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.
what is an advantage of the apoplast pathway being blocked by the casperian strip
means water has to go through a selective membrane that is partially permeable. This gives some control over whether or not substances are able to pass through the plant.
how is water transported through the leaves
water leaves the xylem and moves into cells by the apoplast pathway. Water evaporates from cell walls into spaces between cells in leaf. When stomata open water diffuses down a water potential gradient. process is called transpiration.
outline the cohesion and tension theory
- water evaporates from the leaves at the ‘top’ of the xylem
- this creates tension (Suction) which pulls more water into the leaf
- water molecules are cohesive (form hydrogen bonds with each other, so stick to each other) so when pulled into the leave other water molecules follow meaning the whole column of water in xylem moves upwards
- water enters stem through the root cortex cells
what is adhesion
water molecules are attracted to the walls of the xylem vessels which helps water to rise upwards, supported by capillary action
what is transpiration
the process where plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through stoma in the leaves. Plants open their stomata to let CO2 in however water and oxygen leave through the stomata.
why does water move out of the leave when the stomata opens
there is a higher concentration of water inside the leaf than in the air outside, therefore water moves out of the leave down the water potential gradient when stomata open
what is meant by transpiration stream, what is the use of it
movement of water up the stem,
enables processes such as photosynthesis, growth and elongation as it supplies the plant with water which is necessary for these 3 processes
transpiration stream also supplies the plant with minerals + allows it to control the temperature via evaporation of water
what are the 3 processes involved in transpiration
osmosis - movement of water from xylem to mesophyll cells
evaporation - from surface of mesophyll cells to intercellular spaces
diffusion - of water vapour down a water vapour gradient out of the stomata
what assumption is made when using a potometer to estimate transpiration, why might the assumption not be correct
all water taken up is transpired therefore assume water uptake is equivelant to the rate of transpiration
however water could be used by the plant in other ways for example photosynthesis
or the apparatus may not be sealed properly and have leaks.
what are some factors affecting transpiration (abiotic + biotic)
number of leaves
number/size of stomata
presence of waxy cuticle
(biotic)
light intensity
temperature
humidity
(abiotic)
explain why would temperature increase the rate of transpiration
warmer water molecules have increased kinetic energy so evaporate from cells inside the leaf faster. This increases the water potential gradient inside and outside of the leaf, making water diffuse out of the leaf faster
explain what effect does increased humidity have on transpiration
increased humidity decreases the water potential gradient between inside and outside of the leaf, decreasing transpiration.
explain what effect does increased light intensity have on transpiration
increased light intensity, plants open their stomata to increase the amount of CO2 coming in for photosynthesis, this means more water can diffuse out which increases the rate of transpiration.
outline how to set up a potometer
- cut shoot underwater at a slant
- assemble potometer underwater and insert shoot underwater
- remove apparatus from water but keep end of capillary tube submerged in a beaker of water.
- check apparatus is water tight and air tight
- dry leaves and allow time for shoot to acclimatise, then shut tap
- remove end of capillary tube from beaker of water until one air bubble has formed. Put end of tube back into water
- record start position of air bubble
- start stopwatch and record distance moved by the bubble per unit of time.
when using a potometer, why did the student cut the shoot at a slant, and assemble underwater
assembled underwater:
- prevent air entering the shoot so as to not break the continuous column of water
- prevent air entering the potometer
cut at a slant:
increase the surface area available for water uptake
in a potometer experiment, the student wanted to calculate the rate of water uptake per minute, what measurements does she need to take
length of time and distance travelled by the bubble
the radius of the capillary tube
how does a reservoir in a potometer allow repeats to be made
returns the bubble to the start
describe the relationship between size and surface area to volume ratio of organisms
as size increases, surface area to volume ratio decreases.
what are xerophytes
plants adapted to living in dry conditions by minimising water loss
give 2 examples of a xerophytic plant and explain how it is adapted to live in its climate
marram grass:
- stomata are sunk in pits so are sheltered by wind, traps moist air in pits and so lowers the water potential gradient
- has a layer of hairs on epidermis that traps moist air around stoma
- in hot windy conditions they can roll their leaves trapping moist air and reducing exposed surface area
cacti:
- have a thick waxy layer on epidermis - reduces water lost by evaporation as layer is waterproof
- have spines instead of leaves - this reduces the surface area for water loss
- they close their stoma at hottest times of the day
what are hydrophytes
plants that live in water and must adapt to low oxygen levels
describe and explain how hydrophytes are adapted to live in their climate
- thin or absent waxy cuticle as dont need to conserve water
- stoma are constantly open and are found on the upperside of the leaf to maximise gas exchange
- wide flat leaves gives a large surface area for light absorbtion
- air sacs allow the leaves to float on surface of water (more light)
- flexible leaves and stems - helps prevent damage by water currents
why does water move from the soil into the roots
water potential is higher in the soil than in the root due to dissolved substances in cell sap
what is root pressure
the action of the endodermis moving mineral ions into the xylem via active transport, this drives water into the xylem by osmosis, and pushes water upwards
what is the vascular system
series of transport vessels running through stem, roots and leaves
what is parenchyma
undifferentiated cells that are packing and supporting tissue in vascular bundles with walls permeable to water and dissolved solutes
