Lysosomes and Vacuoles

Lysosomes are small, membrane-bound vesicular organelles, first described by the Belgian cytologist Christian de Duve, that are formed by the packaging activity of the Golgi apparatus. What makes a lysosome distinctive is its cargo: it is loaded with a battery of powerful hydrolytic enzymes -- including lipases (which break down lipids), proteases (which break down proteins) and carbohydrases (which break down carbohydrates) -- all of which function optimally in the acidic environment maintained within the lysosome, quite unlike the near-neutral pH of the surrounding cytoplasm. This acidic internal environment is significant, because it means that even if a lysosome were to leak or rupture, releasing its enzymes into the neutral pH of the general cytoplasm, those enzymes would be far less active there, offering the rest of the cell some protection from accidental self-digestion.

Functionally, lysosomes act as the cell's principal site of intracellular digestion. When a cell such as a white blood cell engulfs a bacterium or other foreign particle by phagocytosis, the resulting food vacuole typically fuses with one or more lysosomes, whose enzymes then break the ingested material down into simple, usable molecules. Lysosomes also carry out autophagy, the routine digestion of the cell's own worn-out or damaged organelles, allowing their components to be recycled. It is this capacity to digest cell material -- ordinarily kept carefully under control -- that has earned lysosomes their popular description as the "suicide bags" of the cell: under certain conditions, such as cell damage or during programmed processes like metamorphosis, lysosomal membranes rupture and release their hydrolytic enzymes into the cytoplasm, digesting the cell's own contents in a controlled process of cell destruction.

Vacuoles are a second type of membrane-bound compartment found within the cytoplasm, but they differ from lysosomes both in their general appearance -- typically a single large, fluid-filled sac rather than a small vesicle -- and in the diversity of roles they play across different organisms and cell types. The membrane bounding a vacuole is given a special name, the tonoplast, which, like the plasma membrane, is selectively permeable and can actively transport substances into or out of the vacuole against a concentration gradient.

In a mature plant cell, the vacuole is typically the single largest structure present, sometimes occupying as much as 90 per cent of the total cell volume. It is filled with a solution called cell sap, containing water along with dissolved amino acids, sugars, organic acids, and, in many cases, some proteins as well. By osmotically drawing water into the cell, the vacuole is chiefly responsible for maintaining the internal turgor pressure that keeps a plant's non-woody tissues firm and upright -- a plant that wilts has lost this turgor pressure as its vacuoles lose water. Plant vacuoles frequently also serve as a storage site for pigments, such as the anthocyanins responsible for many red, purple and blue flower and fruit colours, and for various waste products that the cell needs to sequester safely away from its ongoing metabolism. …