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Cambridge IGCSE0610

Xylem and phloem

Biology 0610 Chapter Notes

What this chapter covers

Xylem and phloemWater uptakeTranspirationTranslocation
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In this chapter

  1. 1Xylem: water, mineral ions and support
  2. 2Phloem: sucrose and amino acids
  3. 3Where xylem and phloem are
  4. 4Xylem vessels: structure and function
  5. 5Root hair cells and their functions
  6. 6Why the surface area matters
  7. 7The pathway of water through the plant
  8. 8Following the water with a stain
  9. 9What transpiration is and the path water takes
  10. 10Investigating temperature and wind speed
  11. 11Surface area, stomata and water vapour loss
  12. 12Transpiration pull in the xylem
  13. 13Temperature, wind speed and humidity
  14. 14Wilting
  15. 15Translocation in the phloem
  16. 16Sources and sinks
  17. 17Source at one time, sink at another

1. Xylem: water, mineral ions and support

Xylem transports water and mineral ions. It also gives support to the plant.

A coloured stain taken up at the cut end of a celery stalk travels up the stalk in the xylem. The xylem is the tissue that stains.

Examiner insight

“It was common to mistake phloem for xylem.” “The function that was commonly missed was the structural support function of xylem.”

Worked example 11 mark

A celery stalk was placed in a beaker which contained a red stain. After 24 hours, the red stain appeared at the top of the celery stalk. Which structures stained red?

A cortex cells · B mesophyll cells · C phloem · D xylem

  1. 1

    D: xylem. The stain moves up with the water. [1]

2. Phloem: sucrose and amino acids

Phloem transports sucrose and amino acids.

Starch is not transported in the phloem. It is a large insoluble molecule. Say sucrose, not glucose or food.

Examiner insight

“The most common incorrect response was starch. Starch is a large insoluble molecule which is not transported in the phloem.” “Some candidates stated amino acids as the carbohydrate translocated in the phloem instead of sucrose.”

Worked example 12 marks

In plants, substances are transported in the xylem and phloem. State the names of two molecules transported by the phloem that are not transported by the xylem.

  1. 1

    sucrose [1]

  2. 2

    amino acids [1]

3. Where xylem and phloem are

In a root, the xylem forms a star shape in the centre. The phloem sits in small groups between the arms of the xylem.

In a stem, the two tissues lie together in vascular bundles. The bundles form a ring near the edge. In each bundle the xylem is on the inner side and the phloem is on the outer side.

In a leaf, the vascular tissue is within the spongy mesophyll.

Examiner insight

“There was some uncertainty about the position of the xylem in the root and the leaf.” “A few candidates placed label lines right on the boundary between the xylem and phloem and could not be credited.”

Worked example 12 marks

Describe the position of the xylem and the phloem in a cross-section of a root.

  1. 1

    the xylem is in the centre, in a star shape [1]

  2. 2

    the phloem is in small groups between the arms of the xylem [1]

4. Xylem vessels: structure and function

A xylem vessel is built to carry water. It has thick walls with lignin.

The vessel has no cell contents, so it is hollow. The cells are joined end to end with no cross walls, which forms a long continuous tube.

A hollow tube with no end walls gives less resistance to the flow of water.

Examiner insight

“There were some inaccuracies in candidate’s responses referring to the lack of cell walls, rather than the lack of end walls.”

Worked example 11 mark

What are the features of the cell walls in a xylem vessel?

end wallside wall
Aabsentthick
Babsentthin
Cpresentthick
Dpresentthin
  1. 1

    A: the end wall is absent and the side wall is thick. [1]

Recap

  • Xylem transports water and mineral ions, and gives support.
  • Phloem transports sucrose and amino acids. Starch is insoluble and is not transported.
  • Root: xylem in the centre, phloem between its arms. Stem: a ring of vascular bundles, xylem on the inner side of each.
  • Extended: xylem vessels have thick walls with lignin, no cell contents and no cross walls.
  • Extended: the cells join end to end to form a long continuous tube.

5. Root hair cells and their functions

A root hair cell is the cell that takes up water from the soil. Its functions are to absorb water and mineral ions.

In a diagram of a root, the root hair cells are the ones in contact with the soil. A root hair cell has a cell wall. It has no chloroplasts.

Write ‘root hair cell’, not ‘root cell’. Transport of water is a function of the xylem, not of the root hair cell.

