Transport in plants Flashcards
(46 cards)
why do plants need a transport system?
small surface area to volume ratio
relatively large – high metabolic rate
exchange of substances by direct diffusion is too slow
need transport system as roots can obtain water and minerals but not all sugars
leaves can produce sugars but not get all water from air
what does the xylem and phloem do?
xylem tissue- transports water and soluble minerals upwards
phloem tissue- moves sugars up or down
where is the location of the xylem and phloem in the roots?
they are in the centre to provide support for the root as it pushes through the soil
where is the location of the xylem and phloem in the stem?
near the outside to provide ‘scaffolding’ that reduces bending
found together in vascular bundles
where is the location of the xylem and phloem in the leaves?
make-up a network of veins which support the thin leaves
what are the adaptations of xylem vessels?
hollow dead cells
no end walls- water passes up through the middle easily
pits (small holes) in the sides of the walls allow water and minerals to move into neighbouring cells
strengthened with lignin
ring or spiral structures allow flexibility
what are the adaptations of phloem tissue?
composed of sieve tube elements and companion cells
sieve tube elements have very few organelles- (no nucleus) thin layer of cytoplasm
At either end they have a sieve plate- lots of holes to allow solutes to pass through
companion cells have organelles to support themselves and the sieve tube elements
what is osmosis?
movement of water down a water potential gradient across a partially permeable membrane
what is the symplast pathway?
water enters the root hair cell via osmosis
travels through the cortex via the living parts of the cells (cytoplasm) until it reaches the xylem vessels
water moves down a water potential gradient
cytoplasms are connected via plasmodesmata
what is the apoplast pathway?
water is absorbed in the cell walls of the root hair cell
travels through the cortex via the non-living parts of the cell (walls and spaces between them) until it reaches the endodermis layer
moving via mass flow from an area of high hydrostatic pressure too low hydrostatic pressure
what is the casparian strip?
waxy waterproof layer
stops the water from travelling any further
what is the movement of water from roots to leaves called?
the transpiration stream
why does transpiration happen?
it happens as a result of gas exchange
a plant needs to open it stomata to let in carbon dioxide so it can produce glucose by photosynthesis
it also lets water out – higher concentration of water inside the leaf than in the air outside so water moves out of the leaf down the water potential gradient when the stomata open
how is water transported from the soil to the leaves?
minerals actively transported into root hair cells, lowering water potential in root hair cells
water moves from soil into root hair cells through osmosis, down WP gradient
water moves from cell to cell down WP gradient via symplast OR apoplast pathway
in symplast pathway, water moves through cytoplasm and between cells through plasmodesmata
in apoplast pathway, water moves between spaces in cellulose cell wall
at the casparian strip in the endodermis, water is forced out of apoplast and into symplast pathway
minerals are actively transported from endodermis cells to xylem, lowering WP gradient in xylem
water vapour leaves stomata by diffusion down concentration gradient
water moves up xylem via mass flow / capillary action (cohesive and adhesive)
water is pulled by xylem because of tension created by loss of water vapour via transpiration
why isn’t transpiration rate the same as rate of water uptake?
some water is used for photosynthesis
some water is used to create turgor pressure
some water is also produced by respiration
what are the factors affecting rate of transpiration?
light intensity temperature air movement humidity number of stomata
how does light intensity affect rate of transpiration?
increasing light intensity- more stomata will open, rate of photosynthesis is greater so more gas exchange occurs
rate of transpiration increases until all stomata are open- allows more water to evaporate out of the leaf
rate of transpiration stays constant
how does temperature affect rate of transpiration?
increasing temperature- the faster water vapour molecules will move because they have more kinetic energy
water evaporates from leaf faster
WP inside the leaf increases compared to outside (steeper WP gradient) so water diffuses out of leaf faster
rate of transpiration increases
how does air movement affect rate of transpiration?
the windier it is, the faster the transpiration rate
lots of air movement blows away water molecules from around the stomata
increases the WP gradient (maintains steep gradient) which increases the rate of transpiration
how does humidity affect the rate of transpiration?
the more humid the air, the more water vapour there is
this reduces the WP gradient between inside and outside of leaf
water molecules diffuse more slowly, decreasing the rate of transpiration
how do you measure rate of transpiration?
using bubble potometer
a bubble is incorporated in the setting up so that the movement of water can be tracked
how do you use a potometer?
1) cut shoot underwater to prevent air being introduced into the xylem
2) insert shoot into potometer underwater so no air can enter system
3) check system is air and water tight
4) dry both sides of the leaves
5) allows shoot to acclimatise- rate will be constant
6) introduce an air bubble into the capillary tube
7) record start position of bubble, start timer
8) record distance and time taken for the bubble to move
9) use these values to calculate rate (D/T)
10) only alter one variable at a time - keep other factors which affect transpiration constant
11) repeat to eliminate anomalous result
12) calculate mean result and/or carry out statistical analysis
what are the limitations of using a potometer?
cutting plant shoot may damage plant
plant has no roots so no resistance to water being pulled
how do you calculate transpiration rate?
rate = volume / time
you need to calculate the volume of a cylinder ( length of capillary tube)
V=πr2h
