4.7 Transport in Plants Flashcards
name the two main transport tissues in plants
xylem and phloem
function of xylem
transport water and dissolved minerals from roots to photosynthetic parts of the plant
movement is upwards
function of phloem
transports substances like glucose to leaves for growth and storage
movement can be up or down
what is cambium?
a layer of unspecialised cells that divide, giving rise to more specialised cells that can form xylem + phloem
how is xylem formed?
- starts as protoxylem (living tissue)
arranged in column + can grow - as stem ages, lignin is deposited in cell walls so cells die
and xylem becomes a hollow metaxylem-impermeable
function of lignin
to strengthen and prevent collapse
what does xylem tissue consist of?
xylem vessels
xylem fibres
xylem parenchyma cells
what happens when lignification is incomplete?
bordered pits can form which allows water to leave and pass elsewhere
what does the phloem consist of + how does it work?
-phloem sieve tubes
many cells joined to make long tubes that become perforated to form sieve plates (living)
-specialised sieve plates
phloem contents flow through holes
sieve tube becomes a tube of phloem sap and phloem clls have no nucleus
-companion cells
active cells linked to sieve tube elements by plasmodesmata
lots of infoldings and have lots of mitochondria - ATP
compare xylem and phloem
xylem made of dead cells thick cell wall made of lignin - impermeable no cross walls or cytoplasm transports water + minerals to leaves flow is upwards
phloem made of living cells thin cell wall of cellulose -permeable perforated cross walls plasmodesmata transports food for growth + storage flows up and down
evidence for movement of water
- eosin dye
- ringing experiments
- autoradiography
how is water taken up through root hairs?
water moves from soil into a root hair cell down a concentration gradient by osmosis
this makes the root hair cell more dilute than its neighbour so water moves from cell to cell by osmosis across root to the xylem
what is the symplast pathway?
water moves by diffusion down the concentration gradient from the root hair cells to the xylem through plasmodesmata
what is the apoplast pathway?
water pulled by attraction between water molecules across adjacent cell walls from root hair to xylem
due to open network structure of cellulose, half of the cell wall can be filled with water
water moves across the cells of root in the cell walls until it reaches endodermis
what happens when water meets the Casparian strip?
they temporarily enter the cytoplasm of the cell.
end result of all pathways is a continuous stream of water cross the root to xylem
define transpiration
evaporation of water from the surface of the spongy mesophyll cells + the loss of water by diffusion down a concentration gradient from a leaf
describe the process of transpiration
- water lost by transpiration from leaves - moves by osmosis across leaf from cell to cell all the way to xylem
- when molecules of water leave xylem to enter cell, creates tension in column of water in xylem + tension is sent to roots
- due to polar nature and h-bonds, water molecules ‘stick’ together, giving column of water a high tensile strength
- loss of water molecule from evaporation causes tension through plant because of cohesiveness, so more water is pulled up by xylem to replace what is lost
explain what cohesion is
forces of attraction between like-molecules
water is strongly cohesive - each molecule can make up to 4 h-bonds
cohesion is responsible for surface tension
explain what adhesion is
force of attraction between unlike- molecules
water forms h-bonds with other surfaces including tiny pores in mesophyll cell walls
define translocation
movement of solutes in phloem of plants
describe mass-flow hypothesis
- dissolved sucrose moves in phloem by pressure gradient between sources and sinks
- high concentration of sucrose reduces mesophyll + sieve cell water potential so water is drawn in - high hydrostatic pressure
- forces mass flow of phloem sap towards sink where hydrostatic pressure is low due to use of sucrose or storage as starch
strengths of mass flow hypothesis
- experiments show aphid mouthparts exude sap; supports idea that content of phloem is under pressure
- phloem + xylem are linked in a way that makes sense
weaknesses of mass flow hypothesis
- movement can be up or down in same sieve tube at different speeds
- phloem sieve plates have no obvious role in this theory
- experiments show that sieve and companion cells need to be alive but theory does not explain why
how does wind affect transpiration rate?
as wind blows, it moves air from near the leaf and increases the diffusion gradient, which increases the concentration gradient
how does temperature affect transpiration rate?
as the temperature rises, water molecules gain kinetic energy + move faster, increasing evaporation and transpiration
how does light intensity affect transpiration rate?
as it becomes brighter, light causes the stomata to open for photosynthetic gas exchange and so allows more water out. transpiration increases with light intensity until all stomata are open.
how does humidity affect transpiration rate?
at lower humidity, the concentration gradient is steeper so the transpiration rate increases. as humidity increases, rate of transpiration decreases
how can you work out rate of uptake from a potometer experiment?
rate of uptake = V/ time taken for bubble to move through distance, h
