Mass transport in plants Flashcards
What transports water in plants?
Xylem
What is transpiration?
Evaporation of water from the leaves once it has been pulled through the xylem vessels in the stem
Why does the xylem have tubes with no end walls?
Water can be pulled up in a continuous column maintaining a transpiration stream
Why is it good that the xylem has no cytoplasm?
Nothing to impeede the flow of water
What is lignin in the shape of in the xylem and why is it useful?
Spiral shapes to provide support and prevent collapse under tension while also being flexible and allowing it to grow
What do the thick walls of the xylem do?
Prevent collapse under negative pressure
Why is the xylem waterproof?
To keep water in the cells
What do the pits in the walls of the xylem do?
Allow lateral movement so water can get around blocked vessels
What does the cohesion tension theory state?
Water is evaporated from the leaf reducing the water potential so water is drawn up the leaf from a high to low water potential which creates tension and negative pressure. There are cohesive forces between the water molecules and adhesive forces between the water and xylem walls which means water can be pulled up in a continuous column
What are the 3 pieces of evidence for the cohesion tension theory?
When transpiration is high the diameter of the tree trunk shrinks, if xylem is broken water can no longer be drawn up as not a continuous column, when vessels are broken water won’t leak as it is under tension
What happens to the diameter of a tree trunk as light increases?
decreases due to increased rate of transpiration due to greater evaporation from stomata so greater flow due to more pull
What happens to the diameter of a tree trunk as temperature increases?
decreases due to increased rate of transpiration water molecules have more kinetic energy so move up the xylem faster
Why does the diameter decrease?
More cohesion between molecules so more adhesion between water and the xylem walls which causes tension and negative pressure pulling the walls of the xylem in decreasing the diameter
What happens to transpiration rates as humidity increases?
decreases transpiration rate as less steep water potential gradient due to outside air being more moist so less water evaporated
What happens to transpiration as wind increases?
Removes moisture increases water potential gradient so increases transpiration
How do you use a potometer?
Cut the leafy shoot to reduce air entering and breaking the continuous column then seal the joints with waterproof jelly to prevent water loss and record the distance the air bubble moves and repeat to find mean by opening the resevoir tap
How are root hair cells adapted?
thin cell wall, Large SA for absorption, long hair like projections and lots of mitochondria for active transport
What does the phloem do?
Transport sugars and minerals around the plant by translocation
What is the structure of the xylem?
Long structures with sieve tubes at the ends supported by companion cells
Where does the phloem transport sugars from and to
source in the leaves to sinks e.g. roots
What is the mass flow hypothesis of translocation?
Sugars are produced in the leaf in photosynthesis and are actively transported into the hollow sieve tubes by companion cells. This lowers the water potential in the sieve tubes so water enters by osmosis increasing pressure causing mass movement towards sink or root
What are the sugars mainly used for?
Respiration or storage
How are sieve tubes adapted for their function?
Few organelles so substances can flow through easily
How are companion cells adapted for their function?
Contain lots of mitochondria which releases ATP for active transport and contains lots of ribosomes to produce proteins needed to act as carrier proteins
What does the ringing experiment show?
Above bulge of sugar above dead tissue below shows it is a continuous column
What does adding radioactive CO2 in the tracer experiment show?
Will blacken under x ray where phloem tissues are
What else can you use to prove it is phloem?
Aphids can suck out sugars
