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level: 7.8 Transport of organic molecules in phloem

Questions and Answers List

level questions: 7.8 Transport of organic molecules in phloem

QuestionAnswer
What is the phloem- Transports organic substances like sugars up + down plant - Formed from cells forming tubes
The cell types that make up the phloem- Sieve tube elements - Companion cells
Sieve tube elements- Living cells that form the tube for transporting solutes - Have no nucleus - Have few organelles
Companion cells- Companion cells for each sieve tube element - Carry out living functions for sieve cells e.g. providing energy needed for the active t. of solutes
Translocation meaning- The movement of solutes - Solutes can be called assimilates - Requires energy, happens in phloem - Moves solutes from sources to sinks
Sources meaning- The source of solute is where its made - So its at a high concentration there
Sinks meaning- The area where the solutes is used up - So its at a lower concentration there
Enzymes role in translocation- Maintain a concentration gradient from the source to the sink by changing the solutes at the sink - Either breaking them down or making them into something else - So it makes sure there's always a lower concentration at sink than at the source
What is the the mass flow hypothesis- Best explains phloem transport - Scientists still aren't sure how the solutes are transported from the source to sink by translocation
The mass flow hypothesis (1)- Sucrose moves from companion cells into sieve tube elements by active transport - Reducing water potential of the sieve tube element
The mass flow hypothesis (2)- Water moves into the phloem by osmosis, increases HSP - There is a pressure gradient with high HSP near the source cell + lower HSP near sink cells
The mass flow hypothesis (3)- Solutes move down the pressure gradient towards the sink end of the phloem - Solutes move into sink cells and are converted into other molecules (e.g. starch)
The mass flow hypothesis (4)- The removal of solutes increases the water potential at the sink end, causing water to move out of the phloem by osmosis - Maintaining the HSP gradient between the source + sink
Active loading- How sucrose is actively transported into the sieve tube elements
Active loading (1)- Companion cell actively transports hydrogen ions into surrounding cells - Creating a hydrogen ion gradient between the surrounding cells + companion cell
Active loading (2)- Hydrogen ions move back into the companion cell down concentration gradient through a co-transporter protein - This also transports a sucrose molecule into the companion cell, against its concentration gradient
Active loading (3)- The same process occurs to transport sucrose from the companion cell into the sieve tube element
Evidence supporting mass flow hypothesis- Suitable water potential gradient between leaves + other parts of plant - Phloem cut sap oozes out, pressure gradient exists
Evidence questioning mass flow hypothesis- Sugar travels to many different sinks, not just the one with the highest water potential as the diagram suggests - Sieve plates create barrier to mass flow, much pressure needed for sugar solution to get thru - Mass flow doesn't require living cells, yet phloem cells are alive + active