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The following points highlight the thirteen important Organelles of Cytoplasm. Some of the organelles are: 1. Endoplasmic Reticulum 2. Ribosomes 3. Golgi Apparatus 4. Lysosomes 5. Mitochondria 6. Cytoplasmic Vacuoles 7. Sphaerosomes 8. Microtubules and Microfilaments 9. Centrosomes 10. Basal Granules or Kinetosomes and Others.
Organelle # 1. Endoplasmic Reticulum:
The cytoplasmic matrix is traversed by a vast reticulum or network of inter-connecting tubules and vesicles which is known as endoplasmic reticulum or ER. The endoplasmic reticulum has a single vast and interconnected cavity which is usually bounded by a single membrane.
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The membrane of endoplasmic reticulum is supposed to be originated by inpushings of plasma membrane in the matrix because like the plasma membrane it has an outer and inner layer of protein molecules sandwiching the middle layer of lipid molecules.
The membranes of endoplasmic reticulum may be either smooth when they do not have attached ribosomes or rough when they have attached ribosomes with them. The membranes of endoplasmic reticulum are found to be continuous with the nuclear membrane and plasma membrane.
The endoplasmic reticulum forms the ultra-structural skeletal framework of the cytoplasmic matrix and it provides mechanical support to it. It also acts as an intracellular circulatory system and it circulates various substances into and outside the cell by membrane flow mechanism.
Further, the endoplasmic reticulum acts as a storage and synthetic organ. For example, it synthesizes lipids, glycogen, cholesterol, glycerides and hormones, etc.
Organelle # 2. Ribosomes:
Many minute spherical structures known as ribosomes remain attached to membranes of endoplasmic reticulpm and form granular or rough type of endoplasmic reticulum. The ribosomes also occur scattered freely in the cytoplasm.
The ribosomes are originated in the nucleolus and consist of mainly the ribonucleic acid (RNA) and proteins. Each ribosome is composed of two structural units, a smaller subunit known as 40S subunit and a larger subunit known as 60S subunit.
The ribosomes remain attached with the membranes of endoplasmic reticulum by the 60S subunit. The 40S subunits occur on the larger unit and form a cap-like structure.
The ribosomes consist of three types of RNA’s known as ribosomal RNA’s or rRNA, viz., 5S, 18S and 28S RNA’s. The 28S and 5S rRNA occur in large (60S) subunit, while 18S rRNA occurs in the smaller ribosomal subunit. Ribosomes are the sites of protein synthesis.
Organelle # 3. Golgi Apparatus:
In cytoplasm, a stack of flattened membrane bounded, parallely arranged organelles occurs in the association of endoplasmic reticulum and is known as Golgi apparatus. The Golgi apparatus is a disc-shaped organelle. Each Golgi apparatus is composed of many lamellae (flattened sacs or cisternae), tubules, vesicles and vacuoles.
The membranes of Golgi apparatus are of lipoproteins and these are supposed to be originated from the membranes of endoplasmic reticulum.
The function of Golgi apparatus is the storage of proteins and enzymes which are secreted by ribosomes and transported by endoplasmic reticulum to them. Further, the Golgi apparatus has the most important secretory function. It secretes many secretory granules and lysosomes.
Organelle # 4. Lysosomes:
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The cytoplasm of animal cells contains many tiny spherical or irregular-shaped membrane bounded vesicles known as lysosomes. The lysosomes are originated by Golgi apparatus and contain many digestive enzymes. Their function is the digestion of food material which comes in the cell by pinocytosis and phagocytosis.
During starvation, lysosomes digest the stored food contents of cytoplasm. Further, the lysosomes also do the extracellular digestion.
Organelle # 5. Mitochondria:
In the cytoplasm of most cells occur large-sized filamentous, rounded or rod-like structures known as mitochondria. The mitochondria occur singly or in groups and their shape and size vary from cell to cell. The diameter of mitochondria ranges from 0.2 µ to 2.0 µ and the length may range from 0.3 µ to 40 µ.
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The number of mitochondria in a cell depends on the type of functional state of the cell. The mitochondria are bounded by two membranes of lipoproteins. The outer membrane forms a bag-like structure around the inner membrane which gives out many finger-like folds in the lumen of the mitochondria. The folds of inner mitochondrial membrane are known as cristae or mitochondrial crests.
