Exchange and transport ( mass transport) in plants Flashcards
describe the structure of xylem vessels/fibres
Xylem consists of two main types of tissue
Called xylem vessels and xylem fibres
Xylem vessels - start as a series of plant cells running up the stem from the roots to the leaves
At a certain point, the carbohydrate lignin forms within the cell walls
Lignin - is impermable + prevents substances from passing through the cell wall
The living contents of the cells die
The end walls between the cells break down
However regions of the cell wall remain free of lignin ( called pits)
These allow water and dissolved substances to pass between vessels
Final xylem vessels consist of non-living hollow tubes
Non-living tissue
describe the functions of xylem vessels/fibres
To carry water and dissolved minerals up the plant from the roots to the leaves
Xylem as the tissue that transports water in the stem and leaves of
plants
where are the two types of transport tissue found
Xylem and phloem are found in the vascular bundles of plants
transport tissue - xylem and phloem
Describe the structure of phloem
The fluid moving in the phloem is referred to as phloem sap
Phloem is a living tissue
Phloem consists of two different types of tissue
Sieve tube element cell.
A sieve tube element consists of a long line of cells arranged end to end
Inside these cells, almost all of the organelles have been lost including the nucleus and vacuole
This leaves the interior of these cells almost entirely free to transport phloem sap
The end walls of these cells have been modified to contain large pores
These modified end walls are called a sieve plate
Sieve plates allow the phloem sap to move between the cells
Since the sieve tube elements cells have lost most of their organelles. This means that they cannot produce large amounts of essential molecules such as ATP
However, next to the sieve tube element cells there are companion cells
Companion cells contain a nucleus as well as large amounts of mitochondria
Microscopic channels link the companion cells to the sieve tube element cells
These channels (openings/tunnels) are CALLED plasmodesmata
Molecules such as ATP and proteins can move through the plasmodesmata into the sieve tube element cells
The role of the companion cells is to provide essential molecules to the sieve tube element cells
Phloem tubes do not contain lignin in the cell walls
Phloem contains two types of tissue which provide support which contain lignin
Describe the function of phloem
In the leaves, the plant carries out photosynthesis which produces the sugar glucose
Glucose is used to form other compounds such as different sugars and amino acids
Phloem as the tissue that transports organic substances in plants
from the leaves to other parts of the plants such as the roots or flowers
This means molecules can be transported both up and down the phloem
Describe how root hair cells are adapted for the absorption of water and minerals
Root hairs grow from cells in the epidermis (Outer layer) of the root
Water moves into the root hair by osmosis
Root hair cells are adapted so that osmosis takes place rapidly
The densely packed root hairs, massively increase the SA/V ratio of the root
Secondly, the surface of the root hair consists only of the cell wall and the cell membrane
This makes the surface extrem
Describe how transpiration takes place in plants
The surface of the cells in the leaf are covered with a thin layer of water (moist surfaces of cells inside the leaf)
This water vapour evaporates from the surface of the cells
Because of this, the internal leaf spaces contain a high concentration of water vapour
Generally, the level of water vapour in the external air is relatively low
So when the stomata are open, the water vapour diffuses out of the leaf to the externa air
This evaporation of water followed by the diffusion of water vapour is called transpiration
Because of the continuous evaporation from their surface, the water potential of the cells in the lead decreases
This causes water to move by osmosis from adjacent cells like this
This now lowers the water potential of these cells, causing water to move in to them
At some point this reaches the xylem with water passing out of the xylem to adjacent cells like this
So when transpiration is taking place, water is continuously being pulled out of the xylem vessels
This pulling effect is called tension
The movement of water from the roots, up the xylem and out of the leaf is called the transpiration stream
Describe the cohesion-tension theory of water movement and the evidence which supports this theory
how water moves from the roots, up the xylem and out of the leaf is called the transpiration stream
Water molecules form hydrogen bonds to each other
This type of attraction is called cohesion
Water can also form hydrogen bonds to molecules in the xylem vessel walls e.g. carbohydrates
This attraction is called adhesion
One effect of cohesion and adhesion is that water can move up very thin tubes against the force of gravity
This is called capillary action
So, when water is removed from the top of the xylem vessels due to transpiration, more water moves up the xylem vessels by capillary action to take its place
This whole process is referred to as transpiration pull
So the combined effect of transpiration pull, coupled with cohesion and adhesion
Is that water is drawn into the roots, moves up the stem and passes out of the leaves
This whole process is called the cohesion-tension theory
describe the evidence which supports this theory - cohesion-tension theory
Evidence which support the cohesion-tension theory
If a plant stem is cut, then air is sucked into the xylem, suggesting that the xylem vessels are under tension
However, the air prevents cohesion between water molecules, so water movement stops
Secondly, if we measure the diameter of a tree trunk
We can see that this reduces when transpiration is at its maximum
This supports the idea of transpiration pull is generating a negative pressure/tension in the xylem
