Transport in plants

• Xylem carries water and mineral ions, and supports. Phloem carries sucrose and amino acids. Transpiration is the loss of water vapour from leaves — never “water moving up the stem”.
• Evaporation is a change of state. Diffusion is a movement of vapour. Osmosis is a movement of liquid water through a membrane. All three happen in a transpiring leaf, in that order, and each earns its own mark.
• Xylem has two jobs; phloem has one. Count them on your fingers before you write: water, mineral ions, support — then sucrose, amino acids.
• Say 8.2.3 out loud as four words: hair, cortex, xylem, mesophyll. Then add the two that 8.3.2 supplies: air space, atmosphere. Then say the processes: osmosis, osmosis, carried up the xylem, osmosis, evaporation, diffusion.
• If a sentence you have written could be true of a pump, a sponge or a drinking straw, it is probably not biology. Name the tissue, name the process, name the gradient.

• State the functions of xylem — transport of water and mineral ions, and support — and the function of phloem: transport of sucrose and amino acids.
• Identify the position of xylem and phloem in diagrams and images of sections of the root, stem and leaf of a non-woody dicotyledonous plant, and say how you knew.
• Identify a root hair cell in a diagram or image, and state its function.
• State that the large surface area of root hairs increases the uptake of water and mineral ions.
• Outline the pathway taken by water through the root, stem and leaf as the four named stages: root hair cells, root cortex cells, xylem, mesophyll cells.
• Describe an investigation, using a suitable stain, that shows the pathway of water through the above-ground parts of a plant — and separate the observation from the inference.
• Describe transpiration as the loss of water vapour from leaves.
• State that water evaporates from the surfaces of the mesophyll cells into the air spaces, and that the water vapour then diffuses out of the leaves through the stomata — two processes, in that order, not merged into one.
• Describe the effects of varying temperature and wind speed on the rate of transpiration, and describe how you would investigate either one.
• Calculate a rate of water uptake from bubble movement, including the volume form when the capillary radius is given, and evaluate a potometer investigation. (Papers 5 and 6, both routes.)
• Relate the structure of a xylem vessel to its function, using the three features the syllabus lists and no others: thick walls with lignin, no cell contents, and cells joined end to end with no cross walls to form a long continuous tube.
• Explain how the loss of water vapour is related to the large internal surface area provided by the interconnecting air spaces between the mesophyll cells, and to the size and number of the stomata.
• Explain the mechanism by which water moves upwards in the xylem: a transpiration pull that draws up a column of water molecules, held together by forces of attraction between water molecules.
• Explain — not merely describe — the effects on the rate of transpiration of varying temperature, wind speed and humidity, naming which of the three mechanisms each one acts through, and say where each explanation stops being true.
• Explain how and why wilting occurs.
• Describe translocation as the movement of sucrose and amino acids in phloem, from sources to sinks.
• Describe a source as a part of the plant that releases sucrose or amino acids, and a sink as a part that uses or stores them.
• Explain why some parts of a plant may act as a source and a sink at different times.

Why wilting is sometimes useful to the plant. A wilted leaf hangs down and often curls, which reduces the surface area exposed to the sun and the moving air. Together with closed stomata, that slows further water loss. Wilting is a plant in trouble, but it is not a plant that has given up.

