Question

How do aquaporins facilitate the passage of water?

Answer 1

To function properly, a cell has to maintain a stable internal environment. Therefore, the cell keeps a check on all the substances that enter and exit it. A selectively permeable cell membrane controls the movement of the substances that enter or leave the cell. This is called cell transport. Sometimes the cell has to expend energy to transport a molecule i.e. active transport. Other molecules can passively enter the cell i.e. passive transport.

Complete answer:
Phospholipid bilayer and proteins make up the cell membrane. The phospholipids are amphipathic in nature i.e. they have a polar hydrophilic component (head) and a non-polar hydrophobic component (fatty acid tail). Because of this property, not every molecule is allowed to enter or exit a cell; it takes up lipid-soluble and water-soluble substances.

Two factors determine the uptake of molecules by the cell; the size of the molecule or the polarity of the molecule. Due to the presence of the non-polar hydrophobic tail, lipid-soluble substances (such as cholesterol) are taken up. While water-soluble molecules (polar) require some help.

Passive transport ensures the movement of molecules down the concentration gradient across a cell membrane. The molecules move from a higher concentration region to a lower concentration region. Passive transport can be of three types- simple diffusion, facilitated diffusion, and osmosis.

Molecules can easily diffuse into the cell through the cell membrane down its concentration gradient. Molecules (gases like oxygen, carbon dioxide, etc.) are small therefore, no energy is expended and no proteins are required for this transport.

Some large molecules (such as glucose, etc.) require proteins to facilitate their movement i.e. they require protein channels in the membrane to help enter or exit a cell. This is called facilitated diffusion.

Solvent molecules, such as water, can enter or exit the cell. These solvent molecules move down the concentration gradient and are transported across the cell membrane. This is called osmosis.

Initially, it was thought that water could simply enter or exit the cell through osmosis. Water can pass through the cell membrane easily because it is a small molecule. But, water is also polar in nature which makes simple diffusion of water difficult. Scientists found out that aquaporins (protein channels) mediated the movement of water in the cell. Water osmotically moves through these aquaporins to enter or leave the cell.
Aquaporins also allow other uncharged small molecules such as glycerol to pass through them.

Note: If the cell requires a molecule, it does not matter if it’s a large molecule or a very polar molecule, the cell membrane will allow the molecule to pass through it. This might be done passively or through active transport.
The study of aquaporins and their function can help understand various skin conditions that occur due to an imbalance in water retention capacity.