GCSE Biology Revision "Transpiration"

Scientific concepts & nature phenomena presented

Transpiration

• Ongoing evaporation of water from the surfaces of leaves.
• Water vapor forms from cells within the leaf, diffuses through internal air spaces (notably the spongy mesophyll), and escapes through stomata.

Transpiration stream

• The linked movement of water from xylem into the leaf to replace water lost via transpiration.
• Water is drawn up from root hair cells through xylem vessels to the leaves.

Leaf tissue roles (context for transpiration)

• Palisade mesophyll: site of photosynthesis.
• Xylem: transports water and dissolved mineral ions from roots to leaves.
• Spongy mesophyll (air spaces): pathway for water vapor diffusion before exiting via stomata.

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Factors affecting the rate of transpiration (methodology / factors list)

• Higher temperature → higher transpiration rate
  • Higher temperature increases the evaporation process.
• Drier air (low humidity) → higher transpiration rate
  • Lower humidity increases the evaporation gradient.
• Windy conditions → higher transpiration rate
  • Wind removes water vapor near the leaf surface, allowing more evaporation.
• Higher light intensity → higher transpiration rate
  • Increased light promotes photosynthesis, which leads to stomata opening.
  • This allows carbon dioxide entry and also increases water vapor loss.

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Roles of stomata and guard cells (gas exchange & water loss)

• Stomata are surrounded by two guard cells.

During the day / high light intensity

• Guard cells swell, changing shape, causing stomata to open.
• CO₂ diffuses into the leaf for photosynthesis.
• Water vapor diffuses out, so transpiration increases.

Under hot conditions

• The plant may close stomata to reduce water loss.
• This also means less CO₂ can enter, so photosynthesis can’t proceed as effectively.

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Importance / outcomes of transpiration

• Brings water to the leaves (needed for photosynthesis).
• Transports dissolved mineral ions (e.g., magnesium) via the transpiration stream.
• Cools the leaf via evaporative water loss, especially important in warm weather.

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Researchers or sources featured

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