• 1. “Water is taken into the root by active transport.” Core 8.2.2 Why wrongActive transport moves particles against a concentration gradient using energy. Water never needs that: it moves by osmosis, from the more dilute solution to the more concentrated one, without any energy being spent. Write, CoreWater enters the root hair cell by osmosis, through the partially permeable cell membrane, because the soil solution is more dilute than the cell sap. Supplement 3.2.7 Write, Extended…from the soil solution of higher water potential to the cell sap of lower water potential, through the partially permeable cell membrane.
• 2. “Osmosis is just water moving from high to low concentration — no membrane needed.” Core 3.2.2 Why wrongWithout a partially permeable membrane the solute would simply diffuse too, and there would be no osmosis to describe. Write, CoreWater diffuses through a partially permeable membrane by osmosis, moving from the more dilute solution to the more concentrated one. Supplement 3.2.7 Write, ExtendedOsmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane.
• 3. “Xylem transports sucrose to the rest of the plant.” Core 8.1.1 Why wrongXylem carries water and dissolved mineral ions only. Sugars are carried by the phloem — a different tissue, in the same bundle. WriteXylem transports water and mineral ions from the roots to the leaves; phloem transports sucrose and amino acids from source to sink.
• 4. “Phloem transports downward, xylem upward.” Supplement 8.4.1 Why wrongDirection in the phloem is set by where the source and the sink are. A sprouting tuber sends sucrose up; a photosynthesising leaf may send it up to a shoot tip and down to a root at the same time. WritePhloem transports sucrose and amino acids from source to sink, which may be upward or downward. Core noteThe xylem half of the sentence is Core and is correct as far as it goes — but xylem carries water and mineral ions, and it supports the plant as well.
• 5. “Xylem vessels pump the water up.” Supplement 8.1.3 Why wrongA mature xylem vessel is dead and empty. It has no membrane, no cytoplasm and no mitochondria, so it cannot do anything that requires energy. WriteWater is drawn up the xylem by the transpiration pull, a tension created when water is lost from the leaves.
• 6. “Transpiration is water moving up the stem.” Core 8.3.1 Why wrongThat is the transpiration stream, which transpiration causes. Confusing the cause with its consequence loses the definition mark. WriteTranspiration is the loss of water vapour from leaves.
• 7. “The water evaporates out of the stomata.” Core 8.3.2 Why wrongTwo separate processes are being merged. Evaporation happens at the moist mesophyll cell surfaces, inside the leaf. What leaves through the stoma is vapour, and it leaves by diffusion. WriteWater evaporates from the mesophyll cell surfaces into the air spaces, and the water vapour then diffuses out through the stomata.
• 8. “Wind blows the stomata shut, so transpiration stops.” Supplement 8.3.6 Why wrongOrdinary wind increases transpiration by sweeping away the humid air just outside the leaf. Only extreme conditions, usually combined with water shortage, cause the stomata to close. WriteMoving air removes water vapour from around the leaf, keeping the concentration gradient steep, so transpiration is faster.
• 9. “Humid air has more water in it, so the plant has more water to lose.” Supplement 8.3.6 Why wrongHumidity is a property of the air outside the leaf. It has nothing to do with how much water the plant has available, and a plant in humid air is not better supplied than one in dry air. What changes is the difference between the amount of water vapour in the leaf air spaces and the amount in the air outside. WriteIn humid air the water-vapour concentration gradient between the leaf air spaces and the atmosphere is smaller, so water vapour diffuses out through the stomata more slowly and the rate of transpiration falls.
• 10. “Humid air holds more water, so the plant transpires more.” Supplement 8.3.6 Why wrongThe direction of diffusion depends on the difference between inside and outside. Humid air outside makes that difference smaller, not larger. WriteIn humid air the water-vapour concentration gradient between the leaf air spaces and the atmosphere is smaller, so transpiration is slower.
• 11. “A potometer measures transpiration.” Practical skill Why wrongIt records the water drawn into the shoot. Water loss is not measured at all; it is inferred. WriteA potometer measures the rate of water uptake, which is used as an estimate of the rate of transpiration.
• 12. “The bubble moved 30 mm, so the plant took up 30 mm3 of water.” Practical skill Why wrongA distance is not a volume. The volume depends on how wide the capillary is. WriteVolume = cross-sectional area × distance, where the area is \(\pi r^{2}\) and \(r\) is the internal radius of the capillary — half its diameter.
• 13. “It does not matter whether the shoot is cut in air or under water.” Practical skill Why wrongCutting in air lets air into the open xylem vessels. The continuous column of water is broken, so water is no longer drawn up past the air lock, and uptake falls or stops. WriteThe shoot is cut under water so that air does not enter the xylem and break the continuous water column.
• 14. “The plant has wilted, so it has died.” Supplement 8.3.7 Why wrongWilting is a loss of turgor, and it is usually reversible. Water the plant and the cells take up water by osmosis and become turgid again. WriteWilting occurs when water loss exceeds water uptake, so cells become flaccid and the tissue loses support; prolonged wilting damages the plant, but drooping alone does not mean death.
• 15. “The cell wall becomes flaccid.” Supplement 8.3.7 Why wrongThe wall is made of cellulose and does not change. What changes is the pressure of the cell contents against it. WriteThe vacuole becomes smaller and turgor pressure falls, so the cell becomes flaccid; the cell wall is still present.
• 16. “All the water a plant absorbs is lost by transpiration.” Practical skill Why wrongMost of it is, but not all. Some is used as a raw material in photosynthesis, some is retained in new tissue as the plant grows, and some stays in the cells keeping them turgid. WriteMost of the water taken up is lost as water vapour, which is why uptake is a good estimate of transpiration; a small amount is used in photosynthesis, in growth, and in maintaining turgor.

