Lysosomes and Peroxisomes Made by: Sara Hassan

Lysosomes Lysosomes are membrane bound organelles that contain digestive enzymes. They are found in eukaryotic cells primarily animal cells and plant cells. The primary function of lysosomes is extracellular and intracellular digestion at acidic pH. Lysosomes were discovered by De Duve a Belgian Cytologist in 1955 who designated them as suicide bags. De Duve discovered lysosomes in a fraction that was intermediate between mitochondria and microsomes.

Anatomy of Lysosomes

Lysosomal Membrane The lysosomal membrane contains highly glycosylated lysosomal associated membrane proteins (LAMP) and lysosomal integral membrane proteins (LIMP) LAMPs and LIMPs coating the inner membrane surface protect the membrane against attack by enzymes retained within lysosomes. An ATP dependant proton pump in the membrane pumps H+ into the lumen, creating an acidic pH necessary for lysosomal enzymes to function.

Morphology of Lysosomes Lysosomes have the remarkable characteristic of pleomorphism i.e size and shape of the particle and irregularities of its inner structure. In a cell ,lysosomes are surrounded by smooth or coated vesicles.

Manufacturing Lysosomes Lysosomes are manufactured by golgi apparatus and budded out into the cytoplasm with enzymes inside them. The lysosomal enzymes are made by ribosomes and sent to E.R. where they are tagged with mannose 6 phosphate for being recognized by receptors of their next destination- the Golgi apparatus. The Golgi Apparatus bud off to form lysosomes.

Types of Lysosomes Primary lysosomes (storage granules) are small saclike structures enclosing enzymes synthesized by Ribosomes and stored in E.R. and transferred to Golgi apparatus. Heterophagosome(Digestive Vacuole) The materials engulfed through phagocytosis are digested by enzymes of primary lysosomes. Residual Bodies are formed in case of incomplete digestion. In amoeba they are removed by defecation otherwise they are accumulated like pigment inclusions in nerve cells. Autophagic Vacuole: A special scenario in normal cells where a part of the cell ( mitochondrion or E.R) is in lysosome for digestion.

Secondary Lysosomes The Secondary lysosomes contain materials engulfed by phagocytosis or pinocytosis fused with primary lysosomes. These materials are subjected to cellular digestion by the primary lysosomes enzymes. Hence, a secondary vacuole can also be designated as a digestive vacuole.

Functions of Lysosomes: Heterophagy. Autophagy. Programmed cell death.  Autolysis. Fertilization.

 Autophagy Autophagy is a physiological process for digestion of cells in the body to maintain homeostasis. It maintains homeostasis by protein degradation and turnover of the destroyed organelles for new cell formation.

Autolysis Autolysis refers to the digestion of parent cells by the lysosomes.  Autolysis occurs during amphibian metamorphosis for instance the autolysis of tadpole tail cells.

  Fertilization During fertilization , the nuclear acrosome of the sperm which is considered a giant lysosome secretes hyaluronidase enzyme on the surface of egg cell. This disperses the cells around the egg. Secondly, protease is also secreted to dissolve zona pelucida making a channel for the sperm to enter the egg.

  Heterophagy Heterophagy is the process of lysosomal digestion of extracellular materials entering the cell by the process of phagocytosis, pinocytosis and receptor mediated endocytosis.

Programmed cell death Lysosomal membrane permeabilization (LMP) induces controlled cell death as it ensures the translocation of lysosomal enzymes into the cytoplasm. Cathepsins B, L and C are proteases implicated in cell death they initiate the cascade leading to PCD.

Lysosomal Storage Diseases LSDs are metabolic disorders where lysosmal enzyme fail to function properly. As a consequence of which the substrate is accumulated. When a lysosomal enzyme is deficient or malfunctioning, the substrate it targets accumulates, interfering with normal cellular activity. These diseases include Tay Sachs disease and Gauchers Disease. Healthy Cell Cell with accumulated Substrate

Peroxisomes Peroxisomes are single membrane bound organelles found in the cytoplasm of eukaryotic cells Peroxisomes originate from Golgi Apparatus. They were discovered by De Duve in 1965 in liver cells. Peroxisomes are the site of synthesis and degradation of Hydrogen Peroxide hence designated as peroxisomes. An enzyme catalase, a type of oxidase, is present in large quantity in peroxisomes. Peroxisomes self replicate through fission.

Anatomy of Peroxisomes Peroxisomes have a lipid bilayer membrane that controls what enters and exits them. Peroxisomes have a Urate oxidase crystalline core with 32 peroxins (peroxisomal proteins), that execute peroxisomal functions inside the organelle.

Functions of Peroxisomes They are involved in many different activities like the degradation of hydrogen peroxide by Catalase. For this purpose peroxisomes need significant amount of Oxygen. Initially many oxidases bind with oxygen and hydrogen to form Hydrogen peroxide. In the next step Hydrogen Peroxide is oxidized by catalase into water and oxygen. Peroxidase detoxify alcohol from liver cells. Peroxisomes execute the ß-oxidation of Long Chain Fatty Acids - a major source of energy.

Peroxisomal Disorders Peroxisomal disorders in humans result due to abnormal function of an enzyme necessary for normal peroxisomal function. This may result in peroxisomal disorders like: Liver dysfunction Retinopathy

References Chapter 10 Lysosomes, Endosomes, Coated vesicles and peroxisomes from cell and molecular biology by De Robertis.