LYSOSOMES, VACUOLES AND MICROBODIES?

In today’s topic, we will discuss three major cell organelles namely lysosomes, vacuoles, and microbodies.

Firstly, I will talk about lysosomes. After that, we will discuss the next two organelles in detail. If you don’t want to read the topic in-depth, you can check out the summary (short notes) of my notes at the bottom of the post.

Lysosomes Definition

Lysosomes are single membrane, dense spherical vesicles that function to degrade material. ‘Lysis’ means digestion and ‘soma’ means body. So, the lysosomes mean ‘digestive bodies’. Hence, lysosomes are the organelles that act as the ‘digestive system’ of the cell. They are also called ‘suicidal bags‘ or ‘suicide organelle’ of the cell.

Lysosomes occur freely in the cytoplasm of the cell. A typical lysosome contains at least 50 different hydrolytic enzymes. Altogether, they can hydrolyze every type of biological macromolecule. Typically, these enzymes can function properly in slightly acidic environments with a pH of 5.

Lysosomal membranes are also rich in cholesterol and lysobiphosphatidic acid, where the latter is believed to prevent the hydrolytic enzymes from digesting the lysosomal membrane.

Why lysosomes are called suicidal bags?

When a cell is destroyed or a few of its organelles become non-functional, the digestive enzymes released by the lysosome digests all of the cell content. Enzymes present inside the lysosomes will eat these organelles or cell’s components. As a result, the cell will die. Therefore, they are called ‘suicide organelle’ or suicidal bags.

The lysosome is part of the cell’s endomembrane system which also contains the ER, Golgi complex, and associated coated and uncoated vesicles.

Lysosomes are most numerous in cells that take up macromolecules or larger substances from their environment. e.g., intestinal epithelial cells have several hundred of lysosomes. These are absent in prokaryotic cells. Lysosomes have their own distinguishing features. It is the ultimate destination of soluble proteins taken in by the cells by endocytosis.

Soluble proteins are present in lysosomes. Enzymes present in the lytic compartment of lysosomes catalyzes the hydrolysis of proteins to their building blocks. After that, they can be used by the cells for cellular energy or anabolism.

Lysosomes Structure

Lysosomes are dense, spherical vesicles and range in diameter from 0.2-0.8µm. They are enclosed within a single membrane that is 10 nm in thickness.

lysosomes structure
structure of lysosome

The lysosomal membrane is very delicate and it prevents the hydrolytic enzymes from reacting with the cytoplasmic substrates.

The internal interior is quite heterogeneous ranging from a dense matrix to granular to flaky.

There are two types of lysosomes:

  • Primary
  • Secondary

Primary lysosomes are pinched off from the trans cisternae of the Golgi complex. Their matrix is homogeneous and dense.

Endocytotic vesicles may fuse with the primary lysosome to form secondary lysosomes. Their matrix is more heterogeneous.

What are the functions of a lysosomes?

Lysosome performs two major functions:

  • Phagocytosis: intracellular digestion of extracellular macromolecules (food or any other foreign substance)
  • Autophagy: (self-feeding) intracellular degradation of organelles or macromolecules. The lysosomal enzymes can breakdown and digest some of the cell organelles like mitochondria, ER, etc at the time of starvation

VACUOLES

A vacuole is a membrane-bound organelle that is present in different types of cells, including plant cells and fungi. However, some protist, animal cells and bacteria also contain vacuoles. Most plant cells contain one or more membrane-bound vacuoles.

vacuole image
a vacuole

Immature plant cells contain several smaller vacuoles. As the cell matures, a large central vacuole forms from the fusion of smaller vacuoles. The central vacuole consumes almost 90% of the cell volume.

The organelle has no basic shape or size. Therefore, its structure varies according to the requirements of the cell.

Tonoplast is the membrane that surrounds the organelle. It is 40% lipid and 60% protein.

Functions

  1. Vacuole contains a variety of hydrolytic enzymes. In addition to hydrolases, plant vacuoles may contain inhibitors of serine endopeptidases.
  2. The second major function of plant vacuole is solute accumulation. These solutes include ions (Cl-, K+, and Na+ ), amino acids, sugars, organic acids (malate and citrate), and secondary metabolites. Vacuole synthesizes Some of these molecules, but most are import from the cytoplasm.
  3. The internal acidic environment of a vacuole aids in the degradation of larger molecules sent to the vacuole for destruction.
  4. Vacuoles remove potentially toxic substances from the cytosol, such as excess heavy metals and herbicides.
  5. They also aid in the lysis and recycling of misfolded proteins that have begun to build up within the cell.

