4.4- transport in plants Flashcards
structure of the xylem
-begins as living structure, protoxylem initially formed which is capable of stretching as walls not fully lignified
-lignified tissue then known as metaxylem
-end walls between cells break down, forming hollow tubes running root to tip
-hollow tube with walls of lignin
-water can move out through specialised pits in walls of xylem vessels
function of the xylem
-transports water and minerals from roots to leaves and shoots in transpiration stream
structure of the phloem
-living tissue
-sieve tube elements made up of many cells joined to make very long tubes that run from highest shoots to end of roots
-walls between cells become perforated to for specialised sieve plates, contents flows through holes in these plates
-companion cells linked to STE by many plasmodesmata, they have large sa to transport sucrose into cell cytoplasm and many mitochondria to supply ATP needed to AT
-companion cells support sieve cells
formation of the xylem
-protoxylem forms first, walls do not contain a lot of lignin so it can stretch and grow
-as stem ages, more lignin and it becomes impermeable to water
-the impermeability kills the xylem cell
-Metaxylem is formed when the contents of the cells have died and the end walls have broken down forming hollow tubes running from roots to tip.
plant cell tissues
-in smaller non-woody plants, support comes from turgid parenchyma cells in the centre (unspecialised cells that act as packing for support in stems and roots)
-sclrenchyma are plant cells that have very thick lignified cell walls and empty lumen with no living contents
-collenchyma are plant cells with areas of cellulose thickening that give mechanical strength and support to tissues
how does water move out of the stomata?
-usually less water in the air then inside the leaf
-therefore there is a water potential gradient
-leads to water vapour molecules diffusing out of the stomata
-this water is replaced by water in cell walls of mesophyll cells
water movement across leaf cells
-water which evaporates into air spaces in the leaf is replaced by water from the xylem
-it can move directly through cell walls or through the cytoplasm
-it does this due to the water potential gradient
water transport in plants
-water absorbed by root hairs
-these are cellular extensions of the root epidermal cells
-they increase the surface area
-waqter uptake is by 2 methods; cohesion tension and root pressure
cohseion tention theory- method of water uptake
-water evaporated from mesophyll cells leading to transpiration
-water molecules form hydrogen bonds with each other (cohseion)
-a column of water is pulled up the xylem (transpiration pull)
-water can even be pulled into dead roots
evidence of cohesion tension theory
-when transpiration is high during the day, tree trunks have a smaller diameter
-If a xylem is broken and air enters, water is not drawn up. This is because the continuous stream of water is broken so the molecules no longer stick together.
root pressure- method of water uptake
-depends on osmotic gradient in roots
-dissolved substances build up in epidermal cells by diffusion and active transport which gives the root hair cell a lower water potent than soil
-therefore water enters root hair cells by osmosis
-endodermal cells actively transport solutes into the xylem to create a concentration gradient large enough to draw water up the xylem. This is called root pressure.
3 possible routes of water uptake from root hair cells to endodermis
- apoplast- freely through cellulose cell walls
- symplast- through cytoplasm of adjacent cells
- vacuolar- through cytoplasm and vacuoles
what is translocation
-sites of sugar production are called sources
-places where they are used or stored are sinks
-movement to sink can be in any direction
-The phloem transports amino acids, sugars and inorganic ions such as potassium, chloride and phosphate.
where does translocation of water and mineral ions take place
the xylem
where does translocation of organic solutes like sucrose take place
the phloem
