Lesson 1, Topic 1
In Progress

03-Movement of Substances Through Cell Membranes

March 28, 2021

Transport processes move substances into and out of cells. There are two types of transport processes: passive transport, which does not require the cell to expend energy, and active transport, which does require the cell to expend energy.

Passive transport processes do not require added energy and result in movement “down a concentration gradient.” Diffusion, osmosis, dialysis, and filtration are the passive transport processes.

In diffusion, substances scatter themselves evenly throughout an available space, with the particles moving from high to low concentration. Solute particles move through channels or carriers in a membrane to reach equilibrium, or equality of concentration, of solution on both sides of the membrane. It is not necessary to add energy to the system for this movement to happen.

Osmosis is the passive movement of water molecules when some solutes cannot cross the membrane. It is similar to diffusion in that water moves in a direction that produces an equilibrium. Because water moves, but not all the solutes, the osmotic pressure may change across the membrane.

Dialysis occurs when some solutes move across a selectively permeable membrane by diffusion, and other solutes do not. This results in an uneven distribution of solute types.

Filtration, on the other hand, is the movement of water and solutes caused by hydrostatic pressure on one side of the membrane.

Active transport processes occur only in living cells. They involve the movement of substances “up the concentration gradient,” which requires energy from ATP. Active transport occurs through ion pumps, phagocytosis, and pinocytosis.

An ion pump is a protein complex in a cell membrane. Ion pumps use energy from ATP to move substances across cell membranes against their concentration gradients. Examples include the sodium-potassium pump and the calcium pump. Some ion pumps work with other carriers so that glucose or amino acids are transported along with ions.

Both phagocytosis and pinocytosis are active transport mechanisms because they require cell energy. Phagocytosis is a protective mechanism often used by the body to destroy bacteria. Pinocytosis is used to incorporate fluids or dissolved substances into cells.

Cystic fibrosis, characterized by abnormally thick secretions in the airways and digestive ducts, results from failed chloride transport. Cholera is a bacterial infection that causes chloride and water to leak from cells lining the intestines, resulting in severe diarrhea and water loss.