Describe how to use a bubble potometer to measure the rate of water uptake in a plant
We can measure the rate of water uptake into a plant by using a bubble potometer
A bubble potometer consists of a fine capillary tube which is filled with water
The tube is connected to a plant which has been cut at the stem
The tube is also connected to a syringe filled with water
We use a needle to place a small air bubble at the end of the capillary tube
As water evaporates from the leaves of the plant, water is drawn into the stem
This causes the air bubble to move towards the plant
By measuring how far the air bubble moves in a given time, we can calculate the rate of water uptake into a plant
We can then see how the rate of water uptake changes if we change the conditions e.g. carrying out the experiment under different light intensities/investigating the effect of wind by using a fan
In between experiments, we can reset the position of the air bubble by adding more water from the syringe (pushing the sryinge down)
This potometer only measures water uptake into the plant
Not all of this water will take part in transpiration
E.g. a small amount of water taken in, will be a reactant in photosynthesis
However, most of the water taken in by the plant will take part in transpiration
What are the things to consider when setting up a bubble potometer
Things to consider
When we take our cutting from the parent plant, air will be sucked into the xylem vessels
These air gaps would prevent water from being taken up the stem
To correct for this, we now place the cut stem of our plant into water and cut off the last 1 cm
Water will now flow into the xylem and we’ll avoid any air gaps
We then need to place the potometer under the water and insert the cut end, again avoiding any air gaps
It’s important when doing this that we handle the plant carefully
We don’t want to damage the plant
We want to avoid getting water unto the underside of the leaves - where most of the stomata is found
Important that the potometer is fully sealed
It is a good idea to smear some petroleum jelly around the connection between the stem and tube
We also need to allow the plant to adapt to its surroundings for 10 mins before starting experiments
Describe the advantages of using a mass potometer
When using a mass potometer
We place our plant in its pot on a balance
As the plant loses water through transpiration, the total mass decreases
With this potometer, we have to prevent evaporation of water from the soil. Otherwise this would contribute to mass loss giving a false reading for transpiration
We do this by covering the soil with plastic wrap
Advantages to using the mass potometer
Firstly, this directly measures the rate of transpiration rather than the rate of water uptake
Secondly, this is much less disruptive to the plant as it does not involve cutting the stem
Top half of the leaf - palisade mesophyll (sites of photosynthesis
Bottom half of the leaf - stomata
Factors that affect the rate of transpiration in plants
Light intensity
Relative humidity
Temperature
Movement of air
How does light intensity affect the rate of transpiration in plants
Light intensity
For transpiration to occur, the stomata must be open
Stomata open in light conditions - to allow CO2 to diffuse into the leaf and take part in photosynthesis
As light intensity increases, the rate of transpiration increases
This is because increasing the light intensity, increases the number of open stomata, allowing more water vapour to diffuse out of the leaf
However at high light intensities the rate of transpiration no longer increases
This is because at high light intensities, almost all of the stomata will be open
How does humidity affect the rate of transpiration
Water vapour diffuses out of the leaf down the concentration gradient
This is because the conc. gradient of water vapour outside the leaf is generally lower than inside
The relative humidity tells us the conc. of water vapour in the air as a percentage of the
maximum possible
e.g. 100% relative humidity means that the conc. of water vapour is as high as it possibly could be
If the relative humidity outside the leaf increases e.g. on a humid day
Then that means that there is a smaller conc. gradient between the inside of the leaf and the outside - as less water vapour diffuses out of the leaf
So increasing the relative humidity outside the leaf reduces the rate of transpiration
Describe how temperature affects the rate of transpiration in plants
The rate of transpiration is increased by temperature
At higher temperatures, the water molecules have more kinetic energy
This means that there is a greater rate of evaporation of water from the internal surfaces of the leaf
Secondly, at higher temperatures the relative humidity of the external air decreases
So due to these two effects, the concentration gradient of water vapour between the inside of the leaf and the external air increases at higher temperatures
This increases the rate of transpiration
How is the rate of transpiration affected by the movement of the air
The rate of transpiration is also affected by the movement of the air
When water vapour moves out of the stomata during transpiration that water vapour can build up around the external surface of the leaf
The effect of this is to reduce the conc. gradient for water vapour between the inside of the leaf and the outside
This reduces the rate of transpiration
Air movement such as wind removes the water vapour as it diffuses out of the leaf
Because air movements increase the conc. gradient of water vapour, the effect of this is to increase the rate of transpiration
The rate of transpiration falls on still days, when there is little air movement
Explain how the rate of transpiration can be affected by the level of water in the soil
In drought conditions, the roots of a plant produce a hormone
This hormone triggers stomata to close
The effect of this is to reduce the rate of transpiration, reducing water loss by plant
What is produced during photosynthesis and what is this product converted to and why?
