7.2 Transpiration and Translocation

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe transpiration as the loss of water vapour from leaves.
  2. Understand that water evaporates from the surfaces of the mesophyll cells into air spaces, then diffuses out through the stomata as water vapour.
  3. Explain: (a) the effects of wind speed, temperature, humidity, and light intensity on transpiration rate, and (b) how wilting occurs.
  4. Investigate the effects of wind speed, light intensity, and temperature variation on transpiration rate.
  5. Explain the mechanism by which water moves upwards in the xylem using the idea of transpiration pull and forces of attraction between water molecules.
  6. Describe translocation as the movement of sucrose and amino acids in the phloem from sources to sinks.
  7. Identify the positions of xylem, phloem, and cortex in transverse sections of non-woody dicotyledonous roots and stems.
  8. State the functions of xylem: transport of water and mineral ions, and support.
  9. Relate the structure of xylem vessels to their function: thick walls with lignin, no cell contents, and cells joined end-to-end with no cross walls.

Main Content

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1. What is Transpiration?

Transpiration is the loss of water vapour from the leaves of a plant.

Plants constantly absorb water from the soil through their roots. This water travels up the plant through special tubes and eventually reaches the leaves. Inside the leaves, some of this water evaporates and escapes into the air. This process of losing water vapour from the leaves is called transpiration.

Think of it like a plant slowly "breathing out" water.

2. How Does Water Leave the Leaf?

Understanding how water actually gets out of the leaf is important. It happens in two stages:

Stage 1 — Evaporation inside the leaf:

Inside the leaf, there are layers of cells called mesophyll cells (the main working cells of the leaf). These cells have wet surfaces — they are surrounded by a thin film of water. The air spaces inside the leaf are not completely dry; they are humid (full of water vapour).

  • Water evaporates from the wet surfaces of the mesophyll cells into the air spaces inside the leaf.
  • "Evaporates" simply means the liquid water turns into a gas (water vapour).

Stage 2 — Diffusion out through the stomata:

The air spaces inside the leaf become full of water vapour. On the underside of the leaf (and sometimes the top too), there are tiny pores called stomata (singular: stoma). These pores are like tiny doors in the leaf surface.

  • Because there is more water vapour inside the leaf than in the air outside, water vapour diffuses out through the stomata.
  • Diffusion means particles move from a region where there are lots of them (high concentration) to a region where there are fewer of them (low concentration). Water vapour moves from inside the leaf (high concentration) to the outside air (lower concentration).

Summary of the pathway: Mesophyll cell surfaces → Air spaces inside leaf → Stomata → Outside air

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