• Read the two lists side by side once before you start. The same content appears in both — transpiration, xylem, the factors — but the verbs change. Core states, identifies, outlines and describes. Supplement explains and relates. That is the whole difference, and it tells you what a Supplement answer has to contain that a Core one does not: a mechanism, and a reason.
• Subtopic 8.4 is Supplement in its entirety, and subtopic 8.2 is Core in its entirety. Those two facts between them explain most of the shape of this chapter: the water-uptake sections carry no tier blocks at all, and the translocation section is one from top to bottom.
• Where the tiering bites in Topic 8 — and it bites hard. Nine statements are Core and eight are Supplement, which is close to an even split, and the Supplement half is not scattered evenly across the topic. It is concentrated in three places: the structure of a xylem vessel (8.1.3), most of transpiration (8.3.4 to 8.3.7), and the whole of translocation (8.4). Recognise those three and you have the map.
• Answer-length trap. Topic 8's Core is built from state, identify and outline verbs, which means most Core answers are short. A five-line answer to a one-mark “state the functions of phloem” is not generous, it is a sign of not having read the command word — and the extra lines are where contradictions creep in and cost the mark that the first line had already earned.
• The shape of the answer. 8.3.4's verb is explain, and the thing to be explained is water vapour loss. So every sentence must end at loss, not at anatomy. “There are air spaces between the mesophyll cells” is a description and scores nothing on its own. “The air spaces between the mesophyll cells interconnect and give a large internal surface area, so more water evaporates from the cell surfaces into the air spaces and more water vapour can be lost” is the explanation.
• “Releases”, not only “produces”. 8.4.2(a) says a source is a part of the plant that releases sucrose or amino acids. That single verb is chosen to cover both cases at once: a photosynthesising leaf makes sucrose and releases it, while a sprouting tuber makes none at all — it breaks down stored starch and releases the sucrose. Write “produces” alone and the tuber stops being a source, which is exactly the case 8.4.3 goes on to ask about.
• A control experiment is not the same as a control variable. A control variable is something you hold constant, such as temperature. A control experiment is a whole second set-up that differs in one respect, and it is what lets you attribute the result to that respect. The stain investigation has a real one — the identical shoot standing in plain water — and without it a pink tinge in the stem proves nothing, because it might have been the plant's own pigment all along.

• Every candidate sits one multiple-choice paper, one theory paper and one practical paper. Papers 5 and 6 require the same experimental skills and the same experimental contexts as each other, so a centre's choice between them changes nothing about what you need to know.
• Answer-length trap. Topic 8's Core is built from state, identify and outline verbs, which means most Core answers are short. A five-line answer to a one-mark “state the functions of phloem” is not generous, it is a sign of not having read the command word — and the extra lines are where contradictions creep in and cost the mark that the first line had already earned.