Translocation Flashcards
What is it
The transport of ions and organic/inorganic molecules
Where does translocation occur in flowering plants
The phloem
What’s the phloem made up of
Living (respiring) cells
E.gs of what the phloem transports
Amino acids Sucrose Glucose Potassium Chloride Magnesium
Where does photosynthesis produce plants
In areas called sources
Why can’t the transport in the phloem be down to diffusion
It’s too fast
What is mass transport
Bulk transport of a substance through a given channel in a given time
Explain the mass flow theory (1-5)
- Sucrose made my cells with chloroplasts
- Moves into companion cells by facilitated diffusion
- Companion cells actively pump hydrogen ions into spaces between cell walls (active transport)
- Hydrogen ions move into sieve tube elements by facilitated diffusion
- Sucrose is transported with hydrogen - cotransport via cotransport proteins
Explain the mass flow theory (6-11)
- Build up of sucrose in sieve tubes lowers their water potential
- Water from xylem moves into sieve tubes by osmosis creating a high hydrostatic pressure
- Conc drops as sucrose is used by respiring plant cells
- Sucrose is actively transported into plant cells from sieve tubes due to low sucrose levels
- Water enters plant cells from sieve tubes by osmosis due to their lowered water potential
- Hydrostatic pressure falls in sieve tubes
What causes a hydrostatic gradient in mass flow theory
What does this help occur
The difference in hydrostatic pressure in the sieve tubes at the sinks (low) and sources (high)
Means mass transport in phloem can occur
What 4 gas exchange process are involved in the mass flow theory
Facilitated diffusion
Active transport
Co-transport
Osmosis
How does sucrose get to phloem
Via facilitated diffusion from palisade cells through the spongy mesophyll -> phloem
What’s the gaps in between cells called
Plasmodesmata
Evidence for mass flow theory
Cell sap is released when phloem is cut due to sieve tubes’ hydrostatic pressure
Sucrose concentration greater in leaves than roots
Downward flow of sucrose during light times of day (photosynthesis)
Increases sucrose in leaves follow by increased sucrose in phloem shortly after
Poisons that hinder respiration/lack of oxygen reduce translocation - no active transport
Companion cells contain mitochondria/ATP
Evidence against mass flow theory
Presence of sieve plates
Not all molecules move at same speed - should in mass flow
Sucrose is delivered to all areas at same rate - not delivered quicker to areas that need it most (conc gradients)
How do aphids help investigate phloem
Aphids (green fly) feed on sucrose in phloem using a proboscis
They insert the proboscis into the phloem
The aphid is anaesthetised and it’s head cut off
Scientist can then collect contents from phloem using proboscis
2 other ways of investigating phloem
Radioactive tracers
Ringing
How do radioactive tracers work
Plant supplied with radioactive carbon in CO2 (14C)
This carbon is used to produce sugars in the plant
These can be traced as they travel
What’s the evidence of the presence of phloem/transport of sugars from radioactive tracers
Areas of radioactivity appearing ‘foggy’ on the photographic film
What is placed on photographic film in the dark to test for phloem
Cross sections of stems that are removed
What can test for radioactivity of sap (aphids)
Aphids feeding at different locations on a plant
What happens in ringing
Cylinders of bark + some tissue from beneath are removed from plant stem
Phloem is removed, xylem is left intact
Contents of phloem, above + below the ring can be sampled and analysed using a similar technique to the aphid
Where do sugars build up when investigating phloem
Above the ring in ringing as they can’t be transported any further
