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Structure of Lysosome and Its Function

    In this structure of lysosome and its function post we have briefly explained about lysosome in animal cell, definition, components, functions and importance.

    Structure of Lysosome and Its Function

    The lysosome in animal cell are important products of the secretary pathway in cells. Lysosome in animal cell are also known as “suicidal bags”. They are rounded, elliptical or highly irregular in shape. They are single membrane bounded bodies having a multiple hydrolytic enzymes capable of digesting all kinds of materials inside or outside the cell.

    Definition

    A lysosome in animal cell is a membrane-bound cell organelle that contains digestive enzymes. Lysosome in animal cell are involved with various cell processes. They break down excess or worn-out cell parts.

    They may be used to destroy invading viruses and bacteria. If the cell is damaged beyond repair, lysosomes can help it to self-destruct in a process called programmed cell death, or apoptosis.

    Centrioles are also another cytoplasmic bodies found in most animal cells. These are located at one pole of the cell just outside the nuclear envelop. Higher plant cells lack centrioles, and the spindle is formed without their aid though lower plants do have centriole.

    They are usually hollow cylinders 3000 to 5000Å long and 1200 to 1500Å in diameter composed of nine sets of hollow triple microtubules arranged in a circle and embedded in a dense granule or amorphous, electron dense matrix.

    These may appear to be a granular disc, called satellites, around the centriole. Each triplet formed of three microtubules run oblique towards the centre. These nine triplets are considered to form the wall of the cylinder since centriole has no outer membrane.

    History

    Lysosome in animal cell is an organelle which unlike other organelles, first became known through the biochemical studies and thereafter their morphological identifications were made. Christian de Duve, a Belgian cytologist and biochemist, in 1955 reported the presence of lysosomes in the cells by biochemical studies. Later on, Novikoff in 1956 observed these lysosomes as distinct cell organelles with the help of electron microscope.

    Lysosome in Animal Cell

    Lysosome in animal cell are round tiny bags filled with dense material rich in acid phosphatase (tissue dissolving enzymes) and other hydrolytic enzymes. Lysosome in animal cell consist of two parts: (i) limiting membrane and (ii) inner dense mass.

    Lysosomes

    Structure of Lysosome and Its Function; Lysosome Formation

    Limiting membrane

    This membrane is single and is composed of lipoprotein. Chemical structure is homologous with unit membrane of plasmalemma, consisting of bimolecular layer.

    Inner dense mass

    This enclosed mass may be solid or of very dense contents. Some lysosomes have a very dense outer zone and a less dense inner zone.

    Some others have cavities or vacuoles within the inner granular material. Lysosome in animal cell are of various types and they help in intracellular digestion. Lysosome in animal cell contents vary with the stage of digestion.

    Types of Lysosomes

    There are four types of lysosome in animal cell: primary, secondary, residual bodies and cytolysosome or autophagosome.

    Primary Lysosome

    It is a small sac like body. Its enzymatic contents are synthesized by ribosomes and accumulated in ER. From there, they enter the Golgi region, where acid phosphatase reaction takes place.

    The GERL region, i.e., acid phosphatase rich region of Golgi maturing face is thought to be involved in the production of lysosomes. The primary lysosome comprises only one type of enzyme or another.

    Secondary Lysosome

    These are produced either from phagocytosis or pinocytosis of foreign material by the cell. Actually within the cell, after phagocytosis or pinocytosis, the foreign bodies or extra-cellular substances are enclosed within the membrane and these membranes bound structures are known as phagosome or pinosomes.

    These ultimately fuse with primary lysosomes, thus forming secondary lysosome. This body having engulfed material within membrane has also full complements of acid hydrolases (hydrolytic enzymes).

    The digested material of these lysosomes passes through the lysosomal membrane and is incorporated into the cell so that they may be reused in metabolic pathways.

    Residual bodies

    These are formed in case the digestion is incomplete. In some cells, such as Amoeba and other protozoa, these residual bodies are eliminated by defecation.

