Transport in plants

Question 1

How is the xylem vessel adapted to its function?

Answer 1

1. No cytoplasm, organelles or nucleus to obstruct the flow of water.
2. Cells form one continuous vessel with no end walls to obstruct flow of water.
3. Walls of xylem are lignified to prevent vessels from collapsing when under pressure from transpiration stream.
4. Vessels are very narrow to allow for effective capillary action and to prevent water column from breaking.
5. Lignin deposited in annular, spiral or reticulate pattern to allow for vertical growth.
6. Fully lignified xylem vessels are pitted to allow for movement of water in and out of xylem vessels.

Question 2

How are phloem sieve tube elements adapted to their function?

Answer 2

1. Cytoplasm and organelles are packed to the sides of the sieve tube elements to minimise their obstruction to the flowing sucrose.
2. Lack of organelles like nuclei, mitochondria, ribosomes… allow for this tight packing.
3. Cells are alive to allow for transport of nutrients in both directions.

Question 3

How are phloem companion cells adapted to their function?

Answer 3

1. Dense cytoplasm packed with mitochondria and ribosomes to create enough nutrients and energy to keep itself and neighbouring sieve tube elements alive.
2. Located adjacent to sieve tube elements and are connected to sieve tube elements by plasmodesmata so nutrients can reach sieve tube elements by diffusion.
3. Has the ability to load lots of sucrose via active transport which subsequently diffuses into sieve tube elements.

Question 4

Why do plants need transport systems?

Answer 4

• Plants have a small surface area to volume ratio, therefore only the epithelial cells are able to obtain water and nutrients from surrounding environment. Cells inside the plant need a transport system in order to have a constant supply of water and nutrients in order to survive.
• Roots can easily obtain water and minerals, but cannot produce sugars. Leaves produce sugars, but cannot easily obtain water and minerals.
• Transport system evenly distributes sugars, water and minerals around the plant.

Question 5

How is vascular tissue distributed in roots of dicotyledons?

Answer 5

• Vascular tissue is found in the centre of the root, surrounded by the cortex.
• Xylem vessels and phloem tissue found in distinctive and different vascular bundles. Xylem tissue usually arranged in a cross-like shape, with phloem tissue located between the arms of the cross.
• Just below the endodermis is a layer of meristem called the pericycle which allows for further growth of the roots.
• This arrangement provides the strength to withstand the large pull forces experienced by the roots.

Question 6

How is vascular tissue distributed in stems of dicotyledon?

Answer 6

• Vascular tissue is usually found in discrete bundles around the peripheral of young stems but become continuous in older stems (rings of trees).
• The vascular bundles are arranged so that there is a layer of support tissue, followed by a layer of phloem tissue, followed by a layer of cambium (produces new vascular tissue) followed by the xylem vessels.

Question 7

How is vascular tissue distributed in leaves?

Answer 7

• Dicotyledons usually have branching vessels in leaves where the vascular tissue is found.
• There is usually a midrib that branches out into smaller vessels.
• The vascular tissue is usually arranged so that a layer of support tissue is followed by xylem tissue which is followed by phloem tissue. The xylem vessels are usually arranged in a C-shape.

Question 8

What is water potential?

Answer 8

A measure of the total potential energy water molecules have in system. It determines how likely it is for water to be lost from a system by diffusion down a water potential gradient .

Question 9

How does water move between cells?

Answer 9

Water moves between cells by osmosis down the water potential gradient from an area of high water potential to an area of low water potential.

Question 10

What is the Symplast pathway?

Answer 10

The symplast pathway involves water moving through the cell membranes and into the cytoplasm of plant cells. The cytoplasm of cells are connected by plasmodesmata, forming a continuous cytoplasm. The water can thus diffuse from one cell to the next down the water potential gradient.

Question 11

What is the Apoplast pathway?

Answer 11

Water never directly enters the cells. Instead, water moves along the water-filled spaces between the cellulose fibres that make up the cell walls by diffusion.

Question 12

What is the Vacuolar pathway?

Answer 12

The vacuolar pathway is similar to the symplast pathway in the sense that it also involves the water crossing the plasma membrane. However, rather than being restricted to the cytoplasm, water also passes through the tonoplast and the vacuoles in plant cells.

