How does water move up a plant in the xylem, how is sugar moved in the phloem, and how do xerophytes limit water loss?
3.1.3 Transport in plants: the structure and function of xylem and phloem; the cohesion-tension theory of water transport in the xylem and the factors affecting transpiration; the mass flow hypothesis of translocation in the phloem from source to sink; and the adaptations of xerophytes for reducing water loss.
A focused answer to the OCR H420 3.1.3 dot point on transport in plants. Covers xylem and phloem structure, the cohesion-tension theory of transpiration, the factors affecting transpiration rate, the mass flow hypothesis of translocation, and xerophyte adaptations.
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What this dot point is asking
OCR wants you to describe the structure and function of xylem and phloem, explain water transport up the xylem by the cohesion-tension theory and the factors affecting transpiration, explain translocation in the phloem by the mass flow hypothesis from source to sink, and describe the adaptations of xerophytes that reduce water loss.
The answer
Xylem and phloem structure
- Xylem carries water and dissolved mineral ions from roots to leaves in one direction (upwards). Xylem vessels are dead, hollow tubes formed from cells whose end walls have broken down; their walls are lignified (waterproof and strong), which prevents collapse under tension and stops water leaking out, leaving pits for lateral movement.
- Phloem carries dissolved organic solutes (mainly sucrose) in either direction, from source to sink. Sieve tube elements are living cells joined end to end through perforated sieve plates; they have little cytoplasm and few organelles to allow flow. Each is supported by a companion cell that provides ATP and metabolic support (the two are linked by plasmodesmata).
The cohesion-tension theory
Water moves up the xylem because of evaporation at the top, not pushing from below:
- Water evaporates from the moist walls of mesophyll cells and diffuses out through the stomata (transpiration).
- This lowers the water potential of the mesophyll cells, so water moves into them by osmosis from the xylem, putting the xylem water under tension (negative pressure).
- Water molecules are attracted to one another by hydrogen bonding (cohesion), so they move as a continuous column; they also stick to the xylem walls (adhesion).
- The tension pulls the whole column up, and water enters the roots from the soil down a water potential gradient. This continuous flow is the transpiration stream.
Factors affecting transpiration
Transpiration rate rises with anything that steepens the water vapour gradient or speeds diffusion:
- Light opens the stomata, so the rate increases.
- Temperature increases the kinetic energy of water molecules and the rate of evaporation, so the rate increases.
- Wind (air movement) removes water vapour from around the stomata, keeping the gradient steep, so the rate increases.
- Humidity raises the water vapour concentration outside the leaf, reducing the gradient, so the rate decreases.
A potometer measures water uptake (taken as a proxy for transpiration).
The mass flow hypothesis of translocation
Translocation moves assimilates (sucrose) through the phloem from a source (where they are made or released, for example a photosynthesising leaf) to a sink (where they are used or stored, for example a root or growing tissue):
- At the source, companion cells actively load sucrose into the sieve tubes (using ATP). This lowers the water potential there, so water enters from the xylem by osmosis, raising the hydrostatic pressure.
- At the sink, sucrose is removed (used or stored), raising the water potential, so water leaves and the hydrostatic pressure falls.
- The resulting pressure gradient drives the bulk flow (mass flow) of the phloem solution from the high-pressure source to the low-pressure sink.
Xerophyte adaptations
Xerophytes are plants adapted to dry conditions, with features that reduce transpiration:
- a thick waxy cuticle to reduce evaporation;
- sunken stomata in pits, and leaf hairs, which trap moist air and reduce the water potential gradient;
- rolled leaves (for example marram grass) enclosing a humid space around the stomata;
- a reduced leaf area (for example spines) to reduce the surface area for water loss;
- some store water in fleshy tissues (succulents).
Examples in context
Example 1. Marram grass on sand dunes. Marram grass rolls its leaves so the stomata face inwards into a humid pocket of trapped air, with hairs and sunken stomata reducing the water potential gradient, a textbook set of xerophytic adaptations.
Example 2. Ringing a tree trunk. Removing a ring of bark (which contains the phloem) stops sugar moving down to the roots; sugars accumulate above the ring and the roots eventually starve, classic evidence that the phloem carries assimilates from source to sink.
Try this
Q1. Explain why xylem vessels are lignified. [2 marks]
- Cue. Lignin waterproofs and strengthens the walls, preventing the vessel collapsing under the tension of the transpiration stream and stopping water leaking out.
