transport of organic substances in the phloem Flashcards
(23 cards)
what is the function of the phloem?
transport organic molecules (sucrose and amino acids) around the plant
what two types of cells is the phloem made up of?
sieve tube elements and companion cells
what is in the sieve tube of a phloem which allows sucrose to be moved between cells?
sieve pores
why does the sieve tube have very little organelles and very little cytoplasm?
less resistance to flow so easy to transport
why do companion cells have lots of mitochondria?
provides sieve tube with ATP for active transport
what is a source and a sink?
source- an area from which sucrose is moving
sink- an area to which the sucrose is moving
in summer, what is the source of sucrose?
fully grown photosynthesising leaves near the bottom of the plant
they have a higher rate of photosynthesis than respiration so make excess sucrose
in summer, where are the 4 sinks of a plant?
- meristem tissue that is tips of roots and shoots (where cell division is occurring for growth)
- starch/sugar storage organs such as fruit and tubors
- roots for respiration, since they can’t photosynthesise themselves
- young leaves higher up plant, these have lower rates of photosynthesis than respiration
in winter, what is the source of sucrose?
storage organs
in winter, what are the 2 sinks?
root cells who need it for resp
meristem tissue for growth
briefly compare the structure of the xylem and phloem?
- xylem made of dead cells and phloem made of living
- xylem is hollow and phloem has some structures and little cytoplasm
- xylem has no end walls for continuous column of water and phloem has sieve plates
- xylem cell wall made of cellulose and lignin (strength and waterproof) and phloem just cellulose
what is the theory used for translocation?
mass flow (bulk movement of substances down pressure gradient)
what process first happens in companion cells which starts translocation?
H+ ions are actively transported out of the companion cell into the surrounding cells
this gives a higher conc of H+ ions outside the cell than inside so they will move back into the cell via facilitated diffusion
what is special about the carrier protein that enables the facilitated diffusion of H+ ions back into the cell?
it has two receptor sites and both hydrogen ions and sucrose must bind for transport to occur
the sucrose will move against its concentration gradient into the cell, along with the H+ ions
THERE IS CO-TRANSPORT OF SUCROSE WITH THE HYDROGEN IONS
where does sucrose go once it has been co-transported into the companion cell?
diffuses into sieve tube element via the plasmodesmata (strips of cytoplasm that run through the cell wall to link one cell to other)
what does the addition of sucrose to the sieve tube element do?
lowers the water potential
water from the xylem moves down the water potential gradient into the sieve tube via osmosis
this increases the hydrostatic pressure so that it is higher at the source than the sink
what does high hydrostatic pressure at the source lead to?
mass flow occurs where the sucrose solution moves down the pressure gradient to the sink (i.e. to the root, young leaf cells, or storage organs), where it is required by cells for respiration or storage
how does sucrose get from the phloem to sink?
at the sink, the sucrose is actively transported out of the phloem and the water returns to the xylem
what is removed during a ringing experiment?
protective layer and phloem
what is observed during ringing experiments?
after a period of time, the region of the stem immediately above the missing ring of tissue swells
samples of the liquid accumulated in these areas are rich in sucrose and other dissolved organic substances
some non-photosynthetic tissues in the region below the ring (towards root) wither and die
what do observations in a ringing experiment indicate about phloem?
that the phloem, rather than the xylem, must be important for translocating sugars in the plant
as the ring of tissue removed did not extend into the xylem, the xylem cannot have been responsible
what can be useful for tracing the movement of substances in the plant?
radioactive isotopes of carbon in carbon dioxide
describe how radioactive tracing works?
-plant leaves are covered in a bag which only contains the radioactive isotope of CO2
-this means that the radioactive CO2 helps produce radioactive glucose and therefore radioactive sucrose
-then an X-ray film/ photographic film is used in autoradiography to see where in the plant the radioactive sucrose travelled to
-blackened regions are found to correspond to locations of the phloem tissue in the stem
