What are the Differences between Diffusion, Osmosis and Active Transport?

Living cells need to take up raw materials and nutrients to carry out complex biochemical reactions. At the same time, waste products need to be disposed of in the cell to prevent toxicity to the living cells. These substances move in and out of the cells by different transport processes such as diffusion, osmosis and active transport, which occur through the cell plasma membrane. The semi-permeable cell membrane and the properties of the substances determine whether diffusion, osmosis, or active transport is involved in the movement of different substances across the cell plasma membrane.

To identify the differences between these three transport processes, consider (1) what are the substances involved, (2) how do these substances move, and (3) whether energy is required in and for each of these processes. This 3 minute-read will provide you with the quick revision you need on how to spell out the differences between diffusion, osmosis, and active transport. 

What is Diffusion, Osmosis and Active Transport? 

• Diffusion is the movement of a substance down a concentration gradient. Diffusion can happen with or without passing through a semi-permeable membrane (cell plasma membrane is semi-permeable). 
• Osmosis is the diffusion of water molecules down a concentration gradient through a semi-permeable membrane. 
• Active transport is the movement of a substance against a concentration gradient. 

What are the Substances Involved?

• Examples of substances transported by diffusion across the cell plasma membrane are gases such as oxygen and carbon dioxide, glucose, and urea.
• Osmosis refers specifically to the movement of water molecules across a semi-permeable membrane. 
• Active transport involves substances such as mineral ions (uptake by plant root) and glucose (taken up in the small intestine).

How do the Substances Move?

In diffusion, the substance moves down the concentration gradient, from high to low concentration (from a region of high substance concentration to a region with low substance concentration). 

Water molecules in osmosis also move down the concentration gradient (from a region where water molecules are high in concentration, to a region with low concentration of water molecules) through a semi-permeable membrane. The concentration of water molecules is high in a diluted solution while the water concentration is low in concentrated solutions. In other words, during osmosis, water molecules move from a diluted solution to a high concentration solution. 

Both diffusion and osmosis act to balance the concentration of substances in a solution.

In active transport, the substance moves against the concentration gradient (from a region where the substance concentration is low to a region with a high concentration of the substance).

Do All these Processes Require Energy?

Broadly speaking, transport across plasma membrane can be passive or active in terms of energy requirement. Passive transport does not require energy while active transport needs energy to take place.

Both diffusion and osmosis do not require energy because the substances move down the concentration gradient, from high to low concentration. The net flow of the substances is caused by random movement of the substances owing to the energy level of the substances. 

However, active transport requires energy as the substances involved move against the concentration gradient, from low to high concentration. This energy is derived from ATP (adenosine triphosphate), energy-rich molecules in living cells.