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).
What are the sources of translocation?
Sources as the parts of plants that release sucrose or amino acids.
What are the sinks of translocation?
Sinks as the part of plants that use or store sucrose or amino acids.
What are the two adaptations 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.
Phloem tubes are made of living cells.
What are root hairs?
Root hairs are single-celled extensions of epidermis cells in the root.
Where do plants absorb water from?
From the roots
What is the transport system made up?
A tissue called xylem and phloem
What does phloem transport?
Phloem transports sucrose and amino acids from the leaves where they are made to other parts of the plant
Describe the features of the xylem.
It is made of hollow dead cells joined end to end
the end walls have disappeared so a long open tube is formed
Where do xylem vessels run from?
From the roots, up through the stem and branch out into every leaf
What do xylem not contain?
Cytoplasm and nucleus
What are the xylem walls made up of ?
Made of cellulose and lignin.
What is lignin?
Lignin is very strong so xylem vessels help to keep plants upright
What is wood made up of?
Lignified xylem vessels
Describe the features of the phloem.
They are made of many cells joined end to end. They have sieve plates which have small holes in them- sieve tube elements
What direction do minerals travel in a xylem?
One way
What direction do organic molecules travel in the phloem?
Two way
What is a root cap?
At the very tip of the root hair cells. It is a layer of cells that protects the root as it grows through the soil.
What adaptation do root hair cells have?
They have a large surface area to increase the the rate of absorption
How do root hair cells absorb water?
By osmosis
How do root hair cells absorb ions?
By active transport
What is the function of the root hairs?
To absorb water and minerals from the soil. Water moves into a root by osmosis.
Describe how water is absorbed by the root hair cell.
The water in the soil is normally a more dilute solution so water diffuses into the root hair, down the concentration gradient and through the partially permeable surface membrane.
How does water travel up the xylem?
The pressure at the top of the vessels is lowered, while the pressure at eh bottom stays high.
How is the pressure at the top of the xylem vessels reduced?
It happens because of transpiration
What is transpiration?
Loss of water vapor from plant leaves by evaporation of water at the mesophyll cells followed by diffusion of water vapor through the stomata.
What is the transpiration stream?
The flow of water through a plant, from the roots to the leaves via the xylem vessels.
Describe the attractive forces in the xylem vessels
The attractive forces between the water molecule result in cohesion. These forces are greater than the forces trying to separate them, so the transpiration stream is pulling thin columns of water molecules, which resist the tendency to break apart.
What increases the rate of evaporation?
The many air spaces inside the leaf means that there is a large surface area of wet cells.
What do stomata do?
They allow water vapor to diffuse easily out of the leaf
What do the stomata reduce?
Reduces water potential inside the leaf which encourages more water to evaporate from the surfaces of the mesophyll cells
What is the rate of transpiration?
The rate at which a plant takes up water depends on the rate of transpiration- the faster a plant transpires the faster it takes up water
What is a potometre?
Records how fast the air/ water meniscus moves along the capillary tube- you can compare how fast the plant takes up water.
Describe the effect of temperature on transpiration.
On a hot day water will evaporate quickly from the leaves of a plant. Transpiration increases as the temperature increases
Describe the effect of humidity on transpiration.
Humidity means the moisture content of the air. The higher the humidity, the less water will evaporate from the leaves. This is because there is not much of a diffusion gradient for the water between the air spaces inside the leaf, and the wet air outside it. Transpiration decreases as humidity increases
Describe the effect of wind speed on transpiration.
On a windy day water evaporates more quickly than on a still day. Transpiration increases as wind speed increases
Describe the effect of Light intensity on transpiration.
In bright sunlight, a plant may open to supply plenty of CO2 for photosynthesis. More water therefore will evaporate from the leaves.
What is the direction of water movement in a plant?
Root hair- cortex- xylem- leaf - stomata
What happens if the plant is in short supply of water?
The plant will close its stomata, decreasing the rate of transpiration.
What happens if the leaves too much water loss?
The plant wilts because cells lose water by osmosis and become flaccid
What is translocation?
The movement of sucrose and amino acids in phloem from regions of production to regions of storage or to region of utilization in respiration or growth.
What can carbohydrates be made into?
Amino acids, proteins
What is a source?
The part of the a plant from which sucrose and amino acids are being trans located- leaves
What is the sink?
The part of the plant to which they are being trans located such as roots and flowers
What do the flowers use sucrose for?
To make fructose for when fruits are developing
What would act as a sink when it is a bud?
The leaves.
What is capillarity?
The adhesion of water to the walls of a small tube(e.g xylem)
What factors affect transpiration
Humidity: low = high trans
Temp: high=high trans
Wind speed: high= high trans (water blown away which increases water gradient + decrease humidity)
What does the xylem do?
It transports water and mineral ions to all parts of plants.
What is the xylem made of?
Dead cells and lighting which strengthens the walls
What does the phloem do?
It transports sucrose, amino acids and plant growth substances
It also flows up and down where as the xylem only flows upwards
What is the phloem made of
The cell wall is made up of cellulose
Describe the movement of water through a plant
Water is absorbed by the roots and moves into the xylem via osmosis. It then travels up the stem due to the tension caused by transpiration and cohesion and capillarity. The water then moves into the palisade cells by osmosis and then into the spongy mesophyll and then diffuses/ evaporates into the air.
The xylem and phloem vessels in plants have
different structures and functions:
Transpiration
loss of water vapor from plant leaves by evaporation of water at the surface of the mesophyll cells followed by diffusion of water vapor through the stomata
Transpiration
loss of water vapor from plant leaves by evaporation of water at the surface of the mesophyll cells followed by diffusion of water vapor through the stomata
Translocation
it is the movement of sucrose and amino acids in phloem from the regions of production (source) to the region of storage or the region where they are used in respiration or growth (sink).
