Water is a polar molecule that will not pass through the lipid bilayer; however, it’s small enough to move through the pores — formed by protein molecules — of most cell membranes. Osmosis occurs when there’s a difference in molecular concentration of water on the two sides of the membrane.

Ions, sugars, amino acids, and sometimes water cannot diffuse across the phospholipid bilayer at sufficient rates to meet the cell’s needs and must be transported by a group of integral membrane proteins including channels, transporters, and ATP-powered ion pumps (see Figure 15-3).

Simple diffusion does not require energy or need the assistance of a transport protein. Other larger or charged molecules that diffuse across a membrane may need assistance from a protein. Oxygen is a molecule that can freely diffuse across a cell membrane.

Water can also move into or out of cells through channel proteins called aquaporins . These proteins molecules act as doorways through which water can pass.

Osmosis is a passive transport process during which water moves from areas where solutes are less concentrated to areas where they are more concentrated.

Osmosis is when water moves from an area of LOW solute concentration (low osmolarity) to an area of HIGH solute concentration (high osmolarity) through a semipermeable membrane. Osmosis helps you get nutrients out of food. It also gets waste products out of your blood.

Osmosis provides the primary means by which water is transported into and out of cells. Osmosis is of prime importance in living organisms as it influences the distribution of nutrients and the release of metabolic wastes products such as urea.

Where Does It Happen? Osmosis occurs in both the small and large intestines, with the majority of osmosis occurring in the large intestine. As your body processes food, it moves from the esophagus to the stomach and then to the small intestine. While there, your body absorbs important nutrients via osmosis.

1: Osmosis: In osmosis, water always moves from an area of higher water concentration to one of lower concentration. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated.

The physical driving force of osmosis is the increase in entropy generated by the movement of free water molecules. There is also thought that the interaction of solute particles with membrane pores is involved in generating a negative pressure, which is the osmotic pressure driving the flow of water.