Transport in plants — OCR A-Level Biology
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Transport in plants explained
Transport in plants covers 9 specification points, including (a) the need for transport systems in… and (b) (i) the structure and function of the….
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Open a point below for its explanation and revision guidance.
(a) the need for transport systems in multicellular plants
Multicellular plants are large, so their surface area to volume ratio is small. Diffusion alone across the outer surface would be too slow to supply every cell with water, mineral ions and sugars, and to remove waste. Many plant cells are also deep inside the plant, far from the surface, and have low metabolic rates but still need a steady supply. Plants therefore need transport systems: the xylem carries water and mineral ions upward from the roots, and the phloem translocates sugars and other assimilates. These systems rely on the plant's own energy and on transpiration pull and pressure flow. The need arises from size, distance and the demand for efficient distribution.
(b) (i) the structure and function of the vascular system in the roots, stems and leaves of herbaceous dicotyledonous plants
Herbaceous dicotyledonous plants rely on vascular tissue to move water and minerals upward and assimilates in either direction. In roots, xylem forms a central star-shaped arrangement with phloem between the arms; in stems, vascular bundles ring the outer edge with xylem inside and phloem outside, often with cambium between; in leaves, xylem lies above phloem in the midrib and veins. Xylem vessels are dead, lignified, hollow tubes that carry water and mineral ions from root to shoot, providing support. Phloem sieve tubes with companion cells carry sucrose and amino acids to sinks such as roots, growing tips and storage organs. Recognising tissue position in each organ lets you link structure to function in multiple-choice questions.
(ii) the examination and drawing of stained sections of plant tissue to show the distribution of xylem and phloem
This practical skill requires you to examine prepared stained sections of plant tissue under a light microscope and produce an accurate, labelled drawing showing where xylem and phloem lie. Stains such as toluidine blue or safranin differentiate lignified xylem walls from other tissues, making distribution visible. Begin at low power to locate the vascular tissue, then increase magnification to observe detail. Draw with a sharp pencil using clear continuous lines, keep proportions correct, and do not shade. Label xylem, phloem and, where visible, cambium, and state the organ and magnification. Your drawing is assessed on accuracy of tissue position, correct labelling and good biological drawing technique, not artistic quality.
(iii) the dissection of stems, both longitudinally and transversely, and their examination to demonstrate the position and structure of xylem vessels
This practical requires you to cut a plant stem in two planes and use the sections to show where xylem vessels lie and how they are built. A transverse section is a cross-section cut at right angles to the stem's long axis; it reveals the ring or scattered arrangement of vascular bundles and the position of xylem within each bundle. A longitudinal section is cut parallel to the long axis, exposing elongated vessel elements running up the stem. Use a sharp blade and a fresh, supported stem, cut thin sections, and mount them in water or stain such as toluidine blue to make lignified walls visible under a light microscope. Identify xylem by thick walls, hollow lumen and position toward the inner side of each bundle. Compare the two views: the transverse section shows position and grouping, the longitudinal section shows the continuous tube-like structure.
(c) (i) the process of transpiration and the environmental factors that affect transpiration rate
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves. Water evaporates from the moist surfaces of mesophyll cells into air spaces, then diffuses out through stomata down a water-potential gradient into the surrounding air. This creates tension that pulls water up the xylem in the transpiration stream. The rate depends on environmental factors: higher temperature increases evaporation and diffusion; increased light intensity opens stomata; greater air movement (wind) removes humid air near the leaf, steepening the gradient; and higher humidity reduces the gradient, lowering the rate. Lower atmospheric pressure can also increase the rate. These factors act by changing the water-potential gradient between leaf air spaces and the external air, or by changing stomatal aperture.
(ii) practical investigations to estimate transpiration rates
Transpiration is the loss of water vapour from aerial parts of a plant, mainly through stomata. Practical investigations estimate the rate indirectly by measuring water uptake, usually with a potometer. A shoot is cut under water and sealed into a capillary tube containing an air bubble; as the shoot transpires, water is drawn along the tube and the bubble moves. The distance moved in a set time, converted to volume using the tube's cross-sectional area, gives an estimate of rate. Learners must control variables: light intensity, temperature, humidity and air movement all affect stomatal behaviour, so only one should be varied while others are held constant. Repeats and a control shoot help reliability. The estimate assumes water uptake equals water lost, which is only approximately true because some water is used in photosynthesis and turgor.
(d) the transport of water into the plant, through the plant and to the air surrounding the leaves
Water moves from soil into root hair cells by osmosis, down a water potential gradient, because the soil solution has higher water potential than root hair cell sap. It travels across the root cortex to the xylem, then up the stem in the transpiration stream. Cohesion between water molecules and adhesion to xylem walls, together with tension created by evaporation at the leaves, pull water upward. In the leaf, water moves by osmosis into mesophyll cells and evaporates from their moist surfaces into air spaces, then diffuses out through stomata as water vapour. The water potential gradient from soil to atmosphere drives the whole pathway, and the transpiration stream also carries dissolved mineral ions.
