transport in plants Flashcards
Plants contain two types of transport vessels
Xylem vessels Phloem vessels
state functions of transport vessels
Xylem vessels – which carry water and minerals (pronounced: zi-lem) Phloem vessels – which carry food materials (sucrose and amino acids) made by the plant (pronounced: flow-em)
These are arranged throughout… in groups called ..
- the root, stem and leaves -vascular bundles
Remember that xylem is always on the … and phloem is always on the…
- inside
- outside.
Root Hair Cells
Root hairs are single-celled extensions of epidermis cells in the root
They grow between soil particles and absorb water and minerals from the soil
Water enters the root hair cells by osmosis
This happens because soil water has a higher water potential than the cytoplasm of the root hair cell
How the Large Surface Area of Root Hair Cells is Useful
The root hair increases the surface area of the cells significantly
This large surface area is important as it increases the rate of the absorption of water by osmosis and mineral ions by active transport
Pathway of Water through Root to Leaf
Osmosis causes water to pass into the root hair cells, through the root cortex and into the xylem vessels:
Pathway of water into and across a root
Once the water gets into the xylem, it is carried up to the leaves where it enters mesophyll cells
So the pathway is:
root hair cell → root cortex cells → xylem → leaf mesophyll cells
investigate Pathway of Water through Root to Leaf
The pathway can be investigated by placing a plant (like celery) into a beaker of water that has had a stain added to it (food colouring will work well)
After a few hours, you can see the leaves of the celery turning the same colour as the dyed water, proving that water is being taken up by the celery
Investigating water movement in plants using a stain
If a cross section of the celery is cut, only certain areas of the stalk is stained with the dye, showing that the water is being carried in specific vessels through the stem – these are the xylem vessels
Cut section of celery stalk showing that only the xylem vessels are dyed with the stain
What is Transpiration?
Water travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration
Transpiration is defined as the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata
Xylem is adapted in many ways:
A substance called lignin is deposited in the cell walls which causes the xylem cells to die
These cells then become hollow (as they lose all their organelles and cytoplasm) and join end-to-end to form a continuous tube for water and mineral ions to travel through from the roots
Lignin strengthens the plant to help it withstand the pressure of the water movement
Movement in xylem only takes place in one direction – from roots to leaves (unlike phloem where movement takes place in different directions)
traspiration is defined as
the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata
Transpiration has several functions in plants:
transporting mineral ions
providing water to keep cells turgid in order to support the structure of the plant
providing water to leaf cells for photosynthesis
keeping the leaves cool – the conversion of water (liquid) into water vapour (gas), as it leaves the cells and enters the airspace, requires heat energy. The using up of heat to convert water into water vapour helps to cool the plant down.
How does Transpiration Occur?
Evaporation takes place from the surfaces of spongy mesophyll cells
The many interconnecting air spaces between these cells and the stomata creates a large surface area
This means evaporation can happen rapidly when stomata are open
How is the Transpiration Stream Created?
Water molecules are attracted to each other by cohesion – creating a continuous column of water up the plant
Water moves through the xylem vessels in a continuous transpiration stream from roots to leaves via the stem
Transpiration produces a tension or ‘pull’ on the water in the xylem vessels by the leaves
As water molecules are held together by cohesive forces (each individual molecule ‘pulls’ on the one below it), so water is pulled up through the plant
If the rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quicker
Wilting
If more water evaporates from the leaves of a plant than is available in the soil to move into the root by osmosis, then wilting will occur
This is when all the cells of the plant are not full of water, so the strength of the cell walls cannot support the plant and it starts to collapse
Factors Affecting Transpiration
Investigating the role of environmental factors in
determining the rate of transpiration from a leafy shoot
Cut a shoot underwater to prevent air entering the xylem and place in tube
Set up the apparatus as shown in the diagram below and make sure it is airtight, using vaseline to seal any gaps
Dry the leaves of the shoot (wet leaves will affect the results)
Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water
Set up the environmental factor you are investigating
Allow the plant to adapt to the new environment for 5 minutes
Record the starting location of the air bubble
Leave for a set period of time
Record the end location of air bubble
Change the light intensity or wind speed or level of humidity or temperature (only one – whichever factor is being investigated)
Reset the bubble by opening the tap below the reservoir
Repeat the experiment
The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa
Investigating transpiration rates using a potometer
Environmental factors can be investigated in the following ways:
Temperature : Temperature of room (cold room and warm room)
Humidity : Spray water in plastic bag and wrap around plant
Temperature & Humidity on Transpiration Rate
Transport of Food
The soluble products of photosynthesis are sugars (mainly sucrose) and amino acids
These are transported around the plant in the phloem tubes which are made of living cells (as opposed to xylem vessels which are made of dead cells)
The cells are joined end to end and contain holes in the end cell walls (called sieve plates) which allow easy flow of substances from one cell to the next
The transport of sucrose and amino acids in phloem, from regions of production to regions of storage or use, is called translocation
Transport in the phloem goes in many different directions depending on the stage of development of the plant or the time of year; however dissolved food is always transported from source (where it’s made) to sink (where it’s stored or used):
During winter, when many plants have no leaves, the phloem tubes may transport dissolved sucrose and amino acids from the storage organs to other parts of the plant so that respiration can continue
During a growth period (eg during the spring), the storage organs (eg roots) would be the source and the many growing areas of the plant would be the sinks
After the plant has grown (usually during the summer), the leaves are photosynthesizing and producing large quantities of sugars; so they become the source and the roots become the sinks – storing sucrose as starch until it is needed again
Comparision between xylem and phloem tissue
