Questions on xylem and the transpiration stream

Question 1

Water reaches the leaves of a fifty-metre tree even though atmospheric pressure could support a column of water only about ten metres high. Explain how, using the cohesion-tension theory.

Accepted Answer

B1 water evaporates from the wet cellulose walls of the mesophyll cells into the leaf air spaces and diffuses out through the stomata B1 the water potential of that mesophyll cell falls, so it draws water osmotically from the cell next to it, and so on back to the xylem in the vein B1 water is therefore pulled out of the xylem vessel, and the column of water in the vessel is put under tension rather than being pushed from below B1 cohesion, from hydrogen bonding between water molecules, stops the column pulling apart, so removing a molecule at the leaf draws one in at the root A1 adhesion, from hydrogen bonding between water and the lignified wall, holds the column against the wall of the narrow vessel

Question 2

The capillary tube of a potometer has an internal diameter of 0.80 mm. The air bubble travels 62 mm in 4.0 minutes. Calculate the rate of water uptake by the shoot in mm³ min⁻¹.

Accepted Answer

M1 radius is half the diameter, so r = 0.40 mm M1 cross-sectional area = πr² = π × 0.40² = 0.503 mm² M1 volume swept out = 0.503 × 62 = 31.2 mm³ A1 rate = 31.2 ÷ 4.0 = 7.8 mm³ min⁻¹, accepting 7.79 from unrounded working, with the unit given

Question 3

A crop plant grown in nitrate-deficient soil shows stunted growth and yellowing that appears first in its oldest leaves. Explain both symptoms, referring to what nitrate is used to build and to its mobility within the plant.

Accepted Answer

B1 nitrate is needed to build amino acids and proteins, so a shortage limits protein synthesis and therefore growth, giving stunted growth B1 nitrate is also needed, through the amino acids and proteins built from it, for chlorophyll synthesis, so a shortage reduces chlorophyll content and causes yellowing B1 nitrogen-containing compounds are mobile in the phloem, so when supply is short the plant moves nitrogen out of older leaves to support new growth B1 this is why the yellowing shows first in the oldest leaves rather than the youngest

Question 4

Describe four precautions taken when setting up a potometer, and the reason for each.

Accepted Answer

B1 the shoot is cut and assembled under water, so that no air enters the xylem, which would break the water column B1 the stem is cut at a slant, giving a larger surface area in contact with the water for uptake B1 the leaves are dried, so that evaporation of surface water does not reduce transpiration from inside the leaf and so lower the rate of uptake B1 every joint is sealed with petroleum jelly, so that the apparatus is airtight and the bubble's movement reflects only water uptake by the shoot

Question 5

Compare the structure and function of a sclerenchyma fibre with those of a xylem vessel.

Accepted Answer

B1 both have thick walls strengthened with lignin, and both are dead at maturity with no contents B1 a sclerenchyma fibre is a single cell with tapering ends that overlap its neighbours, whereas a xylem vessel is a file of cells whose end walls have broken down to form a continuous tube B1 a sclerenchyma fibre has a very narrow lumen, whereas a xylem vessel has a wide lumen, with pits in its walls B1 both give support, but only the xylem vessel transports water and mineral ions

Question 6

Explain how the arrangement of cellulose microfibrils and the secondary thickening of the cell wall give sclerenchyma fibres and xylem vessels their strength.

Accepted Answer

B1 cellulose molecules are held in microfibrils by many hydrogen bonds between neighbouring chains, giving high tensile strength B1 once the cell stops growing, a secondary wall is laid down inside the primary wall in several layers B1 the microfibrils in each layer are parallel to each other but lie at a different angle from those in the next layer, so the wall resists forces from more than one direction B1 lignin is deposited between the microfibrils, binding them together so the wall resists compression and does not buckle, and making it waterproof

Question 7

Describe how a student could prepare and examine a section of a plant stem to identify the sclerenchyma fibres, the phloem sieve tubes and the xylem vessels using a light microscope.