MICROBODIES

The microbodies are membrane-bound vesicles that are ubiquitous components of all eukaryotic cells, except mature mammalian erythrocytes.

They are generally spherical, with diameters ranging from 0.3-1.5µm. They have a dense granular appearance and are enclosed by a 7nm membrane.

Microbodies are cytochemically defined by their catalase activity (the enzymes make up about one-fourth of all the protein in the organelle). This enzyme protects the cell from a potentially dangerous accumulation of hydrogen peroxide. They contain different enzymes, which catalyze the specific type of oxidative reactions.

On the basis of their enzymatic composition, they are of the following types:

  • Peroxisomes (leaf peroxisomes and liver peroxisomes)
  • Glyoxysomes

These organelles appear to arise from pre-existing microbodies.

Glyoxysomes

These are another type of microbodies. They occur in the cells of yeast, Neurospora, and oil-rich seeds of many higher plants. Glyoxysomes are absent in mature and dry seeds.

Structure

Glyoxysomes have a single membrane and their matrix is finely granular. They resemble peroxisomes in morphological details, except that, their crystalloid core consists of dense rods of 6.0 µm diameter.

glyoxysome image
structure of glyoxysome

In addition to oxidase and catalase, they contain a number of other enzymes that are not found in animal cells. The unique enzyme found in glyoxysome is isocitrate lyase and malate synthetase.

Function

  1. One of the main functions of glyoxysome is to convert stored lipids into hexose sugars. This conversion of fat to sugar is known as gluconeogenesis. This hexose sugar is utilized by the emerging seedling which uses it as an energy and carbon source until it is able to produce its own sugar by photosynthesis.
  2. Glyoxysomes also function in nitrogen fixation and photorespiration.
  3. In the glyoxysomes, the glyoxylate cycle converts acetyl-coA to succinate for the synthesis of carbohydrates.

Peroxisomes

Peroxisomes occur in many animal cells and in a wide range of plants. They are present in all photosynthetic cells of higher plants, Protozoa, brown algae, fungi, liverworts, mosses, and ferns.

Peroxisomes are also of two types:

  1. Liver peroxisomes
  2. Plant peroxisomes

Structure of peroxisomes

Peroxisomes usually appear circular and are having a diameter between 0.2 and 1.5µm (0.15 to 0.25 µm diameter in most mammalian tissues; 0.5 µm in rat liver cells).

They have a membrane of lipid and protein molecules, which encloses their granular matrix. In some cases, the matrix contains numerous threads or fibrils, while in others, they contain either an amorphous nucleoid or a dense inner core. In many species, the matrix shows a regular crystalloid structure.

Liver Peroxisomes

A typical hepatocyte contains approximately 1000 peroxisomes in the form of oval or spherical vesicles bound by a single membrane. This number is even larger in fetal and post-natal growth and during liver regeneration. Drugs such as aspirin and clofibrate can induce peroxisomes proliferation.

Peroxisomes contain a wide variety of enzymes, but the most important of them is the flavin oxidase-catalase system. Peroxisomes proteins are synthesized in the cytoplasm and once their translation is completed, they are imported into the organelle.

Leaf peroxisomes

Leaf peroxisomes are present in photosynthetic cells of green plants, especially in palisade cells of C3 leaves and bundle sheath cells of C4 leaves.

They develop at the same time as plastids, and indeed the two organelles often are near to each other in the cell.

function
  • The peroxisomes play an important role in photorespiration ( light-stimulated production of CO2). Photorespiration is a wasteful process for the plant cell, since, it significantly reduces the efficiency of the process of photosynthesis (i.e., it returns a portion of fixed CO2 to the atmosphere).

short notes

Lysosomes

  • act as the ‘digestive system’ of the cell.
  • also called as ‘suicidal bags‘ or ‘suicide organelle’ of the cell.
  • part of the cell’s endomembrane system.
  • abundant in intestinal epithelial cells.
  • the ultimate destination of soluble proteins.
  • absent on prokaryotic cells.

structure

  • dense, spherical vesicles.
  • range in diameter from 0.2-0.8µm.
  • enclosed within a single membrane that is 10 nm in thickness.
  • the membrane prevents the hydrolytic enzymes from reacting with the cytoplasmic substrate.
  • the internal interior is ranging from a dense matrix to granular to flaky.
  • two types of lysosomes
    • primary: pinched off from the trans cisternae of Golgi. Their matrix is dense and homogenous.
    • secondary: endocytotic vesicles may fuse with the primary lysosome to form secondary lysosome. Their matrix is more heterogeneous.

