Transport in plants

• Xylem water is pulled from the top by evaporation; phloem sap is pushed from the source by pressure. Both are mass flow of a solution, and only the phloem costs the plant ATP.
• Evaporation from the internal surfaces, then diffusion of water vapour. The Casparian strip blocks the apoplast. Loaded sucrose lowers the water potential, water follows, pressure pushes the sap from source to sink.

• 7.1.1 I can draw — draw plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides and photomicrographs
• 7.1.2 I can describe — describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants
• 7.1.3 I can draw — draw and label xylem vessel elements, phloem sieve tube elements and companion cells from microscope slides, photomicrographs and electron micrographs
• 7.1.4 I can relate — relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions
• 7.2.1 I can state — state that some mineral ions and organic compounds can be transported within plants dissolved in water
• 7.2.2 I can describe — describe the transport of water from the soil to the xylem through the: • apoplast pathway, including reference to lignin and cellulose • symplast pathway, including reference to the endodermis, Casparian strip and suberin
• 7.2.3 I can explain — explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere
• 7.2.4 I can explain — explain how hydrogen bonding of water molecules is involved with movement of water in the xylem by cohesion-tension in transpiration pull and by adhesion to cellulose in cell walls
• 7.2.5 I can make — make annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration
• 7.2.6 I can state — state that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes
• 7.2.7 I can explain — explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins
• 7.2.8 I can explain — explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink

Precise vocabulary is part of the biology. Water moves down a water potential gradient; an active site is complementary to its substrate; enzymes are denatured, not killed; ATP releases energy when it is hydrolysed, and respiration never produces energy. Give a calculated answer to the same number of significant figures as the least precise data, or one more, with its unit. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as variables, method, recording, graphs and evaluation rather than as theory.

• “Water evaporates from the stomata.” Correct Water evaporates from the wet cell walls of the mesophyll (the internal surfaces of the leaf) into the air spaces; water vapour then diffuses out through the stomata. Two steps, two processes, and the stoma is only the exit.
• “In a root the xylem and phloem form a ring of bundles, as in the stem.” Correct A root has one central vascular cylinder: xylem shaped like a star or cross in the middle, phloem between its arms, surrounded by the pericycle and the endodermis. The ring of bundles belongs to the stem.
• “Cohesion is water sticking to the walls of the xylem.” Correct Cohesion is hydrogen bonding between water molecules; adhesion is hydrogen bonding between water and the cellulose of the walls. Swap them and both statements are wrong.
• “The Casparian strip stops water getting into the xylem.” Correct It blocks only the apoplast. Water still reaches the xylem, but it must first cross a cell surface membrane into the symplast of the endodermis.
• “The proton pump pumps sucrose into the companion cell.” Correct The proton pump moves H+ out, using ATP. Sucrose comes in, with H+, through a separate cotransporter protein.
• “Phloem sap always flows down the plant.” Correct It flows from source to sink, which can be up (to a shoot tip or a developing fruit) or down (to a root), in one direction at a time in any one sieve tube.
• A plan diagram with cells drawn in it. Correct A plan diagram shows the tissues only, each as a region bounded by a line, in the correct proportions, with no cells and no shading.
• “Xylem vessels are living cells.” Repair Mature vessel elements are dead and empty. It is the sieve tube elements of the phloem that are living.
• “In the stem, the xylem is on the outside of each vascular bundle.” Repair Phloem outside, xylem inside, cambium between.
• “In a root, the xylem forms a ring.” Repair Root xylem is central and star-shaped, with phloem between its arms.
• “In the leaf, the phloem is on top of the xylem.” Repair In the midrib and each vein the xylem is uppermost, towards the upper epidermis, and the phloem below it.
• A plan diagram with cells drawn in the cortex. Repair Tissues only, each as a region bounded by a line, in the correct proportions; no cells.
• A cell drawing with each cell wall as a single line, or a nucleus drawn in a sieve tube element. Repair Walls as two lines (three where two cells touch); draw only what you can see, and a mature sieve tube element has no nucleus.
• “Root hairs take up water by active transport.” Repair Water enters by osmosis, down a water potential gradient; it is mineral ions that are mainly taken up by active transport.
• “The Casparian strip is in the xylem.” Repair It is in the radial and end walls of the endodermis, and it blocks the apoplast.
• “The apoplast pathway goes through the cytoplasm.” Repair Apoplast = cell walls and spaces between cells; symplast = cytoplasm and plasmodesmata.
• “Transpiration is the evaporation of water from the stomata.” Repair Water evaporates from the mesophyll cell walls, the internal surfaces, into the air spaces; water vapour then diffuses out through the stomata.
• “Water rises in the xylem by osmosis.” Repair It moves by mass flow, pulled by tension; osmosis happens only across a partially permeable membrane, and there is none along a vessel.
• “Cohesion is water sticking to the walls.” Repair That is adhesion. Cohesion is water to water.
• “Marram grass has its stomata on the outer surface of the leaf.” Repair On the inner surface of the rolled leaf, in grooves; the outer surface has a thick cuticle and no stomata.
• “Proton pumps pump sucrose into the companion cell.” Repair They move H+ out; sucrose enters with H+ through a cotransporter protein.
• “Water enters the sieve tube because there is more water outside.” Repair Loaded sucrose lowers the water potential of the sieve tube, so water enters by osmosis from a region of higher water potential.
• “The leaves need to lose water, so they transpire.” Repair Write the cause, not a purpose: water vapour diffuses out because the stomata are open for gas exchange and there is a water vapour potential gradient to the air.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.
• The two cells work as a unit. A mature sieve tube element has no nucleus and few organelles, which leaves an almost empty lumen for mass flow but means it cannot make its own proteins or much ATP; its companion cell supplies both through the plasmodesmata. That is the answer to why sieve tube elements depend on companion cells: the structure that suits flow is the structure that cannot maintain itself.
• Interleave with the chapters that use this one. Topic 8 (transport in mammals) is the contrast case: when you reach it, explain why blood needs a pump and xylem water does not. Topic 13 (photosynthesis) makes the sucrose this chapter loads: re-answer “why is a mature leaf a source?”. Topic 14 (14.2) explains how stomata open: re-answer “why does light increase the rate of transpiration?” with the mechanism added. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 7 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• A Paper 1 item on Topic 7 can turn on one fact in the right place: which tissue is at the centre of a root, which cell has no nucleus, which pathway the Casparian strip blocks, which protein carries sucrose, which way the hydrostatic pressure gradient runs. A Paper 2 question asks you to describe the distribution of tissues from a drawing, explain transpiration, cohesion–tension or phloem loading as a sequence of causes, compare xylem with phloem, or calculate a rate from a potometer reading.
• Photomicrographs of transverse sections to identify tissues in; electron micrographs of sieve tube elements and companion cells to label; drawings of xerophyte leaves to annotate; tables and graphs of transpiration rate against time of day or against an environmental factor; data on labelled sucrose reaching sinks. The numerical skills are recall: actual size = image size ÷ magnification, graticule calibration, volume πr²l in a capillary, rates, and percentage change.
• Paper 3: a potometer investigation — an environmental factor such as wind speed as the independent variable, the distance moved by the bubble in a fixed time as the dependent variable, the same shoot, light and temperature standardised, a shoot with sealed leaves as the control, and the shoot not reaching a steady rate as the main error; and slides of stems, roots and leaves for plan diagrams and cell drawings. Paper 5: planning or analysing a transpiration experiment, for example leaves with sealed surfaces.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.