translocation 7.8

what does the phloem tissue contain?

• sieve tube elements : living cells,contain no nucleus,containg few organelles

• companion cells:provide atp required for active transport of organic substances

mass flow hypothesis in plants

• transport of organic sunstances in a plant requires energy

• source to sink

• mass flow from the source of production the leaves to the sink ,the site where organic substances such as glucose and sucrose are used in respiring cells

translocation 1:how sucrose transports from the source to the sieve tube

• photosynthesis occurring in the chloroplast of leaves creates organic substances e.g sucrose

• this creates a high concentration of sucrose at the site of production,therefore sucrose diffuses down its concentration gradient into the companion cell via facilitated diffusion

• active transport of h+ occurs from the companion cell into the spaces within the cell using energy

• this creates a concentration gradient and therefore h+ move down their concentration gradient via carrier proteins into the sieve tube elements

• co transport of sucrose with the h+ ions occurs via protein co transporters to transport the sucrose into sieve tube elements

translocation 2:movement of sucrose within the phloem sieve tube element

• the increase of sucrose in the sieve tube element lowers the water potential

• water enters the sieve tube elements from the surrounding xlem vessels via osmosis

• the increase is water volume in the sieve tube element increases the hydro-static pressure causing the liquid to be forced towards the sink

translocation 3 :transport of sucrose to the sink

• sucrose is used in respiration at the sink,or stored

• more sucrose is actively transported into the sink cell,which causes the water potential to decrease

• this results in osmosis of water from the sieve tube element into the sink cell

• the removal of water decreases the volume in the sieve tube element and therefore the hydro-static pressure decreases

• movement of soluble organic substances is due to the difference in hydro static pressure between the source and sink end of the sieve tube element

investigatin transolcation?

tracers

ringing experiments

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how does tracers work?

• tracing involves radioactively labeling carbon.

• plants are provided with only radioactively labelled carbon dioxide and over time this is absorbed into the plant and used in photosynthesis to create sugars which all contain radioactively labelled carbon

• thin slices from the stems are then cut and placed on x ray film that turns black when exposed to radioactive materil

• when the stems are placed on the x ray film the section of the stem containg the sugar turns black and this highlights where the phloem are and shows sugars are trnasported in the phloem

what is ringing experiemnts

• a ring of bark and phloem are peeled and removed off a tree trunk.

• the result of removing the phloem is that the trunk swells above the removed section.

• analysis of the liquid in this swelling shows it contains sugar

• this shows that the phloem is removed ,the sugars cannot be transported and therefore proves the phloem transports sugars.