M3C9 - Transport in plants Flashcards
Why do plants need a transport system?
-Metabolic demands = Substances such as oxygen and sucrose need to be transported from one area of the plant to another, and the products of cell metabolism need to be removed.
-Size = Plants need very effective transport systems to move substances up and down from the tip of the roots to the leaves and stems. (LONG DISTANCE)
-SA:V = Plants have a small SA:V. This means they can’t rely on diffusion alone to supply their cells with everything they need.
Describe the structure of Xylem
-Dead cells
-Reinforced with lignin
-No end walls
-Bordered pits
-Xylem vessels = long, hollow structures.
-Xylem parenchyma packed around cells - storing food, contains tannin.
Describe the function of xylem
-Transports water, minerals, salts.
-Transports in one direction (towards leaves)
-Supports plant
Describe the structure of phloem
-Living cells
-Contains sieve plates
-Contains companion cells
-Sieve tube elements
Describe the function of phloem
-Transports assimilates e.g. sucrose, amino acids, products of photosynthesis.
-Transports in BOTH directions
Describe the structure and function of sieve tube elements
Main transporting vessels of phloem
-Form a long hollow structure
-Not lignified
-Sieve plates (perforated walls) between cells.
-Thin walls
Describe the structure and function of companion cells
-Connected to sieve tubes by plasmodesmata
-Have a large nucleus
-Dense cytoplasm
-Numerous mitochondria
-Carry out metabolic functions for sieve elements.
What is transpiration?
The loss of water vapour from the leaves via stomata.
Describe how transpiration is a consequence of gas exchange
When stomata are open for gas exchange, water vapour also moves out by diffusion and is lost.
What is the transpiration stream?
The movement of water from the roots to the leaves
How does light affect the rate of transpiration
Increased light increases the rate of transpiration as it means more stomata will be open for photosynthesis.
How does humidity affect the rate of transpiration?
High humidity will lower the rate of transpiration because of the reduced water potential gradient between the inside of the leaf and the outside air.
How does temperature affect the rate of transpiration?
An increase in temperature means an increase in the rate of transpiration as it increases the concentration of water vapour that the external air can hold before becoming saturated, WHICH increases the diffusion gradient between the leaf and the air.
How does wind affect the rate of transpiration?
Each leaf has a layer of still air surrounding it where water vapour accumulates therefore decreasing the diffusion gradient. So wind prevents water vapour accumulating, and increases the rate of transpiration.
Describe how water moves into the root
-Water moves into the root via osmosis
-The concentration of solutes in the cytoplasm and sap of root hair cells maintains a Ψ gradient between the soil water and the cell.
How are root hair cells adapted as exchange surfaces?
-Microscopic and can penetrate easily between soil particles.
-Large SA:V ratio and thousands on each growing root.
-Thin surface layer through which diffusion and osmosis can take place quickly.
Describe how water moves from the root to the xylem via the symplast pathway
Water moves by osmosis through the plasma membrane into the cytoplasm and from one cell to the next via plasmodesmata.
Describe how water moves from the root to the xylem via the Apoplast pathway
Water and dissolved minerals move through the gaps between the cellulose fibres in the cell wall due to the cohesive properties of water.
Describe how water moves into the xylem
-When the water reaches the endodermis of the root, the Casparian strip prevents the apoplast pathway and makes water follow Symplast pathway. It stops toxic substances entering the xylem.
-Endodermis moves minerals into xylem by active transport, reducing Ψ, drawing water into xylem by osmosis down Ψ gradient.
-HIGH ROOT PRESSURE AT BOTTOM OF XYLEM
What is the Casparian strip?
A band of waxy material called Suberin that runs around each of the endodermal cells forming a waterproof layer.
Give evidence for the role of active transport in root pressure
-Root pressure increases and decreases with temperature, suggesting a chemical reaction is involved.
-If oxygen or respiratory substrate levels fall, root pressure also falls.
Explain the cohesion-tension theory
-Water molecules form hydrogen bonds with each other causing them to ‘stick’ together and from a chain or column (cohesion).
-Evaporation at the top of the xylem creates tension.
-The column of water molecules is pulled up by tension.
Give some evidence for the cohesion-tension theory
-When a xylem vessel is broken (cutting a flower stem) in most circumstances, air is drawn into the root rather than water leaking out.
-If a xylem vessel is broken and air is pulled in, the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken.
What are xerophytes?
+ Examples
A Xerophyte is a species of plant that has adaptations to survive in an environment with little liquid water. E.g. Marram grass, Cacti.
