植物体内的运输

Plants have two transport tissues that run through the roots, stems and leaves:

• xylem 木质部 — carries water and mineral ions 矿物质离子 up from the roots, and gives the plant support 支撑.
• phloem 韧皮部 — carries sucrose 蔗糖 and amino acids 氨基酸 to wherever they are needed.

In a root, stem or leaf, the xylem and phloem lie side by side in vascular bundles 维管束.

Xylem structure (Supplement)

Xylem is made of long tubes called vessels 导管, well suited to carrying water:

• their walls are thick and strengthened with lignin 木质素, which also helps support the plant.
• the cells are dead and empty (they have no cell contents), so water flows through freely.
• the cells are joined end to end with no walls across the tube, making one long, continuous pipe.

Water and mineral ions enter the plant through root hair cells 根毛细胞. The huge number of root hairs 根毛 gives a very large surface area 表面积, which speeds up the uptake of water and mineral ions.

The water then follows this pathway:

root hair cells → root cortex 皮层 cells → xylem → mesophyll 叶肉 cells in the leaf.

You can show this pathway by standing a plant or a white flower in water that contains a coloured stain 染色剂. The stain is carried up the xylem and colours the veins.

Transpiration 蒸腾作用 is the loss of water vapour 水蒸气 from the leaves.

• Water evaporates 蒸发 from the wet surfaces of the mesophyll cells into the air spaces 气腔 inside the leaf.
• The water vapour then diffuses 扩散 out of the leaf through the stomata 气孔.

What changes the rate of transpiration

A potometer 蒸腾计 measures how fast a leafy shoot takes up water. The shoot is sealed into a tube full of water; as the leaves lose water by transpiration, the shoot draws water along a thin capillary tube, and an air bubble in the tube moves towards the shoot. The distance the bubble moves in a measured time gives the rate.

Worked example. In a potometer the air bubble moves 60 mm in 5 minutes. Find the rate of water uptake, and say what happens when a fan is switched on. Rate = distance ÷ time = 60 ÷ 5 = 12 mm per minute. Always give a rate its full unit, "mm per minute", not just "12". A fan raises the wind speed, which carries water vapour away from the leaf, so transpiration speeds up and the bubble moves further in the same 5 minutes.

How water rises up the xylem (Supplement)

As water vapour leaves the leaf, it pulls more water up behind it. This pull is called the transpiration pull 蒸腾拉力. The water molecules stick to one another by forces of attraction 吸引力, so they are drawn up the xylem together as one long column 水柱 of water.

Wilting (Supplement)

If a plant loses water faster than it can take it up, its cells become soft and the plant wilts 萎蔫 — the leaves and stem droop. This often happens on a hot, dry, windy day.

Translocation 转运 is the movement of sucrose and amino acids in the phloem, from sources 源 to sinks 库.

• sources are the parts that release sucrose or amino acids — for example the leaves, which make sugar by photosynthesis.
• sinks are the parts that use or store them — for example growing roots, or a storage organ.

Common mistakes.

• Saying transpiration is the loss of water through the stomata. Be exact: water evaporates from the mesophyll cell surfaces into the air spaces, and then water vapour diffuses out through the stomata.
• Saying root hairs take up water by active transport 主动运输. Water enters by osmosis 渗透; only mineral ions can need active transport.
• Saying humid air speeds up transpiration. More humid air means a smaller difference in water vapour between the leaf and the air, so transpiration is slower.
• Saying a potometer measures transpiration directly. It measures water uptake; a little of the water taken up is used in the plant (for example in photosynthesis), so uptake is only close to the transpiration rate.
• Saying xylem carries sugar, or that phloem carries water up from the roots. Xylem: water and mineral ions, upward. Phloem: sucrose and amino acids, from source to sink, up or down.
• Giving a rate as a bare number. A bubble that moves 60 mm in 5 minutes gives 12 mm per minute, not "12".

• Xylem: water and mineral ions, upward only, plus support — its cells are dead, with thick lignin walls. Phloem: sucrose and amino acids, in living cells.
• Learn the water pathway: root hair cell → cortex → xylem → mesophyll.
• Transpiration = evaporation from the mesophyll, then diffusion out of the stomata. It is faster when hotter, windier or drier.
• Water rises by the transpiration pull; the water molecules hold together as a column.
• Translocation goes from a source (where sugar is made or stored) to a sink (where it is used). The direction can change with the season.