Lysosomes- Definition, Structure, Functions and Diagram
Lysosomes are specialized cell organelles that contain hydrolytic enzymes and act as the digestive system of the cell. They break down various substances, such as food particles, bacteria, viruses, worn-out organelles, and cellular debris, into simpler molecules that can be recycled or eliminated by the cell. The word "lysosome" comes from the Greek words "lysis" (breakdown) and "soma" (body), indicating their role in cellular catabolism.
Lysosomes are small, spherical organelles that vary in size and shape. They are surrounded by a single membrane composed of phospholipids and proteins. The membrane acts as a barrier to prevent the leakage of the digestive enzymes inside the lysosome and to protect the rest of the cell from being damaged by them. The membrane also contains transporters and channels that allow the movement of substances in and out of the lysosome.
The interior of the lysosome, called the lumen, is filled with a fluid that contains about 40 different types of hydrolytic enzymes. These enzymes are capable of breaking down various biomolecules, such as proteins, lipids, carbohydrates, nucleic acids, and polysaccharides. The enzymes are synthesized in the rough endoplasmic reticulum (ER) and then transported to the Golgi apparatus, where they are modified and packaged into vesicles. The vesicles then bud off from the Golgi and fuse with other vesicles containing acidic hydronium ions (H3O+) to form primary lysosomes.
Primary lysosomes are inactive until they encounter a substrate to digest. This can happen in two ways: either by fusing with another vesicle that contains material taken up by the cell through endocytosis (phagocytosis or pinocytosis), or by engulfing a damaged or worn-out organelle within the cell through autophagy. When this happens, the primary lysosome becomes a secondary lysosome, also known as a phagolysosome or an autolysosome. The acidic environment inside the secondary lysosome activates the hydrolytic enzymes, which then degrade the substrate into smaller molecules that can be reused or expelled by the cell.
Sometimes, lysosomes can also release their contents outside the cell through exocytosis. This process is called exocytosis and it can help in extracellular digestion, defense against pathogens, or tissue remodeling.
Lysosomes are spherical or oval-shaped organelles that are surrounded by a single membrane. The membrane contains proteins that protect the lysosome from being digested by its own enzymes. The membrane also regulates the transport of materials in and out of the lysosome. The interior of the lysosome is filled with a fluid called the lumen, which contains various hydrolytic enzymes that break down different types of biomolecules. The enzymes are synthesized in the rough endoplasmic reticulum and transported to the lysosome via the Golgi apparatus. The lysosome can fuse with other vesicles or organelles that contain materials to be degraded, such as food vacuoles, phagosomes, autophagosomes, or endosomes. The diagram below shows the structure and function of a lysosome.
The diagram shows a lysosome fusing with a food vacuole that contains a bacterium. The lysosomal enzymes digest the bacterium and release the nutrients into the cytoplasm. The diagram also shows a lysosome fusing with an autophagosome that contains a damaged mitochondrion. The lysosomal enzymes degrade the mitochondrion and recycle its components. The diagram also shows a lysosome releasing its enzymes into the extracellular space by exocytosis. This process can help in digesting extracellular materials or defending against pathogens.
Lysosomes are dynamic organelles that can change their size, shape, number, and location depending on the metabolic needs of the cell. They can also interact with other organelles such as peroxisomes, endoplasmic reticulum, or nucleus. Lysosomes play a vital role in maintaining cellular homeostasis, energy metabolism, signal transduction, and cellular defense.
Lysosomes are filled with enzymes called hydrolases that catalyze the breakdown of various biomolecules by adding water molecules. Hydrolases are also known as acid hydrolases because they function optimally in an acidic environment. The lysosomal membrane contains a proton pump that maintains the acidic pH inside the lysosome by pumping hydrogen ions into the lumen.
There are about 40 different types of hydrolases in lysosomes, which can be classified into the following main categories:
- Proteases, which digest proteins into amino acids. Examples of proteases are cathepsins, peptidases and collagenases.
- Lipases, which digest lipids into fatty acids and glycerol. Examples of lipases are phospholipases, sphingomyelinases and cholesterol esterases.
- Amylases, which digest carbohydrates into simple sugars. Examples of amylases are glucosidases, galactosidases and mannosidases.
- Nucleases, which digest nucleic acids into nucleotides. Examples of nucleases are endonucleases, exonucleases and phosphodiesterases.
- Phosphatases, which remove phosphate groups from various substrates. Examples of phosphatases are acid phosphatase, alkaline phosphatase and pyrophosphatase.
Hydrolases are synthesized in the rough endoplasmic reticulum (RER) and transported to the Golgi apparatus, where they are modified and packaged into vesicles called lysosomal precursor vesicles. These vesicles fuse with late endosomes to form primary lysosomes, which store the enzymes until they are needed.
When a primary lysosome fuses with a phagosome or an autophagosome, it forms a secondary lysosome or a digestive vacuole, where the enzymes degrade the ingested material. The products of digestion can then be recycled or released into the cytosol or extracellular space.
Some hydrolases can also be secreted outside the cell by exocytosis, resulting in extracellular digestion. This process is important for some fungi and bacteria that feed on organic matter in the environment.
Lysosomal enzymes play crucial roles in various cellular processes, such as digestion, recycling, defense, development and disease. Defects in lysosomal enzymes can lead to various genetic disorders called lysosomal storage diseases, such as Tay-Sachs disease, Gaucher disease and Pompe disease. These diseases are characterized by the accumulation of undigested material in lysosomes, which interferes with normal cell function and causes various symptoms.
