Mass Flow Hypothesis Flashcards
What is the mass flow hypothesis?
A theory that explains the movement of organic substances (mainly sucrose) in the phloem from sources (e.g. leaves) to sinks (e.g. roots, growing tissues).
What is a ‘source’ in the mass flow hypothesis?
A region where substances are produced and loaded into the phloem, such as photosynthesising leaves.
What is a ‘sink’ in the mass flow hypothesis?
A region where substances are removed from the phloem, such as root cells or developing fruits that use or store sugars.
Describe the first step of the mass flow hypothesis.
Sucrose is actively transported from companion cells into the sieve tube elements at the source using ATP.
What happens after sucrose enters the sieve tube?
The water potential in the sieve tube decreases, so water enters from the xylem by osmosis, increasing hydrostatic pressure.
How is sucrose moved along the phloem?
A pressure gradient is created from source to sink, pushing sucrose along the sieve tubes by mass flow (bulk movement of fluid).
What happens at the sink end of the phloem?
Sucrose is removed from the phloem by surrounding cells, increasing water potential in the phloem, so water moves out by osmosis, lowering pressure.
What causes the pressure gradient in phloem?
The high hydrostatic pressure at the source and low hydrostatic pressure at the sink due to movement of water.
What experimental evidence supports the mass flow hypothesis?
- Aphid studies show higher pressure near sources
- Ringing experiments show phloem transport
- Radioactive tracers track movement of sugars.
What are the criticisms of the mass flow hypothesis?
- Sieve plates should hinder mass flow
- Not all solutes move at the same rate
- Sugar transport is bidirectional, not explained by a simple flow model.
What is the role of companion cells in mass flow?
They actively load sucrose into sieve tubes using ATP via co-transport mechanisms.
How is ATP involved in mass flow?
ATP is used for active transport of sucrose from companion cells into sieve tubes at the source.
Why does osmosis occur in sieve tubes after sucrose is loaded?
The addition of sucrose lowers the water potential inside the sieve tube, causing water to move in from the xylem.
Why is mass flow considered a passive process in the sieve tube itself?
Once the pressure gradient is established, the bulk flow of solution occurs passively without further energy input.
How does temperature affect mass flow?
Higher temperatures increase enzyme activity involved in active loading and decrease water viscosity, thus increasing the rate of flow.
