Substances can diffuse directly into single-celled organisms due to the high surface area to volume ratio. However, for multi-cellular organisms, an exchange system is needed. In insects, the tracheal system which contains tracheoles aid gas exchange in ways such as muscles contracting air in and out, ends of the tracheoles filled with water so faster diffusion. In addition, it is highly branched so short diffusion distance to cells.
Ventilation is the process of air going in or out of the lungs. During inspiration, the diaphragm and external intercoastal muscles contract, leading the diaphragm to go flat and the rib cage to go outwards, this increases the volume of the thoracic cavity hence air goes in down a pressure gradient. The converse occurs for expiration.
Enzymes in our body help digest the key biological molecules such as lipase for lipids, peptidases for proteins and carbohydrases for carbohydrates. The action of micelles help carry monogolycerides and fatty acids towwards the epithelial cells and releases them for the absorption of fats across the cell membrane.