Transport mechanisms

1. Overview

Plants move two main groups of substances over long distances. The xylem carries water and dissolved mineral ions up from the roots, while the phloem carries dissolved assimilates such as sucrose and amino acids from where they are made or stored to where they are used. Water is pulled up the xylem by transpiration, the evaporation and diffusion of water from the leaves. This works because hydrogen bonding makes water molecules stick together (cohesion) and stick to the cellulose walls of the xylem (adhesion). In the phloem, active loading at the source raises pressure so that sap flows by mass flow down a hydrostatic pressure gradient to the sinks.

Key Definitions

• Transpiration: the loss of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves, by evaporation followed by diffusion.
• Apoplast pathway: the route by which water moves through the cell walls and the spaces between cells, without crossing any cell surface membrane.
• Symplast pathway: the route by which water moves through the cytoplasm of cells, passing from cell to cell through plasmodesmata.
• Casparian strip: a waterproof band of suberin in the cell walls of the endodermis that blocks the apoplast pathway and forces water into the symplast.
• Cohesion-tension theory: the explanation that water is pulled up the xylem under tension as a continuous column held together by cohesion between water molecules.
• Cohesion: the attraction between water molecules caused by hydrogen bonding, which holds the water column together in the xylem.
• Adhesion: the attraction between water molecules and the cellulose of the xylem walls, which helps draw water up and resists the pull of gravity.
• Source: a part of the plant that loads assimilates into the phloem, such as a photosynthesising leaf or a storage organ exporting stored food.
• Sink: a part of the plant that removes assimilates from the phloem to use or store them, such as a root, growing bud or developing fruit.
• Translocation: the transport of assimilates such as sucrose and amino acids through the phloem sieve tubes from sources to sinks.

Content

What is carried, and in what

Many of the substances a plant transports are dissolved in water. Mineral ions taken up by the roots (for example nitrate and magnesium ions) and organic compounds such as sucrose and amino acids do not travel as solids or gases over long distances; they move as solutes carried in water. This is why the water-conducting xylem and the sap-filled phloem are central to plant transport.

From soil to xylem: the apoplast pathway

Water moves across the root from the soil to the xylem by two routes. In the apoplast pathway water travels through the cell walls and the spaces between cells without ever entering the cytoplasm. The walls are made of a mesh of cellulose fibres, and water moves freely through the water-filled spaces in this mesh, pulled across by the tension created higher up in the plant. Because no membrane is crossed, the apoplast offers little resistance and carries most of the water through the root cortex.

This route does not continue unbroken all the way to the xylem. When water reaches the endodermis it meets the Casparian strip, a band of waterproof suberin in the walls. Suberin blocks the apoplast, so water is forced to cross a cell surface membrane and enter the cytoplasm. This checkpoint lets the plant control which mineral ions pass into the xylem and stops solutes leaking back out. Mineral ions are then actively transported (pumped) from the endodermis cells into the xylem, which lowers the water potential of the xylem sap so that water follows from the endodermis into the xylem by osmosis.

Once water reaches the xylem vessels themselves, the apoplast pathway resumes inside the hollow, dead vessels. Xylem vessel walls are strengthened by lignin, a hard waterproof material deposited in rings, spirals or sheets. The lignified walls give the vessels the mechanical strength to withstand the tension (negative pressure) of the rising water column without collapsing, while the inner surface of the cellulose-and-lignin wall provides the points to which water molecules adhere as they are drawn upwards.

From soil to xylem: the symplast pathway

In the symplast pathway water moves through the living cytoplasm of the cells, passing from one cell to the next through fine cytoplasmic threads called plasmodesmata that pass through the walls. Water moves along this route from a higher water potential to a lower water potential by osmosis. Because the apoplast is blocked at the endodermis, all water must use the symplast at that point, which is why the symplast pathway is essential even though it is slower than the apoplast.

