7 Xylem & Phloem

Stomata

• The spaces next to the stomata are normally more humid than the atmosphere
• This causes a diffusion gradient from near the stomata into the atmosphere
• If stoata are open, water molecules diffuse out into the surrounding air
• Water lost by diffusion is replaced by water evaporating from the cell walls of mesophyll cells
• Plants can change their rate of transpiration by changing the size of the stomatal pores

Movement of water across the cells of a leaf

• Water is lost by the mosophyll by evaporation from their cell walls
• This is replaced by water reaching the cells from the xylem
• This is because the meophyll's water potential is lowered due to evaporation
• This causes water to enter by osmosis from neighbouring cells which do the same thing
• Altogether, this causes a potential gradient that pulls water from the xylem across the mesophyll layer

• Cohesion-tension
• Water evaporates from mesophyll cells due to heat from the sun leading to transpiration
• Water molecules form hydrogen bonds between each other and so they tend to stick together (cohesion)
• Water then makes a continuous, unbroken column across the mesophyll cells and down the xylem
• As water is evaporated, more water is drawn up due to the cohesion
• A column of water is therefore pulled up - transpiration pull
• Transpiration puts the xylem under tension as there is negative pressure within the xylem - hence the cohesion-tension theory
• Movement of water up the xylem is a passive process
• The cells that the water passes through are dead
• Xylem cells have no end walls which is essential for the cohesion-tension theory as it is one long continuous stream
• Energy is needed for transpiration but this comes from the sum in the form of light energy which is convrted to heat energy

• Change in the diameter of the tree trunk according to the rate of transpiration. During the day, transpiration is at its highest so more water is drawn up which creates more negative pressure which creates more tension. This results in a lthinner diameter of the tree trunk. The opposite happens at night when there is less transpiration pull
• If a xylem vessel is broken and air enters, the tree can no longer draw up water as the continuous column of water has been broken and so there is no cohesion in parts
• When a water vessels breaks, water doesn't leak out. This shows that the vesse isn't under high pressure. Instead, air is drawn in which shows that it is under tension instead of high pressure

1) Transfer of sucrose into sieve elements from photosynthesising tissue

• Sucrose is made in the chloroplasts of photosynthesising cells
• Sucrose diffuses down a concentration gradient by facilitated diffusion from photosynthesising cells into companion cells
• Hydrogen ions are actively transported from the companion cells into the spaces within cell walls, using ATP
• Hydrogen ions diffuse down the concentration gradient through carrier proteins into the sieve tube elements
• Surcose molecules are transported along with the hydrogen ions (co-transport)

2) Mass flow of sucrose through sieve tube elements

• Part 1 causes the sieve tube elements to have a lower water potential
• The xylem has a very high water potential
• Therefore, water diffuses across by osmosis into the sieve tube elements
• This creates a high hydrostatic pressure inside the sieve tube elements

• At the sink cells, sucrose is either being used up or stored as starch
• These cells therefore have a low sucrose concentration and so sucrose is actively transported into them from the sieve tube elements lowering their water potential inside sink cells
• Due to the lowered water potential, water moves into these cells from the phloem by osmosis
• The hydrostatic pressure of this region has therefore lowered

• Altogether the gradual change in hydrostatic pressure from source to sink causes mass flow in the sieve tubes from the source to the sink

3) Transfer of sucrose from the sieve tube elements into storage or other sink cells

• Sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells

For

• There is pressure within the sieve tubes as sap leaks out when cut
• The concentration of sucrose is higher in the leaves (source) than roots (sink)
• Downward flow happens during the day but stops at night or when shaded
• Increase in sucrose levels in the leaf are followed by an increase in sucrose in the phloem
• Metabolic poisons or a lack of oxygen inhibits the translocation of sucrose in the phloem
• Companion cells have lots of mitochondria which is needed for ATP production

Against

• The function of the sieve plates isn't clear as they would hinder mass flow
  • could potentially be for structural integrity under the high pressure of the phloem
• Not all solutes move at the same speed but they should under the mass flow theory
• Sucrose is delivered at generally the same rate to all regions of the plant rather than moving quicker to the regions with a lower sucrose concentration

• Woody stems have an outer layer of bark with the phloem after that and the xylem in the middle
• The phloem and woody bark is removed around the complete circumference
• After some time, the region of the stem directly above where the ring was cut will swell
• Samples of this liquid are rich in sugars and other dissolved substances
• Non-photosynthetic tissues in the region below wither and die whilst the tissues above continue to grow
• This is because the transfer of sugars was interupted by the ringing experiment
• Therefore, the phloem, not the xylem, is responsible for transporting sugars

• Radioactive isotopes such as C14 can be used to make radioactive carbon dioxide with the C14 incorporated
• If the plant is grown in an environment with C14 rich air then this isotope will be incorporated into the sugars produced by photosynthesis
• The radioactive sugars can then be traced as they move down the plant (autoradiography)
• this can show that the sugars produced are being transported inside the phloem and not the xylem

• When phloem is cut, a solution containing organic molecules flows out
• Tracer experiments with C14 in the sugars in the phloem
• Aphids have needle-like mouths whivch pierce into the phloem to extract the sugary contents
• Ringinging experiments cause a swell of sugars above the ring
• Diurnal chnages in surcose content in the leaves and root cells show a short delay