Answer:
Concentration gradient.
Explanation:
Diffusion may be defined as the process of the movement of solute particles down the concentration gradient. This process does not require the expenditure of energy.
The main driving force in diffusion is the concentration gradient. The molecules generally move from its region of the higher concentration towards the region its lower concentration.
Thus, the correct answer is option (3).
The principal force driving movement in diffusion is the concentration gradient.
It is said that there is a chemical or concentration gradient for a solute, when there is a difference in concentration between both sides of a membrane.
A substance can move from the side where it is more concentrated to the side where its concentration is lower, this is equivalent to saying that it moves along the concentration gradient.
This is the case of diffusion, the solute moves spontaneously always in favor of the gradient, which becomes the driving force for the movement.
Those molecules that are less polar (more soluble in lipids) will generally penetrate the membrane more quickly than polar molecules (more soluble in water).
That is, in passive diffusion, hydrophobic molecules and small uncharged molecules lower their concentration gradient directly across the membrane without the expenditure of energy.
Therefore, we can conclude that the principal force driving movement in diffusion is the concentration gradient.
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