What is osmosis, and how do plants take up and transport water?
Osmosis as the movement of water across a partially permeable membrane, turgid, flaccid and plasmolysed plant cells, the roles of xylem and phloem, water uptake by root hair cells, and transpiration and the factors affecting it.
A focused CCEA GCSE Biology answer on osmosis and plant transport, covering osmosis across a partially permeable membrane, turgid, flaccid and plasmolysed cells, xylem and phloem, root hair cells, and transpiration.
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What this dot point is asking
CCEA wants you to define osmosis, describe turgid, flaccid and plasmolysed plant cells, explain the roles of xylem and phloem, describe how root hair cells absorb water, and explain transpiration and the factors affecting its rate.
What osmosis is
Turgid, flaccid and plasmolysed cells
When a plant cell is in a dilute solution, water enters by osmosis until the cell is firm, or turgid (the contents push against the wall, giving support). In a slightly concentrated solution the cell loses water and becomes flaccid. In a strongly concentrated solution so much water leaves that the membrane pulls away from the cell wall: the cell is plasmolysed.
Xylem and phloem
Water uptake and transpiration
Water enters the plant through root hair cells, which have a large surface area and take in water by osmosis from the dilute soil water. Water then travels up the xylem and evaporates from the leaves through the stomata. This loss of water vapour is called transpiration, and it pulls more water up the xylem (the transpiration stream).
Examples in context
Example 1. The factors that speed up transpiration. Transpiration is faster when it is warm (more evaporation), dry (a steeper water gradient out of the leaf), windy (water vapour is blown away, keeping the gradient steep) and bright (stomata open for photosynthesis). On a hot, dry, windy day a plant may lose water faster than its roots can replace it, so it wilts. Recognising and explaining these four factors is a common CCEA transpiration question.
Example 2. Why fresh vegetables go limp. A lettuce leaf left out of water loses water by evaporation and transpiration. Its cells lose water, become flaccid and can no longer support the leaf, so it goes limp. Soaking it in water lets water re-enter the cells by osmosis until they are turgid again and the leaf becomes crisp. This everyday example connects osmosis, turgor and support, exactly the chain CCEA wants you to explain.
Try this
Q1. Define osmosis. [2 marks]
- Cue. The movement of water from a dilute to a more concentrated solution across a partially permeable membrane.
Q2. Name the tissue that carries water up a plant. [1 mark]
- Cue. The xylem.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20205 marksDescribe an experiment to investigate the effect of sugar concentration on osmosis in potato cylinders.Show worked answer →
Five marks for a method that changes concentration and measures a change by mass.
Cut several potato cylinders of the same size and blot and weigh each one.
Place the cylinders in a range of sugar solution concentrations (for example 0.0, 0.2, 0.4 and 0.6 mol per litre), one per concentration, for a set time.
Remove, blot and reweigh each cylinder, and calculate the percentage change in mass.
In dilute solutions the cylinders gain mass (water enters by osmosis); in concentrated solutions they lose mass (water leaves). The concentration where there is no change equals the concentration inside the cells.
Markers reward same-size cylinders, a range of concentrations, percentage change in mass, and the trend explained by osmosis.
CCEA 20194 marksExplain how the structure of a root hair cell helps a plant absorb water.Show worked answer →
Four marks for the adaptation and the osmosis explanation.
A root hair cell has a long, thin extension that increases the surface area in contact with the soil, so more water can be absorbed.
The cell sap inside has a higher solute concentration than the dilute soil water, so water moves in by osmosis across the partially permeable cell membrane.
Many root hair cells together absorb large amounts of water and dissolved minerals.
Markers reward large surface area, osmosis down the water gradient, partially permeable membrane, and the link to absorbing more water.
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Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)