7.8- Transport of organic substances in the phloem

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  • Created by: Megan2413
  • Created on: 16-03-17 09:55
What is translocation?
The process by which organic molecules and some mineral ions are transported from one part of a plant to another is called translocation
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What 3 things make phloem different to xylem structurally?
- Phloem have sieve plates with pores, xylem don't - Phloem are made up of living cells rather than dead cells - Phloem are surrounded by companion cells
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What is the source in translocation?
The site of production
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What is the sink in translocation?
The site at which substances are used dircetly or stored for future use
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Does the mechanism of transloaction transport materials faster or slower than diffusion?
faster
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Whcih theory is sued to explain translocation?
Mass flow theory
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How many phases can the Mass flow theory be divided into?
Three
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What is the first stage of translocation?
Transfer of sucrose into sieve tubes from photosynthesising tissue
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What happens to sucrose in this stage?
Sucrose is produced by photosynthesis in the source and travels down a concentration gradient by facillitated diffusion into a companion cell
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What happens to H+ ions in this stage?
They are actively transported from companion cells into the spaces within cell walls using ATP
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Where dot eh H+ ions travel next and how?
H+ ions diffuse down a concentration gradient through carrier proteins into the sieve tube elements
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How are sucrose molecule transported into the sieve tube elements?
They are co-transported along with H+ ions via co-transport proteins
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What is the second stage of translocation?
Mass flow of sucrose through sieve tube elements
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What is mass flow?
It is the bulk movement of a substance through a given channel or area in a specified time
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As sucrose molecules move into the sieve tubes, its water potential is lowered, what does this cause?
Due to the xylem running alongside the phloem having a less negative water potential, water moves by osmosis from xylem not phloem
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What does this movement of water cause in the sieve tubes?
There to be a high hydrostatic pressure within the sieve tubes
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What happens to sucrose in the sink?
It is either stored as starch or used directily for respiration
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What happens due to the sieve tubes near the sink having a low sucrose content?
Sucrose is actively transportd from the sieve tubes into the sink cells
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Due to lowered water potential....
Water moves into the respiring cells from the sieve tubes by osmsis
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What happens to the hydrostatic pressure in the sieve tubes near the sink?
It decreases
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Why is there therefore a mass flow of sucrose solution into the sieve tubes?
There is high hydrostatic pressure at the source and a low hydrostatic pressure at the sink, hence a mass flow of sucrose solution travels down this hydrostatic gradient
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What is the third stage of translocation?
Transfer of sucrose from the sieve tube elements into storage or other sink cells
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What happens to sucrose during this stage?
It is actively transported by companion cells, out of the sieve tubes and into the sink cells
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Why is mass flow overall an active process?
Eventhough the mass flow mechanism is passive, it occurs as a result of the active transport of sugars which is why it is affected by temperature and metabolic poisons
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State three pieces of evidence supporting the mass flow hypothesis
- There is a pressure within the sieve tubes as shown by the release of sap when they are cut - Metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem - Downward flow in the phloem only occurs in daylight
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State three pieces of evidence questioning the mas flow hypothesis
- The funtion of the sieve plates is unclear as they would seem to hinder mass flow - Not all solutes move at the same speed, they should do so if movement is by mass flow - Sucrose is delivered at more or less the same rate to all regions rather -->
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than going more quickly to the ones with the lowest sucrose concentration, which the mass flow theory would suggest
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Card 2

Front

What 3 things make phloem different to xylem structurally?

Back

- Phloem have sieve plates with pores, xylem don't - Phloem are made up of living cells rather than dead cells - Phloem are surrounded by companion cells

Card 3

Front

What is the source in translocation?

Back

Preview of the front of card 3

Card 4

Front

What is the sink in translocation?

Back

Preview of the front of card 4

Card 5

Front

Does the mechanism of transloaction transport materials faster or slower than diffusion?

Back

Preview of the front of card 5
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