transport in plants Flashcards
Why do plants need a specialised transport system?
- to move products of photosynthesis, water, and oxygen around from their place of origin (high metabolic demand)
- most plants are large and so have to transport substances huge distances from root to tip
- SA: vol is small in plants (even though for leaves SA:vol high)
where is the vascular bundle in the roots
the xylem is in the centre surrounded by phloem to provide support to the roots as it pushes through the soil
it helps the plant withdtand the tugging strains that teults as the stems and leaves ate blown in the wind
where are the vascular bundle in the stem
the xylem and the phloem are around the edges to provide strength and support and prevent bending
where are the vascular bundles in the leaves
xylem and phloem make up a network of veins which support the thin leaves
how are xylem adapted for transport of water and mineral ions
they are very long tuble like structures formed from cells (vessle elements) joined end to end
no end walls on these cells making an uninterupted tube that allows water to pass up through the middle easily
cells are dead so no cytoplasm
walls are thickened by lignin which helps support the xylem vessel and stops them collapsing inwards. lignin can be diposited in xylem walls in different ways e.g in a spiral or as distinct rings
amount of lignin increases as cells get older
water and mineral ions move into and out of the vessel through small pits in the wall where theres no lignin
what do xylem transport
water and mineral ions
what do phloem vessels transport
solutes(dissolved substances) mainly sucrose
features of sieve tube elements
they are living cells that form the tube for transporting solutes through the plants
they are joined end to end to form sieve tubes
the sieve parts are the end walls which has lots of holes to allow solutes to pass through
they have no nucleus, a very thin layer of cytoplasm and a few organelles
they cytoplasm of adjacent cells is connected through the holes in the sieve plates
features of companion cells
the lack of nucleus and other organelles in the sieve tube element means they cant survive on their own so theres a companion cell for every sieve tube element
they carry out the living functions for both themselves and their sieve cell
for example they provide the energy for active transport of solutes
how does water go to the xylem
enters the roots via the root hair cells
then passes through the root cortex including the endodermis to reach the xylem
how does water enter the roots
by osmosis
water mives from an area of high water potential to an area of low water potential down a water potential gradient
soil generally has high water potential and leaves have low water potential as water constantly evaporates of them
thtis creates a water potential gradient that keeps water moving in a plant in the right direction from roots to the leaves
what is the symplast pathway
water flows through the cytoplasm by osmosis . the cytoplasm of neighbouring cells are connected through the plasmodesmata
what is the apoplast pathway
movement of water through the cellulose cell wall. the walls are very absorbant and water can simply diffuse through them as well as pass thriugh the space between them. the water can carry solutes and go from an area of high hydrostatic pressure to an area of low hydrostatic pressure. this is an example if mass flow
what happens when water gets to the endodermis cells
water in the apoplast path is blocked by the casparian strip which is a band of suberin that runs around the endodermis cells forming a waterproof layer so water enters the symplast pathway
this is useful as water has to go through the cell membrane which is partially permeable do can control whether or not substances can pass through
once it has passed this barrier it moves into the xylem
what is the main pathway
apoplast as it provides the least resistance
what happens when water reaches the xylem
xylem vessel transports the water all around the plant
at the leaves water leaves the xylem and moves into the cell mainly by apopolast pathway
water evaporates from the cell wall into the spaces between cells in the leaf
when the stomata is open water diffuses out of the leaf down a water potential gradient into the surrounding air
the loss of water from a plants surface is called transpiration
the loss of water lowes the water potential of the cell so water moves into the cell from an adjacent cell by osmosis along both apoplasy and symplasy pathway
how does cohesion and tension help water move up a plant
water evaporates from the leaves at the top of the xylem
this creates tension which pulls more water into the leaves
water molecules are cohesive so stick together so when some are pulled into the leaf more follow. this means the whole column of water in the xylem from the leaves down to the roots moves upwards
water enter the stem through the root cortex cells
how does adhesion help the movement of water
water is attracted to the walls of the xylem vessel. this helps water rise up through the xylem vessel
how does water move into the xylem
the solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to the cells in the xylem and the endodermal cells move mineral ions into the xylem by active transport.
As a reult the water potential of the xylem cells is much lower than the water potential of the endodermal cells. this increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the symplast pathway
what happens once the water enters the xylem
it returns to the apoplast pathway to enter the xylem itself and move up a plant.
the active pumping of minerals into the xylem to produce movement of water by osmosis results in root lressure and it is independent of any effects of transpiration.
root pressure gives water a push up the xylem but is not the major facot in the movement of water up from the roots to the leaves
evidence for the role of active transport in root presure
some poisons such as cyanide affect the mitochindria and prevent the production of ATP. if cyanide is applied to root cells so theres no energy supply the root pressure disappears
root pressure increases with a rise in temperature and decreases with a fall in temperature suggesting chemical reactions are involved
if levels of oxygen or respiratory substances fall root pressure falls
why is transpirationa consequence of gas exchange
a plant needs to open its stomata to let carbon dioxide in for photosynthesis
but this lets water out as theres a high concentration of water inside the leaf then in the air so water moves out of the leaf down a water potential gradient when the stomata open
evidence for the cohesion tension theory
changes in the diameter of trees. when transpiration is at its height during the day the tension in the xylem vessel is st its highest too. as a result the tree shrinks in diamtere. at night when transpiration is at it’s lowest the dianeter of the tree increases
when a xylem vessel is broken. when you cut flower stems to put them in water in most circumstances air is drawn into the xylem rather than water leaking out
if a xylem vessel is broken and air is pulled in the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken
how does light affect the transpiration rate
light- the lighter it is the faster the transpiration rate. this is beacuse the stomata open when it is light so carbon dioxide can diffuse into the leaf for photosynthesis. when it is dark the stomata are usually closed so little transpitation
