4.9 Lysosomes and peroxisomes
Lysosomes are small spherical organelles, enclosed by a single membrane, which are common in animal cells but rare in plant cells. They measure about 0.5-1.0 µm across, and they contain digestive enzymes. Lysosomes fuse with membrane-bound endosomes (containing nutrients ingested by endocytosis), and the lysosomal enzymes digest large nutrient molecules. Cells can also degrade and recycle the components of their own organelles and structures when they are old or damaged, or if the cell is 'starving' in the absence of nutrients. This process, known as autophagy, usually involves formation of a membrane around the cell component and fusion of the resulting vesicle with lysosomes. Autophagy is thought to have an important role in many processes including cell growth, cell death and infection. Lysosomes contain many different kinds of digestive enzymes, and the inside of the lysosome is acidic, with a pH of around 5. The enzymes of the lysosome are specialised to perform their function only at this low pH; so, should they leak out into the cytosol, which has a pH of around 7.2, they do not do a great deal of damage. The inside of the lysosome is made acidic by the action of specialised transport proteins that lie in the lysosomal membrane and 'pump' hydrogen ions into its lumen. Other lysosomal membrane proteins transport the useful products of digestion out of the lysosome into the cytosol.
What useful materials would be produced by digestion in the lysosome?
The products would depend on the starting material, but could be amino acids, sugars and nucleotides.
Peroxisomes are also small enzyme-containing organelles bound by a single membrane, which are very similar in size to lysosomes, measuring between 0.2 and 1.0 µm in diameter. They are thought to be present in all eukaryotic cells. In mammals, they are particularly plentiful in liver cells and adipocytes but are much less abundant in other cells. Like lysosomes, peroxisomes also have a role in metabolism; they contain enzymes that break down fatty acids and amino acids, resulting in, among other things, the production of the toxic substance, hydrogen peroxide. Peroxisomes therefore also contain high levels of an enzyme known as catalase which breaks down the hydrogen peroxide into harmless products (water and oxygen). Lysosomes and peroxisomes are shown in Figure 21.
For many years, peroxisomes were thought to be identical to lysosomes in their properties. The difference between these organelles was discovered as the result of cell fractionation experiments (Box 1). Although very similar in size, their contents and therefore their densities are different, so under particular centrifugation conditions the two organelles sediment in different fractions. Different enzymes were found to be associated with the two fractions and it is now known that the two organelles are very distinct.
What is the difference between the site of synthesis of lysosomal and peroxisomal proteins?
Lysosomal proteins are synthesised at the RER (Section 4.5); peroxisomal proteins are synthesised by free ribosomes in the cytosol (Section 4.4).
How are the correct proteins delivered from the cytosol to the peroxisomes?
Peroxisomal proteins have a signal sequence to ensure their correct targeting.
Correct targeting of peroxisomal proteins to the peroxisomal membrane or interior occurs because of specific sequences in the proteins, as described previously.
Having studied the organelles and other cellular components involved in the synthesis and delivery of proteins in the cell, in the next section you will consider an organelle that you already know something about, the mitochondrion, which plays a fundamental role in the generation of ATP in eukaryotic cells.