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Transport in Plants (Osmosis in plant cells (. (Plant cell put into pure…
Transport in Plants
Osmosis in plant cells
The cell surface membranes of animal and plant cells are partially permeable
–> so is the membrane around the plant cell's sap vacuole
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Plant cell put into pure water/dilute solution
=> contents of cell have a lower WP than external solution
=> cell will absorb water by osmosis
=> cell swells up and cytoplasm pushes against the cell wall
–> internal pressure KA turgid
Plant cell put into concentrated solution
=> contents of cell have higher WP than external solution
=> loses water by osmosis
=> decreases in volume and cytoplasm no longer pushes against cell wall
–> KA flaccid
Plant cell in very concentrated solution
=> shrinks so much that the membrane and cytoplasm split away from the cell wall
=> gaps appear between wall and membrane
–> KA plasmolysed
Turgor
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Plant loses too much water
=> becomes flaccid
=> plant wilts (leaves droop and collapse)
–> cuts down water loss by reducing the exposed surface area of the leaves and closing the stomata
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Osmosis
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Osmosis is the new diffusion of water across a partially permeable membrane, from a solution with higher water potential to one with lower water potential
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Uptake of water by roots
Root hairs
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Hairs greatly increase surface area of root epidermis – each hair is a single, specialised cell of root epidermis
Long, thin, outer projection of the root hair cell penetrates between the soil particles, reaching the soil water
–> soil water has a higher WP
=> water moves into the cell
=> increased WP
=> moves through the root
If a plant needs a mineral ion that is at a low concentration in the soil, the root hair cells use active transport to absorb it
Transpiration
Epidermis of leaves is covered by a waxy cuticle
–> impermeable to water
=> water passes out of leaves as water vapour through pores KA stomata
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Structure of a stem
Young stem: xylem and phloem grouped together in vascular bundles
–> arranged in a circle around the outer part of the stem
–> phloem on the outside, xylem on the inside
Older stem: vascular tissues grow to form complete rings around the stem
–> inner xylem forms the woody central core with living layer of phloem outside
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