Difference between mass flow hypothesis and cohesion theory
Answers
The pressure-flow hypothesis of phloem: sap moves down the concentration gradient translocated by the water drawn in by osmosis at the top of the gradient and by the water pushed out at the bottom of the gradient.
Water in the xylem moves out laterally, by osmosis, to surrounding vascular, ground and epidermal tissues. The phloem draws this water strongest near the sucrose source (either roots in spring or leaves in summer). This way the phloem direction of flow can be reversed.
Sucrose is actively loaded (energy is used) into the phloem vesicle’s sieve cells creating a high solute concentration. Phloem with a low water potential (high solute concentration) pulls water in. The water being drawn in creates osmotic pressure inside the phloem column so the sap is pushed down the gradient. At the bottom of the gradient the 'sink' removes sucrose for use or storage, depending on season. The phloem's water moves back to the xylem at the bottom of the gradient because the xylem has a higher solute concentration, once the sucrose is removed from the phloem. This model implies some water actually circulates in the plant from xylem to phloem's flow and back to the xylem.