Transport in Plants: Biology 0610 (Cambridge O Level / IGCSE)

A student plants a young sunflower seedling in moist soil. Some of the water taken up by the roots eventually reaches cells inside a leaf, where it evaporates.

Which row shows the correct order of the structures that this water passes through, from the soil to the leaf?

A gardener wants to see where water travels inside a cut stem. She stands the cut stem of a white chrysanthemum flower in a jar of water with red food dye stirred into it, and leaves it overnight. The stem, flower head and petals remain attached to each other as one continuous piece of plant throughout.

(a) Name the plant tissue responsible for carrying the dye up the stem, and state one other substance this tissue normally transports in an intact, growing plant. [2]

(b) The next morning, the gardener slices across the stem. Instead of one solid red patch in the centre, she sees a ring of separate small red dots arranged in a circle just inside the outer layer of the stem. Explain what this pattern tells her about the position of the tissue named in (a) in this stem. [2]

(c) She then examines one of the flower's white petals with a hand lens and sees the red dye has spread along thin veins running through the petal. Describe the pathway the dye follows, in order, from the water in the jar until it reaches these petal veins. [2]

A student examines a longitudinal section of stem tissue under a microscope and identifies a group of long, hollow, tube-like cells joined end to end. Her teacher explains that these are xylem vessels, and that the water they carry is pulled upward mainly by a process happening in the leaves.

(a) Describe three structural features of xylem vessels that can be seen or inferred from this description. [3]

(b) Explain how two of the features you gave in (a) help the xylem to carry a continuous stream of water from the root to the leaf. [2]

(c) Explain the mechanism, called transpiration pull, by which water is drawn upward through the xylem, referring to the loss of water vapour from the leaves and to the properties of water molecules. [3]

A class investigates transpiration using leafy shoots of the same species and size. Each shoot stands in its own small flask of water, and a thin layer of oil floats on the water surface in every flask. Each flask and its shoot are weighed at the start, then reweighed after one hour, and the mass lost is recorded.

Set-up A: room temperature, still air, low humidity. Mass lost in one hour was 4.8 g. Set-up B: room temperature, air moved by a fan, low humidity. Mass lost in one hour was 6.5 g. Set-up C: room temperature, still air, high humidity (inside a humid enclosed box). Mass lost in one hour was 2.1 g.

(a) Using set-ups A and B, describe the effect of wind speed on the rate of transpiration. [2]

(b) Using set-ups A and C, describe and explain the effect of humidity on the rate of transpiration. [3]

(c) Suggest why the layer of oil on the water surface in each flask was necessary for this investigation to give a valid measure of transpiration. [2]

In an actively growing tomato plant, mature leaves lower down the main stem are photosynthesising strongly and loading sucrose into the phloem. Some of this sucrose travels upward through the phloem to a cluster of small, actively dividing leaves near the shoot tip, while at the same time other sucrose travels downward through the phloem to actively dividing cells in the root tips.

Which statement correctly explains why sucrose moves in two different directions within the phloem of this same plant?

A plant is moved from a cool room at 15°C to a warmer room at 28°C. The light intensity, humidity and air movement are kept the same in both rooms.

Which statement correctly describes and explains the effect of this change on the plant's rate of transpiration?

A student digs up a young bean seedling and notices that the region of the root just behind the growing tip is covered in thousands of tiny, thread-like outgrowths, each too small to see clearly without a hand lens. Her teacher explains that these are root hair cells, and that they are responsible for most of the water absorbed by the plant.

(a) Describe two structural features of a root hair cell that adapt it for absorbing water from the soil. [2]

(b) A root of a given length has thousands of small root hair cells covering it, rather than a single continuous absorbing surface of the same total length. Explain why having many small root hairs, rather than one smooth surface, increases the rate at which the root as a whole can take up water. [2]

(c) A pest destroys most of the root hairs on this seedling's roots, leaving the rest of the root surface undamaged. Suggest, with a reason, what effect this would have on the plant's rate of water uptake. [2]

A student cuts a thin cross-section through a plant stem and, under a microscope, identifies two different vascular tissues within the same vascular bundle: one made of dead, hollow, lignified vessels, and the other made of living cells connected end to end through perforated cross-walls, each living cell lying next to a smaller companion cell.

(a) State two structural differences between the xylem vessels and the phloem sieve tube elements described above. [2]

(b) State the main substances carried by each tissue, and the direction in which each transports its main substances through the plant. [2]

(c) Sieve tube elements are living cells, but each one has a companion cell attached to it, connected by many fine strands of cytoplasm. Explain why sieve tube elements need a companion cell alongside them, given the structural differences you described in (a). [3]

On a hot, dry, windy afternoon, a gardener notices that the leaves and stem of an unwatered potted plant have become soft and have drooped downward, even though the same plant looked upright and healthy that morning.

Which statement best explains why the plant has wilted?

A student sets up a potometer, an instrument that measures the rate at which a leafy shoot takes up water by tracking how far a small air bubble moves along a graduated capillary tube in a fixed time. She assumes that, over the course of each short trial, the rate of water uptake gives a reasonable estimate of the shoot's rate of transpiration. The same shoot is used throughout, and she records the distance the bubble moves in five minutes under three different conditions.

Set-up P: high light intensity, 20°C, bubble moved 45 mm in 5 minutes. Set-up Q: low light intensity (shoot covered with a black cloth), 20°C, bubble moved 18 mm in 5 minutes. Set-up R: high light intensity, 30°C, bubble moved 63 mm in 5 minutes.

(a) Using set-ups P and Q, describe and explain the effect of light intensity on the rate of water uptake by the shoot. [3]

(b) Using set-ups P and R, describe the effect of temperature on the rate of water uptake by the shoot. [2]

(c) Explain why the rate at which the bubble moves along the potometer tube gives only an estimate of the shoot's rate of transpiration, rather than an exact, direct measurement of it. [2]