ch8 transport in animals

• Plants have a transport system made up of xylem and phloem vessels that transport nutrients from the roots of the plant to the stem and leaves, and vice versa.
• The xylem is used to transport water through plants from the roots in transpiration.
• The xylem is made from hollowed-out dead cells that have the ends removed to make a tube for water to pass through.
• The phloem is made of living cells and is used to transport sugars and food nutrients in translocation.
• Water is taken up by root hair cells via osmosis, then moves into the root cortex cells by osmosis, before entering the xylem vessel where it is drawn up the stem to the leaves.
• At the leaf, water diffuses into mesophyll cells where it is used in metabolic reactions such as photosynthesis.
• Root hair cells are adapted for efficient water uptake by having a large surface area, a thin wall, and are located in areas of high water potential.
• Transpiration is the loss of water vapour from the mesophyll cell surface due to evaporation, which exits the plant via the stomata.
• Water helps maintain plant structure by keeping cells turgid.
• If the plant loses too much water which is not replaced, it begins to wilt as water moves out of cells and turgor pressure decreases.
• To limit water loss, the plant closes the stomata to prevent water vapour diffusing out.
• Water molecules are drawn up the xylem by transpiration pull (not osmosis), and are cohesive, meaning they stick together, so as the water evaporates at the leaf and diffuses out of the stomata, more water is drawn up the plant from the roots.
• The rate of transpiration depends on the temperature and humidity, with a high humidity decreasing the rate of transpiration as the concentration gradient of water vapour between the inside and outside of the plant is comparatively low, so diffusion out of the plant is slower.
• Translocation occurs in the phloem vessels and involves the transport of amino acids and sucrose.
• Areas where amino acids and sucrose are produced are called sources, and regions where they are stored or used for respiration and growth are called sinks.
• Materials are always transported from source to sink.
• Sucrose and amino acids are produced in the leaves, before being transported to the roots for storage, then transported to regions where they are used in respiration and for growth.
• Some parts of the plant, such as the leaves, can act as both source and sink within a plant’s life as they synthesise molecules and use them in metabolic reactions.
• xylem vessels have thick walls with lignin, no cell contents, cells joined end to end with no cross walls to form a long continuous tube
• transpiration as the loss of water vapour from leaves
• transpiration : that water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour
• how water vapour loss is related to: the large internal surface area provided by the interconnecting air spaces between mesophyll cells and the size and number of stomata
• Explain the mechanism by which water moves upwards in the xylem in terms of a transpiration pull that draws up a column of water molecules, held together by forces of attraction between water molecules