Water-carrying and food-carrying tubes and their functions. How water and food are transported through the plant. Linked to photosynthesis and respiration.
water transportfood transportplant parts
Plant transport system — the two tubes
Water-carrying tubes
Carry water and minerals absorbed by roots Direction: roots → stem → leaves Water used in photosynthesis; exits as vapour through leaves
Food-carrying tubes
Carry food (sugar) made in leaves Direction: leaves → rest of plant Food used for growth and energy throughout the plant
Part A — Multiple choice (Booklet A style · 2 marks each)
Which row in the table correctly describes the two transport tubes in a plant?
A
B
C
D
Water-carrying tubes transport water AND minerals (not water alone). Food-carrying tubes carry the sugar produced by photosynthesis in the leaves. Option B is the common mix-up — students sometimes reverse the two tubes.
Question 2 — MCQ (2 marks)
Booklet ADirection of transport
A plant is cut at the stem and the cut end is placed in red-coloured water. After a few hours, the leaves and petals turn red. Which statement best explains this observation?
AThe food-carrying tubes transported the red water from the roots to the leaves.
BThe water-carrying tubes transported the red water from the stem upwards to the leaves and petals.
CThe food-carrying tubes transported sugar mixed with red water from the leaves downwards.
DThe red water was absorbed through the leaves directly from the air.
This is the classic celery/white carnation experiment. Red-coloured water travels through the water-carrying tubes — upward from stem to leaves and petals — making them turn red. This proves that water-carrying tubes move water upward through the plant.
The diagram shows the transport system inside a plant stem. Two types of tubes — W and F — are present.
(a) State what is transported in tube W and where it comes from. (1 mark)
(b) Explain why the substance in tube F must be transported to all parts of the plant, including the roots. (2 marks)
(a)Tube W carries water and minerals, absorbed from the soil by the roots.
(b)Tube F carries food (sugar) made by the leaves during photosynthesis. All parts of the plant — including the roots — need food to obtain energy through respiration and to grow and repair cells. Since roots are in the soil and cannot photosynthesise (no light), they depend entirely on food transported down from the leaves.
W = water + minerals (up) | F = food/sugar (down to all parts)
The key reasoning for (b): roots are underground — they get no light and cannot make their own food. They must receive it via the food-carrying tubes from the leaves.
A student removes a ring of bark (outer layer) from around the trunk of a tree. After several weeks, the leaves remain green and healthy, but the roots begin to die. The diagram shows the tree before and after ring-barking.
(a) The leaves remain green and healthy. What does this tell us about which type of transport tube is affected by ring-barking? Explain your reasoning. (2 marks)
(b) Why do the roots die after ring-barking? (2 marks)
(a)Ring-barking blocks the food-carrying tubes (which are in the bark/outer layer). The water-carrying tubes (in the inner wood) remain intact, so water and minerals continue to reach the leaves — allowing photosynthesis to continue and keeping the leaves green and healthy.
(b)With the food-carrying tubes blocked, sugar made in the leaves cannot travel downward past the ring to the roots. The roots are starved of food and cannot obtain energy through respiration to carry out life processes. Without energy, the roots die.
A key PSLE fact: The food-carrying tubes are in the bark (outer layer); the water-carrying tubes are in the wood (inner part of stem). Ring-barking removes the bark — so it cuts off food transport but not water transport. This is why leaves survive but roots die.
A student places a well-watered plant in a dark cupboard for 48 hours, then tests a leaf for sugar. No sugar is found. She then moves the plant to a sunny windowsill for 6 hours and tests another leaf — sugar is now present.
(a) Suggest a hypothesis the student could have been testing with this experiment. (1 mark)
(b) After the plant is returned to light, which transport tube carries the sugar away from the leaves? State the direction of transport. (1 mark)
(c) The student notices that after 6 hours in sunlight, the leaves appear slightly wilted (droopy) even though the soil is moist. Using your knowledge of plant transport, explain why this might happen. (2 marks)
(d) Another student claims: "The roots must also contain sugar because sugar travels down from the leaves." Explain how you could test this claim. State the changed variable and what you would measure. (1 mark)
(a) HypothesisLight is needed for plants to produce sugar (through photosynthesis). (Also accept: "If a plant is kept in the dark, it will not produce sugar.")
(b)The food-carrying tubes carry sugar away from the leaves, in a downward direction from leaves to the rest of the plant (stem, roots).
(c)In bright sunlight, the rate of water loss through the leaves (as water vapour) increases. If water is lost faster than the water-carrying tubes can supply it from the roots, the plant does not have enough water in its cells to keep them firm — causing the leaves to wilt. This can happen even when soil is moist, if the demand for water exceeds the supply rate.
(d)Test the roots of a plant for sugar. Changed variable: whether the plant has been in light or dark. Measure: presence or absence of sugar in root tissue (using a sugar test). Compare roots from a plant kept in light vs a plant kept in darkness for 48 hours.
Light → photosynthesis → sugar in leaves → food tubes carry it downward
Part (c) links plant transport to a real observable effect — wilting. The chain is: high sunlight → faster water evaporation from leaves → water demand exceeds supply rate → cells lose firmness → wilting. This multi-step reasoning is exactly what Booklet B high-mark questions test.
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