Exchange and transport in plantsOxford AQA IGCSE Biology: Flashcards

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What is the structure of xylem tissue?

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What is the structure of xylem tissue?: Xylem consists of vessel elements (dead, hollow cells with no end walls) and tracheids (cells with thickened walls). The cells are reinforced with lignin, making them strong and waterproof. They form continuous tubes throughout the plant.
What is the function of xylem?: Xylem transports water and mineral ions from the roots to the leaves and other parts of the plant. It also provides structural support due to its strong, lignified walls.
Describe the structure of phloem tissue.: Phloem consists of sieve tube elements (living cells connected by sieve plates with pores) and companion cells (living cells that provide energy for active transport). The sieve tubes have thin walls and contain cytoplasm.
What is the function of phloem?: Phloem transports sugars (sucrose) and amino acids produced by photosynthesis from the leaves to other parts of the plant that need them for growth, respiration, and storage.
Define transpiration.: Transpiration is the loss of water vapour from the leaves and stems of a plant through the stomata (in leaves) and lenticels (in stems). It occurs mainly through stomata on the lower epidermis of leaves.
Name four factors that affect the rate of transpiration.: Temperature, humidity (water potential of air), wind speed, and light intensity all affect transpiration rate. Higher temperature, lower humidity, higher wind speed, and greater light intensity all increase transpiration rate.
Explain how temperature affects transpiration rate.: Higher temperature increases the kinetic energy of water molecules, causing more water to evaporate from the leaf surface. This increases the water potential gradient, drawing more water up through the xylem and increasing transpiration rate.
How does humidity affect transpiration?: Lower humidity (drier air) increases the water potential gradient between the leaf and the atmosphere. This causes water vapour to diffuse out of the leaf faster, increasing transpiration rate. High humidity decreases transpiration.
Explain the effect of wind speed on transpiration.: Wind speed removes water vapour from around the leaf surface, reducing the water potential of the air surrounding the stomata. This increases the water potential gradient, causing faster diffusion of water vapour out of the leaf.
How does light intensity affect transpiration rate?: Higher light intensity causes stomata to open wider (stomata open in light for photosynthesis). With wider stomata, more water vapour can diffuse out of the leaf, increasing transpiration rate.
Describe the transpiration stream and how water moves through a plant.: Water is absorbed by root hair cells from the soil. It moves through the root cortex into the xylem by osmosis (due to active uptake of mineral ions creating a low water potential). Water then moves up the xylem in a continuous column to the leaves, where it is lost by transpiration. Cohesion between water molecules and adhesion to xylem walls help maintain the water column.
What is a potometer and what does it measure?: A potometer is apparatus used to investigate transpiration rate. It measures the rate of water uptake by a plant, which is approximately equal to the rate of water loss by transpiration. The water level drops in a capillary tube as the plant transpires.
Define translocation.: Translocation is the transport of solutes (sucrose and amino acids) through the phloem from regions of production or storage to regions where they are needed. It occurs from source (e.g. leaves during photosynthesis) to sink (e.g. roots, growing shoots, fruits).
Explain how translocation differs from transpiration.: Transpiration is the passive movement of water vapour out of the plant through stomata. Translocation is the active transport of dissolved sugars and amino acids through phloem sieve tubes, requiring energy in the form of ATP from respiration.
Name the substances transported by phloem during translocation.: Sucrose (a sugar produced by photosynthesis) and amino acids (used for protein synthesis) are transported by phloem. These are moved from source regions (like photosynthesising leaves) to sink regions (like roots and growing tissues).