Examiner insight

“Some candidates gave transport of water as a function, but that is a function of the xylem.” “Often candidates used general terms, such as root instead of root hair.”

Worked example 12 marks

Name the specialised cell that absorbs water from the soil. State one other substance that this cell absorbs.

  1. 1

    root hair cell [1]

  2. 2

    mineral ions [1]

6. Why the surface area matters

The large surface area of root hairs increases the uptake of water and mineral ions.

The root hair is an extension of the cell. The extension gives the cell a larger surface area, so more water and mineral ions can be absorbed in the same time.

Water is absorbed by osmosis, not by active transport.

Examiner insight

“The absorption of water was well know as a function of a root hair cell but absorption of mineral salts was less well known.” “Some candidates incorrectly believed that water is absorbed by active transport.”

Worked example 11 mark

Which statements about root hairs are correct?

1 Root hairs allow water to be absorbed by active transport.

2 Root hairs increase the surface area available for absorption.

3 The rate of absorption of water and ions is increased by root hairs.

A 1, 2 and 3 B 1 and 2 only C 2 and 3 only D 3 only

  1. 1

    statement 1 is wrong: water is absorbed by osmosis, not by active transport

  2. 2

    statements 2 and 3 are correct: the large surface area increases the uptake of water and ions

  3. 3

    answer C [1]

7. The pathway of water through the plant

Water takes one pathway: root hair cells, root cortex cells, xylem, mesophyll cells.

Water first enters the root hair cells. It then crosses the root cortex. Only then does it reach the xylem. It does not go to the xylem directly.

The xylem carries the water up the stem to the leaf. In the leaf, the water reaches the mesophyll cells.

Examiner insight

“Whilst most candidates understood that water first enters root hair cells, many candidates were uncertain whether water then progresses to the xylem or to the mesophyll.” “Others seemed unfamiliar with the section through a root and ended their lines in the cortex. A considerable number of the remaining candidates ended their line on the phloem.”

Worked example 13 marks

Describe the pathway taken by water from the soil to the leaf of a plant.

  1. 1

    into the root hair cell [1]

  2. 2

    across the root cortex [1]

  3. 3

    up the xylem [1]

  4. 4

    then to the mesophyll cells of the leaf

8. Following the water with a stain

A stain added to the water shows the pathway above ground. A cut celery stalk stood in the stain takes it up with the water.

Leave the stalk for a set time, so that the dye is absorbed. Then cut sections across the stalk. The stained tissue is the xylem, and it marks the position of a vascular bundle.

Measure the distance travelled by the stain, not the number of sections. The knife and the dye both carry a risk.

Examiner insight

“Many candidates appreciated that the blue stain is transported in the xylem. The commonest incorrect response was phloem.” “The points most frequently omitted were giving time for the dye to be absorbed and linking the stained xylem with the position of a vascular bundle.”

Worked example 13 marks

A celery stalk is stood in red dye for a set time. State which tissue in the stalk is stained red. Describe how a student can find how far the dye has travelled.

  1. 1

    xylem [1]

  2. 2

    cut sections across the stalk [1]

  3. 3

    measure the distance travelled by the stain, up to the last section that is stained [1]

Recap

  • Root hair cells absorb water and mineral ions from the soil.
  • Write ‘root hair cell’, never ‘root cell’. Root hair cells have a cell wall and no chloroplasts.
  • The large surface area of root hairs increases the uptake of water and mineral ions.
  • Water is absorbed by osmosis, not by active transport.
  • Pathway: root hair cells, then root cortex cells, then xylem, then mesophyll cells.
  • The xylem, not the root hair cell, transports water.
  • A stain shows the pathway: the xylem is stained. Allow time, then cut sections and measure the distance travelled by the stain.

9. What transpiration is and the path water takes

Transpiration is the loss of water vapour from leaves.

Water evaporates from the surfaces of the mesophyll cells into the air spaces. It then diffuses out of the leaves through the stomata as water vapour.

It is water vapour that leaves the plant, not liquid water.

Examiner insight

“The least well-known was how water vapour moves out of the plant, with many incorrectly stating that it moves by osmosis.” “Some candidates correctly completed the sentence about transpiration, but many stated that water evaporates from the surfaces of plant cells rather than from the surface of mesophyll cells.”

Worked example 13 marks

Describe transpiration.

