Transport in Plants: Transpiration (Syllabus 8.3)

Hello Biologists! This chapter is all about how plants manage their water supply—specifically, how they lose it! Transpiration is often described as the "inevitable evil" of plant life. It's a necessary process, but if plants lose too much water, they are in big trouble.
Don't worry if this seems tricky at first; we will break down the movement of water from the tiny cells in the leaf to the massive forces that pull it up the stem.

1. What is Transpiration? (Core Content)

Transpiration is the process where plants lose water. But it's not just any water loss; it's the loss of water specifically as water vapour from the surfaces of the leaves (and sometimes stems).

Core Definition and Location:

1. Water travels up the xylem vessels to the leaf.
2. This water moves into the mesophyll cells (the spongy and palisade layers inside the leaf).
3. Water evaporates from the surfaces of these moist mesophyll cells into the empty spaces inside the leaf, called the air spaces.
4. This water vapour then builds up inside the leaf and eventually diffuses out of the leaf through small pores called stomata.

Analogy: Think of a piece of wet cloth hanging outside. The water evaporates from the wet surface into the air. In a plant, the moist mesophyll cells are the wet cloth, and the stomata are the small windows allowing the vapour to escape.

Quick Review: Key Terms
  • Transpiration: The loss of water vapour from the leaves and stems of plants.
  • Stomata: Small pores, usually on the lower epidermis of the leaf, that allow gas exchange (including water vapour diffusion).

2. Why Do Plants Lose So Much Water? (Supplement Content)

For a plant to survive, it needs to take in carbon dioxide (CO2) for photosynthesis. The stomata must be open for CO2 to diffuse in, but open stomata immediately lead to water vapour rushing out.

The Role of Leaf Structure (Supplement 4)

The internal structure of the leaf is perfectly adapted for photosynthesis, but this same structure dramatically speeds up water loss.

The two main features contributing to high water loss are:

  1. Large Internal Surface Area: The mesophyll cells are loosely packed, creating many interconnecting air spaces. Water evaporates from the vast surface area of all these mesophyll cells.
    (If the cells were tightly packed like concrete, evaporation would be very slow!)
  2. Size and Number of Stomata: Leaves have thousands of stomata. When these pores are open to allow CO2 in, they provide a direct, short path for the abundant water vapour in the air spaces to diffuse out rapidly.

Did you know? A single mature oak tree can transpire hundreds of litres of water on a hot day!

3. The Mechanism of Water Movement: Transpiration Pull (Supplement Content)

We know water is lost at the top (the leaf), but how does that loss cause water to move upwards from the roots against gravity? This movement is explained by the Transpiration Pull theory.

Step-by-Step: The Transpiration Pull (Supplement 5)

The water moves up the xylem vessels in a continuous column, like a long chain.

  1. Evaporation at the Leaf: Water molecules evaporate from the mesophyll cell surfaces and diffuse out of the stomata (transpiration).
  2. Creation of Tension: As water molecules leave, they pull on the molecules immediately behind them in the xylem vessel. This creates a powerful suction force or "tension" at the top of the xylem.
  3. The Cohesion Force: Water molecules are polar (they have slightly charged ends), so they are highly attracted to each other. This attraction is called cohesion. This cohesive force is strong enough to keep the water molecules bonded together, maintaining a continuous column or 'thread' inside the xylem vessels.
  4. The Adhesion Force: Water molecules are also attracted to the walls of the narrow xylem vessels. This attraction is called adhesion. This force helps prevent the water column from breaking and keeps it stuck firmly to the sides of the vessel.
  5. The Pull: Because of cohesion, the pull created by transpiration at the leaf surface is transmitted all the way down the water column, dragging new water molecules up from the root cortex cells and eventually from the soil.

Analogy: Imagine a long chain of people holding hands. If the person at the front (the leaf) steps forward, the entire chain (the water column in the xylem) is pulled forward too, because the links (cohesion) are strong.

Key Takeaway for Mechanism

The energy required to pull the water comes from the Sun, which drives the evaporation process (transpiration) at the leaf surface. The main forces that make the column stick together are cohesion (water-water attraction) and adhesion (water-xylem attraction).

4. Factors Affecting the Rate of Transpiration (Core 3 & Supplement 6)

The rate at which a plant loses water is constantly changing based on environmental conditions. This is a very common topic for practical investigations!

The rate of transpiration is measured using a piece of apparatus called a potometer, which measures the rate of water uptake (which is almost equal to the rate of water loss).

How Environmental Factors Change Transpiration Rate:

We must explain *why* these factors have their effect. It usually relates to speeding up evaporation or making diffusion faster.

Factor Effect on Rate Explanation (WHY?)
Temperature Increase in temperature increases the rate. Higher temperatures give water molecules more kinetic energy, increasing the rate of evaporation inside the leaf and speeding up diffusion of water vapour out of the stomata.
Wind Speed Increase in wind speed increases the rate. Wind blows away the layer of still, humid air surrounding the stomata. This maintains a steep concentration gradient (very dry air outside, moist air inside the leaf), speeding up diffusion.
Humidity Increase in humidity decreases the rate. High humidity means the air already contains a lot of water vapour. This reduces the concentration gradient between the air spaces inside the leaf and the air outside, slowing down the rate of diffusion.
Light Intensity Increase in light intensity increases the rate. Stomata open wider in bright light to allow CO2 in for photosynthesis. Wider open stomata mean more surface area for water vapour diffusion.

Memory Aid: Remember the factors that speed up evaporation in everyday life (like drying clothes): Hot, windy, and dry conditions all speed up drying (and transpiration).

5. Wilting: When Transpiration Goes Wrong (Supplement 7)

If a plant loses water through transpiration faster than it can take water up through its roots, it enters a condition called wilting.

How and Why Wilting Occurs:

When water loss exceeds water uptake, the plant cells start to lose water by osmosis.

  1. The large central vacuole in the plant cell shrinks.
  2. The pressure of the cell contents pressing outwards on the cell wall (the turgor pressure) drops.
  3. The cell becomes flaccid (soft or floppy).
  4. The entire plant tissue loses its rigidity, causing the stems and leaves to droop—this is wilting.

The plant's reaction to Wilting:

Wilting is dangerous, but it also acts as a survival mechanism. When the guard cells lose turgor, they become flaccid and the stomata close. Closing the stomata massively reduces the rate of water loss, which can save the plant's life until water is available again.

Key Takeaway: Wilting

Wilting happens when the rate of water loss > rate of water uptake, causing cells to become flaccid and lose turgor pressure. This forces the stomata to close.