Mitochondria: Mitochondria are very important organelles of eukaryotic cells because they are involved in manufacture and supply of energy to cell. They are also known as powerhouse of cell. Under compound microscope they appear to be vesicles, rods or filaments. Under electron microscope they show complex morphology. Although their number, shape and internal structure vary widely a mitochondrion is bounded by two membranes outer membrane is smooth while inner membrane forms infoldings into inner chamber called mitochondrial matrix. These infoldings are called cristae. Their membranes are similar in structure to other cell membranes. Detailed studies have shown that they also contain DNA as well as ribosomes. Presence of ribosomes and DNA indicates that some proteins are synthesized in them. It is a self replicating organelle. Inner surface of cristae in mitochondrial matrix has small knob like structures known as F1 particles. Mitochondrial matrix contains in it large number of enzymes, coenzymes and organic and inorganic salts which help in several vital metabolic processes like Krebs cycle, aerobic respiration and fatty acid metabolism etc. As result of these metabolic processes energy extracted from organic food is transformed into energy rich compound ATP and ATP then provides energy to cell on demand. Size and number of mitochondria varies and depends on physiological activity of cell. They extract energy from different components of food and convert it in the form of ATP. This energy is used for various cellular activities. Spent energy which is in the form of ADP is regenerated by mitochondria into ATP. Mitochondria are therefore described as power house of the cell.
Plastids: Membrane bound mostly pigment containing bodies present in the cells are called plastids. They are present in plant cells only.
There are three main types of them.
Chloroplasts: In photosynthetic plant cells there are membrane bound structures containing green pigment called chloroplasts. Green pigment is an organic compound which helps cell to absorb light energy and utilize it to manufacture food. Chlorophyll molecule resembles haem group of hemoglobin a protein used in transport of oxygen. Main difference between these two molecules is that chlorophyll has Mg++ while haem has Fe++ as the central atom. Chloroplasts vary in their shape and size. Under light microscope they appear to be heterogenous structures with small granules known as grana embedded in matrix. Under electron microscope it shows three main components envelop, stroma and thylakoid. Envelop is formed by double membrane while stroma covers most of volume. Stroma is fluid which surrounds thylakoid. It contains proteins, some ribosomes and small circular DNA. It is the part where carbon dioxide is fixed to manufacture sugars. Some proteins are also synthesized in this part. Thylakoids are flattened vesicles which arrange themselves to form grana and intergrana. Granum appears to be a pile of thylakoids stacked on each other like coins. On an average there are 50 or more thylakoids piled to form one granum. On layers of thylakoids chlorophyll molecules are arranged and that is why granum appears to be green. Each granum is interconnected with others by non green part called intergranum. Membrane of grana are the sites where sun light energy is trapped and where ATP is formed. Chloroplasts are self replicating organelles.
Chromoplasts: They impart colors to plants other than green. They are present in petals of flowers and in ripened fruits. They help in pollination and dispersal of seeds.
Leucoplasts: They are colorless and they are triangular, tubular or of some other shape. They are found in underground parts of plant and store food.
Nucleus: Presence of cell nucleus was reported in 1831 by Robert Brown. It slightly darker than surrounding cytoplasm. It controls life and activities of cell. In animal cells it generally occupies central space while in case of plant cells it is pushed towards periphery due to presence of large vacuole. It may be irregular or spherical in shape. Generally cells have one nucleus are called mononucleate. On the other hand cells have two are called binucleate and with more than two as multinucleate. It is only visible when cell is in non dividing stage. It contains chromatin network and soluble sap called nucleoplasm. In dividing cells it disappears and chromatin material in it is replaced by chromosomes. Heredity material is in form of chromosomes which controls all activities of cell. DNA, RNA and proteins including enzymes form chemical composition of nucleus. It consists of nuclear membrane, nucleoli, nucleoplasm and chromosomes.
Nuclear membrane: Nucleus is surrounded by nuclear membrane which separates the nuclear material from cytoplasm. Nuclear membrane is actually nuclear envelope composed of two membranes. Outer membrane is at place continuous with endoplasmic reticulum while inner membrane encloses nuclear content. Outer and inner membrane are continuous at certain points resulting in formation of pores, the nuclear pores. Nuclear pores allow exchange of materials between the nucleus and cytoplasm. Number of nuclear pores is highly variable. The undifferentiated cells have numerous pores whereas differentiated cells such as erythrocytes have 3 or 4 pores per nucleus. Each pore has definite structure which controls traffic of substances passing through them.
Nucleolus: It is darkly stained body within nucleus and is without any membranous boundary to separate it from rest of nuclear material. There may be one or more nucleoli in the nucleus. Ribosomal RNA is synthesized and stored in nucleolus. It is composed of two region peripheral granular area composed of precursors of ribosomal subunits and central fibril area consisting of large molecular weight rRNA and DNA. It is nucleolus where ribosomes are assembled and are then exported to cytoplasm via nuclear pores.
Chromosomes: Nucleus is often deeply stained with basic dyes because of chromatin material. During cell division chromatin material is converted into darkly stained thread like structures known as chromosomes. Under compound microscope chromosomes appear to be made of arms and centromeres. centromere is place on chromosome where spindle fibers are attached during cell division. Each chromosome consists of two identical chromatids at beginning of cell division( chromatid is exact replica of chromosome) which are held together at centromere. Chromosome is composed of DNA and proteins. All information necessary to control activities of cell is located on chromosomes in form of genes which are transferred from one generation to other. Number of chromosomes in all individuals of same species remains constant generation after generation. In humans each cell contains 46 chromosomes, frog cell has 26 and chimpanzee has 48 chromosomes. There are 8 chromosomes in fruit fly, 16 in onion, 48 in potato and 14 in garden pea. Number of chromosomes in normal body cell diploid whereas haploid in germ cells.
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