The following points highlight the six main cell organelles of protoplasm. The cell organelles are: 1. Chloroplast 2. Mitochondria 3. Golgi Apparatus 4. Nucleus 5. Endoplasmic Reticulum 6. DNA Model.
Cell Organelle # 1. Chloroplast:
1. Chloroplasts (Figs. 18, 19), responsible for the photosynthesis of the plants, are the characteristic features of the cells of green plants.
2. Around the chloroplast is present a double membrane envelope.
3. Many dark-coloured grana are present. These are interconnected by means of photosynthetic lamellae or thylakoids.
4. A colourless matrix or stroma is present in the chloroplast.
5. Each membrane of chloroplast is 35 to 50 Å thick.
6. There are present some interconnecting intergranal projections called frets.
7. Park and Biggins (1964) have observed some sphere like structures within the membrane component of the granular disc. These are called quantasomes.
8. Quantasomes are structural and functional units of chloroplast. About 250 chlorophyll molecules are present in a quantasome.
Cell Organelle # 2. Mitochondria:
1. Mitochondria (Fig. 20), commonly called “power houses” of the cell, are present in all cells except mature erythrocytes and bacteria.
2. The term ‘mitochondrion’ was first coined by Benda (1897).
3. The mitochondria have an average diameter of 0.5µ. and a length of about 1.5µ. However, some 6-7long mitochondria have also been observed.
4. Mitochondria remain surrounded by two membranes.
5. Each membrane is about 60Å thick and there is a gap of about 80Å in between two membranes.
6. Space between the two membranes is filled by a fluid rich in coenzymes.
7. Outer membrane is continuous and the inner membrane is thrown into many folds into the cavity of the mitochondria. These folds or elongated sacs are called cristae.
8. Chemically, each mitochondrion consists of 70% proteins, 25% phospholipid, 0.5% RNA and a little amount of DNA.
9. Electron microscope has shown that on the inner membrane some racket- like bodies are attached. Each racket-like body consists of a base piece, a stalk about 50Å long, and a round head piece about 80-100Å in diameter.
10. Functions of mitochondria are under-mentioned:
(i) Manufacture and store of energy in the form of ATP.
(ii) To work as respiratory centres of the cell.
(iii) To help in fat metabolism, enzyme activity and gene cell formation.
Cell Organelle # 3. Golgi Apparatus:
1. Camilo Golgi (1898), an Italian anatomist, was awarded Noble prize in 1907 for his discovery of Golgi-complex or Golgi apparatus.
2. Each Golgi body is bounded by a double membrane with an inter-membranous space.
3. Golgi bodies or dictyosomes contain equal amounts of phospholipids and proteins. Some carotenoids and vitamin-C have also been reported.
4. Double membraned structures are plate-like layers arranged in parallel fashion. These are called lamellae.
5. Golgi bodies help in:
(i) Production of hormones.
(ii) Enzyme formation.
(iii) Proteins storage.
(iv) Cell wall development.
(v) Absorption of compounds.
Cell Organelle # 4. Nucleus:
1. Nucleus, discovered for the first time by Robert Brown (1831), is the centre of hereditary activities in the cell.
2. Nucleus is surrounded by a double layered membrane. The two layers are named as inner and outer nuclear membranes.
3. Continuity of the nuclear membrane is interrupted by many pores called nuclear pores.
4. Inside the nuclear membrane is present the nucleoplasm.
5. Nucleus consists of protamines, histones, non-histone proteins, nuclear enzymes and two types of nucleic acids, i.e., DNA and RNA.
6. Nucleolus (one or more) is present in the nucleus.
7. A chromatin network, consisting of euchromatin or heterochromatin (chromosomes) is present in the nucleus.
8. Functions of the nucleus are to control all the activities of the cell and to carry hereditary information’s from one generation to the other in the germ cells.
Cell Organelle # 5. Endoplasmic Reticulum:
1. It forms a series of small canals (Fig. 23) through the cytoplasm.
2. Through these canals various substances pass from the membrane to the nucleus.
3. On these canals are attached small granules called microsomes.
4. Microsomes contain high concentration of RNA. Due to high RNA concentration these are often called ribosotnes or Claude’s particles.
5. Each microsome measures between 60 to 200µ in diameter. These can be observed only under electron microscope.
6. They are essential in protein synthesis.
7. They also help in ATP synthesis.
Cell Organelle # 6. DNA Model:
1. DNA (Deoxyribonucleic acid), the active genetic material responsible for heredity, was discovered by Miecher (1869).
2. Watson and Crick were awarded Nobel Prize in 1962 for their discovery of the double helical model of DNA.
3. DNA consists of sugar, phosphate and bases of two types, i.e., purines and pyrimidines.
4. Purines are adenine (A) and guanine (G), and the pyrimidines are thymine CD and cytosine (C).
5. It is a double helical structure and both the helix coil round a central axis.
6. Each strand is made up of sugar and phosphate groups.
7. The distance between the two helix is 20Å and each pair of base is 34Å long.
8. Each pair consists of one purine and one pyramidine base as mentioned below:
Adenine (A) = Thymine (T)
Cytosine (C) = Guanine (G)