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.
What are Hydrophytes?
Hydrophytes are plants that live in water and have special adaptations to cope with growing in water or in permanently saturated soil. E.g. water lilies.
Give some adaptations of xerophytes
-A thick waxy cuticle: Helps minimise water lost by transpiration.
-Sunken stomata: Reduces air movement, creates an area of still, humid air around the stomata. This reduces the water potential gradient and so reduces transpiration.
-Hairy leaves: Some plants such as marram grass have micro hairs in the sunken stomatal pits. Creates area of still, humid air around the leaf. Reducing the Ψ gradient and minimises the loss of water by transpiration from the surface of the leaf.
-Root adaptations: Long tap roots growing deep into the ground. A mass of widespread, shallow roots with a large surface area able to absorb any available water before a rain shower evaporates is another adaptation.
Give some adaptations of hydrophytes
-Very thin or no waxy cuticle: Hydrophytes don’t need to conserve water as there is always plenty available so water loss by transpiration isn’t an issue.
-Many always-open stomata on upper surfaces: Maximising the number of stomata maximises gaseous exchange. No loss of turgor as always water available. Stomata are usually open all the time for gaseous exchange.
-Reduced structure to the plant: The water supports the leaves and flowers so there is no need for strong supporting structures.
-Wide flat leaves: For example water lilies, to capture as much light as possible.
What is a source?
Any structure in a plant that produces or releases sugars
What is a sink?
Any location in a plant where sugar is delivered to for use.
What are some examples of sources in plants?
-Leaves
-Stems
-Food stores in seeds
-storage organs e.g. tubers, tap roots.
What are some examples of sinks in plants?
-Roots
-Meristems
What is translocation?
The transport of assimilates from a source to a sink
Describe sucrose loading via the apoplast pathway
Apoplast pathway: (Active process)
1. Companion cells pump H+ (protons) out of their cytoplasm using active transport, which requires ATP.
2. This produces a H+ diffusion gradient,
3. H+ diffuse back into companion cells through co-transporter protein (facilitated diffusion).
4. Co-transporter proteins enable H+ to bring sucrose molecules into companion cells.
5. Sucrose concentration builds up in companion cell so diffuses into sieve tube element via plasmodesmata.
Describe the mass flow theory
- Water absorption from nearby cells increases hydrostatic pressure in mesophyll.
- Phloem loading transfers organic food from mesophyll cells to sieve tube.
- Water entry from xylem to sieve elements by osmosis.
- Hydrostatic pressure difference (gradient) drives solute movement from sieve tube to sink.
- Phloem unloading transfer of organic food from sieve tube to sink.
Describe sucrose unloading
Diffusion of sucrose from phloem to
surrounding cells
-Occurs wherever cells need glucose/ sucrose
-Sucrose converted back to glucose (for respiration)
-Or converted to starch for storage
-Concentration gradient of sucrose maintained between phloem and cells
-Loss of sucrose / solutes increases water potential of phloem.
-Water leaves phloem to surrounding
cells / xylem
-Results in lower hydrostatic
pressure.
Explain the importance of lignin
-Strengthens the xylem wall - prevents collapse
-(Spiral) allows flexibility - prevents stem breaking
-Improves adhesion of water molecules - prevents loss of water.
