Final answer:
Passive transport is a process by which molecules move across the cell membrane without energy, from higher to lower concentration areas. It includes diffusion, osmosis, and facilitated diffusion. Molecules like gases may diffuse directly, while others like glucose use transport proteins.
Step-by-step explanation:
Passive Transport in Cells
Passive transport is one of the principal methods by which small molecules and ions move across a cell membrane. It describes the movement of these substances without the need for cellular energy (like ATP) from areas of high concentration to areas of low concentration. There are several types of passive transport including diffusion, osmosis, and facilitated diffusion.
In simple diffusion, small, uncharged molecules like oxygen and carbon dioxide travel directly through the lipid bilayer of the cell membrane due to the existence of a concentration gradient. A concentration gradient exists when the concentration of particles is higher in one area than another. Osmosis is a special case of diffusion where water moves through a semipermeable membrane from a region of lower solute concentration to a higher solute concentration.
Facilitated diffusion is necessary for substances that cannot directly diffuse through the lipid bilayer due to size or charge. These molecules, such as glucose and amino acids, need special transport proteins that provide a pathway through the membrane.
No external energy source is required for passive transport, wherein substances move down their concentration gradient, similar to a ball rolling down a hill. Contrastingly, active transport requires energy as substances are pumped against their concentration gradient.