What is the difference between diffusion, osmosis and active transport?

Diffusion
Diffusion is the process in which particles/ substances spread out in a solution or a gas. The result of this process is that particles move down the concentration gradient from an area of high concentration to an area of low concentration. This process is passive thus requires no energy. Osmosis works on a similar principle whilst active transport is essentially the opposite. An example of a process that uses diffusion is the transfer of oxygen from the lungs into the blood. The diagram below illustrates the diffusion process:



Osmosis 
Osmosis is very similar to diffusion and works on the same principles. Like diffusion the result of this process is that particles, specifically water molecules, move down the concentration gradient from an area of high concentration to an area of low concentration across a partially permeable membrane (i.e. some things can pass through whilst others can’t). The only difference to that of diffusion is that osmosis is specific to the movement of water molecules in a (dilute) solution. An example of a process that uses osmosis is the transfer of water molecules from the ground into the plant. This happens in the root hair cells. The diagram below illustrates the osmosis process:



Active Transport
Active transport is a little different to the other two processes. Unlike diffusion and osmosis, active transport is an active process (as the name suggests) and requires energy. The process involves the movement of substances against the concentration gradient. In this the particles/ substances move from an area of dilute solution to an area of a higher concentration. The energy required to achieve this is produced through respiration. An example of a process that uses active transport is the uptake of glucose in the small intestine. This process happens in the villi, specifically by the epithelial cells. The diagram below illustrates the active transport process: