Answer:
Movement of solutes and solvent molecules across the membrane occurs by 2 methods - active transport and passive transport.
In passive transport energy is not needed. It occurs against the concentration gradient i.e. from higher concentration to lower concentration. The passive transport is 2 types - simple diffusion and osmosis.
The diffusion is the transport method where solute particles move across the cell membrane. This is simple diffusion as the solute particles like proteins, carbohydrates, amino acids, move across the membrane from lower concentration to higher concentration.
Osmosis is another type of diffusion where the solvent molecules / water molecules transport across the membrane. It is also not required energy and happens against concentration gradient. This is the very reason cells show the changes in size in hypotonic and hypertonic solution.
The active transport is the movement of solute/ solvent molecules across the cell from lower concentration to higher concentration. This also includes the diffusion and osmosis. But here diffusion is known as facilitated diffusion as it required carrier proteins for transport of ions, proteins into the cell. This method also requires ATP for transport.
The facilitated diffusion establish the concentration gradient as it deposits more solute particles from lower concentration region to higher concentration region. e.g of such transport are absorption of glucose, amino acids, osmoregulation, and urine formation.
But the passive transport / simple diffusion is saturated by substrate. Because it tries to maintain the equilibrium between the gradient difference.