  1. 1

    Transpiration is the loss of water vapour from leaves. [1]

  2. 2

    Water evaporates from the surfaces of mesophyll cells into the air spaces. [1]

  3. 3

    The water vapour diffuses out through the stomata. [1]

10. Investigating temperature and wind speed

The rate of water uptake by a shoot is approximately equivalent to the rate of transpiration. A potometer measures it by the movement of the bubble in a capillary tube.

Change one factor, such as the temperature or the wind speed, and keep the others constant. A fan on different settings, or at different distances from the shoot, changes the wind speed.

Measure over a fixed period of time. Carry out the investigation at least 3 times at each setting.

A rise in temperature and a rise in wind speed each increase the rate of transpiration.

rate = (distance moved by the bubble)/(time taken)

Examiner insight

“Many candidates identified the need to measure the rate of transpiration, with no explanation of how this would be carried out.” “Some candidates knew that repeat measurements were necessary, although they did not state how many times to repeat, or they described repeating at different wind speeds.”

Worked example 14 marks

A student investigated the effect of wind speed on the rate of transpiration in a plant shoot. As water is taken up by the plant shoot, the air bubble in the capillary tube moves towards the shoot. The student used a fan set at different speeds to change the wind speed. He measured the distance travelled by the air bubble in 300 seconds. The results are in the table.

(i) Calculate the rate of transpiration for a wind speed of 10 m s⁻¹ where the distance moved by the bubble was 135 mm in 300 seconds.

(ii) Describe the effect on the rate of transpiration of increasing the wind speed in this investigation.

(iii) Suggest why the rate of transpiration is only an estimate of water uptake in a plant.

wind speed / m s⁻¹distance travelled by the air bubble in 300 s / mmrate of transpiration / mm s⁻¹
020.01
2120.04
4200.07
6350.12
81150.38
10135?
  1. 1

    (i) rate = 135 ÷ 300 = 0.45 mm s⁻¹ [1]

  2. 2

    (ii) as wind speed increases the rate of transpiration increases [1]

  3. 3

    not a linear relationship: a greater increase in rate at higher wind speeds (from 0.12 to 0.38 mm s⁻¹ between 6 and 8 m s⁻¹) [1]

  4. 4

    (iii) not all of the water taken up by the plant is lost through transpiration [1]

11. Surface area, stomata and water vapour loss

The interconnecting air spaces between mesophyll cells give a large internal surface area. Water evaporates from this surface into the air spaces.

The water vapour leaves through the stomata. So water vapour loss is related to the size and number of stomata.

More leaves means more stomata, so more water vapour is lost through them.

Examiner insight

“Few candidates appreciated that most water evaporation during transpiration occurs from the spongy mesophyll cells.” “Very few made reference to stomata, or the fact that most stomata are located on the underside of the leaf and very few found on the upper surface.”

Worked example 14 marks

Ringing involves removing a circle of the outer tissues from a plant stem. This removes the phloem but not the xylem.

(b) Explain the effect on the rate of transpiration of removing leaves from a plant.

(c) Suggest the effect on the rate of transpiration of ringing a plant. Give a reason for your suggestion.

  1. 1

    (b) reduces the rate of transpiration [1]

  2. 2

    because there are fewer stomata / water is lost through stomata [1]

  3. 3

    (c) no effect on the rate of transpiration [1]

  4. 4

    transpiration occurs through the xylem / the xylem transports water [1]

12. Transpiration pull in the xylem

Water is lost from the leaf by transpiration. This lowers the water potential at the top of the plant and creates a transpiration pull.

The pull acts on a column of water molecules in the xylem. The water molecules are held together by forces of attraction between them, so the whole column is drawn upwards.

Examiner insight

“Common errors included stating that water moves within the xylem vessels by osmosis and making references to water concentration rather than water potential.” “Very few stated that it was caused by transpiration pull, but many attempted to state that cohesion and adhesion were involved.”

Worked example 14 marks

Explain how transpiration causes the movement of water in the xylem.

  1. 1

    water evaporates from the surfaces of mesophyll cells into the air spaces; water vapour is lost by diffusion through the stomata [1]

  2. 2

    loss of water from the leaf lowers the water potential at the top of the plant [1]

  3. 3

    this creates tension (a pull) on the water molecules in the xylem: transpiration pull [1]

  4. 4

    there is a column of water molecules, held together by forces of attraction between them, so water moves up the xylem [1]

13. Temperature, wind speed and humidity

A higher temperature increases the rate of transpiration: more evaporation and more diffusion of water vapour occur.

