Lysosomes are the membrane organelles present in both plant cells and animal cells.
They are also present in protozoa but absent in the bacteria. Instead, a periplasmatic space in between cell wall and cell membrane play the role similar to lysosomes.
Lysosomes are present near to the cell membrane or even attached it.
They have 50 hydrolytic enzymes which play a critical role in cell physiology.
Lysosomes structure
These are polymorphic organelles. Based on structure they are of different types like
- Primary lysosome
- Hetero-phagosome (digestive vacuole)
- Residual bodies
- Autophagic vacuole.
Primary lysosomes are dense particles with a size of 0.4μm and are surrounded by a single membrane.
Secondary lysosomes are formed due to the fusion of primary lysosomes.
Residual bodies contain undigested material and remain as pigment inclusions in the cell.
Autophagic vacuole consists of cell parts which are digested.
Chemical composition of lysosome:
Lysosomes enclose enzymes within the membrane. The pH inside the lysosomes is acidic and is about pH-5. The lysosomes enclose nearly 50 enzymes as mentioned below
| Type of enzymes | Name of enzyme |
| Hydrolases acting on Ester bond | Arylesterase Triglycerol lipase Phospholipase A1 and A2 Cholesterol esterase Acid phosphatase Phosphodiesterase-I Deoxyribonucleae II Ribonucleae II Sphingomyelin phosphodiesterase Arylsulfatases A andB Chondroitinsulfatase |
| Hydrolase acting on glycosylcompounds | Lysozyme Neuroaminidase alpha and beta glucosidase alpha and beta galactosidase Hyaluronoglucosdase alpha and beta mannosidase alpha and beta N-acetyl-glucosaminidase alpha and beta N-acetyl galactosaminidase alpha-L-fucosidase L-idouronidase |
| Hydrolases acting on peptide bonds | Carboxy peptidase A,B and C Dipeptidase Dipeptidylpeptidase Kininogen Elastase Neutral proteinase Plasminogen activator Cathepsin B Cathepsin D Renin Cathepsin E Cathepsin G Collagenase |
| Hydrolases acting on carbon-nitrogen bonds | Asprtylglucosaminidase Amino acid napthylamidase Benzoyl arginine naphthylamidase |
| Hydrolases acting on acid anhydrides | Inorganic pyrophophatase |
| Hydrolases acting on phosphorus nitrogen bonds | Phophoamidase |
| Hydrolases acting on sulfur-nitrogen bonds | Heparin sulfamidase |
Their Functions include
1.Digestion of food: Food is taken into the cell by either phagocytosis (solids) or pinocytosis (liquids). Carbohydrates are hydrolyzed to monosaccharides (monomers of carbohydrates), but disaccharides and polysaccharides are not digested and remain inside the lysosome.
2.Digestion of cell parts through autophagy (self-eating): This is a process of cell renovation and turnover of cell components. Mitochondria are periodically removed from the cell by the lysosomal function. The cell organelles are surrounded by smooth endoplasmic reticulum and the lysosomal enzymes are released to digest the organelles.
3. Breaking down extracellular matter: During development, there is a need for shedding and remodeling of tissues. This involves the removal of cells and also extracellular matter.
Example: Human uterus after delivery of baby weighs 2 kilograms. But in just nine days it returns to 50 gm in weight which is its normal size. This happens as the lysosomes present in phagocytic cells (a type of WBC cells) digest extracellular material and other matter of endometrium.
Further, they control bone strength, fertilization, thyroid hormone release, etc. In plant seeds, lysosomal enzymes remove carbohydrates and proteins and help in germination.