| 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 |
