Lysosomes; Structure, Types and Functions

Lysosomes (also referred to as “suicidal bag” of cell) are membrane-bound, rounded, elliptical, or irregular bodies containing multiple hydrolytic enzymes, enabling them to digest various materials both inside and outside the cell.

The presence of enzymes allows lysosomes to break down intracellular substances, which is why they’re known as lysosomes. Specialized vesicles within cells, they break down large molecules with hydrolytic enzymes. Vesicles, small fluid-filled spheres surrounded by a lipid bilayer, play a crucial role in cellular transport.

They are exclusive to animal cells, with around 300 per human cell, but also found in certain lower plant groups, like euglenoids, and in some saprophytic fungi, where they’re referred to as acidic vacuoles.

Besides digesting large molecules, they help break down and dispose of cellular waste, containing over 60 enzymes to support these functions.

They contain lytic, or destructive, enzymes. When a cell becomes damaged or infected and can no longer function properly, lysosomes within it may rupture and digest the cell itself. This self-digestion helps prevent the spread of infection or damage to surrounding cells, which is why They are often referred to as the “suicide bags” of the cell.

They have the ability to digest a variety of molecules. For instance, they can break down food particles that enter the cell when an endocytic vesicle, which brings particles into the cell, fuses with them. They also play a role in autophagy, a process where malfunctioning organelles are degraded.

Additionally, they contribute to phagocytosis, or “cell eating,” where a cell engulfs and digests a molecule for breakdown.

In 1955, Belgian cytologist and biochemist Christian de Duve discovered it in cells through biochemical analysis. Subsequently, in 1956, Novikoff observed them as distinct cell organelles using an electron microscope.

Structure of Lysosomes

• Lysosomes are typically very small, measuring between 0.1 and 0.5 µm in diameter, but can occasionally grow up to 1.2 µm. They are spherical bodies or vacuoles encased in a single membrane made of a phospholipid bilayer, allowing them to fuse with other membrane-bound organelles.

• This membrane acts as a shield for the cell, as lysosomes contain potent digestive enzymes that could damage cell contents if released.

• They form when vesicles budding from the trans-Golgi apparatus fuse, while their hydrolytic enzymes are produced in the endoplasmic reticulum.

• Lysosomes have two main components: Limiting membrane and Inner dense mass

• Limiting membrane: This single membrane is made of lipoproteins, similar to the unit membrane of the plasmalemma, with a bimolecular layer structure.

• Inner dense mass: This inner region may be either solid or densely packed. Some of them have a denser outer zone and a less dense inner zone, while others contain cavities or vacuoles within their granular material. they vary in type and support intracellular digestion, with their contents changing according to the digestion stage.

• Lysosomal enzymes can break down all major cellular macromolecules, whether they are within the cell or come from external sources, into their basic subunits through hydrolysis.

• Common lysosomal enzymes include proteases, nucleases (such as deoxyribonuclease and ribonuclease), glycosidases, lipases, sulphatases, and phosphatases, which respectively hydrolyze proteins, nucleic acids, polysaccharides, lipids, organic sulphates, and organic phosphates.

Enzymes Present in Lysosomes

More than 60 different types of enzymes have been identified to be present in lysosomes. Based on the types of substrate which they act upon, enzymes present in it can be broadly divided into six main categories.

Types of Lysosomes

There are four types of lysosomes: primary lysosome, secondary lysosome, residual bodies and autophagosomes.

Primary Lysosome (Storage Granules)

• This is a small, sac-like structure whose enzymatic content is synthesized by ribosomes and stored in the endoplasmic reticulum (ER).
• From the ER, these enzymes are transported to the Golgi apparatus, where they undergo an acid phosphatase reaction.
• The GERL region (a region rich in acid phosphatase on the maturing face of the Golgi) is believed to play a role in producing lysosomes.
• Primary lysosomes generally contain only one specific type of enzyme.

Secondary Lysosome (Heterophagosome)

• These lysosomes are formed when the cell engulfs foreign materials by the process of phagocytosis or pinocytosis.
• After phagocytosis or pinocytosis, foreign or extracellular substances are enclosed in membrane-bound structures called phagosomes or pinosomes.
• These structures then fuse with primary lysosomes, creating secondary lysosomes.
• These secondary lysosomes contain engulfed material along with a complete set of acid hydrolases (digestive enzymes).
• The digested materials then pass through the lysosomal membrane and are incorporated into the cell for reuse in various metabolic pathways.