Accepted Answer

B1 cut a thin transverse section of the stem with a sharp blade, cutting away from the fingers, and keep it in water B1 stain it with a stain such as toluidine blue O, so that lignified walls stain a different colour from unlignified ones, and mount it on a slide under a coverslip B1 find a vascular bundle at low power: xylem vessels are the large, empty, thick-walled cells on the inner side and the fibres are small cells with very thick walls and almost no lumen on the outer side B1 the phloem lies between them as thin-walled, unlignified cells, the sieve tubes with small companion cells beside them

Question 8

Wheat seedlings were grown for four weeks in a complete culture solution or in a solution lacking magnesium ions. The mean dry mass per plant was 120 mg in the complete solution and 78 mg in the solution lacking magnesium; the results are invented for this question. Calculate the percentage decrease in dry mass caused by the lack of magnesium, and explain why the lack of magnesium reduced the dry mass.

Accepted Answer

M1 (120 − 78) ÷ 120 × 100 A1 a decrease of 35% B1 magnesium ions are needed to make chlorophyll, so less chlorophyll is made B1 less light is absorbed, so the rate of photosynthesis falls and less carbohydrate is made for growth

Question 9

Describe the apoplast and symplast routes taken by water across the root cortex, including the point on each route at which water has to cross a plasma membrane.

Accepted Answer

B1 on the apoplast route water travels through the cellulose cell walls and the spaces between them, without entering the cytoplasm of any cell B1 on the symplast route water travels through the cytoplasm, passing from one cell to the next through plasmodesmata A1 the symplast route crosses a plasma membrane at the root hair cell, while the apoplast route crosses none until the Casparian strip forces water into the cytoplasm of an endodermal cell

Question 10

A student records a potometer reading and writes it down as the rate of transpiration. Explain why this will not score, and explain whether the true rate of transpiration was above or below the figure she recorded.

Accepted Answer

B1 the bubble moves because the cut shoot is drawing water in, so the quantity measured is the rate of water uptake B1 a small fraction of the water taken up never leaves the shoot: some is a raw material in photosynthesis and some is retained in cells that are expanding or staying turgid A1 uptake therefore slightly exceeds loss, so the true rate of transpiration was below the figure she recorded

Question 11

Air enters one xylem vessel of a transpiring tree and the water column in that vessel breaks, yet the leaves above it go on receiving water. Suggest how.

Accepted Answer

B1 the column in that vessel is no longer continuous, so tension cannot be transmitted through it and it stops carrying water B1 pits are gaps where lignin was not laid down, so water can pass sideways from one vessel into its neighbour A1 water detours through the pits into an intact vessel alongside, which is why many narrow vessels survive damage that would cripple a single wide one

Question 12

Describe the structure of a root hair cell, and describe how this structure increases the surface area available for absorbing water and mineral ions.

Accepted Answer

B1 a root hair cell is an epidermal cell of the root, extended into a long, narrow projection, the root hair B1 thousands of these hairs occur per centimetre of root, greatly increasing the surface area of the root in contact with the soil solution B1 the hairs extend into the spaces between soil particles, reaching water an unmodified root surface could not reach

Question 13

Compare root pressure with the transpiration pull as mechanisms for moving water up a plant, referring to where each is generated and how far each could move water on its own.

Accepted Answer

B1 root pressure is generated in the root, by the active pumping of ions into the xylem lowering its water potential so water follows by osmosis, whereas the transpiration pull is generated in the leaf, by evaporation from the mesophyll cell walls B1 root pressure produces only a modest positive pressure, enough to push water a few metres at most, whereas the tension generated by the transpiration pull is large enough to draw water to the top of a tree many tens of metres tall B1 root pressure depends on respiration and active transport and stops if the roots are short of oxygen or poisoned, whereas the cohesion-tension mechanism needs no ATP for the movement of water itself, only cohesion between water molecules

Question 14

Explain three functions of the lignin deposited in the walls of a xylem vessel.

Accepted Answer

B1 it stiffens the wall, so the tube resists collapsing inwards under the atmospheric pressure squeezing a tension-filled vessel B1 it waterproofs the wall, so water travelling up the vessel does not leak out sideways along its whole length B1 it is mechanically very strong, so lignified xylem also functions as the plant's main supporting tissue, which is why wood is largely old xylem

Question 15

Compare a xylem vessel with a tracheid, referring to their end walls and to how each behaves when an embolism occurs.

