Mass transport in plants Flashcards
How is water moved out through stomata?
The humidity of the atmosphere is usually less than that of the air spaces next to the stomata.
So there is a water potential gradient from the air spaces through the stomata to the air.
When the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air.
Water lost by diffusion from air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.
By changing the size of the stomatal pores, plants can control their rate of transpiration.
How does water move across the cells of a leaf?
Water is lost from mesophyll cells by evaporation from their cell walls to the air spaces of the leaf.
This is replaced by water reaching the mesophyll cells from the xylem either via cell walls or via the cytoplasm.
Why does water movement of the cytoplasmic route occur?
Mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun.
These cells now have a lower water potential and so water enters by osmosis from neighbouring cells.
The loss of water from these neighbouring cells lowers their water potential.
They, in turn, take in water from their neighbours by osmosis.
A water potential gradient is established that pulls water from the xylem, across the leaf mesophyll, and into the atmosphere.
How does movement of water up the stem in xylem occur - cohesion part?
Cohesion tension:
Water evaporates from mesophyll cells due to heat from the sun leading to transpiration.
Water molecules form hydrogen bonds between one another and hence tend to stick together - cohesion.
Water forms a continuous, unbroken column across the mesophyll cells and down the xylem.
How does movement of water up the stem in xylem occur - transpiration pull?
As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of the cohesion.
A column of water is therefore pulled up the xylem as a result of transpiration - the transpiration pull.
Transpiration pull puts the xylem under tension, a negative pressure within the xylem - cohesion-tension theory.
How is the cohesion tension theory supported?
Change in the diameter of tree trunks according to the rate of transpiration.
During the day, when transpiration is greatest, there is more tension in the xylem. This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter.
At night, there is less tension in the xylem and so the diameter increases.
How is the cohesion tension theory supported - breaking?
If a xylem vessel is broken and air enters it, the tree can no longer draw up water. This is because the continuous column of water is broken and so the water molecules can no longer stick together.
When a xylem vessel is broken, water does not leak out. Instead air is drawn in, which is consistent with it being constantly under tension.
Is energy required for water uptake in plants?
Transpiration pull is a passive process and therefore does not require metabolic energy to take place.
The xylem vessels through which the water passes are dead and so cannot actively move the water.
Xylem vessels have no end walls which means that xylem form a series of continuous, unbroken tubes from root to leaves.
Energy is nevertheless needed to drive the process of transpiration, in the form of heat that evaporates water from the leaves and ultimately comes from the sun.
What is translocation?
The process by which organic molecules and some mineral ions are transported from one part of a plant to another.
What are the phloem?
In flowering plants, they are the tissue that transports biological molecules.
It is made up of sieve tube elements, long thin structures arranged end to end.
Their ends are perforated to form sieve plates.
Associated with sieve tube elements are companion cells.
What is the source and sink?
Having produced sugars during photosynthesis, the plant transports them from the sites of production, the sources, to the places were they will be directly used or stored for future use, sinks.
Sinks can be anywhere in the plant - sometimes above and sometimes below the source - so translocation of molecules in phloem can be in either direction.
What does the phloem transport?
Organic molecules to be transported are sucrose and amino acids.
It also transports inorganic ions such as potassium, chloride, phosphate and magnesium ions.
What is the first phase of the mass flow theory?
Transfer of sucrose into sieve elements from photosynthesising tissue:
Sucrose is manufactured from the products of photosynthesis in cells with chloroplasts.
The sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells.
Hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP.
The hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements.
Sucrose molecules are transported along with the hydrogen ions by co-transport, with co-transport proteins.
What is mass flow?
The bulk movement of a substance through a given channel or area in a specified time.
What is the second phase of the mass flow theory?
Mass flow of sucrose through sieve tube elements:
The sucrose produced by the source is actively transported into the sieve tubes.
This causes the sieve tubes to have a lower water potential.
As the xylem have a much higher water potential, water moves from the xylem into sieve tubes by osmosis, creating a high hydrostatic pressure within them.