(e) adaptations of plants to the availability of water in their environment
Plants survive where water supply varies, so their structure and physiology must match the habitat. Hydrophytes, living in or on water, gain buoyancy and rapid gas exchange: air spaces (aerenchyma) in stems and leaves, stomata on upper leaf surfaces, and poor root development. Xerophytes face water shortage and reduce loss while storing or conserving water: thick waxy cuticle, sunken stomata in pits, rolled or reduced leaves, dense hairs trapping humid air, and deep or extensive roots. Mesophytes sit between these extremes. The exam asks you to link each named feature to its function in that water regime, not merely to list features.
(f) the mechanism of translocation.
Translocation is the movement of assimilates, mainly sucrose and amino acids, through the phloem from sources to sinks. Sources are regions where assimilates are produced or released, such as photosynthesising leaves; sinks are regions where they are used or stored, such as growing roots, fruits or storage organs. The accepted mechanism is mass flow driven by a pressure gradient. At the source, sucrose is actively loaded into sieve tube elements, lowering water potential so water enters by osmosis from the xylem, raising hydrostatic pressure. At the sink, sucrose is unloaded, water leaves by osmosis and pressure falls. Sieve plates and companion cells support the flow.
Your focus
- Explain why large multicellular plants need transport systems.
- Identify the substances transported by xylem and phloem.
- Relate plant size and diffusion distance to the need for transport.
Show all 27 objectives
- Identify the arrangement of xylem and phloem in root, stem and leaf cross-sections of herbaceous dicotyledonous plants.
- Relate the structure of xylem vessels to their function in water and mineral transport and support.
- Relate the structure of phloem sieve tubes and companion cells to assimilate transport.
- Examine stained sections of plant tissue using a light microscope at appropriate magnifications.
- Produce an accurate, labelled drawing showing the distribution of xylem and phloem.
- Apply correct biological drawing conventions including clear lines, proportions and labelling.
- Describe how to cut a stem both transversely and longitudinally to reveal xylem.
- Identify xylem vessels in a section by their position and structure.
- Explain how the two planes of section together demonstrate xylem position and structure.
- Describe the process of transpiration from evaporation to diffusion through stomata.
- Explain how named environmental factors change the transpiration rate.
- Relate changes in transpiration rate to the water-potential gradient and stomatal aperture.
- Describe how a potometer is used to estimate transpiration rate from bubble movement.
- Explain how bubble distance and tube cross-sectional area give a volume and hence a rate.
- Identify environmental variables that must be controlled and justify repeats and a control.
- Describe the pathway of water from soil into roots, through the plant and out of leaves.
- Explain how osmosis, cohesion, adhesion and tension move water through the plant.
- Explain how water evaporates in leaves and diffuses out through stomata as vapour.
- Identify adaptations of hydrophytes and xerophytes to their water environment.
- Explain how each named feature reduces water loss or aids water uptake.
- Distinguish adaptations suited to aquatic versus dry habitats.
- Describe the movement of assimilates from source to sink in the phloem.
- Explain how active loading and osmosis create a pressure gradient for mass flow.
- Distinguish translocation from transpiration and identify the tissues involved.
Transport in plants exam tips
Marking Points
- Multicellular plants have a small surface area to volume ratio, so diffusion across the surface is too slow to meet the needs of all cells.
- Many cells are deep within the plant, far from the external surface, so diffusion distance is too great.
- Plants need to transport water and mineral ions from roots to the rest of the plant.
- Plants need to transport sugars and other assimilates from sources (e.g. leaves) to sinks.
- Xylem transports water and mineral ions upward; phloem translocates sugars.
- Transport systems allow efficient distribution and removal of substances despite the plant's size.
- Xylem in roots is arranged centrally, often as a star shape, with phloem between the arms of the xylem.
- In stems, vascular bundles are arranged in a ring near the periphery, with xylem positioned internally and phloem externally.
- In leaves, xylem is located above phloem within the vascular bundle of the midrib and veins.
- Xylem vessels are dead, lignified, hollow structures that transport water and mineral ions and provide support.
- Phloem sieve tubes and companion cells transport assimilates such as sucrose and amino acids to sinks.
- The cambium between xylem and phloem allows secondary thickening in dicotyledonous plants.
- Locate vascular tissue first at low power, then examine at higher magnification before drawing.
- Use a sharp pencil with clear, continuous, unbroken lines and no shading to represent tissue boundaries.
- Maintain correct proportions and relative positions of xylem and phloem as seen in the section.
- Label xylem and phloem accurately, and include cambium or other visible tissues where appropriate.
