Lysosomes

Lysosomes are membrane-limited cytoplasmic organelles which contain enzymes. They are found in a wide variety of cells. They function in the digestion of substances brought into the cell from its environment, and in the degradation of cellular constituents.

Lysosomes are formed when part of the lipid bilayer containing enzymes buds off from a Golgi body. The membrane region demarcating the lysosome has active proton pumps traversing it; this enables the internal environment to be kept optimally acidic for enzyme activity.

Lysosomal enzymes include:

• peptidases
• nucleotidases
• polysaccharidases
• phosphatases
• sulphatases

The primary lysosome becomes a secondary lysosome on fusion with an object, usually membrane-bound, that has to be digested. Examples include:

• autophagocytosis: cell structure to be degraded becomes enclosed within endoplasmic reticulum membrane. The lysosome then fuses with this new vesicle.
• phagocytosis: phagosome is membrane-bound and derived from the environment; lysosome fuses with it directly

Examples of lysosomes include the specific and azurophilic granules of neutrophils.

Defects in lysosomes can present as the lipid storage diseases.