Xylem and Phloem Flashcards
What is the different between the protoplast and the apoplast?
protoplast is everything inside the plasmodesma
apoplast is everything outside
What is the xylem?
the dead cell walls part of the apoplast pathway which create the pathway of water, minerals and nutrients from the soil
What is everything inside the endomersis called?
the stele
Name the 3 structures outside the endodermis?
the cortex and the epidermis and hypodermis
What is the role of cellulose and pectin in the cell wall?
- they control what can pass through teh wall from the outside to the inside as they are relatively permeable to water
What does it mean when the plasmodesmata is open?
molecules can go between cells without crossing a membrane
What is the role of the casprian strip? (3)
- block the cell wall pathway
- force molecules through the membrane
- control what goes into the xylem
Name the 3 pathways of water movement across teh root
apoplastic
cell to cell
symplastic
What is the difference between the apoplastic and the symplastic pathway?
apoplastic; involves the Caspian strip
symplastic; plasmodesmata are open and reach the stele
Name the mechanisms of xylem transport
1) root pressure
2) transpiration pressure
Describe the process of root pressure in xylem transport
- endormisis and caspiran strip make stele act like a single cell
- accumulation of ions increase osmotic pressure and lowering water potential
- this generates transport of water across the xylem
What creates transpiration tension?
cohesion and adhesion create a negative pressure in the leaves under transpiration
When is cavitation caused?
when water bubbles block the xylem
When is cavitation greater?
when transpiration is high
Why is there an absence of cavitation at night?
because the stomata is shut so there’s no tension
whereas during the day, teh sun comes up and there is more evaporation
more tension so more cavitation
How can cavitation events be cleared out?
by root pressure
What are tree rings a consequence of?
- CO2 fixation
- transport effiency vs cavitation resistance
What is the key process decreased by cavitation
transport efficiency
Define the phloem
the vascular tissue in plants which conducts sugars and other metabolic products downwards from the leaves.
Name 3 differences in the structures of the phloem and the xylem
- xylem is made up of dead cells whereas phloem is made up of living cells
- xylem has a thick cell wall, phloem has a thin cell wall
- xylem cell wall made up of lignin whereas phloem is made up of cellulose
What is the differences in transportation between phloem and xylem (2)
- xylem transport is passive, phloem transport is active
- xylem transport is unidirectional, phloem is bidirectional
- xylem carries water from roots to leaves, phloem carries solutes from leaves to growing parts
What is phloem a generic term for?
the translocation system
What is a source in these terms?
a plant organ in which sugar is being produced by either photosynthesis or the breakdown of starch
What is a sink in these terms?
an organ that is a net consumer or store of sugar e.g. a photosynthesising leaf
What is the role of the sieve tube?
specific tube that does teh transport in the translocation system and goes to teh xylem
What is the role of the companion cell?
provides genetic information and proteins, and an important function in maintaining the sieve tube
Why do the sieve tubes need the companion cell?
sieve tube doesn’t have a nucleus but it is alive
What is the role of the plasmodesmata?
they connect cytoplasm of adjacent cells, not like gap junctions, very selective, can open or close
What is the main mechanism of translocation in angiosperms?
pressure flow
What creates a difference in hydrostatic pressure to drive phloem sap from the source to the sink?
- high levels of sugar at source lowers water potential and causes water to flow into the tube
- removal of sugar at the sink increases water potential and causes water to flow out of teh tube
Name the two ways in phloem loading occurs
- apoplastic which involves membranes
- symplastic - no membranes involved