The space between the outer and inner mitochondrial membranes as well as the central space is filled up by a viscous mitochondrial matrix. The matrix, outer and inner membranes are found to contain many oxidative enzymes and coenzymes.
The mitochondria perform most important functions such as oxidation, dehydrogenation, oxidative phosphorylation and respiratory chain of the cell. Their structure and enzymatic system are fully adapted for their different functions.
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They are the actual respiratory organs of the cells where the foodstuffs, i.e., carbohydrates and fats are completely oxidised into CO2 and H2O.
During the biological oxidation of the carbohydrates and fats, large amount of energy is released which is utilised by the mitochondria for the synthesis of the energy rich compound known as adenosine triphosphate or ATP. Because mitochondria synthesise energy rich compound ATP, they are also known as power houses of the cell.
Organelle # 6. Cytoplasmic Vacuoles:
The cytoplasm of many plants and some animal cells (i.e., ciliate protozoans) contains numerous small or large-sized, hollow, liquid-filled structures, the vacuoles. These vacuoles are supposed to be greatly expanded endoplasmic reticulum or Golgi apparatus.
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The vacuoles of animal cells are bounded by a lipoproteinous membrane and their function is the storage, transmission of the materials and the maintenance of internal pressure of the cell.
The vacuoles of the plant cells are bounded by a single, semi-permeable membrane known as tonoplast. These vacuoles contain water, phenol, flavonols, anthocyanins (blue and red pigment), alkaloids and storage products such as sugars and proteins.
Organelle # 7. Sphaerosomes:
In plant cells, certain cytoplasmic organelles contain large amount of lipids (98%) and certain hydrolytic enzymes and are known as sphaerosomes. The sphaerosomes are bounded by the lipoproteinous membrane and are supposed to function as the lipid storage organelles.
Organelle # 8. Microtubules and Microfilaments:
The cytoplasm of plant and animal cells is traversed by numerous ultra-fine tubules of tubulin protein, called microtubules. The function of microtubules is the transportation of water, ions or small molecules and the formation of fibres or asters of the spindle during cell division.
Moreover, they form the structural units of the centriole, basal granules, cilia and flagella. The cytoplasm of most animal cells also contains many ultra-fine proteinous, solid microfilaments which maintain the structure of the cell and form contractile components of the muscle cells.
Organelle # 9. Centrosomes:
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The centrosome contains dense cytoplasm and is located near the nucleus of animal cells. During the cell division, the centrosome is found to contain two rod-shaped granules known as centrioles. The centrioles consist of nine fibrillar units and each fibrillar unit is found to contain three microtubules.
At the time of cell division, the centrioles form the spindle of microtubules which help in the separation movement of chromosomes during last stages of cell division.
Organelle # 10. Basal Granules or Kinetosomes:
The animal or plant cells which are having locomotory organelles such as the cilia or flagella, contain spherical bodies known as basal granules or kinetosomes at the base of the cilia or flagella. The kinetosomes are embedded in the ectoplasm and are composed of nine outer fibrils and two central fibrils.
Each outer fibril consists of three microtubules, out of which two enters in the cilia or flagella. Each central fibril contains single microtubule.
Organelle # 11. Cilia and Flagella:
The cells of many unicellular organisms and ciliated epithelium of multicellular organisms consist of some hair-like cytoplasmic projections outside the surface of the cell. These are known as cilia or flagella and they help in locomotion of the cell.
The cilia and flagella consist of nine outer fibrils around the two large central fibrils. Each outer fibril consists of two microtubules. The cilia and flagella are originated from the basal bodies and chemically consist of proteins and adenosine triphosphate (ATP).
Organelle # 12. Tonofibrils:
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The cytoplasm of most cells contains bundles of fibrils ranging from 30A° to 60A° long in the region of desmosomes. The desmosomes are the apertures of the plasma membrane by which cells of a tissue remain in direct contact with each other.
Organelle # 13. Plastids:
The plastids occur commonly in plant cells and their diameter varies from 4 microns to 6 microns. They may be colourless or coloured. The colourless plastids are known as leucoplastids or leucoplasts, and coloured plastids as chromo plastids or chromoplasts. The leucoplasts usually have storage function and store starch and lipids and may be called amyloplasts and lipoplasts respectively.
The chromoplasts may have many forms and pigments, but those having the chlorophyll are most important. These are known as chloroplasts. They contain DNA, ribosomes and complete synthetic machinery. The chloroplasts help in the biosynthesis of foodstuffs by the process of photosynthesis.