Residual Bodies

• These form when digestion is incomplete. In organisms like amoebas and other protozoa, these residual bodies are expelled by defecation.
• Lysosomes containing undigested materials or debris are called residual bodies, often arising due to a deficiency of certain enzymes.
• These residual bodies are removed from the cell through exocytosis, but in some cases, they remain in cells for prolonged periods, contributing to aging.
• Accumulated residual bodies can also lead to health issues such as fever, hepatitis, pyelonephritis, hypertension, and congestive heart failure.
• If the debris, often lipid in nature, condenses into concentric layers, it forms structures called myelin figures.

Autophagic Vacuole (Cytolysosome or Autophagosome)

• In autophagic vacuoles, the lysosome digests parts of the cell itself, such as mitochondria or portions of the endoplasmic reticulum, through autophagy.
• For example, in liver cells during starvation, numerous autophagosomes form containing remnants of mitochondria.
• This process allows the cell to degrade its own components without causing irreversible damage.

Functions of Lysosomes

They function by using their enzymes to act on various materials, whether these materials originate from within the cell or come from external sources.

• Exocytosis: This process releases enzymes outside the cell to break down surrounding materials. Lysosomal enzymes can digest extracellular substances outside the cell, a method used by saprophytic fungi and other microorganisms to break down complex substrates in their environment into simpler, absorbable forms.
• Autophagy: Refers to the digestion of the cell’s own materials or cytoplasmic components. The digested simpler molecules are then used in synthesizing other substances, a process known as turnover. This includes non-functional parts of organelles, like mitochondria, enclosed in vesicles known as autophagic vesicles or autophagosomes. Autophagy also clears dead cells through self-digestion.
• Pinocytosis: Cells engulf extracellular fluid through pinocytosis, allowing intake of liquid materials from outside the cell.
• Phagocytosis: In this process, cells engulf solid extracellular materials, such as debris, bacteria, or other particles, primarily in specialized cells.
• Hormone Release: Lysosomal enzymes, particularly acid hydrolases, aid in hormone release from secretory cells, such as the release of thyroid hormones by hydrolyzing thyroglobulin.
• Fertilization: Enzymes from the acrosome, a large lysosome in sperm cells, dissolve the cortical granules surrounding the egg nucleus, facilitating sperm entry into the egg.
• Metamorphosis: During embryo development, certain tissues that become nonfunctional are digested by lysosomal enzymes, and the resulting materials are absorbed by neighboring cells.
• Protection: Lysosomes in leukocytes provide defense against infections by digesting bacteria, microbes, and toxins. Mature leukocytes, or white blood cells, contain many of them for this protective role.
• Malfunctioning: Research indicates that irregular lysosomal activity can lead to health issues such as fever, heart failure, hepatitis, kidney inflammation, hypertension, joint injuries, and tissue damage.
• Inflammation: The accumulation of indigestible materials like silica, asbestos, and sodium urate crystals can cause cell inflammation. This inflammation occurs when lysosomal enzymes are released following the breakdown of lysosomes in cells containing these particles.

Summary

Lysosomes, often called the “suicidal bags of the cell,” are membrane-bound, rounded, oval, or irregular structures that contain numerous hydrolytic enzymes, allowing them to break down various substances within and outside the cell.

They contain lytic, or destructive, enzymes which may rupture and digest the cell itself preventing spread of infection, hence, also known as suicidal bag of cell. They also play a role in autophagy and phagocytosis.

Christian de Duve discovered it in cells through biochemical analysis. Subsequently, in 1956.

They are spherical bodies or vacuoles encased in a single membrane made of a phospholipid bilayer, containing single layered lipoproteinous membrane called limiting layer and densely packed inner region containing several enzymes called as inner dense mass.

There are four types of lysosomes: primary (storage granules), secondary (heterophagosomes), residual bodies and autophagosomes (autophagic vacuoles).

The main functions of lysosome includes exocytosis, autophagy, pinocytosis, phagocytosis, hormone release, metamorphosis and defense.