Overview
This lecture covers the mechanisms plants use to transport sugars and water, focusing on translocation via phloem and transpiration via xylem.
Sugar Transport: Translocation
- Plants produce sugars through photosynthesis in the leaves.
- Sugars must be transported from leaves to other parts of the plant for use or storage.
- Translocation is the movement of sugars through the plant, mainly via phloem cells.
- Phloem cells are arranged end-to-end, forming long tubes called phloem tubes.
- Small pores between phloem cells allow the movement of a sugar-water mixture called sap.
- Sugars transported by phloem can be used immediately for energy or stored for later.
- Phloem can transport sugars both up and down the plant.
Water Transport: Transpiration
- Xylem tubes, made from dead cells with no end walls, transport water and mineral ions.
- Xylem tubes are strengthened by lignin and form continuous hollow tubes.
- Water moves from roots up to leaves, primarily driven by evaporation from leaves.
- Evaporation causes a pull, creating a transpiration stream that moves water upward.
- Transpiration is the evaporation of water from plant leaves, mainly through stomata.
Factors Affecting Transpiration Rate
- Light intensity: More light increases photosynthesis, opening stomata and raising transpiration rates.
- Temperature: Higher temperatures give water molecules more energy to evaporate, increasing transpiration.
- Air flow: Greater wind removes water vapor quickly, maintaining a high concentration gradient and increasing transpiration.
- Humidity: Higher humidity reduces the gradient for water diffusion, lowering transpiration.
Key Terms & Definitions
- Translocation — transport of sugars through the phloem in plants.
- Phloem — living tubes that transport sugars and other compounds around the plant.
- Sap — mixture of water and sugars transported in phloem.
- Xylem — dead, lignified tubes transporting water and minerals up the plant.
- Transpiration — evaporation of water from plant leaves.
- Transpiration stream — continuous flow of water through a plant driven by evaporation.
- Stomata — pores on the leaf surface that allow gas exchange and water loss.
- Lignin — substance strengthening xylem cell walls.
Action Items / Next Steps
- Review factors that affect transpiration and be able to explain their effects.
- Study diagrams of phloem and xylem structure.
- Prepare for questions on how water and sugar are transported in plants.