- State the organ examined and the magnification used for the drawing.
- Use the stain to distinguish lignified xylem walls from surrounding tissue when interpreting distribution.
- States that a transverse section is cut at right angles to the stem's long axis and shows the position of xylem within vascular bundles.
- States that a longitudinal section is cut parallel to the stem's long axis and shows elongated, continuous xylem vessels running along the stem.
- Identifies xylem vessels by thick lignified walls and a hollow lumen, distinguishing them from phloem in the same bundle.
- Explains that thin sections mounted in water or a suitable stain are needed for clear light-microscope examination.
- Recognises that combining both planes of section gives positional and structural evidence that neither view alone provides.
- Defines transpiration as loss of water vapour from aerial parts, chiefly via stomata.
- Describes evaporation from mesophyll surfaces into air spaces followed by diffusion out through stomata down a water-potential gradient.
- Explains that rising temperature increases the rate by increasing evaporation and diffusion.
- Explains that increased light intensity opens stomata, raising the rate.
- Explains that greater air movement removes humid air, steepening the gradient and increasing the rate.
- Explains that higher humidity reduces the water-potential gradient and lowers the rate.
- Transpiration is water loss as vapour, mainly through stomata, and is estimated indirectly by measuring water uptake with a potometer.
- A shoot is cut under water and sealed into a capillary tube with an air bubble; the bubble moves as water is drawn in, and distance moved per unit time estimates rate.
- Volume transpired is calculated from bubble distance multiplied by the capillary tube's cross-sectional area, giving a rate per unit time.
- Environmental variables such as light intensity, temperature, humidity and air movement must be controlled so only the chosen variable changes.
- Repeats and a control improve reliability, and the method assumes water uptake approximately equals water lost, though some water is used in photosynthesis and turgor.
- Water enters root hair cells by osmosis from the soil, down a water potential gradient, because soil solution has higher water potential than root hair cell contents.
- Water moves across the root cortex to the xylem and then upward through the plant in the transpiration stream.
- Cohesion between water molecules and adhesion to xylem walls, combined with tension from evaporation, pull water up the xylem.
- In leaves, water moves by osmosis into mesophyll cells, evaporates into air spaces and diffuses out through stomata as water vapour.
- The continuous water potential gradient from soil to atmosphere drives movement, and the transpiration stream also transports dissolved mineral ions.
- Aerenchyma air spaces in hydrophytes provide buoyancy and a reservoir of gases for submerged tissues.
- Stomata on the upper surface of hydrophyte leaves allow gas exchange with air rather than water.
- Reduced or absent roots in hydrophytes reflect that water is absorbed over the whole surface.
- A thick waxy cuticle in xerophytes reduces cuticular transpiration.
- Sunken stomata in pits trap humid air, reducing the water potential gradient and so transpiration.
- Rolled leaves and dense hairs create a humid microclimate around stomata, lowering water loss.
- Extensive or deep root systems in xerophytes maximise water uptake from soil.
- Each adaptation must be linked to the specific water availability of the habitat, not described in isolation.
- Translocation moves assimilates such as sucrose and amino acids through the phloem.
- Sources are regions producing or releasing assimilates; sinks are regions using or storing them.
- Active loading of sucrose into sieve tubes at the source lowers water potential.
- Water enters sieve tubes by osmosis from the xylem, raising hydrostatic pressure at the source.
- Unloading of sucrose at the sink raises water potential, so water leaves by osmosis and pressure falls.
- The pressure gradient drives mass flow of sap from source to sink.
- Companion cells provide ATP for active loading; sieve plates allow flow between sieve tube elements.
Examiner Tips
- 💡Link the need for transport to surface area to volume ratio and diffusion distance.
- 💡Name the substances transported and the vessels involved (xylem, phloem).
- 💡Use the terms 'source' and 'sink' when discussing sugar transport.
- 💡Explain why diffusion alone is insufficient rather than just stating it.
- 💡Sketch a quick labelled cross-section of root, stem and leaf before answering to fix tissue positions.
- 💡Read each option carefully for organ-specific wording, since a true statement about a stem may be false for a root.
- 💡Link each tissue to its transported substance and direction to eliminate distractors quickly.
- 💡Start at low power to orientate yourself before switching to higher magnification for detail.
- 💡Label with straight, ruled lines that touch the structure they identify and do not cross other labels.
- 💡Check that xylem and phloem positions match the organ you are drawing before finalising labels.
- 💡Link each plane of section explicitly to the feature it demonstrates: position (transverse) versus continuous tube structure (longitudinal).
- 💡Use structural identifiers for xylem — thick lignified walls and hollow lumen — rather than colour alone.
- 💡When a question shows a section image, first orient it (which plane?) before naming tissues.
