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Water Transport and Minerals
Water is required for photosynthesis, transportation of minerals, temperature regulation and to hold themselves up.
Transpiration stream - water is taken up through the roots, transported round the plant and finally lost through the leaves.
Transpiration
The loss of water from the leaves of the plant by evaporation, allowing the plant to suck up more water through the roots.
Water is taken in from the soil via the roots. The roots are covered in millions of root hairs that provide a large surface area for water to be absorbed by osmosis.
Water is carried up the plant in tubes called xylem vessels. Xylem vessels have very narrow diameters, they are made of hollw dead cells and are grouped together with pholem vessels in vascular bundles.
Stomata are holes on the underside of leaves, which allow the plant to breathe. the water evaporates in the leaves and escapes through the stomata. As the water escapes, more is drawn up the xylem by suction.
Plants also need water to be able to escape through the leaves to keep the transpiration stream flowing and cool the plant. A balance between water loss and gain is needed.
Plants control water loss through transpiration, opening and closing the stomata. The guard cells surronding the stomata becaome more or less turgid, allowing them to open and close.
The holes open to allow carbon dioxide and oxygen in and out. The holes are closed to reduce water loss.
The Transport System
Two types of tubes make up the plants transportation system
Xylem Vessels - Consist of dead cells with no end walls, contain lignin to form stiff tubes. the vessels are impermeable, strengthened with fibres. The xylem are used to transport water and minerals, to the leaves.
Take water from the roots up the plant to the leaves.
Minerals dissolved in the water taken up the plant to the shoots and leaves.
In larger woody plants the xylem vessels are used to support the plant. Ligin that lines the cells of the vessels makes them strong and impermeable. Each year the cells die and new xylem rings are added.
Phloem Vessels - Living cells lined with cytoplasm, walls made of cellulose and perforated end walls called sieve plates. The cells are permeable and surrounded by companion cells. The pholem are used to transport food to the growing parts and the storage organs.
Take food to any part of the plant that needs it up and down the vessels.
The two vessels are found together but occupy different locations in the root and the stem, they are referred to as the vascular bundle.
Support for most green plants
Dependant on cell turgor, the stiffness to cell walls by being full of water. plant cell walls have a cellulose cell wall surrounding the cell membrane, they are strong and rigid, but also completetely permeable to water and dissolved minerals. The cell membrane is partially permeable so water can enter the cell via osmosis. The water swells the vaculoe, increasing pressure inside the cell, giving the cell stiffness.
The turgid cells in turn press against the cortex of the plant stem, giving stiffness to the stem.
If the plant is lacking water it will wilt, the concentration gradient has being reversed. Water has moved the other way, the cells become flaccid as the pressure inside the cells reduces.
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