Lysosomes Flashcards
What is the structure of lysosomes?
- single membrane bound organelles
- contain ~ 60 different enzymes
- acidic pH of 5
- size ranges from 0.1 micrometers to 1.2 micrometers
How many lysosomes are found in a human cell ?
~300
What is the function of lysosomes ?
Lysosomes act as the digestive system of a cell and digest intracellular proteins and carbohydrates but also molecules and pathogens from outside the cell
Where do lysosomes originate from ?
They bud off of the trans face of the Golgi apparatus
Why are lysosomal membrane proteins glycosilated ?
Because the glycosilation protects the proteins from the proteases present in the lumen
What are the main lysosomal pathways ?
- Endocytosis
- Phagocytosis
- Pinocytosis
- Autophagy
What is heterophagy ?
The digestion of material of extracellular origin
What is autophogy ?
The digestion of material of intracellular origin
How is the internal acidic environment of a lysosome maintained ?
V-ATPase pumps protons from the cytoplasm to the lysosomal lumen which maintains the acidic environment within the lysosome
What is the role of mannose-6-phosphate ?
It targets proteins to lysosomes
It allows lysosomal hydrolases and membrane proteins to be delivered to lysosomes
What are lysosomal storage diseases ?
They are inherited metabolic diseases characterised by abnormal build up of materials in cells as a result of enzyme deficiencies
Give some examples of lysosomal storage diseases
- Hunter & Hurler syndrome
- Tay-Sachs disease
- Gaucher disease
- Inclusion cell disease
Summarise Hunter & Hurler syndrome
It develops as a result of large sugar molecules building up in body tissues
Symptoms include an enlarged head, thickening of the lips and a distended abdomen due to enlarged internal organs
Summarise Tay-Sachs disease
Develops as a result of deficiency of the hexosaminidase enzyme which means lipids accumulate in the brain and other organs
Symptoms include mental challenges, blindness and death within 3 years
Summarise Gaucher disease
Develops due to the accumulation of glycolipids
Symptoms include enlargement of the liver and spleen, anaemia and lung disease
Summarise Inclusion cell disease
Caused by deficiency of N-glucosamine phosphotransferase which is what transfers M6P
Symptoms include psychomotor challenges and death
What is the endosomal-lysosomal system ?
The endosomal-lysosomal system is made up of a set of intracellular membranous compartments which interconvert
It is comprised of early endosomes, recycling endosomes, late endosomes and lysosomes
What are the membrane compartments used for in the endosomal-lysosomal system ?
- for traffic and digestion of endocytosed molecules
- for sorting and recycling cellular components
- for autophagy
What are endosomes ?
Membrane bound vesicles formed by endocytosis and found in every animal cell
Describe the process of endocytosis
1) Endocytosed molecules are delivered in vesicles to small, irregularly shaped intracellular organelles called early endosomes
2) Some of these molecules are recycled to the plasma membrane whilst some pass into late endosomes
3) They are then passed to lysosomes where they are degraded
Describe the process of phagocytosis
1) Phagocytes engulf objects to form a phagosome
2) The phagosome fuses with a lysosome to form a phagolysosome
3) The hydrolysed products enter the cytosol
Describe the process of pinocytosis
Pinocytosis is the intake of liquid particles to form vesicles known as pinosomes
It balances exocytosis because the cell surface area and volume remain constant by recycling cell membrane parts to maintain the size of the cell
Describe the process of autophagy
- the target organelle or protein is wrapped in a double membrane from the ER which produces an auto-phagosome
- the autophagosome fuses with a lysosome to produce a autophagolysosome
- indigestible remnants are retained as granules that accumulate with age in residual bodies
- debris is released from the cell surface via exocytosis