    Hence, lysosomes having undigested material or debris are called residual bodies. These bodies are formed due to lack of certain enzymes in lysosomes.

    These are rejected from the cell by exocytosis and some time in certain cells these bodies remain in cells for long time causing ageing.

    These residual bodies also cause diseases in man such as fever, hepatitis, polynephritis, hypertension, congested heart failure etc. If the debris which is mostly lipid in nature may accumulate and condense into concentric lamella, it forms myelin figure.

    Autophagic vacuole

    In this case, the lysosome digests a part of cell (e.g., mitochondria or portion of ER) by the process of autophagy.

    For example, liver cell shows numerous autophagosome during starvation among which remnants of mitochondria occur. This is a mechanism by which the cell can achieve degradation of its own constituents without irreparable damage.

    Lysosomes

    Formation of lysosomes and intracellular digestion in them

    Chemical Nature

    Chemically lysosomes are defined as a body rich in acid hydrolases. Acid phosphatase has been found in many cells of plant roots, fungi, liver, kidney and endocrine glands.

    The lysosomal enzymes can break down all major biological macromolecules present in the cells or entering the cells from outside into their building block subunits by adding water.

    The common enzymes in the lysosomes are proteases, nucleases (deoxyribonuclease and ribonuclease), glycosidase, lipases, sulphatases and phosphatase, which hydrolyses proteins, nucleic acids, polysaccharides, lipids, organic sulphatases and organic phosphates respectively.

    Functions

    Useful materials

    Intracellular digestion is a regular feature in protozoans and in lower invertebrates such as sponges and coelenterates. In this process the organic substances (food particles) taken up by the cells in vacuoles (pinosomes or phagosome) from the environment are digested.

    Harmful materials

    The foreign particles, such as viruses, bacteria and toxic molecules, are disposed of by hydrolyzing them in certain leucocytes and macrophages. This is called natural defenses of the body. This activity of lysosomes is characteristic of higher animals.

    Unwanted materials

    The dead cells and debris that accumulate at the sites of injury are destroyed in some white blood cells. This is called natural scavenging of the body.

    Renewal of cells

    The old worn out cells and cell organelles are broken down to make the component molecules available for formation of new cells and cell organelles. Thus, the lysosomes facilitate the turn-over of cells in normal tissues and of organelles in normal cells.

    Starving animals

    Food to a starving animal is provided by digesting the stored food materials (proteins, lipids and glycogen) and even the cells. This is called autophagy.

    Autolysis

    Autolysis caused by the lysosomal enzymes plays a role in normal developmental changes in both animals and plants. E.g., in the breakdown and absorption of tail during the metamorphosis of frog’s tadpole. In autolysis, lysosome membrane ruptures and releases the enzymes into the surrounding cytoplasm. This kills and lyses the cell.

    Aid in fertilization

    The lysosome in animal cell of sperms releases their enzymes to dissolve the egg membranes for the entry of the sperm into the ovum in fertilization. This is called extracellular digestion.

    Importance

    1. As lysosome in animal cell store the hydrolyzing enzymes of the cell, they digest the incoming food materials and remove the foreign bodies and their organelles no longer required.

    2. Their membrane prevents the enzymes from escaping into the cytoplasm and destroying it.

    3. Malfunctioning of lysosome in animal cell may lead to diseases. Abnormal rupturing of lysosomal membrane and release of enzymes may cause blood cancer, sunburn and genetic disorders.

    4. The degenerative changes in bones and joints associated with arthritis are suspected to be the result of abnormal release of enzymes from the lysosomes of the bone cells or lymph cells into the extracellular fluid.

    Further Readings

    Reference

    1. https://www.genome.gov/genetics-glossary/Lysosome
    2. https://www.britannica.com/science/lysosome
    3. https://www.ncbi.nlm.nih.gov/books/NBK9953/
    4. https://www.nature.com/articles/s41573-019-0036-1