Lysosomes | Their Structure, Location and Functions

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

  1. Primary lysosome
  2. Heterophagosome (digestive vacuole)
  3. Residual bodies
  4. 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.

Lysosomes

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 enzymesName of enzyme
Hydrolases acting on Ester bondArylesterase
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 glycosylcompoundsLysozyme
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 bondsCarboxy 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 bondsAsprtylglucosaminidase
Amino acid napthylamidase
Benzoyl arginine naphthylamidase
Hydrolases acting on acid anhydridesInorganic pyrophophatase
Hydrolases acting on phosphoru nitrogen bondsPhophoamidase
Hydrolases acting on sulfur-nitrogen bondsHeparin sulfamidase

Their Functions

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.

Involvement in breakdown of tissue

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.

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