Question 13

How are root hair cells adapted to their function?

Answer 13

1. Thin cell walls to decrease distance water and ions need to travel in order to enter the cells.
2. Hair-like projection dramatically increases surface area of root hair cells and thus make them more efficient at active transport and osmosis.
3. Lots of mitochondria to produce ATP for use in active transport of mineral ions from the soil.

Question 14

How does water enter the root?

Answer 14

The root hair cells are constantly loading mineral ions from the soil by active transport, which involves energy from ATP. This process lowers the water potential of the root hair cells. Water moves into the root hair cells, across the plasma membrane by osmosis down the water potential gradient.

Question 15

How does the water move across the cortex and into the xylem?

Answer 15

• Mineral ions are constantly being loaded into the xylem vessels by the endodermal cells through the process of active transport.
• This lowers the water potential inside the xylem vessels and so water moves into xylem vessels by osmosis.
• This subsequently lowers water potential in cells directly adjacent to the xylem vessels and sets up a water potential gradient across the whole cortex which moves water along the symplast pathway from root hair cells to xylem.
• Water from the other 2 pathways eventually join symplast pathway.

Question 16

What is the function of the Casperian strip?

Answer 16

The Casperian strips are waterproof strips across the cell walls of endodermal cells that:

1. Ensures water in apoplast pathway has to go through plasma membrane which controls the entry of substances.
2. Ensures ions are in the cytoplasm of endodermal cells so they can be loaded into xylem by active transport.
3. Ensures water cannot escape xylem via the apoplast pathway.

Question 17

How does the water move up the stem?

Answer 17

1. Root pressure: The constant loading of mineral ions into the xylem means that water is constantly moving into to xylem by osmosis. This constant movement of water create hydrostatic pressure in the xylem at root level which pushes water up the stem.
2. Transpiration stream: Cohesion between the water molecules means that a water column is maintained all the way down the plant. As water evaporates from the leaves, more water is pulled into the leaves from the xylem. This creates tension that pulls the whole column of water up the stem. This process is called the cohesion-tension theory and is the main force that allows movement up the stem.
3. Capillary action: There is also adhesion between the walls of the xylem vessels in the stem and water molecules. This with the narrow xylem vessels actually help pull the water upwards.

Question 18

What is transpiration?

Answer 18

The loss of water by evaporation from the ariel parts of the plant.

Question 19

What processes does transpiration involve?

Answer 19

1. Water enters the mesophyll cells in the leaves by osmosis from the xylem vessel.
2. Water evaporates from the surface of the mesophyll cells and enter the intracellular spaces in the leaf.
3. When the stomata opens, there is a higher water vapour potential in the leaf and a lower one outside the leaf, so water vapour diffuses out of the leaf down the water vapour potential gradient.
4. water can also evaporate from the upper epidermis, but it has to travel through the waterproof waxy cuticle which significantly reduces transpiration.

Question 20

Why is transpiration useful to the plant?

Answer 20

As water is lost in the leaves, more water replaces it which creates tension in the water column and pulls water up the stem from the roots. This provides water to the cells in the leaves for photosynthesis, as wells water for turgor pressure. Movement of water up the plant also allows for mineral ions to be transported up the plant. Evaporation of water also keeps the plant cool.

Question 21

How can transpiration be measured?

Answer 21

Transpiration can be measured using a potometer.

Question 22

What precautions should be taken in order to make sure the readings obtained are valid?

Answer 22

1. Make sure the seal between the plant and the potometer is tight and there are no leaks.
2. Make sure there are no other air bubbles in the apparatus apart from the one that’s part of the experiment.
3. Make sure the environment doesn’t change during the experiment.
4. Make sure the scale is perfectly parallel to the apparatus.
5. Make sure the plant has tie to acclimatise before starting the experiment.

Question 23

Why are potometers not 100% reflective of the rate of transpiration?

Answer 23

Potometers measure the rate of water intake. That water may be used for photosynthesis or to maintain turgor pressure, not just transpiration.

Question 24

How does the number of leaves affect rate of transpiration?

