Transpiration and translocation Flashcards
What is transpiration?
the loss of water vapour via the stomata
What is the transpiration stream?
the stream of water flowing up through a plant
Is the movement of water in a plant passive or active?
passive
Describe and explain the structure of the xylem?
xylem tissue contains tubes called vessels
dead cells are arranged end to end
their walls become impregnated with lignin which is impermeable to water
the living parts of the cell die, leaving a water filled space called the lumen
What direction/s does the xylem transport?
one direction ONLY
How does water move up the xylem?
due to cohesive-tension theory
Explain cohesive-tension theory?
-water evaporates from the stomata, which creates a water potential gradient
-this loss of water by transpiration means that more water molecules are pulled up the xylem to replace it
-due to hydrogen bonds between water molecules (cohesion) a column of water is created
-as the column of water is pulled up it creates tension, pulling the xylem in to be narrower
-water molecules also adhere to the sides of the xylem which helps to pull the column upwards
-finally water enters the roots via osmosis to replace the water moving up the xylem
What are the factors affecting transpiration rate and why?
wind (air movement)- increases transpiration- more wind will blow away humid air containing water vapour, so maintains the water gradient
humidity-decreases transpiration- increases water vapour in the air so the water potential gradient is not as steep
temperature-increases transpiration- more heat=more kinetic energy, faster moving molecules, more evaporation
light-increases transpiration- more light=more stomata to open, larger surface area for evaporation
How can we measure the rate of transpiration?
using a potometer
What is translocation?
the movement of organic substances around a plant via the phloem
What is the most important organic substance moved around a plant?
sucrose (disaccharide, non-reducing sugar, inert)
What is a source?
where substances are being produced, so have a high concentration
What is a ‘sink’?
where substances are being used up, or converted to other molecules, so have a lower concentration
How can radioactive isotopes be used in translocation?
take carbon-14 for example- will be used as normal in photosynthesis, so the c-14 will be incorporated into near sugars, and as c-14 is radioactive it can be traced
Describe and explain the phloem structure
SIEVE TUBE- THE BIT IN THE MIDDLE
it has sieve tube elements (a whole part of the phloem tube, looks like the whole of one cell on a diagram) which are living cells that form the tubes for transporting solutes. they have no nucleus and few organelles
it also has companion cells which are the cells surrounding the sieve tube, and they carry out living functions for sieve cells. they have a lot of mitochondria
What is the direction of flow in translocation?
always from the source to where its being used, but can go in any direction
Explain the two parts to the mass flow hypothesis
at the source-
-active transport is used to load the sugars into the phloem from companion cells
-water potential is lowered in the sieve cells of the phloem, so water moves in too via osmosis from the xylem
-this creates a high hydrostatic pressure at the source at the end of the phloem
at the ‘sink’-
-sugars are removed from the phloem to be used up
-water returns to xylem by osmosis due to an increase in water potential
-hydrostatic pressure is hence reduced in the phloem at the sink end
What causes the flow of solute in the phloem?
the hydrostatic pressure gradient
Why is there cohesion between water molecules?
-water is a dipolar molecule
-enables hydrogen bonds to form
-creates cohesion as they stick to each other
What 3 things make up cohesive-tension theory?
cohesion/adhesion/root pressure
What are the adaptions of sieve tube elements?
living cells, but no nucleus and few organelles
What are the adaptions of companion cells?
provide ATP for the active transport of organic substances
