Transport in Plants
Plants move two things over long distances: water and mineral ions upwards in the xylem, and sucrose and amino acids in the phloem. Almost every question in this topic turns on knowing which tissue does which.
1. Xylem and phloem
| Xylem | Phloem | |
|---|---|---|
| Transports | Water and mineral ions | Sucrose and amino acids |
| Direction | Upwards only — roots to leaves | Both directions — source to sink |
| Cells | Dead, hollow, no end walls | Living cells |
| Walls | Strengthened with lignin | Not lignified |
| Also provides | Support for the plant | — |
Xylem adaptations
- Dead, hollow cells with no end walls, forming a continuous tube — offering little resistance to water flow
- No cytoplasm or organelles — leaves the lumen clear
- Lignin in the walls — provides strength and support, and stops the vessel collapsing under the pressure of the water column
Xylem is dead; phloem is living. This single fact answers a surprising number of questions, and gets stated the wrong way round constantly.
Positions in the plant
The arrangement differs by organ, and diagrams are examined:
| Organ | Arrangement |
|---|---|
| Root | Xylem in the centre (often star-shaped), phloem around it |
| Stem | Vascular bundles near the outside; xylem on the inside of each bundle, phloem on the outside |
| Leaf | In the vein; xylem on top, phloem underneath |
2. Water uptake
Water is absorbed from the soil by root hair cells.
Their adaptations:
| Adaptation | Why |
|---|---|
| Long, thin extension | Greatly increases the surface area for absorption |
| Thin cell wall | Short diffusion distance |
| Many mitochondria | Release energy for active transport of mineral ions |
| No chloroplasts | They are underground — no light |
Water enters by osmosis, moving from the soil (higher water potential) into the root hair cell (lower water potential) through the partially permeable membrane.
Mineral ions enter by active transport, because they are usually in lower concentration in the soil than in the cell — so they move against the concentration gradient and require energy from respiration.
Water by osmosis, mineral ions by active transport. Giving one mechanism for both is a frequent error.
3. Transpiration
Transpiration is the loss of water vapour from leaves, by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata.
The transpiration pull
Water lost from the leaves creates a pull that draws the column of water up the xylem. The water column is continuous because water molecules are attracted to each other by cohesion.
A useful working image: drinking through a straw. The water rises as a continuous column with no gaps — that is cohesion holding it together.
Factors affecting the rate
| Factor | Effect on rate | Why |
|---|---|---|
| Increased temperature | Increases | More kinetic energy, so faster evaporation and diffusion |
| Increased wind speed | Increases | Blows away water vapour, maintaining a steep concentration gradient |
| Increased light intensity | Increases | Stomata open for photosynthesis, so more water vapour escapes |
| Increased humidity | Decreases | Reduces the concentration gradient between leaf and air |
Humidity is the one that goes the other way. More water vapour in the surrounding air means a smaller difference in concentration, so less diffusion out of the leaf. In a table of four factors, humidity is the odd one out.
Measuring transpiration: a potometer measures the rate of water uptake, which is taken as a measure of transpiration.
4. Wilting
If a plant loses water faster than it can absorb it, the cells lose water and become flaccid rather than turgid. Without turgor pressure pushing outwards against the cell walls, the plant loses its support and wilts.
Wilting has a protective effect: the leaves droop and curl, reducing the surface area exposed and helping the stomata close, so less water is lost.
The cell wall is not damaged when a plant wilts. The cells have simply lost water and turgor. Saying “the cell wall breaks” is wrong — the wall stays intact throughout.
5. Translocation
Translocation is the movement of sucrose and amino acids in the phloem, from sources to sinks.
| Term | Definition |
|---|---|
| Source | A part of the plant that releases sucrose and amino acids — where they are made or released from store |
| Sink | A part that uses or stores them |
A structure can be a source at one time and a sink at another — which is exactly what questions test:
| Season | Potato tuber | Growing shoot |
|---|---|---|
| Spring | Source — stored starch is broken down to sucrose and exported | Sink — uses sucrose for growth |
| Summer | Sink — stores sucrose as starch | Leaves are the source, photosynthesising |
In spring the tuber is the source, not the sink. Students almost always reverse this, reasoning that the tuber is a storage organ so it must always be storing. Ask instead: is the sugar going in or coming out right now?
Typical sources: photosynthesising leaves; storage organs releasing their stores in spring. Typical sinks: growing roots and shoots, fruits, seeds, storage organs filling up.
6. Mistakes that cost marks
Saying xylem carries sucrose. Xylem carries water and mineral ions; phloem carries sucrose.
Saying phloem is dead. Phloem is living; xylem is dead.
Saying water enters roots by active transport. Water enters by osmosis; mineral ions by active transport.
Saying humidity increases transpiration. It decreases it.
Explaining wind without the concentration gradient.
Saying wilting damages the cell wall.
Reversing source and sink for a storage organ in spring.
Defining transpiration as “water loss from a plant.” It is loss of water vapour from the leaves, by evaporation then diffusion through the stomata.
Frequently asked questions
What is the difference between xylem and phloem? Xylem carries water and mineral ions upwards in dead, lignified vessels. Phloem carries sucrose and amino acids in both directions in living cells.
How does water get into a plant? By osmosis, into the root hair cells, from a region of higher water potential in the soil.
How do mineral ions get in? By active transport, using energy from respiration, because they move against the concentration gradient.
What is transpiration? The loss of water vapour from the leaves — evaporation from mesophyll cell surfaces, then diffusion out through the stomata.
Which factor decreases transpiration? Humidity, because it reduces the concentration gradient.
Why does a plant wilt? Cells lose water and become flaccid, losing the turgor pressure that supports the plant.
What is the difference between a source and a sink? A source releases sucrose; a sink uses or stores it. The same organ can be either, depending on the season.
Quick revision checklist
- I can compare xylem and phloem across all five rows
- I know xylem is dead and phloem is living
- I can explain how lignin and the absence of end walls suit xylem’s function
- I can describe the positions of xylem and phloem in root, stem and leaf
- I can give four adaptations of root hair cells with reasons
- I know water enters by osmosis and mineral ions by active transport
- I can define transpiration precisely — vapour, evaporation, diffusion, stomata
- I can explain the transpiration pull and cohesion
- I can explain all four factors, and know humidity works the other way
- I can explain wilting in terms of turgor, not damage
- I can define translocation, source and sink
- I can work out whether a storage organ is a source or a sink in a given season
These notes cover topic 8 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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