Transport mechanisms

Syllabus points

• Explain the movement of water from soil to xylem by the apoplast and symplast pathways.
• Explain the cohesion-tension theory of water transport in the xylem.
• Explain transpiration and the factors affecting its rate.
• Explain translocation in the phloem by the mass flow hypothesis.
• Describe the adaptations of xerophytes.

From soil to root xylem

Water enters through root hairs, which are long thin extensions of epidermal cells. They give an enormous surface area and reach between soil particles.

Water enters by osmosis, because the soil solution is dilute and has a higher water potential than the cytoplasm of the root hair cell.

Water then crosses the root cortex to the xylem by two routes.

The apoplast pathway. Water travels through the cell walls and the spaces between them, never entering a cell. Cellulose walls are permeable and there is a continuous network of them, so this route is fast and offers little resistance. Most water travels this way.

The symplast pathway. Water travels through the cytoplasm, moving from cell to cell through the plasmodesmata. This is slower, because it means crossing cytoplasm rather than open wall.

A third route, the vacuolar pathway, passes through the vacuoles as well, and is often treated as part of the symplast route.

The Casparian strip

At the endodermis, the layer of cells surrounding the vascular tissue, each cell wall carries a band of suberin called the Casparian strip. Suberin is waxy and impermeable to water.

This blocks the apoplast pathway. Water travelling through the walls reaches the strip and can go no further, so it is forced into the cytoplasm, joining the symplast pathway to get past.

That is the point of the whole arrangement. Water passing through a cell surface membrane is water the plant can filter, because the membrane is partially permeable and the transport proteins in it are selective. The plant can therefore control which mineral ions reach the xylem and exclude what it does not want. Water crossing in the walls could not be controlled at all.

Note the two materials and do not swap them: suberin is in the Casparian strip, and lignin is in the xylem wall.

Up the stem: cohesion-tension

Water is not pushed up the plant. It is pulled, and the pull comes from the leaves.

1. Water evaporates from the wet cell walls of the spongy mesophyll into the air spaces of the leaf, and diffuses out through the stomata. This is transpiration.
2. This lowers the water potential of the mesophyll cells, so they draw water from neighbouring cells and ultimately from the xylem.
3. Water is pulled out of the top of the xylem, putting the whole column under tension.
4. Because water molecules are polar, they hydrogen bond to one another. That is cohesion, and it means the column behaves as a single thread: pull on the top and the whole column moves.
5. Water molecules also hydrogen bond to the lignin and cellulose of the vessel wall. That is adhesion, and it helps support the column.
6. Water is drawn in at the roots to replace what left.

The evidence for tension is that a tree trunk has a smaller diameter during the day, when transpiration is fastest, because the vessels are being pulled inward. Lignin is what stops them collapsing entirely.

Root pressure exists and can push water a short distance, but it cannot account for water reaching the top of a tall tree. Cohesion-tension can.

Transpiration

The loss of water vapour from the aerial parts of a plant, mainly through the stomata.

It is an unavoidable consequence of gas exchange: a leaf must open its stomata to let carbon dioxide in for photosynthesis, and water inevitably escapes through the same pores.

The factors

Every one of these works through the same underlying quantity: the water potential gradient between the leaf air spaces and the atmosphere. An answer that names that gradient is stronger than one that lists effects.

A potometer measures the rate of water uptake, which is not quite the same as transpiration, since a small proportion of water is used in photosynthesis and to keep cells turgid. Uptake is a very close approximation and is what the apparatus actually records.

Translocation

The transport of assimilates, chiefly sucrose, through the phloem from a source to a sink.

A source is anywhere sucrose is made or released, such as a photosynthesising leaf or a storage organ in spring. A sink is anywhere it is used or stored, such as a root, a growing shoot or a developing fruit.

Because sources and sinks change with the season, phloem transport is bidirectional, whereas xylem transport is always upward.

Loading at the source

Sucrose is loaded into the sieve tube actively, not by diffusion, and the standard mechanism is co-transport:

1. ATP in the companion cell is used to actively pump hydrogen ions out of the companion cell into the surrounding cell walls.
2. This builds up a high concentration of hydrogen ions outside.
3. The hydrogen ions diffuse back in through a co-transport protein, which carries a sucrose molecule with them as they move down their gradient.
4. Sucrose accumulates in the companion cell and passes into the sieve tube element through the plasmodesmata.

This is why companion cells carry so many mitochondria.

Mass flow

1. Loading sucrose at the source makes the sieve tube contents very concentrated, so water potential falls sharply.
2. Water enters the sieve tube from the nearby xylem by osmosis, raising the hydrostatic pressure at the source end.
3. At the sink, sucrose is unloaded and used or converted to starch, so the water potential rises there, water leaves, and the hydrostatic pressure falls.
4. There is now a hydrostatic pressure gradient from source to sink, and the solution flows down it, carrying the sucrose with it. That bulk movement is mass flow.

The energy is used at the loading step. The flow itself is driven by the pressure difference, which is why the mechanism is sometimes described as passive flow with active loading.

Evidence for it: aphids inserting their stylet into a single sieve tube produce sap that continues to flow under pressure after the aphid is removed, and the flow is faster nearer the leaves, where the pressure is highest. Metabolic poisons that stop ATP production stop translocation, which shows the loading is active.

Xerophytes

Plants adapted to dry conditions, which reduce transpiration by attacking the same water potential gradient from several directions:

• Thick waxy cuticle, so almost no water escapes through the epidermis.
• Sunken stomata, sitting in pits where humid air is trapped, reducing the gradient.
• Hairs on the leaf surface, trapping a layer of humid air for the same reason.
• Rolled leaves, as in marram grass, enclosing the stomata in a humid chamber.
• Fewer stomata, and stomata on the lower surface only.
• Reduced leaf area, as in the spines of a cactus, which cuts the surface for evaporation.
• Deep or widely spread roots, reaching water others cannot.
• Water storage tissue in stems or leaves, as in succulents.

Each of these has a cost: reducing water loss also reduces carbon dioxide uptake and so limits photosynthesis. That trade-off is the point a good answer makes.

Common mistakes

• Saying water is pushed up the xylem by root pressure. It is pulled by transpiration.
• Confusing cohesion with adhesion. Cohesion is water to water; adhesion is water to the vessel wall.
• Putting lignin in the Casparian strip. It is suberin.
• Saying phloem transport is upward. It runs source to sink, in either direction.
• Saying mass flow requires ATP along the whole tube. ATP is used at loading; the flow follows a pressure gradient.
• Saying humidity increases transpiration. High humidity reduces the gradient and slows it.