Microbodies Microtubules and Microfilament

Microbodies

Spherosomes

These are very small, spherical bodies with a single unit membrane. These are found in all the plant cells especially in endosperm cells of oil seeds shape and size. These are spherical in shape and about 0.5-2.5µm in size.

Structure

The spherosomes are membrane-bound spherical organelles occurring in most plant cells. They contain hydrolytic enzymes such as the hydrolase, protease, ribonuclease, phosphatase and esterase, etc. so called plant lysosomes. They have fine granular structure internally rich in lipids and proteins. They originate from the endoplasmic reticulum.

Functions

• The spherosomes are involved in lipid metabolism.(i.e. synthesis and storage of lipids).
• They may also act as a lysosome.

Peroxisomes

Peroxisomes are the microbodies found in many animal cells and in a wide range of plants. These are present in all photosynthetic cells of higher plants, liver and kidney cells of vertebrates. These have also been reported in the cells of the brain, small intestine, testis, adrenal cortex, protozoa, brown algae, fungi, liverworts, mosses and ferns. These are variable in shape and size, but usually appear circular in cross section having the diameter between 0.2mµ and 1.5mµ.

Structure

These are small spherical bodies having single unit membranes made up of lipid and protein molecules. They contain finely granular or crystallised protein catalyses and oxidases. The enzyme catalyses, catalyses the decomposition of hydrogen peroxide(H₂O₂) to water (H_2O) and oxygen(O₂), whereas the enzymes oxidases oxidise organic compounds.

Functions

• They protect the cellular organelles from the toxic effect of hydrogen peroxide(H₂O₂).
• Plant peroxisomes are also involved in photorespiration(in the synthesis of glycine and serine).
• In animal cells, these are involved in ß-oxidation of fatty acids.

Microtubules

Microtubules are the slender, proteinaceous threads first discovered by Robert and Franchi. They are called as microtubules.

Occurrence

The microtubules are electron microscopic structures found in the cytoplasm of eukaryotic cells at following seven sites;

• Cilia and flagella
• Centrioles and basal bodies
• Nerve processes
• The mitotic apparatus
• The cortex of meristematic plant cells
• Elongating cells such as during the formation of the lens or during spermatogenesis of certain insects.
• Axostyle of parasitic flagellates, the axoneme of Echinosphaerium etc.

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Structure

A microtubule is a hollow cylindrical structure of about 250Å in diameter and about the 150Å lumen. Its wall is about 50Å thick. Its wall is formed of 13 parallel, proto-tubules, each being formed of a linear series of globular protein molecules. Chemically a microtubule is formed of the tubulin protein. A tubulin protein is formed of α-tubulin and β-tubulin.

Functions

• Microtubules form a part of the cytoskeleton and help in cell shape and mechanical support.
• They form the mitotic apparatus and during cell division and gets attached with the kinetochore of the chromosomes. Their contraction causes the movement of the chromosomes towards the opposite poles of the cells.
• The microtubules of cilia and flagella help in the locomotion and feeding.
• These help in distribution of pigment in the chromatophores, so help in skin coloration.
• The microtubules of neuraxis help in the conduction of nerve impulses.
• These control the orientation of cellulose microfibrils of the cell wall of plants.

Microfilament

Microfilaments are electron-microscopic, long, narrow, cylindrical, contractile and proteinous structures found only in the eukaryotic cytoplasm. These are present in the microvilli, muscle fibres, etc. These are also associated with the pseudopodia, the plasma membrane of fibroblasts, etc. They are long, thin, cylindrical and very fine protein filaments which are about 5-6nm in diameter.

Structure

Each microfilament is a solid filament and is formed of a helical series of globular protein molecules. These usually occur in bundles and have been observed in the path of streaming cytoplasm in the plant cells or below the plasma membrane. They form a network in the cytoplasm and extend up to the core of microvilli. Chemically they are mainly formed of actin protein and a small amount of myosin protein.

Functions

• These form a part of cytoskeleton so help in cell shape and mechanical support.
• These helps in cyclosis of cytoplasm.
• Microfilaments of muscle fibres help in movements and locomotion.
• Microfilaments of microvilli help in their movements and absorption of food.
• These help in endocytosis through the cell membrane.
• These help in cleavage during the cell division.

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Cilia and Flagella

The cilia and flagella are microscopic, hair or thread like motile structures present extracellularly but originate intracellularly from the basal bodies and helps in the movement locomotion, feeding, circulation, etc. The presence of flagellum was first to report the structure of sperm flagellum.

Occurrence

Cilia are found in all the ciliate protozoans, flame cells of flatworms, in some larval forms e.g. Bipinnaria larva of starfish, in some body structures eg; windpipe, fallopian tubes, etc, ciliated epithelium of the metazoa, etc.

Flagella are found in all the flagellate protozoans, collar cells of sponges, gastrodermal cells of coelenterates, spermatozoa of animals and lower plants, zoospores of algae etc.

Vacuoles

True or sap vacuoles are a fluid-filled sac-like structure which is bounded by a membrane called tonoplast. The vacuoles contain water, minerals, sugar, salts amino acids, pigments, waste materials, etc. These vacuoles occur both in plant and animal cell.

Functions

• Help in the storage.
• Help in cell elongation and growth.
• Give turgidity and mechanical support.
• Modified vacuoles like contractile vacuoles help in osmoregulation.
• Food vacuoles help in intaking and digestion of food.
• Gas vacuoles or pseudo vacuoles help in gases exchange and give buoyancy.