1. Reduced leaves: leaves of some plants are spiny or needle-shaped to decrease the surface area of the leaves, reducing water loss.
2. Rolled leaves: the stomata inside rolled leaves are enclosed in a still, humid environment. The concentration gradient of water vapour between the air in the air space and the surrounding atmosphere is less steep, which reduces the rate of diffusion of water vapour from inside the leaf outward to the atmosphere through the stomata.
3. Sunken stomata: the sunken stomata are enclosed in a still, humid environment. The concentration gradient of water vapour between the air in the air space and the surrounding atmosphere is less steep, which reduces the rate of diffusion of water vapour from inside the leaf outward to the atmosphere through the stomata.

Question 16

State and explain the difference in stomatal density for upper and lower epidermis of most terrestrial monocotyledonous plants.

Answer 16

The stomatal density of the lower epidermis is similar to that of the upper epidermis. As the leaves are oriented vertically, both sides of the leaves receive similar sunlight and lose water at a similar rate.

Question 17

State the difference in stomatal density for upper and lower epidermis of submerged aquatic plants.

Answer 17

There are no stomata in the epidermis.

Question 18

State the difference in stomatal density for upper and lower epidermis of floating plants.

Answer 18

There are no stomata in the lower epidermis, but there are stomata on the upper epidermis.

Question 19

Name two devices which can be used to estimate the rate of transpiration of a plant.

Answer 19

Bubble potometer, weight potometer

Question 20

Describe and explain the change in rate of transpiration as light intensity increases.

Answer 20

The rate of transpiration is low in darkness. This is because in the darkness, when the need of gas exchange decreases in the absence of photosynthesis, most stomata open to a lesser extent to reduce water loss from the leaves.
As light intensity increases, the rate of transpiration increases. This is because as light intensity increases, more stomata open to a greater extent. The cross-sectional area for the diffusion of water vapour increases. As a result, water vapour diffused out of the leaves at a higher rate through the stomata.
The rate of transpiration levels off at high light intensities.

Question 21

Describe and explain the change in rate of transpiration as temperature increases.

Answer 21

As temperature increases, the rate of transpiration increases. This is because as temperature increases, the rate of evaporation of water from the surfaces of mesophyll cells increases. This steepens the concentration gradient of water vapour between the air space in the leaves and the surrounding air. As a result, water vapour diffuses more rapidly through the stomata.

Question 22

Describe the change in rate of transpiration as wind speed increases.

Answer 22

As wind speed increases, the rate of transpiration increases initially. After the rate of transpiration reaches a maximum value, as wind speed increases further, the rate of transpiration decreases.

Question 23

Compare and explain the rate of transpiration in still air and that in gentle wind.

Answer 23

The rate of transpiration is relatively low in still air compared to that in gentle wind.
In gentle wind, the water vapour that has diffused out of the leaves are blown away. However, in still air, water vapour that has diffused out of the leaves accumulates around the stomata. This causes the concentration gradient of water vapour between the air space in the leaves and the surrounding air to become less steep compared to that in gentle wind. Therefore, the rate of diffusion of water vapour out of the leaves is lower in still air compared to gentle wind.

Question 24

Describe and explain the rate of transpiration when the surrounding wind speed increases from zero to gentle wind.

Answer 24

The rate of transpiration increases when the wind speed increases.
The gentle wind blows away the water vapour that has diffused out of the leaves to the air around the stomata. This causes the concentration gradient of water vapour between the air space in the leaves and the surrounding air to be maintained steeper compared to that in still air. As a result, water vapour diffuses more rapidly through the stomata.

Question 25

Explain the rate of transpiration of plants in very strong wind.

Answer 25

The rate of transpiration **decreases** when the wind becomes too strong. Strong winds cause **more stomata to open to a lesser extent** to prevent excessive and rapid water loss of the plant by transpiration. The cross-sectional area for the diffusion of water vapour decreases. As a result, water vapour diffused out of the leaves at a lower rate through the stomata.

Question 26

Describe and explain the change in rate of transpiration as relative humidity of the surrounding air increases.

Answer 26

As the relatively humidity of the surrounding air increases, the rate of transpiration decreases. Since the air space inside the leaves is **saturated with water vapour**, an increase in relative humidity causes the **concentration gradient of water vapour** between the air space and the surrounding air to be **less steep**. Therefore, less water vapour in the air space diffuses out to the atmosphere through the stomata per unit time and the transpiration rate decreases.

Question 27

State 3 factors, other than environmental factors, that affect the rate of transpiration of a plant.

Answer 27

Total surface area of leaves, thickness of cuticle, abundance of stomata.

Question 28

State the difference between the net amount of water lost and the net amount of water absorbed by a healthy plant in 24 hours, and explain the significance of this observation.

