Mass transport in plants Flashcards
how is water taken up the plant
Water is taken in through the roots (high SA root hairs) from the by osmosis from a high to low water potential and continues to pass into the centre of the root from cell to cell
how does water travel through the cells of the plant (2)
symplastic pathway
apoplastic pathway
symplastic pathway
through the cytoplasm and plasmodesma of the root cells
apoplastic pathway
through the permeable cell walls (faster) but will stop at the impermeable casparian strip so is forced upwards to the symplast pathway
what does the xylem transport
• Dead cells which transport water and dissolved mineral ions up the stem
cohesion tension
water is pulled up the xylem by transpiration pull as a continuous column because water molecules form hydrogen bonds with one another
how is the xylem adapted to transport the things
Contains lignin which is strong and waterproof
where is the xylem in the roots and why
one big xylem in the middle so it is protected
where is the xylem in the stem and why
xylem is branched out into vascular xylem so stem is stronger from having many water cylinders to keep it upright
types of cells in the phloem
Living cells have cellulose cell walls and sieve plates which are controlled by companion cells
what does the phloem transport
• Transports amino acids, sucrose and inorganic acids (e.g. magnesium, phosphate, chloride, potassium) throughout the plant
translocation
the process by which organic molecules and some minerals are transported from one part of a plant to another
mass flow theory
- Sucrose passes from the source into companion cells by facilitated diffusion
- H+ ions pass from companion cells into the phloem by active transport
- H+ ions diffuse into sieve elements and carries sucrose by co-transport which lowers water potential in the sieve tubes
- Higher water potential in the xylem means water moves into the phloem by osmosis, increasing the hydrostatic pressure
- Respiring cells have a low concentration of sucrose so sucrose enters which lowers water potential causing water to move in and lower the hydrostatic pressure in the sieve tubes
- High hydrostatic pressure at the source and a low hydrostatic pressure at the sink
- Mass flow of sucrose solution from source to sink
- Sucrose is actively transported by the companion cells out of the sieve tubes and into the sink cells
evidence to support mass flow theory (4)
Solution leaks out when phloem is cut suggests high pressure inside
Concentration of sucrose is higher in leaves than roots
↑ sucrose concentration in leaves also ↑ in phloem
Companion cells have many mitochondria
evidence against mass flow theory (3)
Sucrose is delivered at the same rate throughout but mass flow would suggest areas with low concentrations are quicker
Unclear function of sieve elements
Not all solutes move at the same speed
ringing experiments
Remove the phloem and bark from a tree
Swell above the region and area is rich on sucrose suggests phloem is the site
of sugar transport
tracer experiments
Can label CO2 used in photosynthesis by radioactive isotopes and trace it when incorporated into sugars and transported
transpiration
the passive movement of water up a plant from the roots through the stem and out of the leaves
potometer
what does it measure
issue in the experiment
measures the rate of transpiration
- hard to form a bubble, difficult to control variables e.g. leaves are wet, draft, temperature, sunlight
factors affecting transpiration
temperature
wind speed
light intensity
humidity
how does temperature increase affect transpiration
o rate of transpiration increases
o particles have more energy so move faster so more water particles evaporate from the leaf
how does wind speed increase affect transpiration
o rate of transpiration increases
o wind moves away water particles that surround the leaf so increases concentration gradient
how does light intensity increase affect transpiration
o rate of transpiration increases
o more stomata are open for photosynthesis
o at a certain light intensity, it will not increase due to another limiting factor
how does humidity increase affect transpiration
o Rate of transpiration decreases
o Humid air has a higher concentration of water molecules so a shallow concentration gradient
