A-Level Biology: Plant Transport and Transpiration

Understanding how plants move water and sugars is fundamental to grasping their survival and growth. This complex internal transport system, operating without a pump, not only supports individual plants but also drives global water cycles and carbon distribution. Mastering the mechanisms of xylem and phloem transport is key to explaining plant adaptations, agricultural practices, and responses to environmental change.

Water Uptake: From Soil to Root

Water enters the plant primarily through the root hairs, which provide a large surface area for absorption. The driving force for this uptake is osmosis—the net movement of water molecules from a region of higher water potential (the soil) to a region of lower water potential (the root hair cell cytoplasm) across a partially permeable membrane. Root hair cells actively pump mineral ions into their cytoplasm, lowering their water potential and ensuring a continuous osmotic gradient for water influx.

Once inside the root hair cell, water must travel across the cortex to reach the xylem vessels in the stele. This occurs via two main pathways. The apoplast pathway involves water moving through the non-living spaces between cellulose cell walls and the spaces between cells. This route is fast and does not cross any membranes. In contrast, the symplast pathway involves water moving through the living cytoplasm of cells, connected by plasmodesmata (microscopic channels). Water crosses cell membranes via osmosis in this route. The Casparian strip, a waterproof band of suberin in the endodermal cell walls, forces water and solutes from the apoplast into the symplast, allowing selective control of what enters the xylem.

The Transpiration Stream and Cohesion-Tension Theory

The journey of water from roots to leaves is known as the transpiration stream. Transpiration is the loss of water vapour from the aerial parts of a plant, primarily through stomata in the leaves. This evaporation creates a tension or pull on the column of water in the xylem. The cohesion-tension theory explains how this pull is transmitted all the way down to the roots.

Water molecules are cohesive (they stick together via hydrogen bonds) and adhesive (they stick to the hydrophilic walls of the xylem vessels). As water evaporates from the mesophyll cells, it draws more water out of the xylem. This creates a tension that pulls the entire continuous column of water upwards. Think of it like drinking through a very long straw: your suction (transpiration pull) creates a tension that lifts the fluid due to the cohesion between its molecules. This process is entirely passive and driven by the sun's energy evaporating water.

Measuring and Analysing Transpiration Rates

The rate of transpiration is not constant; it is influenced by several environmental factors that alter the diffusion gradient for water vapour from the leaf. Light intensity, temperature, humidity, and wind speed all play critical roles. Increased light opens stomata, higher temperature increases evaporation, low humidity steepens the diffusion gradient, and wind removes water vapour, maintaining that gradient.

A potometer is used experimentally to estimate transpiration rate by measuring water uptake. It’s important to remember it measures uptake, which is assumed to closely correlate with water loss. A standard procedure involves cutting a shoot underwater to prevent air entering the xylem, assembling the potometer under water, and then using the calibrated capillary tube to measure the distance an air bubble moves over time. For example, you might calculate the rate in cm³/min by measuring the volume of the capillary tube (πr² × distance moved). Analysing data from potometer experiments under different conditions allows you to directly link environmental variables to the plant's water transport.

Phloem Transport: Translocation of Sucrose

While the xylem transports water and minerals upwards, the phloem transports products of photosynthesis, mainly sucrose and amino acids, from source to sink. This process is called translocation. A source is an area of production or release (e.g., a photosynthesising leaf). A sink is an area of use or storage (e.g., a growing root or fruit).

The most widely accepted model for this transport is the mass flow (pressure flow) hypothesis. It proposes that sucrose is actively loaded into the sieve tubes of the phloem at the source. This active transport, often involving companion cells and hydrogen ion co-transport, lowers the water potential in the sieve tube, causing water to enter by osmosis from the xylem. This influx creates a high hydrostatic pressure. At the sink end, sucrose is actively unloaded for use or storage, increasing the water potential in the sieve tube, so water leaves by osmosis. This creates a lower hydrostatic pressure. Sap therefore flows, en masse, down this pressure gradient from source to sink.

Evidence for Transport Mechanisms

Our understanding of these systems is supported by key experimental evidence. For the cohesion-tension theory, observations include xylem vessels under tension producing a 'popping' sound when broken, and trees dying if air blocks (embolisms) form in the xylem, breaking the continuous water column.

For phloem translocation, evidence is more direct. Experiments using radioactive tracers, such as carbon-14 labelled carbon dioxide, show that radioactive sucrose moves through the phloem. A plant exposed to ${}^{14}C{O}_2$ will incorporate it into sugars, which can then be traced using autoradiography as they move to sinks. Even more elegant evidence comes from aphid stylets. Aphids pierce single sieve tube elements with their mouthparts (stylets) to feed on sap. If the aphid is anaesthetised and its body carefully removed, the remaining stylet acts like a tiny tap, exuding phloem sap for hours. Analysis of this sap confirms it is rich in sugars, and the slow but continuous flow demonstrates the presence of pressure within the sieve tube.

Common Pitfalls

A frequent mistake is to overstate the role of root pressure. While root pressure can contribute to guttation (water droplets at leaf edges) in some plants, it is not the main driver of water transport in tall plants. The cohesion-tension mechanism is the primary force for water movement upwards.

Another common error is confusing the direction of transport in xylem and phloem. Remember, xylem transport is unidirectional (upwards only), whereas phloem transport is bidirectional—but not in the same tube at the same time. Different sieve tubes will carry sap to different sinks.

Students often misinterpret potometer data. The apparatus measures water uptake, which is a very good estimate for transpiration, but some water is used in photosynthesis and to maintain turgor. It does not measure transpiration directly.

Finally, when describing mass flow, avoid stating that sucrose is "pumped" through the phloem. The flow itself is passive, driven by the pressure gradient. The active processes are the loading at the source and unloading at the sink, which establish that gradient.

Summary

• Water enters root hairs by osmosis and moves to the xylem via the apoplast and symplast pathways, with the Casparian strip forcing a symplastic checkpoint.
• Transpiration generates a pull, and the cohesion-tension theory explains how the cohesive and adhesive properties of water allow a continuous column to be drawn up the xylem.
• Transpiration rate is measured with a potometer and is increased by high light, temperature, wind, and low humidity due to their effects on the diffusion gradient.
• Phloem transports sucrose by translocation from source to sink via the mass flow hypothesis, driven by active loading/unloading and resulting hydrostatic pressure gradients.
• Key evidence includes radioactive tracer studies and the analysis of sap from aphid stylets, which support the mechanism of phloem transport.