Ad Banner Placeholder

Cambridge IGCSE Biology · 0610

Chapter 8: Transport in plants (Part 2)

Transpiration

Definition
The loss of water vapour from leaves.
Mechanism
Water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour.
Internal surface area
The interconnecting air spaces between mesophyll cells create a large internal surface area, increasing the amount of water that can evaporate.

Exam Traps

  • Avoid saying water leaves directly from xylem vessels through the cuticle as the main route.

The transpiration pull

  • Water molecules are cohesive, meaning they stick together.
  • As water evaporates at the leaf, it creates a transpiration pull that draws up a column of water molecules from the roots through the xylem.

Examiner Report Insights

  • Link transpiration pull to xylem transport — phloem is not involved in this mechanism.

Exam Traps

  • Do not say the roots push water up by active transport as the main force in the xylem.
  • Avoid confusing cohesion (water molecules sticking together) with adhesion to xylem walls alone.

Factors affecting transpiration rate

  1. Temperature: Higher temperatures increase the rate as water evaporates more easily.
  2. Wind speed: Higher wind speed increases the rate by blowing away water vapour from the leaf surface.
  3. Humidity: High humidity decreases the rate because the concentration gradient of water vapour between the inside and outside of the leaf is low.
  4. Stomata: A greater number or larger size of stomata increases the rate of evaporation.

Exam Traps

  • Do not say high humidity increases transpiration — it slows diffusion of water vapour out of the leaf.
  • Avoid claiming wind decreases transpiration; it removes humid air from the leaf surface.

Wilting

  • Water maintains plant structure by keeping cells turgid.
  • If water loss is not replaced, turgor pressure decreases and the plant begins to wilt.
  • To limit water loss, plants close their stomata.

Translocation

Definition
The movement of sucrose and amino acids in the phloem from sources to sinks.
Sources
Parts of the plant that release sucrose or amino acids (e.g., leaves during photosynthesis).
Sinks
Parts of the plant that use or store them (e.g., roots for storage, or regions of active growth and respiration).
Variable roles
Some parts can act as both a source and a sink at different times. For example, leaves synthesise molecules (source) but also use them for their own metabolism (sink).
Summary of translocation showing arrows from a source leaf to sinks such as roots and developing flowers
Diagram 1: Summary of translocation. Arrows move from a source leaf to sinks like roots or developing flowers, illustrating the bidirectional nature of phloem transport compared to xylem.

0/15

Ad Banner Placeholder