52. Transpiration & Translocation

Plants use two distinct transport systems to move essential substances like water, minerals, and sugars between their roots and leaves.

1. Translocation (Sugar Transport)

Translocation is the process of moving sugars (produced during photosynthesis) from the leaves to the rest of the plant for immediate energy or storage.

• Tissue: Carried out by phloem tubes.

• Structure: Phloem is made of living cells arranged end-to-end. Between adjacent cells are small pores that allow cell sap (a mixture of water and sugar) to flow through.

• Direction: Phloem can transport substances in both directions (up and down the plant).

2. Transpiration (Water & Mineral Transport)

Transpiration involves the movement of water and mineral ions from the roots, up the stem, and to the leaves.

• Tissue: Carried out by xylem tubes.

• Structure: Xylem is made of dead cells that form a hollow, continuous tube. These tubes are strengthened with a tough material called lignin.

• The Transpiration Stream: This is the constant flow of water through the plant. It is driven by the evaporation of water from the leaves (through the stomata). As water evaporates, it pulls the chain of water molecules up the xylem from the roots to replace what was lost.

• Direction: Xylem only transports substances in one direction (upwards from roots to leaves).

3. Factors Affecting the Rate of Transpiration

The rate of transpiration is essentially the rate of evaporation from the leaf's surface. Four main factors influence this:

1. Light Intensity: Higher light intensity increases the rate of photosynthesis, which keeps the stomata open for longer to let in CO₂. This allows more water to evaporate.

2. Temperature: Warmer temperatures provide water particles with more energy, making them evaporate and diffuse out of the stomata more quickly.

3. Air Flow (Wind): Stronger air flow blows away water vapor as it exits the leaf. This maintains a steep concentration gradient between the inside and outside of the leaf, increasing the rate.

4. Humidity: High humidity means there is already a lot of water vapor in the air. This reduces the concentration gradient, slowing down the rate of diffusion and evaporation.

Summary Table: Xylem vs. Phloem