Translocation

• What is most glucose transported around the plant as?

  Sucrose
• What is most glucose stored in a plant as?

  Starch
• What are assimilates?


  The products of photosynthesis moving around the plant
• How much higher is sucrose concentration in phloem sap compared to normal cell sap?

  40 to 60 times higher
• What is translocation?

  The movement of assimilates dissolved in water via the phloem from sources to sinks
• What is a source of assimilates?

  Places where assimilates are made
• What are the 3 main sources of assimilates in a plant?

  1. Storage organs such as tubers or tap roots when they are unloading their assimilates

  2. Green leaves and stems

  3. Food stores in seeds when they germinate
• What is a sink of assimilates?

  The parts of a plant which require assimilates, and have assimilates delivered to them
• What are the 3 main sinks of assimilates in a plant?

  1. Actively dividing meristems

  2. Roots which are growing and/or actively absorbing mineral ions

  3. Any parts of the plant laying down food stores such as seeds, storage organs or fruits
• What is an example of a plant organ which can act as both a source and a sink for assimilates?

  Storage organs such as tubers
• What is co-transport?

  Where a molecule such as glucose must be transported across a carrier/channel protein with an ion, such as Na⁺, in order for the protein to allow the molecule across
• What does the mass flow hypothesis state happens at sources?

  Assimilates are actively loaded into the sieve tubes of the phloem. This lowers water potential, so water enter the phloem from the xylem via companion cells.
• According to the mass flow hypothesis, what is the pressure at the top of the plant (the sources)?

  High
• According to the mass flow hypothesis, what is the pressure at the sinks?

  Low
• What does the mass flow hypothesis state happens at the sinks?

  Assimilates usually leave via diffusion due to being used up here. Removal of solutes causes water potential to rise, and water leaves by osmosis to lower it.
• How does loading at the source work?

  1. H⁺ ions are actively pumped out of companion cells into mesophyll cells

  2. A high concentration of H⁺ builds up in mesophyll cells, causing the ions to move back into companion cells, taking sucrose with it via a co-transporter protein

  3. This causes a high concentration of sucrose in companion cells, so sucrose moves by diffusion into sieve tube elements, lowering water potential

  4. Water moves by osmosis into the sieve tube elements from surrounding cells such as the xylem, increasing turgor pressure in the elements and causing movement of water and assimilates by mass flow to the sinks
• What adaptation do companion cells have to aid active loading at the source?

  Many mitochondria to provide the necessary ATP for the active transport of hydrogen
• How does unloading at the sink work?

  1. Water and assimilates arrive in sieve tube elements in the sink

  2. Sucrose diffuses into surrounding cells, which convert them into other substances such as starch

  3. This lowers sucrose concentration, so more sucrose diffuses out of the sieve tube

  4. The reduced sucrose in the sieve tube leads to an increase in water potential, so water moves out into surrounding cells by osmosis.

  5. This reduces turgor pressure in the sieve tubes, and maintains mass flow in the phloem
• What is some evidence for active loading?

  1. If the mitochondria in companion cells are poisoned, active loading stops

  2. Advances in microscopy mean we can see it happen, and see adaptations of companion cells to help it

  3. Rate of movement of sucrose is 10,000 times quicker than by diffusion
• What is some evidence for translocation occurring in the phloem (as opposed to anywhere else)?

  1. Girdling- if you cut a ring of bark from a tree, thereby cutting off all phloem, which are found in the bark, the tree will bulge out above the cut as a new sink is formed

  2. Aphids put their stylets into the bark of a tree where the phloem are and, if the fluids inside them are analysed, they can be found to contain many sugars
• What are two reasons we are unsure about the mass flow hypothesis?

  1. Unsure of the role of sieve plates in translocation

  2. Sucrose seems to move through the phloem at the same rate regardless of concentration at the sink