Both active and passive transport are the movement of molecules across the cell membrane, or concentration gradient, but there is a key distinction between active and passive transport. Active transport is the movement of molecules against the gradient, while passive transport is the molecular movement with the gradient. Two differences exist between active vs passive transport: energy usage and concentration gradient differences.
Use of Energy
The main difference between active and passive transport is the use of energy during cell transport of materials. Active transport uses energy and passive transport does not. In active transport, molecules are moving against a concentration gradient (or membrane), meaning the cell moves materials from an area of low concentration to an area of high concentration. The cell uses ATP (adenosine triphosphate) as its energy source to move substances in and out of the cell membranes. Passive transport, on the other hand, is the movement of molecules from higher to lower concentration. Because material is moving with the gradient, energy is not required.
Active and passive transport also have a difference in the concentration gradient. The substances that gather on either side of the cell membrane are different. The cell’s contents have a higher concentration gradient than the outside of the cell. For example, should the cell desire to bring more substances toward itself, then it needs energy to do this. Therefore, active transport accomplishes its task by going against this gradient by using some of the cell’s energy.
Role of Diffusion
Diffusion is a type of passive transport in which molecules move from an area of high concentration to low concentration. Diffusion occurs along the concentration gradient, or the gradual difference in the concentration of substances between two areas. Facilitated diffusion is how molecules move down a concentration gradient with the help of proteins. When certain molecules can’t get past the membrane, the special proteins undergo a change to allow the molecule to pass through.
Osmosis is the other type of passive transport where water is diffused through a membrane. Water always moves along the osmotic gradient, or the difference in the concentration of particles on either side of the membrane. If there is an equal amount of particles on both sides of the membrane, then the cell is isotonic and water will not move by osmosis. However, if the particle concentration is higher inside the cell, then the cell is hypertonic. If the cell has a lower particle concentration than the outside, then the cell is hypotonic.