22. Lysosomes and Endosomes

Question 1

General points of lysosomes

Answer 1

They are particularly abundant in cells with great phagocytic activity (e.g. macrophages).

• Lysosomes contain hydrolytic enzymes – optimally active at an acidic environment at a PH of about 4.5
• hydrolytic enzymes capable of breaking down virtually all kinds of biomolecules, including proteins, nucleic acids, carbohydrates, lipids, and cellular debris
• These enzymes are synthesised in the RER
• Membrane receptors for M6P-proteins ensure correct packaging in GA in small vesicles (endosomes) creating lysosomes
• Synthesis of lysosomes is controlled by nuclear genes.
• Thus lysosomes act as the waste disposal system of the cell by digesting unwanted materials (worn-out organelles, food particles, and engulfed viruses or bacteria ) in the cytoplasm
• Once a lysosome has broken down waste material  fuses with autophagic (self eating) vacuoles= dispense their enzymes into the autophagic vacuoles - digesting their contents. destroy itself
• The size of lysosomes varies greatly based on their function.

Question 2

Lysosome definition?

Answer 2

Lysosomes are responsible for breaking down waste material and cellular debris within the cell. A lysosome is a membrane-bound cell organelle found in most animal cells (they are absent in red blood cells).

Question 3

Why do they have a role in homeostasis?

Answer 3

Lysosomes are also responsible for cellular homeostasis for their involvement:

• secretion
• plasma membrane repair
• cell signalling
• energy metabolism

Question 4

Endosomes?

Answer 4

Endosome is a membrane-bound compartment inside eukaryotic cells. Endosomes also contain hydrolytic enzymes but the difference is, it is formed due to invagination of the cell membrane

It is a compartment of the endocytic membrane transport pathway originating from the plasma membrane.

Molecules/ligands internalized from the plasma membrane can follow this pathway all the way to lysosomes for degradation, or they can be recycled back to the plasma membrane.

Molecules are also transported to endosomes from the trans-Golgi network and either continue to lysosomes or recycle back to the Golgi.

Endosomes can be classified as early, sorting, or late depending on their stage post internalization.

Endosomes represent a major sorting compartment of the endomembrane system in cells.

Endosomes provide an environment for material to be sorted before it reaches the degradative lysosome. For example:

• LDL is taken into the cell by binding to the LDL receptor at the cell surface.
• Upon reaching early endosomes, the LDL dissociates from the receptor, and the receptor can be recycled to the cell surface.
• The LDL remains in the endosome and is delivered to lysosomes for processing.
• LDL dissociates because of the slightly acidified environment of the early endosome, generated by a vacuolar membrane proton pump V-ATPase.