What is an assimilate and what is translocation
Plants produce the monosaccharide glucose during photosynthesis in the leaves
All parts of the plant require glucose for respiration
The glucose produced in the leaves is first converted to the disaccharide sucrose
Sucrose is less reactive than glucose and is less likely to react with other molecules
Molecules such as sucrose which are made as a result of photosynthesis are called assimilates
Assimilates can also include amino acids
Assimilates such as sucrose are transported around the plant in the phloem
The transport of assimilates in the phloem is referred to as translocation
What is a source and a sink
Assimilates such as sucrose are transported from sources to sinks
Sources are where the assimilates are produced e.g. photosynthesising leaves
Other sources include storage organs such as tubers which can release their carbohydrate stores when they are needed
Sinks are regions where assimilates are required
Sinks include roots which carry out active transport and therefore have a high rate of respiration
Storage organs can also act as sinks when they are refilling their carbohydrate stores
Other sinks include growing regions such as shoots which contain dividing meristem tissue
Describe translocation of solutes in the phloem
Active loading
Mass Flow
Xylem vessel Sieve tube element cell companion cell source of sucrose
Source e.g. photosynthesising leaf
The source is connected by the phloem to a sink which in this case is the root
At the source, sucrose is loaded into the phloem by an active process
A protein on the cell membrane of the companion cell uses ATP to pump hydrogen ions out of the cytoplasm and into the spaces of the cell wall (of the companion cell)
This process is active transport
This creates a concentration gradient for hydrogen ions with more hydrogen ions on the outside of the cell membrane.
The hydrogen ions can now flow through a cotransporter protein down the concentration gradient back into the cell
The inward flow of hydrogen ions is coupled to an inward flow of sucrose into the companion cell
Companion cells have a large number of mitochondria which provide the ATP needed for the active transport of hydrogen ions
Foldings on the cell membrane also increase the surface area for the proteins involved
So because of this transport process, the concentration of sucrose in the companion cells is high
The sucrose can now diffuse through the plasmodesmata from the companion cells into the sieve tube element cells
This means that we now have a high conc. of sucrose in the sieve tube element cells
The effect of this is to lower the water potential inside the sieve tube element
Water now moves into the sieve tube element by osmosis from nearby tissues including the xylem vessels
This now increases the hydrostatic pressure inside the sieve tube element
As a result the phloem sap now moves up or down the sieve tube element towards the sink
This bulk movement of phloem sap is called mass flow
At the sink, the sucrose moves out of the sieve tube element and is converted to glucose for use in respiration
Or in the case of storage organs, the sucrose is converted to starch
As the sucrose leaves, this increases the water potential in the sieve tube element causing water to move out of the sieve tube element by osmosis
Some of this water will go back into the xylem and join the transpiration stream
Describe some of the evidence which supports this active model of movement in the phloem
There are several lines of evidence to support this active model of movement in phloem
For example, the rate of flow of sucrose in the phloem is much faster than could take place by diffusion alone
If we inhibit the companion cell mitochondria, then translocation stops
