Enzymes can function both inside cells (intracellular) or
outside cells (extracellular).
Extracellular enzyme activity
For example, the enzymes that function in our digestive systems
are manufactured in cells - but work extracellularly. Spiders and
flies are two examples of animals that have taken extracellular
digestion a step further. They secrete an enzyme soup into or on
their food. In spiders, this is injected into the prey's body. The
enzyme soup digests the prey's body contents (specific enzymes
breaking down proteins to amino acids, lipids into fatty acids and
glycerol and polysaccharides into monosaccharides) and the spider
simply sucks up the resulting already digested food. Saprophytic
fungi also secrete enzymes through their hyphal tips in order to
digest their food.
Intracellular enzyme activity
Enzymes that act inside cells are responsible for catalysing the
millions of reactions that occur in metabolic pathways such as
glycolysis in the mitochondria and in the
photosynthetic pathway in the chloroplast. The
lysosome contains many enzymes that are mainly responsible for
destroying old cells.