Transport In Plants Flashcards
What do plant leaves have?
Stomata on the underside of the leaves
What happens if you cute the leaves off the plant?
You decrease the number of stomata meaning the mean rate of water uptake/transpiration rate decreases.
Explain the cohesion-tension theory.
Water evaporates from mesophyll cells due to heat from sun leading to transpiration.
There is cohesion between water molecules due to hydrogen bonds.
Water forms a continuous column across mesophyll cells down the xylem.
When water evaporates from mesophyll to surrounding air spaces below stomata (water vapour), more water molecules are drawn up behind it due to cohesion.
Column of water is pulled up xylem from transpiration creating the transpiration pull.
This transpiration causes tension + negative pressure, so is called the cohesion-tension theory.
List 3 pieces of evidence that support the mass flow hypothesis.
- There is pressure within sieve tubes, shown by sap being released when they are cut.
- Conc. of sucrose = higher in the leaves (source) than in the roots (sinks).
- Downward flow in the phloem occurs in daylight, but stops when leaves are shaded or at night.
- Increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem a little later.
- Companion cells have many mitochondrion and readily produce ATP.
- Metabolic poisons/lack of oxygen inhibits translocation in the phloem.
List 3 pieces of evidence questioning the mass flow hypothesis.
- Function of sieve plates = unclear, as they would seem to hinder the mass flow theory (but, they may have structural function as helping to prevent the tubes from bursting under pressure).
- Not all solutes move at the same speed - they would if movement was by mass flow.
- Sucrose is delivered at more or less the same rate to all regions, rather than going straight to the ones with the lowest sucrose concentration, which is what the mass flow theory would suggest.
Is the mass flow theory a passive process?
Is a passive process but occurs as a result of active transport of sugars, so therefore process as a whole is active.
So it is affected by, for example, temperature + metabolic poisons.
What is translocation?
The method that organic molecules (e.g. Sucrose + amino acids and some inorganic ions e.g. Potassium, chloride, phosphate + magnesium ions) + some mineral ions are transported from one part of the plant to another.
Transportation of sugars produced during photosynthesis from photosynthetic cells - sources in xylem - to places where they can be used/stored for future use - sinks in xylem.
Sinks = sometimes above the sources, so therefore translocation of molecules in the phloem can be in either direction.
Why did they come up with the mass flow theory?
The rate of mouvement of materials in the phloem was too fast to be explained by diffusion so they came up with the mass flow theory.
What are the 3 parts of the mass flow theory?
1) Transfer of sucrose into sieve elements from photosynthesising elements.
2) Mass flow of sucrose through sieve elements.
3) Transfer of sucrose from sieve tube elements to storage/sink cells.
How is sucrose transferred from photosynthesising elements into sieve elements of the xylem?
Sucrose is produced by photosynthesis in cells with chloroplasts.
Sucrose diffuses down concentration gradient by facilitated diffusion from photosynthesising cells to companion cells.
Hydrogen ions are actively transported into companion cells into cell wall using ATP.
Hydrogen ions diffuse down concentration gradient through carrier proteins into sieve tube elements.
Sucrose are transported at same time as hydrogen molecules across co-transport proteins into sieve tube elements.
How does the mass flow of sucrose through sieve tube elements occur?
Sucrose from photosynthesising cells = actively transported into sieve tubes, which lowers the sieve tubes’ water potential (=more -ve). Xylem have a much higher water potential (less -ve), so therefore water moves across from the xylem to the phloem by osmosis creating a high hydrostatic pressure.
At respiring cells (sinks) sucrose = used up in respiration/converted to starch for storage. .: these cells have a lower sucrose content , so sucrose is transported from the sieve tube (sinks) to them lowering sinks’ water potential. .: Water moves by osmosis to respiring cells from sink sieve tube elements. Hydrostatic pressure of sieve tubes = lowered.
Water enters sieve tubes at source + leaves at sink meaning high (sink) to low(source) hydrostatic pressure. So there is a mass flow of sucrose down a hydrostatic gradient in sieve tubes.
How is sucrose transported from sieve tube elements into storage/sink cells?
Sucrose = actively transported by companion cells out of sieve tubes and into sink cells.
What is the function of the phloem and what type of plant are they found in?
In flowering plants.
Transport biological molecules.
List 3 factors of the phloem’ structure adapting it to it’s function.
Made of sieve tube elements - long thin strictures arranged end to end.
End walls = perforated to form sieve plates.
Next to sieve tube elements are companion cells to aid the active transport of sucrose into and out of the phloem/sieve tube elements.
What is transpiration?
The transport of water up the stem through the xylem from the roots to the leave a
