pack 7 transport in plants Flashcards
(14 cards)
what is mass transport
mass transport is the movement of materials over large distances, due to pressure differences.
features of xylem vessels
1) their walls contain lignin - strengthens the xylem walls against the tension within them and makes them waterproof
2) the lignified vessel walls cause the cell contents to die - this leaves a hollow lumen with no cytoplasm that offers little resistance to the mass flow of water and minerials
3) walls contain tiny holes called pits - if a vessel becomes blocked or damaged, the water can be diverted, so the upward movement of water can continue in a adjacent vessel
4) vessels also lose their end walls - they form a continuous column for water movement from root to leaves
transpiration
- occurs in the leaves: water is evaporated from the mesophyll cells of the leafs, reducing their water potential
- water vapour forms in the air spaces which then diffuse out through open stomata
- a water potential gradient is formed across the leaf. water leaves the xylem vessels in the leaf and diffuses into the mesophyll cells by osmosis, replacing the lost water
- this creates a negative tension at the top of the xylem vessels
- the remaining water in the xylem is under tension and is pulled up towards the leaves
- continuous columns of water are maintained due to cohesion between water molecules
- there is also adhesion of water molecules to the walls of the xylem
- creates an inward pull on the vessel walls as the water is pulled up, causing the xylem vessel to decrease in diameter
evidence of transpiration
- tension has been measured in xlyem as plants transpire
- if a column of water in the xylem is broken, air bubbles in the xylem from and this stops any further upward movement of water in the xylem vessel as the air bubbles prevent cohesion
- respiratory inhibitors, such as cyanide or lack of oxygen, don not inhibit this process- passive
- the diameter of trees decrease when they are transpiring, and more so when temp and light intensities are higher. this can be measured with a denodrometer
xerophytic adaption- trapped humid air with high water potential
water vapour is trapped within the rolled leaf, thus reduces the water potential gradient between air spaces and the atmosphere. a lower rate of diffusion from the stomata occurs
xerophytic adaption- hairs on lower epidermis of leaf
water vapour is trapped between the hair so reducing the water potential gradient between air spaces and the atmosphere.a lower rate of diffusion from the stomata occurs
xerophytic adaption- stomata sunken pits
water vapour is held above the stomata pore so reducing the water potential gradient between air spaces and the atmosphere.a lower rate of diffusion from the stomata occurs
xerophytic adaption- thick waxy cuticles
The cuticle is waxy so reduces water loss (from the epidermis). Greater thickness increases the length of the diffusion pathway for water to reach the atmosphere, and so decreases rate of diffusion of water through the cuticle.
Structure of the phloem tissue
· Each sieve tube element links to the next via a sieve plate which is perforated with pores.
· The sieve tube has little cytoplasm, no nucleus, no vacuole and few organelles other than a small number of mitochondria.
· The sieve tubes are alive because of cytoplasmic connections (plasmodesmata) with the companion cell.
· Each companion cell has a nucleus, many mitochondria and other organelles.
Sources are:
· Where the organic solutes are produced and are therefore at a high concentration.
· The source for sucrose is usually the mesophyll cells of the leaves where it is formed by the condensation of fructose and glucose
Sinks are:
· Where organic solutes are used up and are therefore at a low concentration.
· The sinks are the other parts of the plant, especially the growing points
(meristems) of roots, stems, flowers and leaves where the sucrose is hydrolysed
to glucose and fructose and then respired to provide metabolic energy in the form of ATP.
Fruits, seeds, roots and other storage organs act as sinks when sucrose is
converted into starch and stored.
translocation
1 – At the Source
· Sucrose is actively transported from mesophyll cells of the leaf into the phloem via the companion cells
· The increased concentration of sucrose in the sieve tubes lowers the water potential, so water enters by osmosis from the xylem.
· This increases the volume within the sieve tubes around the source, which increases the hydrostatic pressure.
· This results in the mass flow in the sieve tubes from high pressure at the sources to low pressure at the sinks (e.g. the roots) where the sucrose can be used in respiration or in storage
2 – At the sink
· Sucrose is actively transported from sieve tube elements through companion cells into sink cells.
· This increases the water potential of the sieve tube cells and so water leaves by osmosis entering sink cells (and xylem).
· This decreases the volume which decreases the hydrostatic pressure.
Evidence supporting the mass flow theory
· There is high hydrostatic pressure in the phloem as shown by the release of sap when they are cut
· The concentration of sucrose is higher in the leaves (source) than in the roots (sink).
· Downward flow in the phloem occurs in daylight, but ceases when leaves are shaded, or at night.
· Increases in sucrose levels in the leaf are followed by increases in the phloem a little later.
· Metabolic inhibitors and/or a lack of oxygen inhibit translocation of sucrose in the phloem.
· Companion cells have many mitochondria.
Evidence against the mass flow theory
· The theory leaves the function of sieve plates unclear as they would hinder mass flow of sucrose – they may have a function in preventing bursting of sieve tubes under pressure.
· Not all solutes move at the same speed – they should if moved by mass flow.
· Sucrose is delivered at more or less the same rate to all sinks, rather than going faster to those with the lowest sucrose concentration as the mass flow theory would suggest.