A higher wind speed increases the rate of transpiration. A higher humidity decreases it.

Humidity and wind speed change the concentration gradient of water vapour between the air spaces in the leaf and the air outside. High humidity lowers it. High wind speed raises it.

Examiner insight

“Some candidates did not access the mark as they stated that an increase in the rate of transpiration would result in an increase in temperature.” “While many candidates chose the correct option, some thought that high humidity raises the concentration gradient and high wind speed lowers the concentration gradient.”

Worked example 11 mark

A decrease in which factor normally causes transpiration to increase?

A humidity · B light intensity · C number of stomata · D temperature

  1. 1

    A: a decrease in humidity increases transpiration. [1]

14. Wilting

If a plant absorbs less water than it loses, its cells lose water and become flaccid. The plant wilts.

Closing the stomata reduces water loss, and so helps to prevent wilting.

Examiner insight

“The best answers explained that less water would be absorbed, and cells would become flaccid, and the plant would wilt.” “Many candidates correctly stated that closing the stomata would reduce water loss in the plant with a minority describing the prevention of wilting.”

Worked example 14 marks

A potted plant is left on a warm windowsill and is not watered. Its leaves droop. Explain how and why the plant wilts.

  1. 1

    Water vapour continues to be lost from the leaves by transpiration. [1]

  2. 2

    Less water is absorbed than is lost. [1]

  3. 3

    The cells lose water and become flaccid. [1]

  4. 4

    The plant wilts. [1]

Recap

  • Transpiration is the loss of water vapour from leaves.
  • Water evaporates from the surfaces of mesophyll cells into the air spaces, then diffuses out through the stomata as water vapour.
  • Higher temperature and higher wind speed increase the rate of transpiration.
  • Measure the bubble in a potometer over a fixed time, and repeat at each setting.
  • Extended: the large internal surface area of the air spaces and the size and number of stomata affect water vapour loss.
  • Extended: transpiration pull draws up a column of water molecules held together by forces of attraction.
  • Extended: higher humidity decreases the rate of transpiration. Wilting happens when cells lose water and become flaccid.

15. Translocation in the phloem

Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks.

Glucose made in photosynthesis is converted to sucrose before it is transported. Starch is insoluble, so it is not translocated.

Examiner insight

“Several candidates imprecisely described phloem as the transport tissue for glucose or food.” “Some candidates incorrectly believed that starch is transported by translocation, not appreciating that starch is an insoluble substance.”

Worked example 11 mark

Which carbohydrate is transported in the phloem?

A cellulose · B glycogen · C starch · D sucrose

  1. 1

    D: sucrose. [1]

16. Sources and sinks

A source is a part of a plant that releases sucrose or amino acids. A sink is a part of a plant that uses or stores sucrose or amino acids.

In the leaf, glucose is converted to sucrose. The leaf is the source. The roots, which use or store it, are the sink.

Examiner insight

“This question asked for two examples of sinks for sucrose. The most common incorrect answer was leaf – the main source of sucrose during plant growing seasons.” “Very few answers referred to the leaf as the source of the sucrose or the shoot tips and roots as the sinks.”

Worked example 15 marks

Ringing involves removing a circle of the outer tissues from a plant stem. This removes the phloem but not the xylem.

Plant A was ringed and plant B was not ringed. Most of the leaves were removed from the plants. Plastic bags filled with carbon dioxide containing radioactive carbon (¹⁴C) were tied around the remaining leaves. After a set time, radioactive carbon (¹⁴C) was detected in different parts of the plants.

In plant A the ring was on the stem between the leaf and the top of the stem. The table gives the results.

Explain why radioactive carbon (¹⁴C) was found in the roots but not in the top of the stem in plant A.

Part of plantPlant A (ringed)Plant B (not ringed)
Top of the stemno ¹⁴C detected¹⁴C detected
Leaf¹⁴C detected¹⁴C detected
Roots¹⁴C detected¹⁴C detected
  1. 1

    carbon dioxide containing ¹⁴C is taken into the leaf (by diffusion) [1]

  2. 2

    photosynthesis occurs, so the glucose made contains ¹⁴C [1]

  3. 3

    glucose is converted to sucrose [1]

  4. 4

    sucrose containing ¹⁴C is transported in the phloem to the roots, but not past the ring to the top of the stem [1]

  5. 5

    the leaf acts as a source and the roots act as a sink [1]

17. Source at one time, sink at another

Whether a part is a source or a sink depends on the time. A part that stores sucrose is a sink. The same part is a source when it releases the sucrose again.