- 💡Always link each environmental factor to the mechanism — evaporation, diffusion gradient or stomatal aperture.
- 💡Use the term water-potential gradient rather than vague 'concentration' when explaining diffusion of water vapour.
- 💡For data questions, read the axis and units carefully before comparing rates.
- 💡State clearly that the potometer estimates rather than measures transpiration, and explain the assumption behind that estimate.
- 💡Name the controlled variables explicitly (light intensity, temperature, humidity, air movement) and say why each must be held constant.
- 💡Describe how bubble distance is converted to a rate using the tube's cross-sectional area, and mention repeats for reliability.
- 💡Describe the pathway in order: soil, root hair cell, cortex, xylem, leaf mesophyll, air spaces, stomata, atmosphere.
- 💡Link each stage to osmosis or to cohesion, adhesion and tension, using the water potential gradient to explain direction.
- 💡Mention that the transpiration stream also carries mineral ions, showing understanding of the xylem's dual role.
- 💡Read the stem to identify the habitat (hydrophyte, xerophyte or mesophyte) before choosing an option.
- 💡Reject options that name a real feature but attach the wrong function.
- 💡Where two options seem plausible, check which one correctly links structure to reduced transpiration or improved uptake.
- 💡Identify the source and sink in the scenario before reasoning about direction of flow.
- 💡Link each step to water potential and pressure changes rather than describing events vaguely.
- 💡Check that any option about the phloem names assimilates, not water or mineral ions.
Common Mistakes
- Thinking plants do not need transport because they are simple: large multicellular plants have a small surface area to volume ratio and need transport.
- Confusing xylem and phloem functions: xylem carries water and mineral ions, phloem carries sugars.
- Assuming diffusion is always sufficient: diffusion is too slow over the distances involved in large plants.
- Ignoring that many cells are deep inside the plant, increasing diffusion distance.
- Assuming xylem is always on the outside of a bundle: in stems the xylem lies towards the inside and phloem towards the outside, so check the organ before deciding position.
- Confusing the direction of phloem transport as always upward: phloem moves assimilates bidirectionally to sinks, which may be roots or growing tips.
- Describing xylem as living cells: xylem vessels are dead and lignified, which is why they form hollow tubes for water movement.
- Shading or using sketchy lines: biological drawings require clear continuous lines without shading, so redraw with a sharp pencil.
- Drawing from memory rather than observation: the drawing must reflect the actual section, so check tissue positions against the slide.
- Omitting the magnification or organ identity: these labels are needed to make the drawing scientifically meaningful.
- Confusing the two planes: a transverse section is a cross-section at right angles to the stem axis, not a lengthwise cut; correct this by linking each plane to what it reveals.
- Assuming xylem lies on the outer edge of the bundle: xylem is typically toward the inner side, with phloem outward; correct the position using the section image.
- Treating a thick hand-cut slice as adequate: thin sections are needed for transmitted light, so correct by cutting thin sections and mounting them suitably.
- Describing transpiration as a passive 'pumping' of liquid water: it is loss of water vapour, with the transpiration stream driven by tension; correct the wording.
- Saying wind 'dries' the leaf without mechanism: wind removes humid air, increasing the diffusion gradient; state the gradient.
- Claiming humidity increases transpiration: high humidity reduces the gradient and lowers the rate; correct the direction.
- Thinking a potometer measures transpiration directly: it measures water uptake, which only approximates transpiration because some water is retained for photosynthesis and turgor.
- Failing to cut the shoot under water or to seal joints, which lets air enter and breaks the water column so the bubble does not move reliably.
- Changing several environmental factors at once, so any change in bubble movement cannot be attributed to the variable being tested.
- Saying water is actively pumped up the xylem: movement is driven by tension from evaporation plus cohesion and adhesion, not by a pump.
- Reversing the water potential gradient, claiming water moves from lower to higher water potential; osmosis moves water from higher to lower water potential.
- Treating transpiration as liquid water leaving the leaf: water evaporates inside the leaf and leaves as vapour diffusing through stomata.
- Listing features without linking each to its function: the correction is to state how the feature reduces water loss or aids uptake.
- Assuming all plants reduce stomata: hydrophytes often have many stomata on the upper surface, so match the feature to the habitat.
- Confusing sunken stomata with closed stomata: sunken stomata remain open but sit in pits that trap humid air.
- Treating aerenchyma as a water-storage tissue: its main roles are buoyancy and gas storage.
- Confusing translocation with transpiration: translocation moves assimilates in phloem, transpiration moves water in xylem.
- Reversing the pressure gradient: pressure is higher at the source and lower at the sink, so flow goes source to sink.
- Saying sucrose moves by diffusion alone: the mechanism is mass flow driven by active loading and a pressure gradient.
- Treating a source and sink as fixed: the same organ can be a source or sink depending on season and stage of growth.