Diffusion is an example of passive transport by which gases like carbon dioxide and oxygen and solutes in a solution get transported. Molecules move from where they are plentiful (higher concentration) to where they are less in amount (lower concentration). When a soluble substance is placed in water it gets dissolved or in other words it is dispersed in water. For example, when a few crystals of potassium permanganate (KMnO4) are added to water, they get dissolved and the colouration of water becomes uniform throughout the beaker.
This is due to diffusion of KNn4 in water. You will observe several deep-colored lines originating from the crystals inside the water before diffusion is completed. When you go to a party, you may smell perfume which is due to diffusion from the body or clothes of a person who has used it.
Diffusion may be defined as the process of movement of gases, liquids or solutes from the region of higher concentration to the region of lower concentration till the molecules are uniformly distributed throughout the available space.
The process of diffusion in gases is faster than that in liquids. Gases, liquids or solutes can diffuse simultaneously in different directions and at different rates in the same place without interfering with each other. In unicellular organisms, diffusion occurs throughout the body surface or cell membrane like Chlamydomonas, Paramoecium, Amoeba. In multi-cellular organisms like Hydra, Sponges, transport of oxygen and nutrients take place through diffusion. Diffusion takes place in all directions. Larger surface area of leaf provides great advantage in diffusion.