Phloem

Made up from 4 different parts:

• Phloem fibres
• Phloem parenchyma
• Sieve tubes
• Companion cells

The sieve tubes and companion cells are both involved with the mass flow hypothesis.

Sieve Tubes

• Cell membrane broken down
• Fluid cytoplasm
• No vacuole
• No nucleus

With the end of the sieve wall broken down it allows inter cellular movement.

• Have plasmodesmata.
• Very metabolically active.
• High number of mitochondria.
• May provide energy for translocation (mass movement).

Osmotic Pressure

The movement of solutes from a high hydrostatic pressure to a low hydrostatic pressure.

Simplified diagram of water and energy movement in a Plant

Evidence to support Mass Flow hypothesis

• Solution under pressure.
• Evidence for concentration gradient.
• Observation of sieve tubes.
• Movement of virus through plant.
• No movement of virus when no photosynthesis.

Evidence against Mass Flow hypothesis

• Get the impression of steady flow, this is wrong.
• Certain things move different ways.

Phloem in a Root

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