Transport of Water in Plants (Chapter 7)

Question 1

Why do transport systems needed in plants?

Answer 1

1) to move substances from where they are absorbed to where they are needed e.g. water and mineral ions from roots to other parts of the plant
2) to move substances from where they are produced to where they are needed for metabolism e.g. glucose from leaves to all parts of the plant
3) to move substances to different parts of the plant for storage

Question 2

Why are transport systems not needed to transport CO2 and O2?

Answer 2

Because it is relatively easy for CO2 and O2 to diffuse into and out of the bodies of plants, reaching and leaving every cell quickly enough down diffusion gradients

Question 3

For transport of what substances are transport systems needed?

Answer 3

Distribution of water, inorganic and organic nutrients, and plant hormones

Question 4

What are the two transport systems in plants?

Answer 4

1) xylem

2) phloem

Question 5

Describe the characteristics of the xylem

Answer 5

• Carries mainly water and inorganic ions from roots to above ground
• The xylem sap can only move in one direction

Question 6

Describe the characteristics of the phloem

Answer 6

• Carries substances made by photosynthesis from to leaves to other areas of the plant
• Phloem sap can be moving in different directions in different parts of the plant

Question 7

What are the 3 main organs involved in plant transport?

Answer 7

Stems, roots and leaves

Question 8

What are stem, roots and leaves composed of?

Answer 8

More than one tissue, each a collection of cells specialised for a particular function

Question 9

What are the two types of angiosperms (flowering plants)?

Answer 9

Monocots and dicots

Question 10

What are the leaves of monocotyledons like?

Answer 10

Long and narrow e.g. grass

Question 11

What do the leaves of dicotyledons typically have?

Answer 11

Blades and stalks

Question 12

What is the difference between monocots and dicots?

Answer 12

There are differences in the distribution of xylem and phloem in their roots, stems and leaves

Question 13

What is xylem made up of?

Answer 13

Tubes (vessels) made from dead cells (xylem vessel elements)

Question 14

Describe the walls of xylem vessel elements

Answer 14

Reinforced with a strong, waterproof material called lignin

Question 15

What is the function of xylem?

Answer 15

1) long distance transport of water and mineral salts

2) provides mechanical support and strength

Question 16

Describe the structure of vascular tissue in roots

Answer 16

Xylem is at the centre with a series of ‘arms’ between which phloem is found

Question 17

Describe the structure of vascular tissue in stems

Answer 17

The xylem and phloem are found in vascular bundles around the pith (xylem on the inside, phloem on the outside

Question 18

Describe the structure of vascular tissue in leaves

Answer 18

The xylem and phloem are around in vascular bundles linearly (xylem at the top, phloem at the bottom

Question 19

What do the outside of vascular bundles have?

Answer 19

Caps made of sclerenchyma fibres which provide extra support for the stem

Question 20

What are sclerenchyma fibres?

Answer 20

Long, dead, empty cells with lignified walls to provide support (do not transport water)

Question 21

Why is the distribution of xylem and sclerenchyma different in roots and stems?

Answer 21

Because the organs are subjected to different stresses and strains:

• stems need to be supported in air
• roots are usually spreading through soil and subject to pulling strains from parts above the ground

Question 22

How is wood formed in trees and shrubs?

Answer 22

Extra xylem is made in roots and stems

Question 23

Describe the pathway of water as it is transported through a plant

Answer 23

1) transpiration of water vapour through open stomata into the air (mainly from the underside of the leaf)
2) evaporation of water into leaf air spaces
3) water moves from xylem into leaf cells
4) water moves up xylem
5) water enters xylem
6) water uptake near root tips

Question 24

How is the movement of water passive?

Answer 24

Because it is driven by evaporation in the leaves

Question 25

Describe how water moves from roots to leaves

Answer 25

1) the energy of the sun causes water to evaporate from the leaves (transpiration)
2) this reduces the water potential in the leaves and sets up a water potential gradient
3) water moves down this gradient from soil into the plant e.g. through its roots hairs
4) water then moves across the root into the xylem tissue in the centre
5) once inside the xylem vessels, the water moves upwards through the root to the stem and from there into the leaves

Question 26

What is transpiration?

Answer 26

• The loss of water vapour from a plant to its environment, by diffusion down a water potential gradient
• It is an inevitable consequence of gas exchange in plants

Question 27

Where does most transpiration take place?

Answer 27

Through stomata in leaves

Question 28

Why is the air inside the leaf usually saturated with water vapour?

Answer 28

Because the walls of the mesophyll cells are wet and some of this water evaporates into the air spaces

Question 29

When will water vapour diffuse out of the leaf?

Answer 29

When there is a higher water potential inside the leaf than outside the lead and ∴ a water potential gradient

Question 30

Describe how water movement in a mesophyll cell leads to water moving up the xylem vessel

Answer 30

1) water vapour diffuses from an air space through an open stomata (transpiration) and is carried away from the lead surface by air movements, reducing the WP in the leaf
2) water evaporates from a mesophyll cell wall into the air space
3) water moves out of the mesophyll cytoplasm into the mesophyll cell wall
4) water leaves a xylem vessel through a non-lignified area (pit), entering the mesophyll cell wall or cytoplasm
5) water moves up the xylem vessels to replace water lost from the leaf

Question 31

What 4 factors affect transpiration?

