II: 7.1 - Transport in plants

Question 1

What is the function of the xylem?

Answer 1

Transports water and dissolved mineral salts from roots (source) to all parts of the plant (sinks).

Also helps to support the plant.

Transport in the xylem only goes one way: up the stem from the roots

Question 2

List the adaptations of the xylem

Answer 2

• Narrow lumen to maintain high pressure
• Non-living and hollow - has no cytoplasm so greater capacity for water, does not use up water itself
• Lignified cells/has lignin rings to provide strength
• Waterproof bands to prevent water loss
• Thin walls in some places to allow water to diffuse out to nearby cells
• No cross-walls, so nothing hinders water flow

Question 3

What is the function of the phloem?

Answer 3

Transports dissolved nutrients, e.g. amino acids and sucrose, from areas of production (sources), e.g. leaves and stem, to areas of use and storage (sinks), e.g. roots and storage organs.

Transport in the phloem can happen in both directions.

Question 4

List the adaptations of the phloem

Answer 4

• Has cytoplasm to help maintain a concentration gradient for the diffusion of nutrients
• Sieve plate has pores to allow food to pass through
• Sieve plate can help to regulate movement of food
• Thin walls to allow diffusion
• No nucleus to allow more space for food (has a companion cell containing a nucleus)

Question 5

What is the distribution of the xylem and phloem in the roots (i.e. where is it positioned in a transverse section of a root)?

Answer 5

Question 6

What is the distribution of the xylem and phloem in the stem (i.e. where is it positioned in a transverse section of a stem)?

Answer 6

Question 7

What is the distribution of the xylem and phloem in the leaf (i.e. where is it positioned in a transverse section of a leaf)?

Answer 7

Question 8

What is the function of root hair cells?

Answer 8

To absorb water and mineral salts from the earth.

Question 9

List the adaptations of root hair cells

Answer 9

• Elongated shape to increase surface area
• Large vacuole for storage
• Contains dissolved salts to lower the water potential, promoting osmosis
• Semi-permeable membrane for osmosis, diffusion and active transport
• Thin, porous cell wall to allow subtances through

Question 10

Describe an experiment to investigate the path water takes up a plant

Answer 10

Apparatus:

Shoots of leafy plants (e.g. celery), water, dye (e.g. methylene blue), beakers

Method:

1. Dye the water
2. Place the shoots in the solution
3. Leave in light for 30 minutes or more
4. Cut the stems to see results

Result:

Dye should be seen in the vascular bundles, indicating the path of water

Question 11

State the pathway taken by water through root, stem and leaf

Answer 11

1. Soil
2. Root hair
3. Root cortex cells
4. Xylem
5. Mesophyll cells

Water moves by osmosis through the cells and by diffusion between the cells.

Question 12

Define

transpiration

Answer 12

The evaporation of water at the surfaces of the mesophyll cells, followed by a loss of water vapour from the plant leaves through the stomata.

Question 13

Describe how water vapour loss occurs (related to cell surfaces, air spaces and stomata)

Answer 13

1. Water evaporates from surfaces of mesophyll cells…
2. …into the air spaces created by the irregular shape of spongy mesophyll cells…
3. …and diffuses out of the stomata, as the concentration of water vapour in the atmosphere is lower than that in the leaf air spaces.

Question 14

Explain the transpiration stream (mechanism of water uptake)

Answer 14

• As the leaves lose water, a water potential gradient is created in the mesophyll cells
• Water enters the mesophyll cells from the xylem in the leaf, creating a tension (‘pull’) in the xylem of the stem
• The water potential gradient in the xylem draws cohesive water molecules up the plant from the roots

Columns of water are held together by cohesive forces (attractive forces betwen water molecules) and adhesive forces (attractive forces between water and xylem vessels).

Question 15

Describe the effect of variation of temperature on transpiration rate

Answer 15

increase in temperature → increase in transpiration rate

Explanation: An increase in temperature promotes evaporation and lowers the water potential around the leaf. As there is a high water potential within the leaf, water is lost. Also, diffusion is faster as particles have more energy.

Question 16

Describe the effect of variation of wind on transpiration rate

Answer 16

increase in wind → increase in transpiration rate

Explanation: Water molecules around the leaf are blown away, lowering the water potential. As there is a high water potential within the leaf, water is lost.

Question 17

Describe the effect of variation of light intensity on transpiration rate

Answer 17

increase in light intensity → increase in transpiration rate

Explanation: Light promotes photosynthesis, which causes more stomata to open to allow CO₂ to enter. More water can be lost through the stomata.

Question 18

Describe the effect of variation of humidity on transpiration rate

Answer 18

increase in humidity → decrease in transpiration rate

Explanation: More water molecules surrounding the leaf increases the water potential outside the leaf. Lower water potential gradient means less water is lost.

Question 19

Describe how wilting occurs

Answer 19

• Occurs when water loss is greater than water uptake
• Leaf cells become flaccid (opposite of turgid)
• This is because there is no internal water pressure
• Wilting helps the plant survive dry conditions by reducing the surface area from which evaporation can occur
• If water loss continues to be greater than water uptake, the plant will eventually die

Question 20

Describe how plants that live in water (e.g. in ponds) are adapted

Answer 20

• Wide, broad leaves on surface of water to exchange gases
• Stomata on upper side of the leaf to be in contact with air
• Little lignin in xylem, since leaf is supported by water
• Very thin cuticle since water is plentiful

Question 21

Describe how plants that live in the desert are adapted

Answer 21

• Waxy cuticle to reduce water loss
• Small leaves/spiny leaves to reduce surface area for water loss
• Rolled leaves/leaf hairs and stomata sunk in pits to trap moisture - lowers water potential gradient
• Reduced number of stomata
• Thick, fleshy stem stores water
• Shallow roots absorb lightest rainfall
• Deep roots penetrate to very low water table

Question 22

Describe how plants that live in a garden are adapted

Answer 22

• Leaves collapse and stomata close to reduce heat absorption and transpiration of water
• In severe conditions, plants allow leaves to fall off so no water loss can occur. No photosynthesis takes place, but plants can remove chlorophyll from leaves for storage (yellow/red leaves)

Question 23

Define

translocation

Answer 23

The movement of sucrose and amino acids in phloem;

• from regions of production
• to regions of storage OR to regions of utilisation in respiration or growth

Nutrients must be transported in soluble forms.

Question 24

Describe how the translocation throughout the plant of applied chemicals (such as systemic pesticides) is useful

Answer 24

• Aphids (greenfly) insert mouthpiece into phloem to take nutrients
• Systemic pesticides/insecticides are sprayed onto plants and absorbed into phloem due to translocation
• Used to kill only pests (the aphids) instead of killing useful insect species (pollinators)

Question 25

Compare the role of transpiration and translocation in the transport of materials from sources to sinks, within plants at different seasons

Answer 25

• Winter & early spring: sucrose transported from stores in roots to leaves
• Summer & early autumn: sucrose goes from photosynthesising leaves to root stores