Xylem:
In plants, water is absorbed by the roots through extensions (root hairs).
There is a water potential gradient from the air spaces inside a leaf to the surrounding air. Provided stomata are open, water vapour diffuses out. This lost water is then replaced by water evaporating from the cell walls of the mesophyll cells. This is replaced by water entering the mesophyll cels from the xylem via the cell walls/cytoplasm. A water potential gradient is established that bring water from the xylem. The rate of transpiration can be controlled by changing the size of the stomatal pore.
The movement of water up the xylem results mainly from cohesion-tension:
- Water evaporates from the mesophyll cells establishing a water potential gradient
- Water molecules form hydrogen bonds with one another sticking together (cohesion)
- This forms a continuous unbroken column fo water down the xylem and across the mesophyll cells
- As water evaporates from the mesophyll cells into air spaces next to the stomata water molecules are drawn up (this is as a result from the cohesion) - a column of water known as the transpiration pull is pulled up the xylem
- The transpiration pull puts the xylem under tension (negative pressure)
Evidence to support the cohesion tension theory includes changes in the diameter of tree trunks between day and night (coding to the rate of transpiration)/trees can no longer draw up water if a xylem vessel is broken/water does not leak out when a xylem vessel is broken, this is consistent with it being under tension.
Transpiration pull is a passive process so no metabolic energy is needed but energy in the form of heat that evaporates the water comes from the sun which drives the process.
Xylem are well adapted for this purpose as they are dead and have no end walls meaning they form a series of unbroken tubes from root. As they mature their walls incorporate lignin and the cells die. The lignin forms spirals/rings around the vessel.
Phloem:
Translocation is the process by which organic molecules and some inorganic ions are transported from one part of a plant to another. The phloem is the tissue that transports biological molecules in flowering plants. It is comprised of sieve tube elements whose end walls are perforated to form sieve plates. Companion cells are associated with sieve tube elements. Plants must move sugars from their site of production (sources) to the site of use/store (sinks). Sinks can be situated above or below sources so translocation can occur along or against gravity. Okay so this bit gets a bit confusing because we don’t actually know how the sugars are transported…we think it might be the mass flow theory:
- Sucrose is manufactured and diffuses down a concentration gradient by facilitated diffusion from photosynthesising cells into companion cells
- Hydrogen ions are actively transported from companion cells into spaces within cell walls (since it’s active transport, this uses ATP)
- The same hydrogen ions diffuse down a concentration gradient from companion cells into sieve tube elements and carry sucrose molecules with them (co-transport)
- This causes the sieve tube elements to have a lower water potential (remember it’s just more negative as 0 is the highest water potential you can have)
- Xylem has a much higher (less negative) water potential so water moves from the xylem into sieve tube elements by osmosis
- This creates a high hydrostatic pressure in the sieve tube elements
- The sinks have a low sucrose content as it has been used up or converted to starch for storage
- Sucrose is actively transported into the sinks from the sieve tubes lowering the water potential of the sinks
- Due to this water also moves into the sinks
- Hydrostatic pressure of this region is lowered
- Mass flow of sucrose solution down the hydrostatic gradient
NOTE: if anyone can help this would be great…..why is the sucrose actively transported into the sinks if there is a lower concentration there??????? thanks xx
This mass flow is passive but occurs as a result of the active transport of sugars. This means that overall it is an active process. This means it is affected by things such as metabolic poisons and temperature.
We need to know like for and against points on the mass flow theorem…
For:
- Sap is released when sieve tubes are cut = there is pressure within the sieve tubes
- Sucrose concentration is higher in sources (e.g leaves) than sinks (e.g roots)
- Downward flow in the phloem ceases when leaves are shaded/at night
- Increases in sucrose in sources (e.g leaves) are shortly followed by similar increases in sucrose levels in the phloem
- Metabolic poisons/lack of o2 inhibit translocation of sucrose in the phloem
- Companion cells contain many mitochondria to readily produce a lot of ATP
Against:
- Sieve plate functioning/structure is unclear
- Not all solutes move at the same speed but they should do if they move by mass flow
- Sucrose is delivered practically at the same rate to all regions rather than going to ones with a low sucrose concentration more quickly
Last bit, we need to know about ringing experiments and tracers that investigate transport in plants…
Ringing experiments:
On the inside of bark is phloem (on the inside of the phloem layer is the xylem). The bark and phloem layers are removed around the circumference of the stem. After a short while the section of stem immediately above the ring will swell and samples of liquid in this region are rich in sugars (as well as other dissolved substances). Non-photosynthetic tissue below the ring may die/wither whilst those above the ring do not (they continue to grow). This suggested that removing the phloem lead to sugars accumulating above the ring (swelling) and an interruption of the flow of sugars below the ring (death). From this it was concluded that it is the phloem (not the xylem) that is responsible for sugar translocation.
Tracer experiments:
Radioactive isotopes can be used to trace the movement of substances in plants. 14C can be used to make radioactive carbon dioxide (14CO2). Plants grown in atmosphere containing 14CO2 will incorporate it into it’s sugars which can be traced using autoradiography when moving through the plant. This can be done by taking a cross section of the stem and placing it on X-ray film. Where it has been exposed to radiation, the film becomes exposed to the radiation which is produced by 14C. The blackened areas correspond to phloem meaning phloem are responsible for translocation of sugars.
More evidence that the translocation of organic molecules occurs in phloem:
- When phloem is cut a solution of organic molecules flows out
- Plants provided with radioactive carbon have radioactively labelled carbon in phloem after a short time
- Aphid mouthparts can be used to show daily variations in sucrose concentration in leaves. This is later mirrored by identical changes in the phloem
- Removal of a ring leads to accumulation of sugars above the ring and death below (no sugars below)
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