Answer 24

Increased number of leaves also increases total surface area of plants which increases the area over which water vapour can be lost.

Question 25

How does stomata density affect rate of transpiration?

Answer 25

Increased stomata density increases rate of transpiration because there are more pores available for the water vapour to diffuse out of the leaves.

Question 26

How does light intensity affect rate of transpiration?

Answer 26

Rate of transpiration increases as light intensity increases because with greater light intensity means greater rate of photosynthesis. This means that the stomata will be open wider, which increases rate of diffusion of water vapour out of leaves.

Question 27

How does temperature affect rate of transpiration?

Answer 27

As temperature increases, rate of transpiration increases because higher temperatures mean increased rate of evaporation of water from walls of mesophyll cells. Water vapour molecules also have more kinetic energy so rate of diffusion of water vapour out of leaves also increases.

Question 28

How does humidity affect rate of transpiration?

Answer 28

Increased outside humidity decreases rate of transpiration. As humidity outside increases, water vapour potential gradient decreases between inside and outside of leaves. This decreases rate of diffusion of water vapour out of leaves.

Question 29

How does air movement/ wind affect rate of transpiration?

Answer 29

Increased air movement/ wind increases rate of transpiration. This is because air movement/ wind carries away saturated air outside of leaves, maintaining a high water vapour potential gradient between inside and outside of the leaves, increasing rate of diffusion of water vapour outside of leaves.

Question 30

What happens if the plant loses too much water?

Answer 30

Plant cells lose their turgidity and become plasmolysed, resulting in the plant wilting and eventually dying.

Question 31

What are xerophytes?

Answer 31

Plants adapted to living in arid conditions.

Question 32

How are xerophytes adapted structurally to reduce water loss?

Answer 32

Reduced leaf size: Reduces surface area from which water can evaporate.
Thicker waxy cuticle: Reduces water loss by transpiration from the upper epidermis.
Densely packed spongy mesophyll: Reduces surface area of cells exposed to air spaces, thus reducing the amount of water evaporating into the air spaces.
Low stomata density: Reduced number of ways for water vapour to diffuse out of the leaves.
Stomatal pits: Traps air that quickly become saturated with water vapour. This reduces water vapour potential gradient between inside and outside of leaves, reducing rate of diffusion of water vapour out of leaves.
Stomatal hairs: Traps air outside of stomata that quickly become saturated with water vapour.
Hairs on leaves: Traps a layer of air above surface of leaves, reducing water vapour potential gradient.
Rolled leaves: Lower epidermis not exposed to external environment and traps air.
Roots: Roots can be long and extensive to tap into water deep in soil.
Fleshy stems: Increases amount of water the plant can store.

Question 33

How are xerophytes adapted behaviourally to reduce water loss?

Answer 33

1. Stomata closed when water is short.
2. Stomata only open at night.
3. Leaves roll when water is short.
4. Stomata open when water is short.

Question 34

How do xerophytes obtain water?

Answer 34

Their roots are often maintained at a very low water potential gradient so water can move in from the outside by osmosis.

Question 35

How does translocation happen?

Answer 35

1. Active transport used to load sucrose into the phloem sieve tube at source.
2. The water potential in sieve tube decreases so water moves in by osmosis from surrounding.
3. Movement of water into sieve tube at source increases hydrostatic pressure at source.
4. At sink, sucrose moves out of sieve tube by diffusion or active transport.
5. Water potential in sieve tube at sink increases.
6. Water moves out of sieve tube by osmosis.
7. Movement of water decreases hydrostatic pressure at sink so pressure gradient is created between source and sink.
8. Sucrose is pushed along sieve tube from source to sink as result of pressure gradient.

Question 36

How is sucrose loaded into sieve tube?

Answer 36

1. Active transport is used to pump H+ ions out of companion cells which requires ATP.
2. H+ ion concentration outside companion cell increases which creates a H+ ion concentration gradient.
3. H+ ions diffuse back into companion cells by facilitated diffusion.
4. Sucrose moves into sucrose with H+ ions due to co-transport proteins.
5. Sucrose diffuses from companion cell cytoplasm into sieve tube by diffusion through plasmodesmata.