Contents
1. Xylem and Phloem
| Xylem | Phloem | |
|---|---|---|
| Substance transported | Water and dissolved mineral ions | Sucrose, amino acids and other organic solutes |
| Direction | Upward only (root → stem → leaf) | Both directions (source to sink) |
| Cells | Dead, hollow, lignified tubes | Living sieve tube cells + companion cells |
| Driving force | Transpiration pull (cohesion-tension) | Active loading using ATP |
2. Transpiration
The loss of water vapour from the aerial parts of a plant, mainly through the stomata of leaves.
How transpiration drives water movement (cohesion-tension theory)
- Water evaporates from mesophyll cells into air spaces and diffuses out through stomata.
- This creates a water shortage in the mesophyll cells.
- Water is pulled from xylem vessels in the leaf veins by osmosis.
- Water molecules stick together (cohesion) and are pulled up the xylem as a continuous column.
- Water is absorbed from the soil into root hair cells by osmosis, replacing water lost.
Use a potometer — it measures water uptake (not directly transpiration, but water uptake ≈ transpiration rate). A bubble of air moves along a capillary tube; the faster it moves, the greater the transpiration rate.
3. Factors Affecting Transpiration Rate
| Factor | Effect on transpiration rate | Reason |
|---|---|---|
| Light intensity ↑ | Increases | Stomata open wider → more water vapour escapes |
| Temperature ↑ | Increases | More kinetic energy → faster evaporation; also steeper concentration gradient |
| Humidity ↑ | Decreases | Concentration gradient between leaf and air is smaller → slower diffusion |
| Wind speed ↑ | Increases | Removes water vapour from around stomata → steeper gradient maintained |
High humidity means the air already contains a lot of water vapour, so the concentration gradient between the inside of the leaf and the outside air is smaller. Less water vapour diffuses out per unit time.
4. Translocation
The transport of sucrose, amino acids and other organic solutes through the phloem from sources (where they are made or released) to sinks (where they are used or stored).
- Source: leaf (where glucose is made by photosynthesis and converted to sucrose for transport).
- Sink: growing regions (roots, fruits, seeds), storage organs.
- Unlike transpiration, translocation requires energy (ATP) — it is an active process.
- Translocation can move substances up or down the plant simultaneously.
- Xylem: dead cells, carries water + minerals, only upward (one-way), passive (transpiration pull)
- Phloem: living cells, carries sucrose + amino acids, bidirectional, requires energy (active)
- Factors increasing transpiration: high light intensity, high temperature, low humidity, high wind speed, large leaf surface area
- Stomata open in light (CO2 enters for photosynthesis) - more transpiration in day
- Guard cells become turgid in light (water enters by osmosis) - stomata open
5. Common Exam Traps
Never swap these. Xylem = water and minerals, one direction (up). Phloem = sucrose and amino acids, both directions. A common question asks about which vessel is blocked or damaged.
A potometer measures the rate of water uptake by the cut stem. This is a close estimate of transpiration rate — but strictly it measures uptake, not loss from leaves. State this distinction if asked.
Transpiration is a biological process involving stomata, guard cells and the structure of the leaf. Simple evaporation from a water surface has no biological control. Guard cells can close stomata to reduce transpiration.
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.