Phloem – Plant Tissue

Join now

If you're ready to pass your A-Level Biology exams, become a member now to get complete access to our entire library of revision materials.

Join over 22,000 learners who have passed their exams thanks to us!

Sign up below to get instant access!

Join now →

Or try a sample...

Not ready to purchase the revision kit yet? No problem. If you want to see what we offer before purchasing, we have a free membership with sample revision materials.

Signup as a free member below and you'll be brought back to this page to try the sample materials before you buy.

Download the samples →

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.

Translocation from Source (e.g. sugar in leaves) to Sink (roots)
  • 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.

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.