Accepted Answer

B1 a vessel's end walls break down completely, forming one continuous open tube, whereas a tracheid keeps its tapered end walls and water must pass through pits between cells B1 a vessel offers lower resistance to flow because of its continuous open bore, whereas a tracheid, with end walls in the way, resists flow more B1 an embolism in a vessel can disable a long length of tube at once, whereas in a tracheid an embolism spreads less easily, because each cell is a separate short unit connected only through pits

Question 16

A researcher wants to use potometers to estimate the rates of transpiration of two different shoots, one from a sun-loving plant with small leaves and one from a shade plant with large leaves. Suggest why comparing the raw rate of water uptake from each potometer would not by itself show which species transpires faster per unit of leaf area, and suggest what additional measurement would fix this.

Accepted Answer

B1 a shoot with a larger leaf area has more stomata and more surface for evaporation, so it is likely to take up more water simply because it is bigger, regardless of any real difference between the species B1 comparing the raw uptake rates therefore confounds species identity with leaf area, so a higher reading for one shoot might reflect its size rather than a real difference between the species B1 measuring the leaf area of each shoot and expressing the rate as water uptake per unit leaf area allows a fair comparison between the two species

Question 17

Describe the positions of the xylem, the phloem and the sclerenchyma fibres in a transverse section through a young dicotyledonous stem.

Accepted Answer

B1 the xylem, phloem and fibres lie together in vascular bundles arranged in a ring near the outside of the stem, around a central pith B1 in each bundle the xylem is on the inner side, towards the pith, and the phloem lies outside it, with cambium between them B1 the sclerenchyma fibres form a cap on the outer side of the phloem, towards the epidermis

Question 18

The table describes three types of cell, A, B and C, seen in sections of a plant stem. Identify each type of cell.

Accepted Answer

B1 A is a xylem vessel element B1 B is a phloem sieve tube element B1 C is a sclerenchyma fibre

Question 19

A student investigating plant mineral deficiencies grew seedlings in culture solutions in boiling tubes. Explain why the student wrapped each tube in aluminium foil, made up and topped up every solution with distilled water rather than tap water, and waited four weeks before recording the results.

Accepted Answer

B1 the foil keeps light out of the solution, so algae cannot grow in it and take up the mineral ions B1 tap water contains mineral ions, which could supply the ion that a solution is meant to lack B1 the seed contains its own store of mineral ions, so deficiency symptoms take weeks to appear

Question 20

A potted plant shows yellowing between the veins of its oldest leaves, while the youngest leaves stay green. Identify the mineral ion most likely to be in short supply, and account for the older leaves being affected first.

Accepted Answer

A1 magnesium ions, which sit at the centre of every chlorophyll molecule B1 magnesium travels in the phloem, so a plant running short strips it out of old leaves to supply new growth

Question 21

Describe two pieces of observational evidence, other than the existence of tension itself, that support the cohesion-tension theory of water movement in xylem.

Accepted Answer

B1 any one of: a tree trunk is measurably narrower at midday than at night, showing the xylem is being pulled inward under tension rather than pushed outward as root pressure would; cutting a stem of a transpiring plant draws air in rather than forcing water out, showing the sap is below atmospheric pressure; or letting an air bubble into a vessel stops water above it moving, showing the mechanism depends on a continuous column B1 any second, different piece of evidence from the same list

Question 22

In a young herbaceous stem the sclerenchyma fibres lie in a ring near the outside of the stem. Suggest why this position gives more support than the same fibres would give at the centre of the stem.

Accepted Answer

B1 when the stem bends, for example in wind, the tissue near the surface is stretched or compressed the most B1 stiff fibres placed in a ring near the outside resist that stretching and compression where it is greatest, so the stem resists bending better

Question 23

Name the band of waterproof material in the walls of the endodermal cells that blocks the apoplast route into the xylem.

Accepted Answer

A1 the Casparian strip, a band of suberin

Cell	Living when working	Wall lignified	Ends of the cell
A	No	Yes	End walls absent; joined into a long open tube
B	Yes	No	End walls perforated with pores
C	No	Yes	Tapering, overlapping the ends of neighbouring cells