Living organisms can be divided into two categories based on their method of acquiring food.
Heterotroph – Cannot synthesize nutritive substances from inorganic compounds so must feed on organic compounds that have originated in other plants or animals.
We, humans, are heterotrophs.
Heterotrophs undergo four main stages in order to acquire energy from food.
1. Ingestion – the process of acquiring food
2. Digestion – the process of mechanically and chemically breaking down food
3. Absorption – the process of transferring nutrients to an organisms internal environment
4. Egestion – the process of excreting waste molecules
Refer to single-celled (like an amoeba) and simple multi-celled (like a hydra) organisms.
Their processes of ingestion, digestion, absorption, and excretion can vary widely.
Engulf food by phagocytosis (the cell invaginates food in the cell membrane). The amoeba then stores food in a vacuole. At this point the food is not considered inside the cell. It is stored in a vacuole, but because it has not been absorbed into the cytoplasm, it is not inside the cell. The amoeba digests the food with digestive enzymes and then absorbs the nutrients.
Note that most chemical digestion occurs by digestive enzymes that are actually proteins. As such, the rate of enzymatic digestion is dependant on temperature and pH.