Lysosomes are membrane-bound cell vesicles found in the cytoplasm of all animal cells. They were discovered and named by Christian de Duve in 1955. de Duve used lysosomes to describe an organelle containing hydrolytic enzymes capable of breaking down biological polymers such as nucleic acid, proteins, carbohydrates, and lipids.
When activated, they function as the cell’s digestive system that destroys invading viruses and bacteria, repairs worn-out cells, and in programmed cell death or apoptosis. If lysosomes are absent in a cell, the cells tend to gather these substances and become poisonous. They are hence called the ‘trash can’ or the ‘recycle bin’ of the cell and are essential to maintain the health of a cell.
Do Plant Cells Have Lysosomes
Although predominantly found in animal cells, recent discoveries show that the function of vacuoles in plant cells is similar to the lysosomes in animal cells.
How Many Lysosomes are in a Cell
There are 50 to 1,000 lysosomes in a typical mammalian cell. However, plants and fungi have a single large or multilobed vacuole.
They are dense, spherical vacuoles bounded by a single membrane. The area within the membrane (the lumen) houses all the hydrolytic enzymes. The membranous sac helps to keep the enzymes separate from the rest of the cell, protecting the cell from destruction. The lysosome size typically ranges between 0.1 – 0.6 μm, with the largest being 1.2 μm.
The lysosome membrane comprises a lipid bilayer similar to a cell membrane containing hydrophilic phosphate group heads, a glycerol molecule, and hydrophobic fatty acid tails. The pH of the lumen lies between 4.5 and 5.0 and thus is acidic. It is similar to the pH of the acids found in the stomach.
Why are Lysosomes Called Suicidal Bags
As we know, lysosomes mainly work to remove unwanted substances from within the cell by processing unwanted materials and degrading them. However, sometimes the digestive enzymes end up damaging the lysosomes. It can be fatal and can cause the cell to die. The process is performed through autolysis (‘auto’ means ‘self’, and ‘lysis’ means ‘the disintegration of the cell by destroying its cell membrane’).
How are Lysosome Formed
Lysosomes are formed by the union of transport vesicles that bud off from the trans-Golgi apparatus as sacs or vesicles with endosomes, which contain molecules taken up by endocytosis at the plasma membrane. The hydrolytic enzymes within the lysosome are formed in the endoplasmic reticulum.
Lysosomes are responsible for three main functions:
1. Digestion of Macromolecules
One of the significant purposes of the lysosome is the digestion of foreign molecules taken up inside the cell by endocytosis. Lysosomes contain about 50 hydrolyzing enzymes that digest macromolecules (nucleic acid, proteins, carbohydrates, and lipids) by hydrolysis.
Nucleic acids are broken down into nucleotides by nucleases, proteins into amino acids by peptidases, carbohydrates into simple sugars by amylases, and lipids into fatty acids and glycerol by lipases. These processes release energy that the cell requires for performing various cellular activities.
These recycled monomers are also used to synthesize macromolecules when needed by the cell or are released outside the cell as waste products.
2. Intracellular Transport by Endocytosis
As a part of the endomembrane system, the lysosome, Golgi apparatus, and endoplasmic reticulum exchange and digest materials within and outside the cell. The lysosome also maintains the cell’s internal milieu. The steps are:
- Materials outside the cell are taken up in clathrin-coated endocytic vesicles that bud from the plasma membrane and fuse with early endosomes.
- The cell membrane recycles membrane components, and the early endosomes gradually mature into late endosomes, which act as lysosome precursors. During endosomal maturation, the internal pH reaches almost 5.5, which helps deliver lysosomal acid hydrolases from the trans-Golgi network.
- Acid hydrolases move towards lysosomes by mannose-6-phosphate residues recognized by mannose-6-phosphate receptors in the trans-Golgi network and packaged into clathrin-coated vesicles.
- After removing the clathrin coat, the vesicle fuses with the late endosomes. The acidic pH causes the hydrolases to dissociate from the mannose-6-phosphate receptor and release into the lumen of the endosome.
- Late endosomes then mature into lysosomes acquiring the acid hydrolases, which digest the foreign molecules initially taken up by endocytosis.
3. Phagocytosis and Autophagy
Apart from degrading the foreign molecules by endocytosis, digesting materials through phagocytosis and autophagy helps to protect the cell from large particle invasions such as pathogens and remove aged cells and cell debris.
In phagocytosis, specialized antigen-presenting cells like macrophages take up and degrade large molecules that need immediate removal from the body. Such molecules are taken up in phagocytic vacuoles known as phagosomes which then fuse with lysosomes, resulting in the digestion of the particle. The lysosome helping in such digestion is called phagolysosome.
Lysosomes are also involved in autophagy, a process by which cells destroy old, abnormal, or damaged proteins and other substances in their cytoplasm that helps recycle cellular components in an organism.
Besides the above functions, lysosomes are also involved in various cellular processes such as secretion, plasma membrane repair, apoptosis, cell signaling, and energy metabolism.
4. Platform for Intracellular Signal Transduction
Lysosomes can sense nutrients and thus regulate cell proliferation and growth with the help of signaling cascades like PI3K/AKT by activating receptor tyrosine kinases (RTKs) and epidermal growth factor receptors (EGFRs), among others.
Mutations in the genes encoding the hydrolytic enzymes cause more than 30 human genetic diseases, called lysosomal storage diseases (LSD). Such disorders are caused due to accumulation of undegraded material within the lysosome.
Most of these diseases occur due to deficiencies in single lysosomal enzymes. For example, Gaucher’s disease, the most common LSD, results from a gene mutation that encodes for an enzyme required to break down glycolipids.
Ans. A lysosome is responsible for the breakdown of the waste products of cells. In contrast, vacuoles are storage organs that help maintain the cell’s osmotic or turgor pressure.
Ans. Since prokaryotic organisms like bacteria do not have membrane-bound organelles, they do not have lysosomes.
Ans. Both lysosomes and Golgi bodies break down food and release energy.
Ans. Lysosomes fuse with food vacuoles inside a cell to help digest food molecules.
Ans. If lysosomes rupture in a cell, the cell dies due to the release of hydrolytic enzymes, which destroys the cellular components.
Ans. Phagocytic white blood cell is best for studying lysosomes.
Article was last reviewed on Thursday, May 18, 2023