Module 3: Transport in plants Flashcards
Why do plants need a transport system?
Plants need to take in CO2 and nutrients and the waste products need to be released.
Why are single celled organisms efficient at exchanging substances with their environment in comparison to larger organisms?
Single celled - large SA:volume, diffusion distance is small, these organisms have lower levels of activity so smaller metabolic demands.
Larger - increased transport distances, small SA:volume, increased metabolic rate.
Adaptations of plants to increase SA:volume?
Plants have a branching body shape.
Leaves are flat and thin.
Roots have root hairs.
What is mass flow?
Bulk movement of materials to allow efficient transport of nutrients and waste.
Why is mass transport important in plants?
=> Brings substances quickly from 1 exchange site to another.
=> Maintains diffusion gradients at exchange sites.
What is a vascular bundle?
Contains both xylem and phloem and other protective tissues that transport nutrients and sugars around the plant.
What is the cambium in the vascular bundle?
A meristematic tissue between xylem and phloem that is involved in plant growth.
Describe how the vascular bundles are arranged in:
Dicots
Monocots
Leaves
Roots
Stems
Answer could be wrong.
Dicots: Vascular bundles are typically arranged in a circular pattern in the stem.
Monocots: Vascular bundles are scattered throughout the stem.
Leaves - vascular bundles form the midrib and veins and so spread from the centre of the leaf in a parallel line. Xylem usually found on upper surface (closest to upper epidermis) and phloem usually found closer to the lower surface.
Roots - vascular bundles are found in the centre and the centre core is the xylem. This helps the roots to resist forces that could pull the plant out of the ground due to water transport up and the plant growth.
Stems - vascular bundles are arranged near the edge of the stem, with the phloem on the outside and the xylem on the inside (to support the plant).
What is the function of the xylem?
Xylem is a vascular tissue that transports water and mineral ions UP the plant. It also gives structural support.
Describe the structure of the xylem.
=> Made up of dead cells
=> Movement occurs through transpiration
=> No end walls
=> Cell wall material is lignin and cellulose.
Describe the structure and function of the phloem.
Function: transports sucrose and other nutrients (assimilates) UP and DOWN the plant. From source (like leaf) to sink (like roots).
Structure: it’s made up of sieve tube elements and companion cells. Other cell types of phloem tissue also include parenchyma for storage and strengthening fibres
The location of the vascular bundles is dependent on which organ they are in as the different organs are under different stresses:
In the roots the vascular bundle is found in the centre and on the edges of the centre core is the phloem tissue
In the stems, the vascular bundles are located around the outside and the phloem tissue is found on the outside (closest to the epidermis)
In the leaves, the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line. The phloem tissue is found on the lower side of the bundles (closest to the lower epidermis)
Xylem tissue is made up of 4 cell types.
Tracheids (long, narrow tapered cells with pits).
Vessel elements (large with thickened cell walls and no end plates when mature).
Xylem parenchyma.
Sclerenchyma cells.
Most of the xylem tissue is made up of tracheids and vessel elements, which are both types of water-conducting cell.
Relating structure and function of the xylem.
=> Lignified cell walls add strength to withstand the hydrostatic pressure so vessels don’t collapse. They are also impermeable to water.
=> No end plates allow mass flow of water and dissolved solutes as …?
=> No protoplasm doesn’t impede the mass flow of water and dissolved solutes?
=> Pits in walls (non-lignified sections) allow lateral movement of water. this ensures air bubbles don’t form in vessels.
=> Small diameter of vessels help prevent water column from breaking and assists with capillary action.
Adaptations of sieve tube elements.
=> Sieve pores allows continuous movement of organic compounds.
=> Cellulose cell wall strengthens the wall to withstand the hydrostatic pressure that moves the assimilates.
=> No nucleus or vacuole maximises space for translocation of assimilates.
=> Thin cytoplasm reduces friction to facilitate movement of assimilates.
Adaptations of companion cells.
Each sieve tube element has a companion cell associated with it as companion cells control the metabolism of their associated sieve tube member
They also play a role in loading and unloading of sugars into the phloem.
=> Nucleus and other organelles present to provide metabolic support for sieve tube elements and helps with loading/unloading assimilates.
=> Transport proteins in plasma membrane moves assimilates into and out of sieve tube elements.
=> Lots of mitochondria to provide ATP for above.
=>Plasmodesmata links the sieve tube elements which allow organic compounds to move from companion cell to sieve tube elements.
Compare xylem and phloem.
Phloem - living companion cells, movement occurs through active translocation, they have end walls (sieve plates with sieve pores), cell wall is cellulose.
What are dicotyledonous plants?
Have seeds that contain:
=> 2 cotyledons (seed leaves)
=> Network of veins
=> Tap root with lateral branches
Herbaceous dicots have a relatively short life cycle (one growing season) and non-woody tissue.
Define transpiration.
Loss of water vapour from a plant to its environment by evaporation and diffusion. Transpiration is a consequence of gaseous exchange at the stomata
Advantages of transpiration?
It provides a means of cooling the plant via evaporative cooling.
The transpiration stream is helpful in the uptake of mineral ions.
The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants.
