Difference between diffusion and bulk flow?

Diffusion: Moment of a solute from one solution to another solution but there will not be any moment of solvent. Diffusion is nothing but a random moment of individual molecules, from less concentration are to another concentration area. If you have two solutions of a particular material and when you mix them the solute will flow from higher concentration area to lower concentrate area even when they are separated by a membrane.
Bulk flow: Bulk flow is the movement of the whole solution from one area to another area due to the pressure gradient.
In bulk flow moment of many molecules will happen due to the pressure gradient. in diffusion only one molecule movement will happen till the concentration is uniform. Diffusion is due to the concentration gradient of a particular material. Generally, in plants, these two types of transportation will take place.

The bulk flow of the liquids is one where the liquid mass moves as a single entity in the direction of flow. Due to viscosity the liquid in contact with the solid surfaces might flow with a lower speed than that of the liquid which is far from the solid surface but for all practical purposes it can be assumed as a single unit moving in the direction of flow. This is the most normal flow of liquids.

On the other hand in diffusion, one liquid comes in contact with other and depending upon their relative concentration they will diffuse in each other. The diffusion is a slow process and there are many factors associated with the rate of diffusion and other consideration. Generally the high concentrate solution will try to enter the low concentration solution. The two liquids will try to satisfy the condition of diffusion to a certain extent and the movement will stop only after an equilibrium is reached and no fresh relative flow is expected.

The bulk flow in liquids is categorised by an average movement of the fluid and depending upon the boundary conditions it will be assumed to be moving with some average pace. Diffusion is not bulk flow. It is in fact a more deeper connotation and means that a liquid is diffusing in other liquid and slowly mixing with it depending upon the difference of concentration between the two.

Diffusion is useful when certain materials are dissolved in it and monitoring is also done by the people undertaking the research in this area. Bulk flow is more useful in understanding the hydrodynamic principles.

Bulk - flow takes place only when both the molecules and solvent are in the state of motion and there exists a differential gradient allowing to deplete both molecules and solvent from a higher gradient to lower one. In bulk flow, both the molecules and solvent remain under perpetual motion and an erosion of concentration of both solute and solvent takes place due to difference set up in their velocity gradient.
Diffusion a slow process of absorption of the solute in the given solvent. This may termed as a kind of osmosis where there is continious flow of the solute till a threshold is achieved.
Both the terms are quite relevant in understanding plant mechanics.