transport in plants Flashcards
why do plants need transport systems
large plants have a small surface area to volume ratio, therefore cannot absorb the nutrients they need from the environment.
need to meet metabolic demands
what is the name given to plants that have transport systems
vascular plants
what are the 2 structures that make up a vascular system
xylem
phloem
what are some adaptations of the xylem vessel
- they have no cell walls making an uninterrupted column for continuous flow of water
- cells walls contain lignin which supports the walls. Lignin can be deposited in spirals or distinct rings, this variety allows for flexibility + prevents stem breaking.
- pits in the wall do not contain lignin and allow water and mineral ions to move in or out
- cells are dead so do not contain a cytoplasm
what is the xylem vessel, what does it transport
long tube like structures formed from cells joined end to end.
transports water and mineral ions upwards.
what is the phloem vessel, what does it transport
phlpem is formed from cells arranged in tubes, transports sucrose in both upwards and downwards directions.
what are some adaptations of the phloem vessel
- contains sieve tube elements - living cells that form the tube, these are joined end to end. the end walls contain holes to allow solutes to pass through.
- sieve tube elements have no nucleus, thin layer of cytoplasm -creates more space for solutes
- absence of organelles means sieve tube elements cant survive on their own, so are accompanied by companion cells which carry out the living functions.
- companion cells are packed with mitochondria to provide lots of energy for the active loading of sucrose into the sieve tube element.
- The sieve tube element and the companion cell are connected through plasmodesmata (channels in the cell wall) which allows the two cells to communicate.
what is translocation
the movement of assimilates from source to sink end.
what is one way a concentration gradient is maintained to ensure there is always more assimilates at the source end
enzymes change the dissolved substances at the sink eg. by breaking them down to converting to substances. This ensures a lower conc. of assimilates at the sink.
outline the mass flow hypothesis
- active transport actively loads solutes into sieve tubes at the source. Increase in sucrose decreases water potential.
water moves into phloem from the xylem by osmosis.
There is high hydrostatic pressure at the source end - at the sink end solutes are removed from the phloem to be used up. Always a higher conc. in the phloem than surrounding tissue so solutes move via facillitated diffusion into surrounding tissue.
decrease in sucrose, increases water potential. water leaves the phloem + returns to the xylem
there is low hydrostatic pressure at the sink end - this creates a pressure gradient from source to sink which pushes solutes along the phloem.
what occurs during the process of active loading
- companion cells use ATP to actively pump hydrogen ions out of the cell into surrounding tissue cells. This sets up a proton gradient
- H+ ions bind to a co-transport protein and re-enter the cell down their concentration gradient. Sucrose molecule binds to the co-transport protein at the same time. The movement of H+ ions is used to move the sucrose molecule into the cell against it’s concentration gradient
- sucrose molecules are transported out of the companion cell into the sieve tube element down the proton gradient.
what is some evidence for translocation
- metabolic inhibitors - inhibit translocation suggesting active processes (active loading) is occuring.
- radioactive labelling - plants are suplied with radioactively labelled CO2 and light. Radioactivity is later detected in the phloem. Thin slices of stem can be placed on x-ray film and turn black showing they contain sugar.
- aphid feeding - aphids feed on sugars by penetrating sieve tubes with stylets. If the body is removed leaving the stylet, sap continues to exhude the stylet
- ringing a tree - involves removing the bark layer and phloem, causes trunk to swell and sugars collect above the ring. shows when phloem is removed sugars cannot be transported.
what are some points against mass flow
function of sieve tubes is unclear - would appear to hinder mass flow
not all solutes move at the same speed, we would expect this if mass flow is true.
sucrose is delievered at mostly the same rate to areas of different concentrations
if a sample of a plant is stained with TBO, what colours should be observed
lignin in walls of xylem will turn blue
phloem looks pink/purple
what are the 2 pathways water can take through a plant
the symplast pathway
the apoplast pathway
what is the symplast pathway
goes through the living parts of the cell- the cytoplasm. Cytoplasm connects through plasmodesmata.
Each successive cell’s cytoplasm has a lower water potential allowing water to move via osmosis.
there is some resistence to flow of water so is slower.
what is the apoplast pathway
goes through the non living parts of the cell - the cell wall.
walls are very absorbant so water can diffuse quickly through via mass flow
however water in apoplast pathway will reach endodermis cells in the root, the pathway is blocked by a waxy strip called the casperian strip. This is made of suberin which is waterproof. Water then takes the symplast pathway.
