8. Transport in Plants Flashcards
What two types of transport vessels do plants have?
Xylem vessels
Phloem vessels
Function of the xylem vessels
transport water and minerals from the roots to the stem and leaves
Function of the phloem vessels
transport food materials (mainly sucrose and amino acids) made by the plant from photosynthesising leaves to non-photosynthesising regions in the roots and stem
How are the transport vessels in a plant arranged?
These vessels are arranged throughout the root, stem and leaves in groups called vascular bundles
What are root hairs?
Root hairs are single-celled extensions of epidermis cells in the root
Where do root hairs grow and what do they do?
They grow between soil particles and absorb water and minerals from the soil
By what process does water enter the root hair cells?
Water enters the root hair cells by osmosis
How does water move by osmosis into the root hair cells?
This happens because soil water has a higher water potential than the cytoplasm of the root hair cell
What adaptation does the root hair cell have and how does this help?
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
What is the path by which water travels from root hair cell to the leaf?
root hair cell → root cortex cells → xylem → leaf mesophyll cells
How can the pathway of water in a plant be investigated?
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)
What can you see after a few hours? (investigating pathway of water)
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
What happens when a cross-section is done of the celery?
(investigating pathway of water)
If a cross-section of the celery is cut, only certain areas of the stalk is stained the colour of the water, showing that the water is being carried in specific vessels through the stem – these are the xylem vessels
Why does water have to keep travelling to the leaf?
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 definition
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
What substance helps with the adaptation of xylem vessels and what does it do?
A substance called lignin is deposited in the cell walls which causes the xylem cells to die
Why is it useful that the cells in the xylem vessels are dead? How does this help with the adaptation?
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
How does lignin help the adaptation of the xylem vessels?
Lignin strengthens the plant to help it withstand the pressure of the water movement
Does the movement in the xylem take place in many directions?
NO
-Movement in xylem only takes place in one direction – from roots to leaves
Does movement in the phloem take place in many directions?
YES
movement takes place in different directions
What are the main ways in which transpiration helps plants? (4)
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
How does transpiration help to keep 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
When can evaporation happen rapidly?
This means evaporation can happen rapidly when stomata are open
What are water molecules attracted to each other by and what does this create?
Water molecules are attracted to each other by cohesion – creating a continuous column of water up the plant
What does the water move by through the xylem vessels?
Water moves through the xylem vessels in a continuous transpiration stream from roots to leaves via the stem
How does transpiration help the transpiration stream?
Transpiration produces a tension or ‘pull’ on the water in the xylem vessels by the leaves
How is water pulled through the plant?
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
What happens to the transpiration stream when the rate of transpiration increases?
If the rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quicker
When does wilting occur?
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
What happens inside the plant that causes wilting?
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
Difference between what is carried by the xylem and phloem
X - water and mineral ions
P - sucrose and amino acids
Difference between what process is carried out by the xylem and phloem
X - transpiration stream
P - translocation
Difference between cells in the xylem and phloem
X - dead
P - living
What are the 2 factors affecting transpiration?
Temperature
Humidity
What is the effect of temperature on the rate of transpiration?
Increases with increasing temperature
What is the effect of humidity on the rate of transpiration?
decreases with increasing humidity
What are the soluble products of photosynthesis?
sugars (mainly sucrose) and amino acids
How are the soluble products of photosynthesis transported around the plant?
These are transported around the plant in the phloem tubes which are made of living cells
What is the structure of phloem vessels?
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
What is translocation?
The transport of sucrose and amino acids in phloem, from regions of production to regions of storage or use, is called translocation
What does the direction of translocation depend on?
Transport in the phloem goes in many different directions depending on the stage of development of the plant or the time of year
Despite the fact that translocation can take place in many directions, what is the one way in which this process has to take place?
dissolved food is always transported from source (where it’s made) to sink (where it’s stored or used):
What is the direction of translocation during winter?
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
What is the direction of translocation during a growth period?
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
What is the direction of translocation after the plant has grown?
After the plant has grown (usually during the summer), the leaves are photosynthesising 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
What do the xylem vessels transport/ What is the function of xylem vessels?
Transport water and minerals from the roots to the stem and leaves and support.
What do the phloem vessels transport/ What is the function of phloem vessels?
Transport food materials (mainly sucrose and amino acids) made by the plant from photosynthesizing leaves to non-photosynthesizing regions in the roots and stem.
Plants contain two types of transport vessels/ What are the two vessels of the vascular system: Name them.
Xylem vessels.
Phloem vessels.
What are the three adaptations of the xylem?
Cells are joined end to end with no cross walls to form a long continuous tube.
Cells are dead, without cell contents, to allow free passage of water.
Outer walls are thickened with a substance called lignin, strengthening the tubes, which help support the plant.
Water enters the root hair cells by osmosis. How exactly does this happen?
This happens because soil water has a higher water potential than the cytoplasm of the root hair cell.
What is the pathway of water from the root hair cell?
root hair cell → root cortex cells → xylem → leaf mesophyll cells.
What is transpiration?
Transpiration is defined as the loss of water vapor from plant leaves by evaporation of water through the surface of mesophyll cells and diffusion through the stomata.
How does transpiration happen?
Transpiration is defined as the loss of water vapor from plant leaves:
By evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapor through the stomata.
The root hair increases the surface area of the cells significantly. Why is this large surface area important?
This large surface area is important as it increases the rate of the absorption of water by osmosis and mineral ions by active transport.
What is the direction of movement in the xylem and phloem?
Movement in the xylem only takes place in one direction (from roots to leaves) unlike phloem where movement takes place in different directions.
What are the four functions of transpiration 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.
Respiration.
What is the effect of temperature on transpiration rates?
The higher the temperature, the greater the transpiration rate, because water vapor molecules will have more kinetic energy and hence move out of the leaf faster.
What is the effect of humidity on transpiration rates?
The higher the humidity of the air outside the leaf the lower the transpiration rate because water usually diffuses down a concentration gradient, but if the air outside is already saturated with water, there is more likely to be a net movement of water into the leaf (providing the stomata are open) rather than out.
Evaporation takes place from the surfaces of spongy mesophyll cells. How do the interconnecting air spaces between these cells increase the rate of evaporation?
The many interconnecting air spaces between these spongy mesophyll cells and the stomata create a large surface area meaning evaporation can happen rapidly when stomata are open.
Explain the transpirational pull.
As water is lost from the plant by transpiration, this creates tension in the xylem.
Water moves upwards as it replaces the water that is lost. It travels up the xylem due to the cohesion within the water, this meaning the property of water molecules that make them stick to each other.
NOTE - Water molecules are drawn up the xylem by transpiration pull (NOT OSMOSIS).
Define transpirational pull.
Transpiration pull is the force that aids in drawing water upwards from roots to leaves.
What is the effect of wind speed on transpiration rates?
The faster the wind speed, the faster the water is moved away from the plant, creating a steeper gradient and increasing the transpiration rate.
What is wilting and what is the effect of this?
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.
How and why does wilting occur?
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.
Define translocation.
The transport of sucrose and amino acids in the phloem, from sources to regions of storage or use (sinks).