Lysosomes can be classified into different types based on their origin, function and morphology. Some of the common types of lysosomes are:
Primary lysosomes: These are newly formed lysosomes that contain only hydrolytic enzymes and no other material. They are derived from the Golgi apparatus and have a uniform appearance. They are also called storage granules as they store enzymes until they are needed.
Secondary lysosomes: These are formed by the fusion of primary lysosomes with other vesicles or vacuoles that contain material to be digested. They are also called digestive vacuoles as they carry out the intracellular digestion of various substances. They have a heterogeneous appearance as they contain both enzymes and digested or undigested material.
Residual bodies: These are the remains of secondary lysosomes after the digestion is completed. They contain indigestible material such as lipofuscin, which gives them a brownish color. They may remain in the cell or be expelled by exocytosis.
Autophagic vacuoles: These are formed by the engulfment of cell organelles or parts of cytoplasm by primary lysosomes. They are involved in the process of autophagy, which is the self-degradation of cell components for recycling or disposal.
Heterophagic vacuoles: These are formed by the engulfment of extracellular material such as bacteria, viruses, debris or foreign particles by primary lysosomes. They are involved in the process of heterophagy, which is the digestion of exogenous material for defense or nutrition.
Lysosome-related organelles (LROs): These are specialized structures that share some features with lysosomes but have distinct functions and compositions. They include melanosomes (pigment granules), platelet dense granules (storage vesicles), lamellar bodies (secretory vesicles) and MHC class II compartments (antigen-presenting vesicles).
Lysosomes serve two major functions:
Intracellular Digestion: Lysosomes help to digest food particles, bacteria, viruses, and other foreign substances that enter the cell. They do this by fusing with the membrane of a food vacuole or a phagosome and releasing their enzymes into it. The enzymes break down the macromolecules into smaller units that can be absorbed by the cell. This process is also called heterophagy.
Autolytic Action: Lysosomes also help to recycle the cell`s own components that are damaged, worn out, or no longer needed. They do this by engulfing the organelles or parts of the cytoplasm in a membrane-bound vesicle called an autophagosome. The autophagosome then fuses with a lysosome and the contents are digested by the enzymes. This process is also called autophagy.
Some other processes in which lysosomes play crucial roles are:
Extracellular Digestion: Some lysosomes can secrete their enzymes outside the cell by exocytosis. This allows them to digest extracellular materials such as dead cells, debris, or pathogens. This process is common in some fungi and immune cells.
Autolysis: Sometimes, lysosomes can cause the death of an entire cell by releasing their enzymes into the cytoplasm. This can happen when the cell is injured, infected, or undergoing programmed cell death (apoptosis). This process is also called self-digestion.
Fertilization: The acrosome of a sperm cell is a specialized lysosome that contains enzymes that help to penetrate the egg cell during fertilization. The acrosome releases its enzymes when it contacts the egg`s zona pellucida, a protective layer around the egg. The enzymes digest a hole in the zona pellucida and allow the sperm to enter the egg.
As Janitors of the Cell: Lysosomes help to keep the cell clean and healthy by removing any unwanted or harmful substances that may accumulate in the cell. They also prevent diseases by destroying pathogens and abnormal proteins that may cause infections or disorders.
Lysosomes are involved in various cellular processes that require the breakdown of substances or the recycling of materials. Some of these processes are:
- Heterophagy: The taking into the cell of exogenous material by phagocytosis or pinocytosis and the digestion of the ingested material after fusion of the newly formed vacuole with a lysosome. This process helps the cell to obtain nutrients, eliminate pathogens, and remove debris from the extracellular environment. For example, white blood cells use heterophagy to destroy bacteria and viruses that enter the body.
- Autophagy: A normal physiological process that deals with the destruction of cells in the body. It is essential for maintaining homeostasis, for normal functioning by protein degradation, turnover of destroyed cell organelles for new cell formation. Autophagy is triggered by various signals, such as stress, starvation, infection, or aging. It involves the formation of double-membrane vesicles called autophagosomes that engulf damaged or unwanted cellular components and fuse with lysosomes for degradation. Autophagy plays a role in cellular quality control, energy metabolism, immunity, and development.
- Extracellular Digestion: Primary lysosomes secrete hydrolases outside by exocytosis resulting in degradation of extracellular materials. This process is used by some organisms to digest their food externally or to degrade extracellular matrix components during tissue remodeling. For example, saprophytic fungi secrete lysosomal enzymes to break down organic matter in their environment and absorb the nutrients.
- Autolysis: It refers to the killing of an entire set of cells by the breakdown of the lysosomal membrane. It occurs during amphibian and insect metamorphosis, where certain tissues or organs are no longer needed and are digested by their own lysosomes. Autolysis also occurs in some pathological conditions, such as ischemia, necrosis, or apoptosis, where cells die due to lack of oxygen, injury, or programmed cell death.
- Fertilization: The acrosome of the sperm head is a giant lysosome that ruptures and releases enzymes on the surface of the egg. This provides the way for sperm entry into the egg by digesting the egg membrane. The acrosomal reaction is essential for successful fertilization and subsequent embryonic development.
- As Janitors of the Cell: Lysosomes remove ‘junk’ that may accumulate in the cell helping to prevent diseases. They degrade excess or defective proteins, lipids, carbohydrates, nucleic acids, and organelles that may impair cellular function or cause toxicity. They also participate in the clearance of senescent or apoptotic cells by a process called efferocytosis. Lysosomal dysfunction can lead to various disorders, such as lysosomal storage diseases, neurodegeneration, cancer, and inflammation.
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