XYLEM
-Transports water and mineral ions
-Unidirectional transport
-At the center of the vascular bundle in roots
-Consist of tracheids and vessel elements
-Tubular shape with no cross walls
-Dead tissue at maturity so it is hollow with no cell contents
-Waterproof gives strength and support to the
plant due to the presence of lignin
phloem
-Transports sucrose and amino acids from sources to sinks
- Bidirectional transport
-Located on the outer side of the vascular bundle
- Consist of sieve tubes and companion cells
- Elongated tubular shape with thin-walled sieve tubes
-Living tissue with little cytoplasm but no nucleus/tonoplast
-Sieve tubes have pores at each end in the cross walls
both the xylem and phloem are present in
ROOTS, STEMS, AND LEAVES.
Root hairs have numerous smaller hairs and themselves are finger-like extensions of the
cytoplasm,greatly increasing the surface area for the absorption of substances
Mineral ions enter the root hairs by
active transport
As the water potential now in the roots is lower than in the soil water, water enters the
root hair by osmosis down its potential gradient.
Placing a celery stalk in a solution of red food coloring,
and then cutting it in half allows us to observe the path of water above the roots
a red stain is visible
along the xylem.
Water isn’t pushed up the xylem, it is pulled by
the evaporation of water
transpiration pull
Water is pulled up the xylem
from the roots to the leaves This pull is caused due to the constant loss of water
from the leaves.
Transpiration depends on 2 properties of water
1) cohesion
2) adhesion
cohesion
water molecules inside the xylem cells are strongly attracted to each other
adhesion
the water molecules tend to stick to the inside of the xylem vessel
rate of transpoiration is greater in the
morning than night
stomata open only in
the morning in the
presence of light and to allow CO 2 to diffuse into the plant.
At night the stomata are closed to reduce
transpiration.
why do stomatas close
They also close in hot, dry conditions when water loss is not being replenished from the roots.
When the stomata have closed and the leaves still don’t get water
the
plant wilts. Cells lose their turgidity and become flaccid - the plant
becomes soft and weak. Stem cannot remain upright and leaves droop
benefits of wilting
Plants do not get too hot as leaves
droop, reducing the number of rays of sunlight that fall on them.
When the temperature drops, later on, the lost water can quickly be
replenished
factors that affect the rate of transpiration
1) light
2) humidity
3) temperature
how does light intensity affect transpiration
The rate of transpiration increases with
increasing light intensity up to a certain
maximum point (this varies and depends on other limiting factors - humidity, and temperature). Stomata open wider as light increases, therefore, allowing more water vapor to leave the plant
how does humidity affect transpiration
This is the measure of water vapor in the air. If the humidity of the air is high, the rate of
transpiration is low due to the greater
concentration in the air. If the humidity of the air is low, the rate of transpiration is high due to the greater concentration in the leaves. Therefore the water vapor leaves the leaf by diffusion from higher to lower concentration
how does temperature affect transpiration
In higher temperatures, the rate of transpiration is higher as water vapor molecules have more kinetic energy. This increases the rate of evaporation from the surface of the mesophyll cells
Uses of sucrose
● Broken down into simple sugars used in respiration
● Changed to starch for storage in cortex or seeds
● Used to make cellulose for cell walls of new cells made at the root tip
● Stored in some fruits to sweeten and attract animals.
translocation is an
active process
the different parts of a plant may act as a source or a sink at
different times of the life of a plant.
circulatory system
a system of blood vessels with a pump (the heart) and valves that ensure the
one-way flow of blood.
Function of xylem is to
transport water and mineral ions and provide support.
Function of phloem is to
transport sucrose and amino acid
Xylem have:
thick walls with lignin
no cell contents
cells joined end to end with no cross walls to form a long continuous tube.
The large surface area of root hairs increases
the uptake of water and mineral ions.
Pathway of water from soil:
root hair cells, root cortex cells, xylem, mesophyll cells.
Transpiration is the
loss of water vapour from leaves.
Water evaporates from the surfaces of the
mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapor
Water vapour evaporates from the surface
of the mesophyll cell out of the leaf. This
reduces the pressure in the vessels. The pressure of the at the top of the Xylem Vessel is therefore less than the pressure at the bottom in the roots. Due to this pressure difference, water flows upwards. It is called transpiration pull. Because it is caused by loss of water vapour by transpiration.
At higher temperature rate of transpiration increases because
at a higher temperature,Molecules have greater kinetic energy, which means the rate of evaporation is higher from the surface of the mass of cells.
On a windy day, the air around the leaf – which contains a lot of water vapour that
has just diffused out of the leaf – is quickly moved away
This means that there is
always a diffusion gradient for the water vapour, because there is less of it outsidethe leaf than in the air spaces inside the leaf. So, water vapour
diffuses out of the leaf faster on a windy day than on a day when the air is still.
As humidity increase
rate of transpiration decrease because there is a smaller
diffusion gradient for the water between the air and inside leaf.
Cells loose water by
transpiration down the water potential gradient by osmosis. The
pressure of water on the cell wall decreases. The turgor pressure decreases. The
leaf become soft and fluffy. This is called wilting.
Translocation
movement of sucrose and amino acids in phloem from sources to sinks.
Sinks
parts of plants that use or store sucrose or amino acids
Sources
parts of plants that release sucrose or amino acids.
During summers
when sunlight is available plant photosynthesis, in this case, the
leaves are the source and the roots are the sink as they are the storage of starch.
During winters
the plant cannot photosynthesize. In this case, the roots are the
source and leaves are the sink because root contains stored starch, which can be
converted to glucose and used by the leaves.
