M3- Chapter 9 - Transport in plants Flashcards
Why do plants need a transport system
Metabolic demands
Size
Surface are : volume ratio
Metabolic Demands for reasons why do plants need a transport system
Plants need to make their own food, by photosynthesis. But the internal and underground parts of the plant don’t photosynthesise.
Size for reasons why do plants need a transport system
Because plants continue to grow their entire life, they are large. They need a very effective transport system to move all the substance around from tip to roots etc.
SA: V ratio for reasons why do plants need a transport system
Leaves are large to allow a large sa: v ratio, for the exchange of gases. However, when the stems, roots, and trunks are taken into account, they still have quite a low sa:v ratio. Hence, they can’t rely on diffusion alone to supply their cells.
Xylem
The transport of mineral ions and water.
From the roots up to the leaves.
They are made up of dead cells.
They are made up of long hollow structures made up by several columns of cells fusing together end to end.
How does the lignin in the walls of xylem vessels enable the vessels to carry out their function?
They are thick and this gives them support and strength. They are also waterproof, and so they don’t allow the water in or out.
Explain why the xylem vessels have flexibility and the ability to elongate as well as being very strong
The lignin that is in the spirals in the vessels allows the vessels to stretch as the plant grows and enables it to bend.
Function of the phloem
It transports food in the form of organic solutes around the plant. The phloem supplies the cells with sugars and amino acids needed for cellular respiration.
Main transporting vessels of the phloem
Sieve tube elements. They are made up of many cells joined end to end to form a long, hollow structure. In some areas, they form sieve plates, and let the phloem contents through.
Tonoplast
Vacuole membrane
Do mature phloem cells have nuclei?
No
Companion cells
They are closely connected to the sieve tube elements. They are linked through the plasmodesmata.
Compare differences for companion cells and phloem
Sieve tube:
- sieve plates
- no nucleus
- no rough ER
- lumen
- no vacuole
- no ribosomes.
- no phloem proteins.
Companion cells;
- no chloroplast
- has a rough ER
Are xylem cells in the middle or outside
They are generally on the inside, and phloem is on the outside.
Why is water so important in plants
Turgor pressure (result of osmosis)
Turgor alos drives cell expansion- a force that enables plant roots to force their way through tarmac etc.
Loss of water by evaporation helps cool down plants.
Mineral ions and the products of photosynthesis are transported in aqueous solutions.
Water is needed for photosynthesis.
Movement of water into the roots.
Root hair cells are a long, thin extension from a root hair cell.
- They can penetrate easily between the soil particles, because they are so small.
- Large SA:V ratio
- Thin surface layer
- Concentration of solutes in the cytoplasm maintains the water potential gradient.
Explain the concentration of solutes in the cytoplasm
Soil water has a low conc. of dissolved minerals and a high water potential.
Cell sap are full of other solutes, so they have a low water potential. so water moves in.
Movement of water across the root
The symplast pathway
The apoplast pathway
Symplast pathway
The continouos cytoplasm of the living plant cells that is connected through the plasmodesmata- by osmosis. Root hair cells have higher water potential than the next cell. So water moves in to the next cell, and continues till it reaches the xylem.
As water leaves a cell, the water potential remains low again, so the movement of water continues.
Apoplast pathway
The cell walls and the intercellular space.
Water fills the spaces between the loose, open network of fibres in the cellulose cell wall. As water moves, it pulls molecules behind it (cohesion). The pull creates a tension so there is a continuous flow of water, giving very little resistance.
Movement of water into the xylem.
it reaches the endodermis. It is noticeable due to the effect of the Casparian Strip. Any water in the apoplast pathway is forced into the cytoplasm, and joins the symplast pathway.
Endodermis
Layer of cells surrounding the vascular tissue of the roots
Casparian Strip
A band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer. It forces the apoplast water into the cytoplasm.
Why is the diversion to the cytoplasm by the apoplast pathway so important?
Because water passes through the selectively permeable cell surface membranes to get there, excluding any toxic solutes in the soil.
How does the symplast pathway get water into the xylem?
The symplast moves minerals ions into the xylem by active transport, to decrease the water potential. This way, the rate of osmosis increases, so that the water can move in quickly down a water potential gradient.
What happens when water is inside the vascular bundle?
The water return to the apoplast pathway to enter the xylem itself and move up the plant.
Pumping mineral ions into the xylem results in root pressure.
Evidence for the role of active transport in root pressure
- Poisions affect the mitochondria and prevent the production of ATP. If the poison is applied to root cells, there is no energy supply because the root pressure disappears.
- Root pressure increases with a rise in temperature. Suggesting chemical reactions are involved.
- Levels of o2 of respiratory substrates fall, root pressure falls.
- Xylem sap may exude from the cut end of stems at certain times.
What is guttation?
When xylem sap is forced out of special pores at the ends of leaves in some conditions.
What is photosynthesis
The process by which green plants make their own food, and it takes place mainly in leaves.
How does CO2 come into a plant
It diffuses in through a concentration gradient.
How does water leave the plants?
It evaporates out from the surface on the the leaf cells into the air spaces.
