Unit 2.3 b

• The survival of a plant depends on balancing water uptake and water loss.
• Cellular uptake is the uptake and release of water and solutes by individual cells.
• Tissue transport is the short distance transport of substances from cell to cell. This includes the transport of water and ions across the root cortex into xylem vessels
• Whole plant transport is the long-distance transport of sap within the xylem and phloem.
• The epidermis of the root is found on the outer layer. It contains root hair cells for the uptake of water amd mineral ions. They also protect roots as they grow through the soil.
• The cortex parenchyma of the root can act as a storage organ. The intercellular spaces allow the movement of water and ions.
• The endodermis layer is found between the cortex parenchyma and the pericycle. It has a waterproof layer which forces water and ions into the cell cytoplasm of the endodermal cells and hence controls transport into the xylem
• The pericycle of the root is the site of lateral root growth and is directly after the endodermis. It has a role in controlling transport into the xylem.
• The xylem transports water and ions from the root to the stem and leaves. Provides support for the plant.
• The phloem transports the products of photosynthesis to the roots from the leaves.
• The cambium is a meristematic tissue that cam undergo mitosis to produce more xylem and phloem.
• The xylem forms a 'cross' in the middle of the root.
• Root hair cells are found on the outer epidermis layer to increase surface area, as this will increase absorption.
• Toot hair cells have a thin cellulose cell wall so the diffusion pathways is short. There is a large number of root hairs each with a long, thin extension to increase the surface area. They contain a large number of mitochondria to produce ATP for active ttransport Ions.
• Wayer can be absorbed in two ways: either it is absorbed passively into the cell wall which acts as a sponge, or it can be moved into the cell cytoplasm via osmosis.
• Ions can be absorbed: passively with water into the cell wall or by active transport into the cell cytoplasm.
• The cuticle on the outside of the cell reduces water loss through water evaporation. It is transparent to allow light through for photosynthesis.
• The epidermis on the stem is for protection and may have hairs to deter animals/insects from eating them.
• Collenchyma cells have thick cell walls to strengthen the cell while remaining flexible.
• The pith parenchyma are thin walled cells that act as a packing tissue. It often breaks down in older stems.
• The sclerenchyma cells are lignified cells that provide support and strength to the stem.
• The vasculature bundle is made up of the xylem, the cambium, the phloem and the sclerenchyma cells.
• A number of different cell types are found in the xlyem: the xylem vessels, the tracheids and the xylem parenchyma.
• The xylem vessels transport water and minerals from the roots to the other parts of the plant.
• Tracheids provide strength to the tissue of the xylem.
• The xylem parenchyma are thin walled, living cells which act as a packing tissue between the xylem vessels.
• Tracheids and xylem vessels are dead cells and have no cell content. Their walls are made from lignin, which is impermeable to water. Water leaves the vessels through holes in the walls called pits.
• All cells in the phloem are alive. They transport the products of photosynthesis throughout the plant. The phloem is made up of sieve cells and companion cells.
• Phloem sieve tubes contain little cytoplasm and few to no organelles. Unlike the xylem, the walls are made of cellulose.
• Companion cells accompany the sieve tubes and provide support needed to keep them alive. Companion cells are involved in the transport of substances in and out of the sieve tubes. They contain many mitochondria, which produces ATP for active transport. The companion cells and sieve tubes are connected via plasmodesmata.
• The end walls of phloem sieve tubes are perforated by small pores. These walls are called sieve plates. Strands of cytoplasm pass through these pores to connect the phloem sieve tubes.
• Water moves across the cortex of the root to the endodermis by three routes: the apopladt route, the symplast route and the vacuolar route.
• The apoplast route is the fastest route of transport across the cortex as there is less resistance and no membranes to cross. It is the diffusion between cell walls which is fully permeable to water. At the endodermis, the casparian strip prevents the apoplast route as it is water proof. It forces water into the cytoplasm.
• The symplast route is the movement of water via osmosis from the cytoplasm of the cells. Water can move passively to other cells via plasmodesmata.
• The vacuolar route is when water moves between the cytoplasm and the large central vacuole. Water moves between cells via plasmodesmata. This is the slowest route for transport as each time water crosses membranes, there is a resisitance to movement and the speed of transport decreases.
• The concentration of available ions in the soil is usually lower than that inside plant cells. This is because clay and other soil particles attract ions and effectively reduces their concentration. This lowers the water potential in the roots so that water enters the epidermal cells by osmosis
• Ions are absorbed by the roots by a combination of active transport and co-transport.
• Active uptake of ions into plant cells results in the concentration of ions being higher in the plant cells that in the soil.
• Ions move into the xylem along the symplast route. This increases the concentration of ions in the stele and therefore lowers the water potential. This means that more water will move into the cortex via osmosis.
• Root pressure can push water throught the xylem against gravity.