Answer 31

1) humidity
2) wind speed and temperature
3) light intensity
4) very dry conditions

Question 32

How does humidity affect transpiration?

Answer 32

If the water potential gradient between the air spaces in the leaf and the air outside becomes steeper (low humidity), the rate of transpiration will increase

Question 33

How does wind speed and temperature affect transpiration?

Answer 33

The rate of transpiration increases with an increase in wind speed or temperature

Question 34

How does light intensity affect transpiration?

Answer 34

• In most plants, stomata open during the day and close at night ∴ bc most transpiration takes place in the stomata, roughly no transpiration takes place at night
• Stomata must be open during the day to allow CO2 to diffuse into the leaf for photosynthesis ∴ during hours of high light intensity, the rate of transpiration increases
• Closing at night, when photosynthesis is impossible, reduces unnecessary water loss

Question 35

How do very dry conditions affect transpiration?

Answer 35

• The water potential gradient between the internal air spaces and external air is steep, a plant may have to compromise by closing its stomata during the day to prevent its leaves from drying out, even if this means reducing the rate of photosynthesis

Question 36

How does transpiration help to cool leaves in hot conditions?

Answer 36

As water evaporates from the cell walls inside the leaf, it absorbs heat energy from these cells ∴ reducing their temperature

Question 37

What happens if the rate at which water vapour is lost by transpiration exceeds the rate at which a plant can take up water from the soil?

Answer 37

• The amount of water in its cells decreases ∴ the cells become less turgid and the plant wilts as the soft parts (leaves) lose the support provided by turgid cells
• The plant also closes its stomata

Question 38

What can we measure to give a very good approximation of the rate of transpiration and why?

Answer 38

• We can measure the rate at which a plant takes up water bc a very high proportion of the water taken up by a stem is lost in transpiration ∴ the rate of transpiration directly affects the rate of water uptake

Question 39

What is a potometer?

Answer 39

An apparatus used to measure the rate at which water is taken up by a plant

Question 40

What 3 things must you do when using a potometer?

Answer 40

1) everything must be completely water and air tight so that no leakage occurs and that no air bubbles break the continuous water column
2) need to insert plant stem into apparatus with everything submerged in water and cut the end of the stem under water with a slanting cut before placing it in the potometer so that air bubbles are less likely to get trapped against it
3) apply petroleum jelly around joints to keep apparatus airtight

Question 41

How does a potometer work?

Answer 41

1) as water evaporates from the plant’s leaves, it is drawn into the xylem vessels that are exposed at the cut end of the stem
2) ∴ water is drawn along the capillary tubing
3) then record the position of the meniscus at set time intervals and plot graph of distance moved against time
4) if you expose the plant to different conditions, you can compare the rates

Question 42

How do you obtain and use epidermal peels?

Answer 42

1) strip lower epidermis from underlying tissues of leaves by grasping epidermis with a pair of fine forceps and peel the epidermis away from the leaf or make slits in the concave surface of the leaf shaped like a small square using a scalpel and peel it off
2) mount the epidermis un water on a slide and view through a microscope

Question 43

What are possible investigations to carry out using an epidermal peel?

Answer 43

1) whether stomata are open/closed in different conditions
2) estimating stomatal density (number of stomata/unit area)
3) factors affecting stomatal density e.g. age/habitat

Question 44

How do you estimate stomatal density?

Answer 44

1) count the number of stomata in one field of view
2) use a calibrated eyepiece graticule to measure the radius of the field of view or the epidermis used and calculate its surface area

Question 45

How do you obtain an epidermal impression?

Answer 45

Paint clear nail varnish onto epidermal surface, allow to dry and peel off to see if the stomata are open or closed

Question 46

Why might you want to know the surface area of a whole leaf?

Answer 46

To measure the rate of transpiration per unit area, allowing you to compare different species or different leaves of the same plant

Question 47

How do you measure the surface area of a whole leaf on grid paper?

Answer 47

1) draw outline of leaf on grid (smaller grid=more accurate area)
2) identify whole squares within outline and calculate combined area
3) only count squares partially occupied by leaf if more than half a square is occupied or divide the number of squares with part of the leaf by 2

Question 48

What are xerophytes e.g. marram grass?

Answer 48

Plants that live in places where water is in short supply and have often evolved special adaptations of their leaves that keep water loss down to a minimum

Question 49

What are adaptations of xerophytes that reduce water loss?

Answer 49

1) having spines and needles as leaves reduce the surface area available for transpiration
2) some leaves can roll up exposing a thick, waterproof cuticle to the air outside the leaf, opening the stomata on the upper epidermis to the enclosed, humid space inide the roll
3) hairs/trichomes help to trap a layer of moist air close to the leaf surface, reducing the steepness of the diffusion gradient for water vapour
4) sunken stomata on the upper epidermis and no stomata on the lower epidermis reduces surface area for transpiration
5) very small leaves with swollen or flattened photosynthetic stems store water and are coated with wax