A plant organ that stores food, such as a root or a bulb, can be both. Leaves are a source when they photosynthesise. They can also be a sink.

Examiner insight

“Many had some idea that part of the plant was a source in the summer and a sink in the winter, but could not explain any further.” “Some candidates incorrectly believed that at stage 3, the bulb was a source and at stage 6, a sink.”

Worked example 13 marks

A plant stores food in an underground bulb. In summer the bulb grows larger. In spring the bulb shrinks as new leaves grow. Explain why the bulb is a sink in summer and a source in spring.

  1. 1

    in summer the leaves photosynthesise and sucrose is translocated to the bulb [1]

  2. 2

    the bulb stores the sucrose, so it is a sink [1]

  3. 3

    in spring the bulb releases sucrose to the growing leaves, so it is a source [1]

Recap

  • Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks.
  • Glucose is converted to sucrose before it is transported. Starch is not translocated.
  • A source releases sucrose or amino acids. A sink uses or stores them.
  • The leaf is a source. The roots and shoot tips are sinks.
  • A part that stores sucrose is a sink. The same part is a source when it releases the sucrose.
  • Removing the phloem stops sucrose reaching the sink.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Name the tissue that transports water and mineral ions in a plant.1 mark
  2. State the names of the two substances transported by the phloem.2 marks
  3. State two functions of the xylem.2 marks
  4. A celery stalk is stood in blue stain for 12 hours. The stalk is then cut across. State which tissue is stained blue and what this shows it transports.2 marks
  5. Describe how the xylem and phloem are arranged in a section through a non-woody stem.3 marks
  6. Copy and complete the table by writing xylem or phloem in each blank. | substance transported | tissue | | --- | --- | | water | ... | | sucrose | ... | | mineral ions | ... | | amino acids | ... |4 marks
  7. A ring of tissue containing the phloem, but not the xylem, is removed from around the outside of a stem. Explain what can still move past the ring and what cannot.3 marks
  8. State two features of the walls of xylem vessels.2 marks
  9. A poison stops living cell contents from working. A cut shoot is stood in water that contains the poison and a red stain. Some hours later the red stain has reached the leaves. Sucrose made in the leaves has stopped moving to the rest of the plant. (a) Name the tissue that carried the red stain to the leaves. (b) Explain why the poison did not stop the stain from moving up the shoot. (c) Name the tissue that has stopped working, and state one other substance that it would no longer transport.5 marks
  10. A young non-woody stem is stood in blue stain and then cut across. In each vascular bundle the stain is found only in the part on the inner side. A student writes: ‘The stained tissue is phloem. Its cells have no cell walls, so the stain flows through easily. This tissue only transports water.’ (a) The student has made three mistakes. Correct each one. (b) Explain how the structure of the stained tissue lets water flow through it easily.6 marks
  11. Name the specialised cell that absorbs water from the soil.1 mark
  12. State two functions of root hair cells.2 marks
  13. A root hair cell has an extension. State the effect of the extension on the surface area of the cell. State the effect on the uptake of water and mineral ions.2 marks
  14. Put these in the order that water passes through them: xylem, root hair cells, mesophyll cells, root cortex cells.1 mark
  15. A student says that water enters a plant through the stomata. Name the cells through which water does enter the plant.1 mark
  16. Name the part of the pathway that each description gives. | Description | Part of the pathway | | --- | --- | | Water enters here from the soil | | | Water crosses this tissue in the root before it reaches the xylem | | | Water moves up through the stem in this tissue | |3 marks
  17. A seedling is pulled from the soil and most of its root hairs are rubbed off. It is put back. Predict what happens to its uptake of water and mineral ions, and explain why.2 marks
  18. A leafy stem is stood in red dye for an hour. A thin section is cut across the stem. State the tissue that is stained red. State what this shows about that tissue.2 marks
  19. Two celery stalks stand in a stain. Each stalk is then cut into sections from the bottom upwards. The table shows the results. Calculate the rate of travel of the stain in each stalk in mm / min. State the stalk in which the stain travelled faster. | Stalk | Time in the stain | Length of each section | Number of stained sections | | --- | --- | --- | --- | | X | 20 min | 4 mm | 12 | | Y | 30 min | 1.0 cm | 9 |4 marks