What is the transpiration stream?
Movement of water from rots to leaves. The evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules causes the movement of water through a plants xylem
It is the gradient in water potential that is the driving force permitting the movement of water from the soil (high water potential), to the atmosphere (low water potential), via the plant’s cells
Factors affecting rate of transpiration.
The transpiration rate is dependent on the concentration gradient of water vapour between the inside of the leaf and the surrounding air
A larger concentration gradient results in a faster rate of diffusion
Air movement, temperature, light intensity and humidity all affect the rate of transpiration in a plant
Air movement:
There is usually a lower concentration of water molecules in the air outside the leaf
When the air is relatively still water molecules can accumulate near the leaf surface. This creates a local area of high humidity which lowers the concentration gradient and the rate of transpiration
Air currents can sweep water molecules away from the leaf surface, maintaining the concentration gradient and increasing the rate of transpiration
Temperature:
An increase in temperature results in an increase in the kinetic energy of molecules. Therefore an increase in temperature will increase the rate of transpiration as water molecules move out of the leaf (down the concentration gradient) at a faster rate
If the temperature gets too high the stomata close to prevent excess water loss. This dramatically reduces the rate of transpiration
Light intensity:
Stomata close in the dark, their closure greatly reduces the rate of transpiration
When the light is sufficient for the stomata to open, the rate of transpiration increases
Once the stomata are open any increase in light intensity has no effect on the rate of transpiration
Stomata will remain open at relatively low light intensities
Humidity:
If the humidity is high that means there is a large concentration of water molecules in the air surrounding the leaf surface
This reduces the concentration gradient between inside the leaf and the outside air which causes the rate of transpiration to decrease
At a certain level of humidity, an equilibrium is reached; there is no concentration gradient and so there is no net loss of water vapour from the leaves
Movement of water in the transpiration stream.
Water and minerals enter the roots as follows:
=> Minerals are taken up from the soil by either active transport or diffusion, depending on soil mineral concentrations
=> Mineral ions lower the water potential of the root hair cells, and water enters the cells from the soil by osmosis
=> Once water has entered the root hair cells it must be transported across the root cortex to the xylem.
There are two pathways that water can take across the cortex:
The apoplast pathway - Water moving on the apoplast, or apoplastic, pathway travels by diffusion within the cell walls and intercellular spaces of plant tissue. Note that this is not osmosis because the water does not cross any cell membranes. Water is drawn across the root via the apoplast pathway due to cohesive forces between water molecules:
Water moves upwards in the xylem due to transpiration
Cohesion between water molecules means that more water is drawn along the apoplast pathway within the root to replace the water that has moved upwards
The symplast pathway - Water moving on the symplast, or symplastic, pathway travels via cell cytoplasm and vacuoles.
Water enters the symplast pathway and moves between cells, and into cell vacuoles, by osmosis. Water can also move from cell to cell by diffusion via the plasmodesmata. Water is drawn across the root via the symplast pathway as follows:
water moves into root hair cells from the soil by osmosis, increasing the water potential of the root hair cell
water moves down its water potential gradient into neighbouring root cells, increasing their water potential
water continues to move across the root from high to low water potential
What happens when water reaches the centre of the root?
It must cross the endodermis to enter the xylem. The cells of the endodermis are surrounded by a waxy band known as the Casparian strip, which forms an impassable barrier to water
The waxy material is known as suberin
The Casparian strip blocks the cell walls of the endodermis cells, preventing water from entering the xylem via the apoplast pathway and instead forcing it into the symplast pathway.
it is thought that this may help the plant control which mineral ions reach the xylem
Water movement in the xylem?
Water is drawn upwards in the xylem due to transpiration as follows:
water evaporates from the surface of cells in the leaves, lowering the water potential of leaf cells
water is drawn out of the xylem and into leaf cells by osmosis down its water potential gradient
more water molecules are drawn upwards in the xylem in a continuous column due to forces of cohesion between water molecules
attractive forces of adhesion between water molecules and the sides of the xylem aid this process
The upward movement of water in the xylem is known as the transpiration stream
Movement of water in the leaves.
Water moves through the leaves of plants due to transpiration as follows:
Water vapour diffuses out of leaf air spaces and into the surrounding environment down a water vapour potential gradient
The loss of water vapour from the air spaces creates a water potential gradient between leaf mesophyll cells and the leaf air spaces, so more water moves from the leaf mesophyll cells into the air spaces
Water first moves from the cell cytoplasm to the cell surface, before evaporating into the air space
Losing water lowers the water potential of the leaf mesophyll cells, so water moves into the cells by osmosis from neighbouring cells and the xylem
Note that water movement through the leaf also occurs via the apoplast and symplast pathways.
- Water vapour diffuses from air spaces through a stoma by transpiration, lowering the water potential.
- Water evaporates from a mesophyll cell wall into air spaces, creating a transpiration pull.
- Water moves through the mesophyll cell wall (apoplastic pathway) or out of the mesophyll cytoplasm into the cell wall (symplastic pathway).
- Water leaves a xylem vessel through a non-lignified area.
- Water moves up the xylem vessels (transpiration stream) to replace the water lost from the leaf.
