3.3 organism exchange- mass transport in plants Flashcards
how can water move through cells
Water can move up through the xylem due to tension, cohesion and adhesion.
As water evaporates from the leaf, it creates tension in the xylem tissues ( a suction effect pulling water molecules upwards)
Water molecules are cohesive. They stick together. This means as one water molecules move up the xylem, it pulls another water molecule up with it.
Water molecules are also adhesive. They stick to certain surfaces, like the walls of xylem vessels.
what is transpiration
when the plant opens its stomata to let carbon dioxide in, water on the surface of the cells evaporates out.
how does light affect transpiration
more light means more transpiration. Stomata open when its light to photosynthesis.
More open stomata= more evaporation
how does humidity affect transpiration
lower humidity means more transpiration. If the air is dry, there is a high concentration between the plant and air
how does temperature affect transpiration
higher temperatures mean more transpiration. Water molecules have more energy, so they evaporate quicker.
how does wind affect transpiration
more wind means more transpiration. Water molecules are blown away from the stomata
what is the xylem
Non-living tissue which transports water and mineral ions from roots to the leaves
what is the structure of the xylem
The vessels are long hollow tubes and fused end to end.
No cytoplasm, organelles or wall, allows continuous flow of water
Lignin is a waterproof substance which stops water leaving the xylem and gives the xylem vessels strength. No vacuole.
Pits are found around xylem vessels to control water exit.
Are continuous hollow tubes that run throughout the plant.
There are holes in the xylem with no lignification bordered pits- allow water to make lateral movement and change vessel
how to measure water uptake
Potometer- measures the rate at which a plant draws up water
Cant measure the rate of transpiration
Measures movement of the air bubble
Can investigate uptake of water under different conditions
what is a potometer
Leaf shoot cut under water. Fill the potometer completely with water ensuring no air bubbles are present. Fit the plant shoot to the potometer with a length of rubber tubing underwater. Seal all joints with waterproof jelly. Introduce air bubble into capillary tube. Measure the distance moved by the air bubble in a given time.
what is the phloem
Transfers sugar (movement of sugar- translocation Transports solutes ( mostly sugars) around the plant Movement is in both directions
what is the structure of the phloem
Sieve tube elements form tube structures throughout the plant
Sieve plate is between allowing movement from one to the next
Companion cells found alongside sieve elements carry out living functions for cells and help with loading and unloading
what is translocation
Movement of solutes throughout plant
Sugars are made in leaves during photosynthesis and need to be moved to places in the plant where they are needed
Eg- areas of growth
Moves solutes from the source ( where made eg- leaves) to the sink ( where needed eg- respiration)
what is the mass flow hypothesis
Stage 1- solutes are loaded from companion cells to sieve elements by active transport. Happens by the source, lowers water potential, water will move by osmosis into sieve elements from xylem
Stage 2- entering of water creates a high pressure in the sieve tubes at the source end
Stage 3- at sink end, solutes are taken from phloem and loaded into the companion cells. Water potential increases, so water leaves the sieve tubes into the xylem.
Stage 4- as water leaves phloem at sink end, the pressure decreases
Stage 5- a pressure gradient is set up from the source to the sink end of the phloem, pushing solutes along the phloem.
what are points against the mass flow hypothesis
- Sieve plates found between sieve tube elements create a barrier. Although sieve tubes are permeable, they do pose a barrier to movement through sieve tube elements. Very high pressure is required to move solutes through them.
- Studies have found different solutes can be found moving in different directions within the same sieve tube element. For mass flow to work, movement in 1 sieve tube must be in 1 direction
- Sugars travels to many different sinks at the same time, regardless of water potential. Evidence suggests sugars can move to any sink. Mass flow hypothesis suggests it would move to a higher water potential.
