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Plants are large, multicellular organisms too, so they face the same scaling-up problem as animals: substances cannot reach every cell by diffusion alone. A tall tree must move water from its roots, deep in the soil, all the way up to leaves many metres above, and move sugars made in the leaves to wherever they are needed. Plants solve this with two transport tissues: xylem and phloem. This lesson, part of Topic B2 of OCR Gateway Science A, compares these tissues, looks at the leaf and its stomata, and explains transpiration — the loss of water from a plant — and how its rate is measured and affected.
By the end of this lesson you should be able to compare xylem and phloem, describe the leaf and the role of stomata and guard cells, explain transpiration and the factors affecting its rate, and describe how transpiration is measured with a potometer.
Plants have two separate transport tissues, each carrying different things in different directions. Keeping them straight is essential.
| Xylem | Phloem | |
|---|---|---|
| Carries | Water and dissolved mineral ions | Dissolved sugars (e.g. sucrose) — "food" |
| Direction | Up only (roots → leaves) | Both directions (up and down) |
| Living or dead? | Dead cells, hollow tubes | Living cells |
| Structure | Hollow tubes strengthened with lignin; no end walls | Cells joined by sieve plates (pores) |
| Process | The transpiration stream | Translocation |
Exam Tip: A reliable memory aid: xylem carries water (both start with sounds you can link) up only, and is dead; phloem carries food (sugars) both ways and is living. Use the proper terms: the xylem flow is the transpiration stream; the phloem flow is translocation.
It is worth pausing on why a plant needs xylem and phloem, because it is the same "scaling up" argument you met for animals. A large plant — a tree, say — has a small surface area to volume ratio, and most of its cells are a long way from where the substances they need are gathered. Water and minerals are absorbed at the roots, deep in the soil, but they are needed in the leaves, possibly tens of metres up. Sugars are made by photosynthesis in the leaves, but they are needed in the roots, flowers and growing tips. Diffusion alone could never move these substances quickly enough over such distances. So the plant, like a large animal, needs transport systems — here the xylem and phloem — to carry substances between where they are taken in or made and where they are used. This is exactly why plants are part of Topic B2: scaling up creates the same supply problem in plants as in animals, and transport tissues are the solution.
Most water loss, and all gas exchange, happens at the leaf. The leaf is adapted for photosynthesis but this creates a problem: to let carbon dioxide in, it must have openings, and water vapour escapes through the same openings.
On the underside of the leaf are tiny pores called stomata (singular: stoma). Each stoma is surrounded by two guard cells, which can change shape to open or close the pore:
This lets the plant control water loss: in hot, dry conditions the stomata can close to reduce water loss, though this also stops carbon dioxide entering for photosynthesis — a trade-off.
Exam Tip: Learn the link between guard cells and the stoma: turgid guard cells → stoma open; flaccid guard cells → stoma closed. Most stomata are on the lower (under) surface of the leaf, where it is cooler and shadier, which helps reduce water loss.
Transpiration is the loss of water vapour from a plant, mostly by evaporation from the leaves and diffusion out through the stomata. It is an unavoidable consequence of having stomata open to let carbon dioxide in.
Although transpiration loses the plant water, the transpiration stream it drives is useful:
The chain of events is: water evaporates from the surfaces of cells inside the leaf and diffuses out through the stomata; this water is replaced by water drawn up the xylem from the roots, creating a continuous transpiration stream from root to leaf.
It is worth tracing the whole path water takes, because it ties together several B2 ideas. Water is first absorbed from the soil by the root hair cells by osmosis (the soil water has a higher water concentration than the cell contents). It then crosses the root to the xylem, and travels up the xylem vessels — the dead, hollow, lignified tubes — as part of the transpiration stream. In the leaf, the water moves into the leaf cells, evaporates from their surfaces into the air spaces, and finally diffuses out through the open stomata as water vapour. The loss of water from the top of this chain is what keeps the whole stream moving, rather like drinking through a straw pulls liquid up from the bottom. This single journey therefore connects osmosis (root uptake), the xylem (transport) and diffusion (loss at the leaf) — a neat illustration of how the parts of B2 fit together.
Exam Tip: Be ready to put the journey of water in order: soil → root hair cell (osmosis) → xylem (up) → leaf cells → evaporation → diffusion out of stomata. Questions sometimes give the steps jumbled and ask you to sequence them.
Anything that speeds up the evaporation or diffusion of water from the leaf increases the rate of transpiration. There are four main factors:
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