Vacuoles structure

  • a membrane-bound organelle that is present in different types of cells, including plant cells and fungi.
  • However, some protists, animal cells and bacteria also contain vacuoles.
  • a large central vacuole form from the fusion of smaller vacuoles as the cell mature.
  • the organelle has no basic shape or size. Therefore, its structure varies according to the requirements of the cell.
  • tonoplast is the membrane that surrounds the organelle. It is 40% lipid and 60% protein.

function

  1. Vacuole contains a variety of hydrolytic enzymes. For example, inhibitors of serine endopeptidases.
  2. The second major function of plant vacuole is solute accumulation. Vacuole synthesizes Some of the molecules, but most are imported from the cytoplasm.
  3. aids in the degradation of larger molecules sent to the vacuole for destruction.
  4. Vacuoles remove potentially toxic substances from the cytosol, such as excess heavy metals and herbicides.
  5. They also aid in the lysis and recycling of misfolded proteins.

Microbodies

  • membrane-bound vesicles that are present in all eukaryotic cells, except mature mammalian erythrocytes.
  • generally spherical, with diameters ranging from 0.3-1.5µm.
  • its enzymes protect the cell from a potentially dangerous accumulation of hydrogen peroxide.
  • on the basis of their enzymatic composition, they are of following types:
    • peroxisomes
      • leaf peroxisomes
      • liver peroxisomes
    • glyoxysomes.

Glyoxysomes

These are another type of microbodies. They are found to occur in the cells of yeast, Neurospora, and oil-rich seeds of many higher plants. Glyoxysomes are absent in mature and dry seeds.

Structure

  • have a single membrane.
  • matrix is finely granular.
  • their crystalloid core consists of dense rods of 6.0 µm diameter.
  • the unique enzyme found in glyoxysome is isocitrate lyase and malate synthetase.

Function

  1. Glyoxysome convert stored lipids into hexose sugars. This conversion of fat to sugar is known as gluconeogenesis.
  2. Glyoxysomes also function in nitrogen fixation and photorespiration.
  3. the glyoxylate cycle converts acetyl-coA to succinate for the synthesis of carbohydrates.

peroxisomes

  • occur in many animal cells and in a wide range of plants.
  • are present in all photosynthetic cells of higher plants, Protozoa, brown algae, fungi, liverworts, mosses, and ferns.
  • Peroxisomes are also of two types:
    • Liver peroxisomes
    • Plant peroxisomes.

Structure of peroxisomes

  • usually appear circular and are having a diameter between 0.2 and 1.5µm(0.15 to 0.25 µm diameter in most mammalian tissues; 0.5 µm in rat liver cells).
  • In some cases, the matrix contains numerous threads or fibrils, while in others, they contain either an amorphous nucleoid or a dense inner core.
Liver Peroxisomes
  • typical hepatocyte contains approximately 1000 peroxisomes in the form of oval or spherical vesicles bound by a single membrane.
  • they have a wide variety of enzymes, but the most important of them is the flavin oxidase-catalase system.
  • firstly, their proteins are synthesized in the cytoplasm. after that, they are imported to the organelle.
Leaf peroxisomes
  • present in photosynthetic cells of green plants, especially in palisade cells of C3 leaves and bundle sheath cells of C4 leaves.
  • They develop at the same time as plastids, and indeed the two organelles often are near to each other in the cell.
function

1. The peroxisomes play an important role in photorespiration ( light-stimulated production of CO2). Photorespiration significantly reduces the efficiency of the process of photosynthesis (i.e., it returns a portion of fixed CO2 to the atmosphere).

Source of information

Other cell organelles

If you want to read more cell organelles, then click on the specific link below

cell wall

cytoskeleton system

Endoplasmic reticulum

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