Answer 28

The net amount of water absorbed should be more than the net amount of water lost, indicating the plant has a net uptake of water. It can be inferred that some water is retained for the plant, and it is likely being used for photosynthesis, formation of new cells, or other **metabolic activities**.

Question 29

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

Answer 29

It is the outermost layer of the root. It is **not covered by cuticle**. It is made up of a layer of **closely-packed thin-walled cells**. It protects the inner tissues from injury and infections. Some epidermal cells have outgrowths called **root hairs**.

Question 30

State the significance of root epidermal cells not being covered by cuticle.

Answer 30

This ensures the absorption of water and minerals is not blocked by the cuticle.

Question 31

State the properties and functions of the cortex in the roots of the dicotyledonous plants.

Answer 31

It is the region beneath the epidermis. It is made up of layers of **loose-packed thin-walled cells**. It stores food in the form of **starch**. It allows the passage of **water and minerals** across the root.

Question 32

In which form is food mainly stored in the cortex of roots of dicotyledonous plants?

Answer 32

Starch

Question 33

State the position of the vascular bundle in roots of dicotyledonous plants.

Answer 33

It is located in the centre of the root.

Question 34

Which vascular tissue is located in the centre of the roots of dicotyledonous plants?

Answer 34

Xylem

Question 35

State the properties and functions of the root cap in roots of dicotyledonous plants.

Answer 35

It covers and protects the root tip. Its cells are constantly worn away when the root grows through the soil. They are replaced by new cells produced in the root tip.

Question 36

State the significance of growing root hairs on roots of dicotyledonous plants.

Answer 36

This provides a large surface area for absorption of water and minerals from the soil.

Question 37

Describe the process of absorption of water by the roots of a dicotyledonous plant.

Answer 37

1. Water is lost from the leaves continuously by **transpiration**, creating **transpiration pull**. 2. Water is **drawn up the xylem vessels** from the roots to the leaves by **transpiration pull**. 3. Water in the cortex cells near the xylem vessels of the roots enters the xylem vessels. This decreases the water potential of these cortex cells. A **water potential gradient** is set up across the whole cortex. 4. There is net movement of water from the root hair cells to the cortex cells closest to the xylem vessels. 5. As water is drawn away from the root hair cells to the neighbouring cortical cells, the water potential of the root hair cells **decreases** to a value **lower than soil water**. Then, water in the soil enters the root hair cells by osmosis.

Question 38

State the major way in which water in the soil enters root hair soils of roots of dicotyledonous plants.

Answer 38

Osmosis

Question 39

Describe three ways in which water flows from cell to cell in the cortex.

Answer 39

1. Water moves along a water potential gradient through the **cytoplasm** of the cells by **osmosis**. 2. Water moves along the same water potential gradient through the **vacuoles** of the cells by **osmosis**. 3. Water travels from one cell to another freely through the **fully permeable cell wall**.

Question 40

State the major way in which water flows from cell to cell in the cortex.

Answer 40

Water travels from one cell to another freely through the **fully permeable cell wall**.

Question 41

State the major way in which water in the soil enters root hair soils of roots of dicotyledonous plants.

Answer 41

Osmosis

Question 42

State the major way in which minerals in the soil enters root hair soils of roots of dicotyledonous plants.

Answer 42

Active transport

Question 43

State a possible way in which minerals in the soil enters root hair soils of roots of dicotyledonous plants other than active transport.

Answer 43

Diffusion

Question 44

Explain how active absorption of minerals by root hair cells facilitate the water absorption in roots.

Answer 44

Active absorption of minerals by root hair cells **lowers the water potential gradient** of the cells. This **steepens the water potential gradient** between the root hair cells and the soil water, facilitating the movement of water from the soil into the root hair cells by osmosis.

Question 45

Explain why a plant may wilt if too much fertiliser is applied to it.

Answer 45

If too much fertiliser is added to soil, the mineral concentration in the soil is **greatly increased**, hence the water potential of the soil is **greatly decreased**. The **water potential gradient** between the root hair cells and the soil water becomes **less steep**, thus there is little or no net movement of water into the root hair cells by osmosis. As the **rate of water absorption becomes lower than the rate of water loss by transpiration**, the plant experiences a **net loss of water** and wilts.

Question 46

State 4 adaptive features of roots of dicotyledonous plants for absorption of water and minerals.

Answer 46

1. Highly branched root with numerous root hairs 2. Root hairs are long and fine 3. Epidermis of root is not covered by cuticle 4. Root hair cells contain many mitochondria

Question 47

Explain the significance for dicotyledonous plants to have highly branched roots with numerous root hairs on the root.

Answer 47

This provides a large surface area for absorption of water and minerals.

Question 48

Explain the significance for dicotyledonous plants to have long and fine root hairs.

Answer 48

The root hairs can easily grow between soil particles to absorb water and minerals; it can also provide good anchorage and support to the plant.