  20. A student wants to find the position of the vascular bundles in a leafy shoot. The student writes this plan: ‘Dip the cut end of the shoot in red stain and take it straight out. Cut sections across the shoot. Count the red sections to find how far the stain travelled. The red tissue is the phloem.’ Find three mistakes in the plan and correct each one. State what the position of the red tissue shows. State two things in the method that carry a risk.6 marks
  21. Define transpiration.1 mark
  22. Complete the sentences. Water evaporates from the surfaces of ......... cells into the air spaces. It then diffuses out of the leaf through the ......... as .........3 marks
  23. The air bubble in a potometer moves 45 mm in 5.0 minutes. Calculate the rate of water uptake in mm / s.2 marks
  24. Leaves were hung from a string for one hour at three temperatures. The table gives the mass each lost. Describe the effect of temperature on transpiration. | temperature (°C) | mass lost (g) | | --- | --- | | 10 | 0.8 | | 20 | 1.5 | | 30 | 2.9 |3 marks
  25. A student uses a fan and a potometer to investigate the effect of wind speed on transpiration. State two variables to keep constant. State how the student should make the results more reliable.3 marks
  26. The graph shows how the rate of water uptake of a shoot changes with wind speed. Use the graph to find the increase in the rate when the wind speed rises from 4 m / s to 8 m / s.3 marks
  27. Two plants of the same species are kept in the same conditions. Plant X has twice as many stomata per unit area of leaf as plant Y. Predict which plant loses water vapour faster and explain your answer.3 marks
  28. Explain how water moves up the xylem from the roots to the leaves of a tall plant.4 marks
  29. A student sets up a potometer with a leafy shoot. The temperature is kept constant. She records how far the air bubble moves in three conditions. P: still air. Q: a fan blowing on the shoot. R: still air, with the shoot enclosed in a clear plastic bag. The air inside the bag becomes very humid. (a) Calculate the rate of water uptake in each condition, in mm / s. (b) Explain the results for Q and for R, compared with P. Use ideas about the concentration gradient of water vapour. | condition | distance moved by the bubble / mm | time | | --- | --- | --- | | P | 18 | 4.0 minutes | | Q | 63 | 300 seconds | | R | 9 | 6.0 minutes |7 marks
  30. Two fields of the same crop grow in dry soil. On a hot, dry, windy afternoon the plants in field A close most of their stomata. The plants in field B have been sprayed with a substance that keeps their stomata open. By the evening the plants in field B have wilted. The plants in field A have not. Explain why the plants in field B wilted and the plants in field A did not.6 marks
  31. Name the two types of substance moved by translocation.2 marks
  32. Name the plant tissue in which translocation takes place.1 mark
  33. Describe translocation.2 marks
  34. Describe what is meant by a source and a sink in a plant.2 marks
  35. A student says that starch is carried from the leaves to the roots in the phloem. Explain why this is wrong.2 marks
  36. A potato plant makes sucrose in its leaves. The sucrose is moved to the tubers, where it is stored. State which part is the source and which part is the sink.2 marks
  37. A ring of outer tissue, including the phloem but not the xylem, is removed from the stem of a plant. The ring lies between the leaves and the roots. Explain why sucrose made in the leaves does not reach the roots.3 marks
  38. A leaf halfway up a stem makes sucrose containing radioactive carbon. Later, radioactive sucrose is found in the shoot tip above the leaf and in the roots below the leaf. Explain what this shows about the direction of translocation. Use the terms source and sink.3 marks
  39. A carrot plant stores carbohydrate in its root. The plant is kept in the dark for several days. During this time sucrose is still found in the phloem of the stem and the shoot tip keeps growing. Explain where this sucrose comes from and where it goes. Use the terms source and sink.4 marks
  40. A tree stores carbohydrate in its roots. In spring its leaves are small and growing. In summer its leaves are full size. A scientist records the direction in which sucrose moves in the phloem of a leaf stalk and the change in the carbohydrate stored in the roots. (a) State whether a leaf is a source or a sink in each season. Explain each answer. (b) State whether the roots are a source or a sink in each season. Use the results to support each answer. | Season | Sucrose movement in the leaf stalk | Carbohydrate stored in the roots | | --- | --- | --- | | Spring | into the leaf | decreasing | | Summer | out of the leaf | increasing |6 marks

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