Q2. State two factors that increase the rate of transpiration and explain one of them. [3 marks]
- Cue. Increasing light, temperature or wind (and decreasing humidity) increase the rate; for example wind removes water vapour from around the stomata, keeping the water potential gradient steep so diffusion is faster.
Q3. Name the process by which sucrose is loaded into the phloem at the source. [1 mark]
- Cue. Active loading (active transport) by the companion cells.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/02 20196 marksExplain how water is transported from the roots to the leaves of a plant according to the cohesion-tension theory.Show worked answer →
Start at the leaf, build the tension, then explain the unbroken column.
Water evaporates from the moist cell walls of the mesophyll and diffuses out through the stomata (transpiration). This lowers the water potential of the mesophyll cells, so water moves into them from the xylem by osmosis, creating a tension (negative pressure) that pulls the column of water up the xylem.
Water molecules are held together by hydrogen bonding (cohesion), so they move as a continuous column; they also adhere to the lignified xylem walls. The tension is transmitted all the way down to the roots, where water is drawn in from the soil down a water potential gradient.
Markers reward evaporation creating tension, cohesion between water molecules forming a continuous column, and the resulting transpiration stream from root to leaf.
OCR H420/02 20214 marksA student measured the rate of water uptake by a shoot using a potometer at different wind speeds. Explain the effect of increasing wind speed on the transpiration rate.Show worked answer →
Link wind speed to the water vapour gradient at the stomata.
Increasing wind speed increases the transpiration rate. Moving air removes the water vapour that accumulates just outside the stomata, maintaining a steep water potential (concentration) gradient between the moist air spaces inside the leaf and the drier air outside.
A steeper gradient means water vapour diffuses out of the stomata faster, so more water evaporates from the mesophyll and more is pulled up and taken in by the shoot.
A potometer measures water uptake, which is assumed to be approximately equal to transpiration. Markers reward removal of water vapour, a maintained or steeper gradient, and a faster rate of diffusion out of the stomata.
Related dot points
- 3.1.1 Exchange surfaces: the need for specialised exchange surfaces as size and metabolic rate increase and surface-area-to-volume ratio falls; the features of an efficient exchange surface; the structure and function of the mammalian gas-exchange system, the counter-current system in fish gills, and the tracheal system of insects.
A focused answer to the OCR H420 3.1.1 dot point on exchange surfaces and gas exchange. Covers why surface-area-to-volume ratio drives the need for exchange surfaces, the features of an efficient surface, the mammalian lung, the fish counter-current system and the insect tracheal system.
- 3.1.2 Transport in animals: the structure and functions of arteries, arterioles, capillaries, venules and veins; the formation of tissue fluid from plasma at the arterial end of a capillary bed and its return at the venous end and via the lymphatic system, explained in terms of hydrostatic and oncotic (osmotic) pressure.
A focused answer to the OCR H420 3.1.2 dot point on blood vessels and tissue fluid. Covers the structure and function of arteries, arterioles, capillaries, venules and veins, and how hydrostatic and oncotic pressure form tissue fluid at the arterial end and return it at the venous end and via the lymph.
- 2.1.2 Biological molecules: the properties of water and their importance to living organisms; the structure of monosaccharides, the formation of glycosidic bonds by condensation, and the structure and function of starch, glycogen and cellulose; the biochemical tests for reducing and non-reducing sugars and for starch.
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- 5.2.1 Photosynthesis: the structure of the chloroplast; the light-dependent stage (photolysis of water, photophosphorylation and the reduction of NADP); the light-independent stage (the Calvin cycle, fixing carbon dioxide using RuBP, forming GP and TP and regenerating RuBP); and the effect of limiting factors (light intensity, carbon dioxide concentration and temperature).
A focused answer to the OCR H420 5.2.1 dot point on photosynthesis. Covers chloroplast structure, the light-dependent stage (photolysis, photophosphorylation and reduced NADP), the light-independent stage (the Calvin cycle with RuBP, GP and TP), and the effect of limiting factors.
- 4.2.2 Evolution: the process of evolution by natural selection acting on variation; the role of mutation in generating variation; the types of natural selection (directional, stabilising and disruptive); the evidence for evolution from fossils, comparative anatomy and molecular biology; and examples such as antibiotic resistance and industrial melanism.
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Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)