Plants are mostly made up of water. Lettuce is about 94% water and a potato is about 77% water. Plants need water for three main purposes: ------photosynthesis, -support
-transport of chemicals
Water is a raw material for photosynthesis. If water is in short supply, the rate of photosynthesis will be limited. Plants need water for support, otherwise they wilt. Water is needed to transport many chemicals within plants. Chemicals, such as mineral salts, dissolve in water and they can then be moved within the plant to the cells that need them.
Plants obtain water from the soil through their roots. The roots are adapted to absorb large volumes of water by having many tiny root hairs, which increase the surface area of the roots. Each root hair is an extension of an individual cell on the outside of a root, called a root hair cell. Water enters the root hair cell by osmosis. Osmosis is the movement of water from a dilute solution to a more concentrated solution, through a partially permeable membrane. The solution inside the root hair cell is more concentrated than that of the soil water, so water moves from the soil into the cell. The cell membrane is partially permeable. It has tiny holes which allow water molecules to pass through. These holes are too small for larger solute particles to pass through. Once inside the root hair cell, the water dilutes the solution in the cytoplasm, so the solution is less concentrated than that of cells closer to the centre of the root. Water therefore passes from cell to cell by osmosis across the root, until it reaches the xylem vessel at the centre of the root. The xylem vessel carries water up the plant.
Plants need minerals to stay healthy. Minerals are found dissolved in the soil water around the plant roots. When minerals dissolve, they form ions. These are very small and can pass through holes in the cell membrane. Some mineral ions diffuse from the soil into the root hair cells, because their concentration in soil water is higher than that in cell sap. Diffusion is a passive process. This means it does not use energy.
When the concentration of minerals is higher in soil water than in root hair cells, the minerals can diffuse into the cells passively. An example of this is when a farmer puts fertilizer onto the soil. Sometimes minerals are in a higher concentration inside the cells than they are in the soil water. This would favor diffusion of minerals out of the plant into the soil. However, this doesn't happen. Instead, the plant pumps minerals into the root hair cells against the concentration gradient by a process called active transport. Active transport uses energy. This energy is provided by respiration. The site of respiration inside cells is the mitochondria. Root hair cells contain many mitochondria. If soil becomes waterlogged the plants may die. This is because water takes the place of air in the soil, so the roots cannot obtain oxygen. Without oxygen, plants cannot respire to release energy for active transport, so they cannot take in minerals.
Glucose sugar is produced when a plant photosynthesises. Some of the sugar is used for respiration to release energy. The sugar is also used to make different types of chemicals needed by the plant. Many sugar molecules are joined together to form starch, which is an insoluble carbohydrate stored in the cells. Starch can later be broken down into glucose again when the plant is not photosynthesising. The glucose molecules can join together in a slightly different arrangement to form cellulose molecules. These are used to make cell walls. Glucose can be converted to fatty acids and glycerol. These make up fat molecules that are stored in many types of seeds. Glucose is combined with nitrogen and other elements to make amino acids, the building blocks of proteins. These other elements are obtained from the soil as minerals. Energy from respiration is needed for all these reactions.
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