Exchange and Transport - Plants Flashcards
What is the role of the xylem?
Transport water and mineral ions unidirectionally.
What is the xylem composed of, and which components are living?
Vessels, tracheids, xylem fibres. (lignified)
Xylem parenchyma
Why are most cells in the xylem dead?
No nucleus, cytoplasm, end walls preventing flow.
What is the purpose of pits in the xylem?
Allows water to move laterally to supply cells/tissues with water.
What is the benefit of the xylem having a wide lumen?
Allows more water to flow.
What strengthens and waterproofs the xylem?
Spiral lignin
spiral shape is flexible and strong
What is the role of the phloem?
Transport photosynthetic products(sucrose+amino acids) multidirectionally.
What is the phloem comprised of, and which components are living?
Sieve tube elements, companion cells, phloem parenchyma. Phloem fibres (lignified)
Describe the sieve tube element/companion cell arrangement.
Nuclei and almost all organelles are in the companion cells which are connected to the sieve tube elements via plasmodesmata.
This decreases the obstruction of flow through the column of elements.
What is parenchyma?
Packing tissue with a support/storage function.
Draw a transverse cut of a plant root and stem.
See bio book.
What is transpiration?
The loss of water vapour via evaporation from the aerial parts of a plant.
Water leaves cells in the spongy mesophyll layer.
Water evaporates into spaces.
Water diffuses out of the leaf, via stomata, down a water potential gradient.
What is the transpiration stream?
Water moving up the xylem vessels in a continuous stream.
What role does transpiration play in the transpiration stream?
Transpiration reduces pressure at the top of the xylem, causing hydrostatic pressure
What causes water to move up the xylem as a continuous stream?
Water molecules pull each other up the xylem through water cohesion due to hydrogen bonding.
Water molecules also move up the xylem through capillary action (water adhesion).
What causes water adhesion in the xylem?
The lignin in the walls of xylem vessels.
How does water first enter a plant?
By osmosis (down a water potential gradient) through root hair cells.
What are the three ways water travels through root hair cells?
Apoplast pathway (cell wall) Symplast pathway (cytoplasm) Vacuolar pathway (vacuole)
What is the casparian strip?
Present in the cell walls of endodermal cells (at the endodermis). Contains suberin (waxy and waterproof). Forces water to use symplast pathways.
What is the purpose of the casparian strip?
Forces water to pass through a partially permeable membrane which filters out any potentially toxic solutes.
What environmental factors affect transpiration? (5)
Light intensity Humidity Temperature Wind speed Soil water availiability
When do stomata open and why only then?
In high light intensity, so that water is not wasted when photosynthesis rate is slow.
What are xerophytes? Can you give some examples?
Plants with structural and physiological adaptations that allow them to survive in dry conditions.
e.g: cacti, marram grass
What are 2 examples of the features of a xerophyte?
Thick waxy cuticle Sunken stomata (reduces air movement so less water loss)
What are hydrophytes? Can you give some examples?
Plants with structural and physiological adaptations that allow them to survive in wet conditions.
e.g: water lilies
What are 4 examples of the features of a hydrophyte?
Thin/no waxy cuticle
Always open stomata (on upper surfaces)
Less structure (water supports them)
Wide flat leaves (for floating)
What is translocation?
The transport of assimilates through the plant in the phloem tissue.
What are the assimilates that are transported via translocation?
Organic solutes, molecules the plant has made such as the products of photosynthesis.
e.g: sucrose, amino acids
What are the production sites in translocation?
(Sources) leaves, storage organs
What are the destination sites in translocation?
(Sinks) roots, meristem
Why was the pressure flow hypothesis for translocation necessary?
The rate of assimilate movement is too fast to be caused only by diffusion.
Outline active loading at the source (translocation).
Companion cell uses ATP to pump H+ ions to the surroundings.
H+ diffuses back in via cotransporter proteins, bring sucrose with it.
Concentration of sucrose rises in companion cells.
Sucrose diffuses into sieve tube elements via plasmodesmata.
Outline mass flow (translocation).
Sucrose is actively loaded at the source, reducing water potential.
Water enters by osmosis, increasing hydrostatic pressure.
Water with dissolved sucrose moves towards sink from source down hydrostatic pressure gradient.
Sucrose is actively transported out of the phloem sieve tubes by surrounding sink cells, increasing water potential in the sieve tube.
Water moves out by osmosis, reducing hydrostatic pressure.
What happens during phloem unloading (translocation) and why?
Sucrose is actively transported out of sieve tube elements by companion cells and then converted into glucose or starch.
This conversion happens to maintain the concentration gradient between the contents of the phloem and surrounding sink cells.
What is the name of the protein which allows sucrose to diffuse into companion cells along with H+ ions?
Cotransporter protein
What piece of equipment could be used to measure the transpiration rate of plants?
A potometer
