K325 / 2027

Nutrition and transport in plants

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Which route correctly describes carbon dioxide entering a photosynthesising leaf?

  1. Root hair -> xylem -> phloem -> mesophyll cell -> chloroplast in the leaf
  2. Cuticle -> sieve tube -> nucleus only
  3. Stoma -> air space -> moist mesophyll cell wall -> cell -> chloroplast
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Stoma -> air space -> moist mesophyll cell wall -> cell -> chloroplast

Carbon dioxide diffuses down its concentration gradient and dissolves at the moist cell surface before entering cells.

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Which statement describes a typical herbaceous dicot stem bundle?

  1. Xylem sieve tubes contain companion cells.
  2. Xylem is on the inner side, while phloem is on the outer side.
  3. Phloem lies nearer the centre of the stem and carries water up from the roots.
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Xylem is on the inner side, while phloem is on the outer side.

Use these positions within each bundle, which forms part of a ring in a typical dicot stem.

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Which condition favours net water entry into a root hair by osmosis?

  1. Soil water has higher water potential than the cell contents.
  2. Soil water has lower water potential than the cell contents.
  3. The cell uses ATP to pump each water molecule across its membrane.
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Soil water has higher water potential than the cell contents.

Water moves down this water-potential gradient through the partially permeable membrane.

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Which statement correctly follows photosynthesis into a food chain?

  1. Plants create new energy from sunlight, and consumers create more energy when they digest the plants.
  2. Mineral ions are the glucose molecules taken up from soil.
  3. Plants store absorbed light energy as chemical energy in organic food, which consumers can obtain.
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Plants store absorbed light energy as chemical energy in organic food, which consumers can obtain.

Photosynthetic producers supply organic matter and chemical energy to most food chains.

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Why is oxygen volume usually a better measure than bubble count?

  1. Bubbles can differ in size, so equal counts need not represent equal gas volumes.
  2. Measuring volume removes the need to time the experiment, so fewer readings can go wrong.
  3. Every bubble always contains a different gas.
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Bubbles can differ in size, so equal counts need not represent equal gas volumes.

Measuring volume accounts for size differences, although other practical limitations remain.

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Hydrogencarbonate indicator is red at normal carbon dioxide levels, yellow when carbon dioxide is higher and purple when it is lower. A leaf in bright light turns the indicator purple. Why?

  1. Light shining on the tube turns the indicator purple whatever the leaf does
  2. Photosynthesis takes in carbon dioxide faster than respiration releases it
  3. Respiration releases carbon dioxide faster than photosynthesis uses it
  4. The leaf has stopped respiring because light is shining on it
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Photosynthesis takes in carbon dioxide faster than respiration releases it

In bright light the net uptake of carbon dioxide lowers its level, so the indicator turns purple.

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A light-response graph reaches a positive plateau. What can be concluded from that graph alone?

  1. More light is not increasing the rate there; another factor may be limiting.
  2. Carbon dioxide is now the limiting factor, because light can no longer be limiting.
  3. Photosynthesis has completely stopped.
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More light is not increasing the rate there; another factor may be limiting.

More evidence is needed to identify which other factor limits the rate.

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Invented data: at 0.01% carbon dioxide, the rate of photosynthesis stops rising above light intensity 3 units. At 0.04%, it rises until 6 units. What limits the rate at 0.01% above 3 units?

  1. Carbon dioxide concentration
  2. Water lost by transpiration
  3. Light intensity
  4. The amount of chlorophyll
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Carbon dioxide concentration

Raising carbon dioxide raised the plateau, so carbon dioxide was limiting at 0.01%.

Check your understanding

A leaf in a sealed tube of hydrogencarbonate indicator is kept in dim light. After an hour the indicator is still red, the same as a control tube without a leaf. What is the best explanation?

  1. The leaf has stopped both photosynthesis and respiration in dim light.
  2. The leaf is photosynthesising faster than it respires.
  3. The rate of photosynthesis equals the rate of respiration.
  4. The leaf is respiring but not photosynthesising.
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The rate of photosynthesis equals the rate of respiration.

This is the compensation point: carbon dioxide used equals carbon dioxide released, so there is no net change.

Check your understanding

Which sequence correctly describes the final stages of transpiration?

  1. Water evaporates from mesophyll surfaces, then vapour diffuses through stomata.
  2. Guard cells pump liquid water out through the open stomata, using energy from ATP.
  3. Water vapour enters phloem and turns into sucrose.
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Water evaporates from mesophyll surfaces, then vapour diffuses through stomata.

Keep the change of state and the subsequent movement through the pore as separate steps.

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Why does higher humidity usually reduce transpiration when other conditions are held constant?

  1. The leaf loses all its stomata.
  2. The water-vapour concentration difference between leaf air spaces and the outside air is smaller.
  3. Water vapour in humid air enters the leaf and refills the xylem, so less water needs to be drawn up.
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The water-vapour concentration difference between leaf air spaces and the outside air is smaller.

A smaller gradient reduces outward diffusion of water vapour.

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In a potometer the air bubble moves 36 mm in 4 minutes. What is the rate of water uptake?

  1. 144 mm per minute
  2. 0.11 mm per minute
  3. 36 mm per minute
  4. 9 mm per minute
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9 mm per minute

Rate = distance / time = 36 / 4 = 9 mm per minute.

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The largest bubble distance in a table is 45 mm and the grid has 10 large squares up the y-axis. Which y-axis scale is best?

  1. 5 mm per large square, so the axis reaches 50 mm.
  2. 1 mm per large square, so the axis reaches 10 mm.
  3. 20 mm per large square, so the axis reaches 200 mm.
  4. 7 mm per large square, so the axis reaches 70 mm.
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5 mm per large square, so the axis reaches 50 mm.

It is an even step, it reaches 45 mm and the points fill most of the grid.

Check your understanding

Which chain best explains wilting?

  1. Stomata close at night, which stops respiration and so the cells stop taking up water.
  2. Loss exceeds uptake -> cells lose water -> turgor falls -> soft tissues droop.
  3. Water enters cells -> turgor rises -> all tissues become flaccid.
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Loss exceeds uptake -> cells lose water -> turgor falls -> soft tissues droop.

This links the whole-plant imbalance to the cell-level mechanism and visible effect.

Check your understanding

A mature leaf supplies a developing fruit above it. Which description is correct?

  1. The fruit is the source because sources are always above sinks.
  2. Sucrose is translocated upwards in phloem from the leaf source to the fruit sink.
  3. Sucrose travels up in the xylem, because the xylem carries substances up and the phloem down.
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Sucrose is translocated upwards in phloem from the leaf source to the fruit sink.

Source-to-sink transport can be upwards or downwards in different pathways.

Explain your thinking

Write a short answer, then compare it with a sample and check for missing ideas.

Question 1 of 12

Identify transport tissue from evidence

4 marks

A herbaceous dicot stem section shows bundles in a ring. Within a bundle, tissue A is nearer the centre and contains large thick-walled vessels; tissue B is nearer the outside and contains sieve-tube elements beside companion cells. Identify A and B and explain their functions. State their typical upper/lower arrangement in a leaf vein. [4]

Use position, cell features and cargo together.

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