Lysosomes are formed from vesicles which bud off which of the following organelles?

Structure

Lysosomes are acidic membrane-bound organelles found within cells, usually around 1 micrometre in length. Lysosomes contain numerous hydrolytic enzymes which catalyse hydrolysis reactions.

Índice Show

Clinical Relevance
I-cell disease
Lysosomal Storage Disease
Where do lysosomes bud off from?
Which of the following organelles help in forming lysosomes?
Do lysosomes bud from the ER?
Which organelle do vesicles come from?

The membrane surrounding the lysosome is vital to ensure these enzymes do not leak out into the cytoplasm and damage the cell from within. In order to maintain the acidic pH of the lysosome, protons are actively transported into the organelle across the lysosomal membrane.

Synthesis

The lysosome and the enzymes within it are synthesised separately. Lysosomal proteins are formed in the same way as any other protein. The first step is the initiatiation of mRNA strand production from relevant DNA segments. The mRNA strands proceed to the rough endoplasmic reticulum, where ribosomes construct the hydrolytic enzymes.

Importantly, these are tagged with mannose-6-phosphate within the Golgi apparatus to target them to the lysosome. As a result, vesicles containing these enzymes bud off from the Golgi apparatus. Two enzymes are responsible for the attachment of the mannose-6-phosphate tag: N-acetylglucosamine phosphotransferase and N-acetylglucosamine phosphoglycosidase.

This vesicle, now in the cytoplasm, then binds with a late endosome which is another acidic, membrane-bound organelle. The late endosome has proton pumps within its membrane that keep its internal environment acidic. The low pH causes dissociation of the protein from the mannose-6-phosphate receptor. This receptor can then be recycled back to the Golgi apparatus.

The phosphate group is also removed from the mannose-6-phosphate tag, to prevent the whole protein returning to the Golgi apparatus. The late endosome can eventually mature into a lysosome, after it has received the enzymes from the Golgi apparatus.

By Mariana Ruiz LadyofHats [Public domain], via Wikimedia Commons

Figure 1 – Diagram of the endomembrane system

Function

The hydrolytic enzymes contained within the lysosome allow foreign particles to be destroyed. Lysosomes play an important role in phagocytosis. When macrophages phagocytose foreign particles, they contain them within a phagosome. The phagosome will then bind with a lysosome to form a phagolysosome.

These enzymes are critical in oxygen-independent killing mechanisms. Lysosomes also help to defend against pathogen entry via endocytosis by degrading pathogens before they reach the cytoplasm.

GrahamColm via Creative Commons

Figure 2 – The role of the lysosome in phagocytosis

Clinical Relevance

I-cell disease

This is caused by genetic defects in the N-acetylglucosamine phosphotransferase enzyme. This enzyme is vital for the addition of mannose-6-phosphate to lysosome-targeted proteins. This results in lysosomal enzymes not being properly targeted. As a result significant amounts are found in both the urine and the blood stream.

Lysosomal Storage Disease

These are a group of genetic conditions affecting lysosomes. The condition vary widely in signs, symptoms and patient demographics. There are several classifications; the most common of these conditions is Gaucher’s disease. It is caused by a deficiency of the beta-glucocerebrosidase. This enzyme is required to break down glucocerebroside. Without this enzyme, the glucocerebroside is able to accumulate within cells, which can damaging them. Symptoms include hepatosplenomegaly and anaemia.

By Nephron (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Fig 3 – Micrograph of necrotic bone tissue in Gaucher Disease

Where do lysosomes bud off from?

In particular, lysosomes are formed by the fusion of transport vesicles budded from the trans Golgi network with endosomes, which contain molecules taken up by endocytosis at the plasma membrane.

Which of the following organelles help in forming lysosomes?

In the formation of lysosomes, the endoplasmic reticulum, and Golgi body are involved.

Do lysosomes bud from the ER?

The endomembrane system separates the cell into different compartments, or organelles, such as the nucleus, the endoplasmic reticulum (ER), the Golgi apparatus, and lysosomes (see Table 2.2). The endomembrane system is derived from the ER and flows to the Golgi apparatus, from which lysosomes bud.

Which organelle do vesicles come from?

Many vesicles are made in the Golgi apparatus and the endoplasmic reticulum, or are made from parts of the cell membrane by endocytosis. Vesicles can also fuse with the cell membrane and release their contents to the outside.