植物体内的运输

Plants move substances through two transport tissues 组织:

• xylem 木质部 carries water and dissolved mineral ions 矿物离子 up from the roots.
• phloem 韧皮部 carries dissolved foods (called assimilates 同化物, mainly sugars) to wherever they are needed.

In a transverse section 横切面 (a cut straight across) of a dicotyledonous plant 双子叶植物:

• in the stem, xylem and phloem sit together in bundles near the outside, with xylem on the inside of each bundle.
• in the root, the xylem is in the centre, often in a star shape, with phloem between the arms.
• in the leaf, both are found in the veins.

When you draw a plan diagram, you draw only the outlines of the tissues, not the single cells.

Xylem vessels

Xylem water-carrying tubes are called vessels 导管. They are made of dead, empty cells joined end to end, with the end walls gone, so they form one long open pipe. Their walls are thickened and waterproofed with lignin 木质素. So the structure suits the job: the hollow, open tube with no contents lets water flow fast, and the lignin gives strength and support.

Phloem sieve tubes and companion cells

Phloem food-carrying tubes are called sieve tubes 筛管. They are living cells joined end to end, but their end walls are not gone — they become sieve plates 筛板 with many holes that sap flows through. To leave room for flow, a sieve tube cell loses most of its contents and has no nucleus.

Beside each sieve tube is a companion cell 伴胞. It keeps its nucleus 细胞核 and has many mitochondria 线粒体. It does the living work for the sieve tube and loads sugars into it.

This is what a real vascular bundle 维管束 looks like under the microscope, in a stained section of a young sunflower stem:

Water enters a root hair cell 根毛细胞 by osmosis 渗透, because the root hair has a lower water potential than the soil water. Water then crosses the root to the xylem by two pathways:

• the apoplast pathway 质外体途径 — water moves through the cell walls 细胞壁 (made of cellulose 纤维素) and the spaces between cells, without entering the cytoplasm. This is fast.
• the symplast pathway 共质体途径 — water moves through the cytoplasm of cells, passing from cell to cell through the plasmodesmata 胞间连丝.

At a ring of cells called the endodermis 内皮层, the apoplast pathway is blocked by the Casparian strip 凯氏带, a waterproof band of suberin 木栓质. This forces all the water through the cell membranes, which lets the plant control what enters the xylem.

Transpiration 蒸腾作用 is the loss of water vapour from a plant. Water evaporates (turns to vapour) from the wet cell surfaces inside the leaf — this is evaporation 蒸发. The water vapour 水蒸气 then diffuses 扩散 out through the stomata 气孔 into the atmosphere 大气.

This loss at the top pulls water up the xylem in a continuous column. It works because of hydrogen bonding between water molecules:

• water molecules attract each other through hydrogen bonds 氢键, so they stick together. This sticking is cohesion 内聚力, and it lets the whole column be pulled up under tension 张力 (the cohesion–tension idea).
• water molecules also stick to the cellulose of the cell walls. This is adhesion 附着力, which helps hold the column in place.

Worked example. A plant cell with a water potential of $-800\ \text{kPa}$ is placed in a solution of water potential $-400\ \text{kPa}$. Which way does water move, and what happens to the cell? Water always moves down a water potential gradient - from the less negative (higher) value to the more negative (lower) one. The solution at $-400$ is higher than the cell at $-800$, so water moves into the cell. The cell swells, the protoplast presses on the cell wall, the pressure potential rises, and the cell becomes turgid. Two traps: $-400$ is greater than $-800$, which is the sign error that reverses half of all answers; and pure water is the maximum at 0, so every solution is negative and a water potential can never rise above zero.

A xerophyte 旱生植物 is a plant adapted 适应 to live where water is scarce. Its leaves reduce water loss by transpiration in several ways: a thick waxy cuticle 角质层, stomata sunk in pits, hairs that trap moist air, and leaves that can roll up. You should be able to draw a labelled leaf section showing these features.

Assimilates such as sucrose 蔗糖 and amino acids 氨基酸 are carried in the phloem from a source 源 to a sink 库.

• a source is where the assimilate is made or released (for example a photosynthesising leaf).
• a sink is where it is used or stored (for example a growing root).

Loading at the source

Companion cells load sucrose into the sieve tubes against its concentration gradient. They use proton pumps 质子泵 to pump hydrogen ions out, then cotransporter proteins 协同运输蛋白 bring sucrose back in together with those ions. This is a form of active transport 主动运输.

Loading sucrose lowers the water potential 水势 inside the sieve tube, so water follows by osmosis. This raises the hydrostatic pressure 静水压 there.

Mass flow

At the sink, sucrose is removed, so the water potential rises, water leaves, and the pressure falls. The result is a pressure difference between source (high) and sink (low). Sap flows from high to low pressure down this gradient. This pressure-driven flow is called mass flow 集流.

Common mistakes.

• Saying water evaporates through the stomata. Water evaporates from the wet cell walls inside the leaf; the water vapour then diffuses out through the stomata.
• Saying the roots push or suck water up the stem. The main force is the pull from transpiration at the top, passed down the column by cohesion.
• Saying sucrose diffuses along the phloem. It is carried by mass flow of the whole sap, from high hydrostatic pressure at the source to low pressure at the sink.
• Saying phloem carries sugar only downwards. It carries assimilates from source to sink, which can be up (to a growing shoot tip) or down (to a root).
• Saying the Casparian strip stops water entering the root. It blocks only the apoplast route at the endodermis, so water must cross a cell membrane there.

• State the tissue for each direction: xylem carries water up (dead, lignified), phloem carries assimilates both ways (living sieve tubes + companion cells).
• Explain water movement by cohesion-tension: transpiration pulls a continuous water column held by cohesion (H-bonds) and adhesion.
• List the factors affecting transpiration rate (light, temperature, humidity, air movement) and how a potometer measures uptake.
• For xerophytes, link each adaptation (thick cuticle, sunken stomata, rolled leaves, hairs) to reduced water loss.