10. Exchange in Plants Flashcards
What is parenchyma?
Packing tissue that fills spaces between other tissues.
What is sclerenchyma?
Sclerenchyma cells have lignified walls and are used to strengthen stems and leaf midribs.
What is collenchyma?
Collenchyma cells have thick cellulose walls and are used to strengthen the vascular bundles and outer parts of stems whilst allowing some flexibility
What does the transport system in plants consist of?
Specialised vascular tissue
Water and soluble mineral ions travel upwards in xylem tissue
Assimilates, such as sugars travel up or down in phloem tissue
How is the vascular bundle arranged in the root?
The vascular bundle is found in the centre of the root
There is a central core of xylem in an X shape.
The phloem is between the arms of the X-shape which gives the root tensile strength.
There is a sheet of cells surrounding the bundle called the endodermis.
How is the vascular bundle arranged in the stem?
The vascular bundle is found on the outer edge which provides strength and flexibility to withstand the bending forces that the stems are exposed to.
The xylem is found on the inside of each bundle then a layer of cambium then phloem.
What is cambium?
A layer of meristem cells that divide to produce xylem and phloem
What is the structure of xylem?
Lignin impregnates cell walls killing them.
Lignin makes the cells waterproof and ensures they won’t collapse.
The end of walls decay leaving a long column of cells.
When lignification is not complete it creates boarded pits aligned into adjacent vessels to allow water to move in and out of the vessel.
How is xylem adapted to its function?
The tubes are narrow so that the water column does not break easily
Boarded pits in the walls allow water to move side waves from one vessel to another
The flow of water is not impeded because there are no cross walls, there is no cell contents, and the lignin thickening prevents the walls from collapsing.
What is the function of xylem?
Transports water and mineral ions from the roots to the rest of the plant.
What is the function of the phloem?
Transports assimilates around (up and down) the plant
What is the structure of phloem?
Elongated sieve tube elements are lined up to form sieve tubes
They contain a nucleus and very little cytoplasm, leaving space for mass flow.
The end of the sieve tube elements have perforated cross-walls called sieve plates.
Between the sieve tubes are small companion cells, with a large nucleus and dense cytoplasm.
Their many mitochondria produce the ATP needed to actively load assimilates into the sieve tubes.
What are the 3 pathways water can take across plant cells?
The apoplast pathway
The symplast pathway
The vacuolar pathway
What is the apoplast pathway?
Water passes through the spaces in the cell walls and between the cells.
It does not pass through any plasma membrane into the cells.
This means that the water moves by mass flow rather than by osmosis.
What is the symplast pathway?
Water enters the cell cytoplasm through the plasma membrane.
It can pass through plasmodesmata from one cell to the next
What is the vacuolar pathway?
Similar to the symplast pathway, but the water is not confined to the cytoplasm of the cells.
It also passes through the vacuoles
How does water move from cell to cell in plants?
Water moves by osmosis from a region of higher water potential to a region of lower water potential. In a plant cell, the cytoplasm contains mineral ions and sugars that will reduce the water potential so water moves by osmosis into the cell. As water diffuses by osmosis into the cell it increases this water potential so it will then diffuse by osmosis into the next cell which has a lower water potential.
How is water taken up into the root of the plant?
Because plant cells have low water potential, water molecules will move down the water potential gradient into the cell.
Once the cell is full of water it is described as turgid.
The water inside the cell starts to exert pressure on the cell wall, called the pressure potential.
This reduces the influx of water.
How is water lost from the plant?
If plant cells have a higher water potential than the solution, water will diffuse by osmosis out of the plant.
This causes the cytoplasm and vacuole of the plant to shrink.
The plasma membrane will lose contact with the wall in a process called plasmolysis.
The tissue is now flaccid.
What is transpiration?
The loss of water from the upper parts of the plant, particularly leaves
How does water usually leave the leaves of plants?
Water enters the leaf from the xylem and moves by osmosis into the cells of the spongy mesophyll of along the cell walls via the apoplast pathway.
Water evaporates from the cell walls of the spongy mesophyll.
This lowers their water potential, causing water to enter them by osmosis from neighboring cells.
Water diffuses out of the leaf through the open stomata down the water vapour potential gradient.
What is the importance of transpiration?
Transports useful mineral ions up the plant
Maintains cell turgidity
Supplies water for growth, cell elongation, and photosynthesis.
Water can keep the plant cool on a hot day
What factors effect the rate of transpiration?
Light intensity, temperature, relative humidity, air movement, water availability in the soil
How does light intensity affect the rate of transpiration?
In light, more energy is supplied to carry out photosynthesis, the stomata open to allow gaseous exchange for photosynthesis.
Higher light intensity increases the rate of transpiration.