Question 49

Explain the significance for the roots of dicotyledonous plants to have epidermis not covered by cuticle.

Answer 49

This allows water to and minerals to pass through the epidermis into the root easily.

Question 50

Explain the significance for root hair cells of dicotyledonous plants to have many mitochondria.

Answer 50

This ensures that enough energy is supplied to absorb minerals from the soil by active transport.

Question 51

State the functions of xylem in flowering plants.

Answer 51

It transports water and minerals, and provides support for plants.

Question 52

State and explain the adaptive features of xylem.

Answer 52

1. It mainly consists of **hollow** xylem vessels: It allows the passage of water with **little resistance**. 2. The walls of the xylem vessels are **thick and lignified**: It prevents the vessels from collapsing under the great tension of transpiration pull.

Question 53

State whether the cells making up xylem vessels are living.

Answer 53

Immature xylem vessels consist of living cells, but mature xylem vessels consist of dead cells only.

Question 54

State the functions of phloem in flowering plants.

Answer 54

It transports organic nutrients, mostly sucrose.

Question 55

State two types of cells which the phloem consists of.

Answer 55

Sieve tubes and companion cells.

Question 56

State and explain three adaptive features of the phloem.

Answer 56

1. Sieve tube contains little cytoplasm and no nucleus: This allows organic nutrients to move along the sieve tube with **little resistance**. 2. Sieve plate has **pores**: This allows organic nutrients to pass through. 3. Companion cell has **dense cytoplasm** and **many organelles**: This supports the metabolism of the sieve tube.

Question 57

Describe the distribution of vascular tissues in the leaf of a young dicotyledonous plant.

Answer 57

Vascular bundles are found in the large central **midrib** and the network of small **veins**. Xylem lies on top of phloem.

Question 58

Describe the distribution of vascular tissues in the stem of a young dicotyledonous plant.

Answer 58

Vascular bundles are arranged in a **ring** at the **periphery**. In each bundle, xylem is located in the **inner** region and phloem the outer region.

Question 59

Describe the distribution of vascular tissues in the root of a young dicotyledonous plant.

Answer 59

Vascular bundles are locate at the **centre**. Phloem is found between the "arms" of xylem.

Question 60

State the definition of translocation.

Answer 60

Translocation is the transport of **organic nutrients** in plants.

Question 61

Name three regions where organic nutrients of plants are translocated to.

Answer 61

Organic nutrients are transported **from leaves**, to 1. Other parts of the plants for direct use (eg. respiration) 2. Growing region (eg. buds and shoot tips) 3. Storage organs (eg. fruits and vegetative organs)

Question 62

Compare the energy requirement for transport of water and translocation in plants.

Answer 62

Transport of water is a **passive** process which does not require energy, while translocation is an **active** process which requires energy.

Question 63

State 3 significances for terrestrial plants to stand upright and stretch our their branches.

Answer 63

1. It enables the leaves to be held at the best position to **receive the maximum amount of sunlight** for photosynthesis. 2. It enables the flowers to be lifted up, facilitating **pollination**. 3. It enables the **fruits** and **seeds** to be lifted up, facilitating their **dispersal**.

Question 64

Name two types of support provided by cells in plants.

Answer 64

Turgidity of thin-walled cells and rigidity of thick-walled cells

Question 65

List two examples of plant parts which have a large proportion of thin-walled cells.

Answer 65

**non-woody** plant parts like leaves and herbaceous stems.

Question 66

Explain how thin-walled cells support the plant.

Answer 66

**When water supply is adequate**, thin-walled cells in the stems and leaves gain water by **osmosis**. They become **turgid** and press against each other. The turgidity of the cells makes the whole stem strong enough to stand upright.

Question 67

Explain why a herbaceous plant wilts when water supply is inadequate.

Answer 67

When water supply is inadequate, thin-walled ells in the stems and leaves **lose water** as the rate of transpiration is higher than the rate of water uptake. The cells become **flaccid** and can no longer support the plant. The plant **wilts**.

Question 68

Name two types of thick-walled cells in plants.

Answer 68

Mature **xylem** cells, and mature **sclerenchyma** cells.

Question 69

Explain how thick-walled cells support the plant.

Answer 69

The cell walls of thick-walled cells contain **lignin**, which makes the cells **hard and rigid**. The rigidity of these cells help support the plant, and still provide support **even when water is temporarily inadequate**.

Question 70

Explain how wood is formed.

Answer 70

In woody stems, more xylem is formed when the plant grows. The older xylem is pushed inwards by the newly formed xylem, forming **annual rings**. More lignin is deposited in the cell walls of the older xylem, which finally becomes **wood**.

Question 71

Explain the significance of xylem vessels being in the centre of the root in relation to root anchorage.

Answer 71

Xylem vessels being in the centre of the root **increases the tensile strength** of the root to resist the uprooting force produced by the pulling effect of wind.