Comparing the two pathways

The quickest way to revise the two routes is to compare them side by side:

Feature	Apoplast pathway	Symplast pathway
Route taken	Through cell walls and spaces between cells	Through the cytoplasm of living cells
Membrane crossed?	No (until the endodermis)	Yes, water enters the cytoplasm by osmosis
Passes through	The cellulose mesh of the walls	Plasmodesmata, from cell to cell
Where it is blocked	At the endodermis, by the Casparian strip	Not blocked; carries all water across the endodermis
Relative speed	Faster; carries most of the water in the cortex	Slower, but essential at the endodermis

Transpiration: evaporation then diffusion

Transpiration happens in two steps that are easy to confuse. First, water evaporates from the wet cellulose walls of the mesophyll cells lining the internal air spaces of the leaf, turning liquid water into water vapour. Second, this water vapour diffuses out of the air spaces, through the open stomata, down a concentration (water-vapour) gradient into the drier air outside. Evaporation and diffusion are different processes, and a good answer names both in the right order.

Hydrogen bonding, cohesion-tension and adhesion

As water evaporates from the leaf, it pulls on the water below it, creating a tension that is transmitted all the way down the continuous column of water in the xylem. This is the cohesion-tension theory, and the resulting pull is the transpiration pull.

The column does not break because water molecules are polar and form hydrogen bonds with one another. This attraction between water molecules is cohesion, and it keeps the water as one continuous thread from root to leaf. Hydrogen bonds also form between water molecules and the cellulose of the xylem and cell walls; this attraction is adhesion, which helps draw water up the narrow vessels and resists the downward pull of gravity. Together, cohesion and adhesion allow water to be lifted many metres without the column snapping.

Xerophytes: adaptations to reduce water loss

Xerophytes are plants adapted to dry habitats, and they show features that cut transpiration. When you draw and annotate a transverse section of a xerophyte leaf, you should label each adaptation and explain how it reduces water loss:

• a thick waxy cuticle, which reduces evaporation directly through the upper surface;
• sunken stomata sitting in pits, which trap moist air and so reduce the water-vapour gradient that drives diffusion out;
• hairs (trichomes) around the stomata, which also trap a layer of humid air close to the leaf;
• a rolled or folded leaf, which encloses the stomata in a humid chamber and shelters them from moving air;
• fewer stomata, or stomata restricted to the lower surface, reducing the number of exit routes;
• a reduced surface area (for example needle-like leaves or spines), which lessens the area available for evaporation. In every case, link the structure to the idea of either reducing evaporation or reducing the steepness of the diffusion gradient.

Worked example

Exam-style question: A student uses a potometer to measure the volume of water taken up by a leafy shoot. In still air, the air bubble moves 60 mm along the capillary tube in 5 minutes. A fan is then switched on, and the bubble moves 144 mm in the next 4 minutes. The cross-sectional area of the tube is 1.0 mm². Calculate the rate of water uptake in each condition and explain the difference. [5]

Model answer:

• Still air: distance per minute $= 60 \div 5 = 12$ mm min⁻¹. Volume rate $= 12 \times 1.0 = 12$ mm³ min⁻¹.
• With fan: distance per minute $= 144 \div 4 = 36$ mm min⁻¹. Volume rate $= 36 \times 1.0 = 36$ mm³ min⁻¹.
• The rate is three times higher with the fan (36 compared with 12 mm³ min⁻¹).
• Moving air blows away the water vapour that builds up just outside the stomata, keeping the air around the leaf drier.
• This makes the water-vapour gradient steeper, so vapour diffuses out faster, transpiration increases, and more water is drawn up the xylem to replace it.
• Note: a potometer measures the rate of water uptake, which is taken as an estimate of (not identical to) the transpiration rate, because a little of the water taken up is used in photosynthesis and to keep cells turgid rather than being transpired.

Sources, sinks and the assimilates that move

Assimilates are the products of photosynthesis and related metabolism that the plant moves around, mainly sucrose and amino acids, dissolved in water. They travel through the phloem sieve tubes from a source, where they are loaded into the phloem (such as a photosynthesising leaf or a storage organ releasing food), to a sink, where they are removed for use or storage (such as a root, a growing shoot tip or a developing fruit). A part of the plant can act as a source at one time and a sink at another, so the direction of phloem transport can change.

Loading the phloem: companion cells, proton pumps and cotransporters

Sucrose is loaded into the sieve tubes by an active mechanism centred on the companion cells. The mechanism works as a clear sequence of steps:

1. ATP-powered proton pumps in the companion cell membrane actively pump hydrogen ions (H⁺) out of the cell, building up a high concentration of H⁺ outside.
2. These hydrogen ions then diffuse back in through cotransporter (symporter) proteins, moving down their concentration gradient.
3. As each H⁺ moves in, it drags a sucrose molecule in with it (cotransport), so sucrose is carried against its own concentration gradient using the energy stored in the H⁺ gradient.
4. The sucrose then passes through plasmodesmata into the neighbouring sieve tube.

