Lysosomes and endosomes. Peroxisomes

Lysosomes are the organelles of intracellular digestion, 0.05 to 0.5 μm, spherical membrane-limited vesicles containing more than 40 different hydrolytic enzymes. Their matrix is at pH 5, which is what those enzymes need. They are synthesised in the rER and modified and packaged in the Golgi.

A primary lysosome holds inactive enzymes; it fuses with a phagosome to become a secondary lysosome, which is larger and includes the substance to be degraded. Autophagy is self-eating: the controlled digestion of damaged organelles.

Endosomes are intracellular sorting organelles important in endocytosis. Absorbed particles are either passed on to lysosomes or returned to the plasma membrane as recycling endosomes.

Peroxisomes are the sites for oxygen utilisation. They carry oxidase, which produces hydrogen peroxide, and catalase, which destroys it.

Lysosomes are organelles of intracellular digestion, measuring 0.05 to 0.5 μm.

• They are spherical membrane-limited vesicles that contain more than 40 different hydrolytic enzymes.
• They are abundant in cells with great phagocytic activity, for example macrophages.
• The lysosomal matrix has a pH of 5, which is favourable for enzymatic activity.
• Lysosomes are synthesised in the rER and get modified and packaged in vesicles in the Golgi apparatus to form lysosomes.

The acid pH is a safety mechanism as well as a working condition: enzymes that need pH 5 do very little damage if they leak into a cytoplasm held near neutral.

Primary lysosomes contain inactive enzymes. They fuse with a phagosome to become a secondary lysosome.

Secondary lysosomes are larger and include the substance to be degraded. That material arrived by endocytosis.

Autophagy is self-eating.

• It is the process of controlled digestion of damaged organelles, in which the cell uses lysosomes to dispose of excess or non-functioning organelles.
• Autophagosomes are produced, which fuse with lysosomes for digestion of the enclosed material.
• Amino acids, nucleotides and so on are released for reuse, as sources of energy or in new synthetic reactions.

Yoshinori Ohsumi received the Nobel Prize for discovering autophagy, in relation to intermittent fasting.

Endosomes are intracellular sorting organelles that are important in endocytosis.

The process:

• A particle strikes the membrane from the extracellular side.
• The membrane inverts the particle inside, until it is completely enclosed.
• This results in a vesicle that fuses with the endosome.
• Finally the absorbed particles are either transported further to lysosomes, or returned as recycling endosomes back to the plasma membrane.

The word sorting is the important one. An endosome is a decision point: keep and digest, or send back.

Peroxisomes are the sites for oxygen utilisation, measuring 0.5 to 3 μm.

They are spherical organelles enclosed by a single layer membrane.

They contain two enzymes: oxidase and catalase.

This is the part worth understanding rather than memorising, because the two enzymes exist to undo each other.

A peroxisome is a compartment for oxidation reactions. Oxidases oxidise substrates by removing hydrogen atoms, in a reaction that produces hydrogen peroxide, H₂O₂, which is toxic for the cell.

That is why peroxisomes contain catalase, which decomposes H₂O₂ to H₂O and O₂, neutralising the toxicity.

So the organelle carries out a reaction whose by-product would poison the cell, and carries the antidote in the same compartment. The membrane keeps both inside.

• β-oxidation of long chain fatty acids.
• Involvement in lipid biosynthesis, the plasmalogens.
• An important role in cellular respiration.
• In plants, peroxisomes also carry out the recycling of carbon from phosphoglycolate during photorespiration.