This active loading raises the solute concentration in the sieve tube, which lowers its water potential.

Mass flow down a pressure gradient

Because active loading lowers the water potential at the source, water follows by osmosis from the nearby xylem into the sieve tube, raising the hydrostatic pressure there. At the sink, sucrose is unloaded and used, so its water potential rises, water leaves the sieve tube, and the hydrostatic pressure falls. The result is a hydrostatic pressure gradient from the high-pressure source to the low-pressure sink, and the phloem sap is pushed along this gradient by mass flow — the bulk movement of the whole solution, carrying the dissolved assimilates with it. Sieve tube elements lose their nucleus and most organelles but keep a thin layer of cytoplasm, leaving the sieve tubes open enough for sap to flow through the sieve plates.

Worked example

Exam-style question: A plant is photosynthesising rapidly in a fully expanded leaf, while sucrose is being stored in a root. Explain how sucrose is moved from the leaf to the root through the phloem. [5]

Model answer:

• The leaf acts as the source and the root as the sink.
• In the companion cells, ATP-driven proton pumps move H⁺ out of the cell, building up an H⁺ gradient.
• H⁺ then re-enters through cotransporter (symporter) proteins, bringing sucrose in with it (cotransport), so sucrose is actively loaded into the sieve tubes.
• This loading lowers the water potential in the sieve tube at the source, so water enters by osmosis and raises the hydrostatic pressure there.
• The higher pressure at the source than at the sink (where sucrose is unloaded) creates a hydrostatic pressure gradient, so the sap moves by mass flow from leaf to root.

Key Equations

This topic is mostly qualitative, but you may be asked to calculate a rate from potometer data:

$$\text{rate of water uptake} = \frac{\text{volume moved}}{\text{time}}$$

where volume moved $=$ distance the bubble travels $\times$ the cross-sectional area of the tube. Remember this gives the rate of water uptake, which is used as an estimate of transpiration rate rather than being exactly equal to it. The rest of the topic relies on water potential gradients, pressure gradients and the cohesion-tension model rather than on set equations.

Common Mistakes to Avoid

• Writing "concentration of water" when explaining osmosis. Always use water potential, and describe movement from a higher water potential to a lower water potential. Avoid terms like "hypotonic" or "hypertonic" and compare water potential values instead.
• Treating transpiration as a single step. It is evaporation of water from the mesophyll cell walls followed by diffusion of water vapour out through the stomata. Name both processes in the correct order.
• Saying sieve tubes have no cytoplasm because they lack a nucleus. Sieve tube elements lose the nucleus and most organelles but retain a thin layer of cytoplasm; do not describe them as completely empty.
• Forgetting lignin when describing the xylem. The apoplast route ends in vessels whose walls are strengthened by lignin; mention that the lignified walls resist collapse under tension, rather than referring only to cellulose.
• Confusing cohesion with adhesion. Cohesion is the attraction between water molecules; adhesion is the attraction between water and the cellulose walls. Both depend on hydrogen bonding.
• Mixing up the proton-pump mechanism. Hydrogen ions are pumped out of the companion cell using ATP, then re-enter with sucrose via a cotransporter; do not say sucrose itself is directly pumped by the ATP.
• Listing xerophyte features without explaining them. For each adaptation, state how it reduces water loss (less evaporation, or a less steep water-vapour gradient), not just what it is.

Exam Tips

• For water movement, structure answers around water potential gradients and name the pathway (apoplast or symplast) the question is asking about.
• When asked about the xylem, work in the key terms lignin, cohesion, adhesion and transpiration pull to show you understand cohesion-tension fully.
• In phloem questions, sequence your answer: active loading at the source → low water potential → water enters → high hydrostatic pressure → mass flow → unloading at the sink.
• For annotated xerophyte drawings, draw clearly, label each adaptation and add a short explanation of how it reduces water loss next to the label.
• Use precise vocabulary consistently throughout: "evaporation then diffusion", "cotransporter (symporter)", and "hydrostatic pressure gradient" rather than vague phrases.
• Remember that plants respire continuously, in light and dark; do not write that they stop respiring when photosynthesising.