Cells contain various organized structures, collectively called as cell organelles. When the cell membrane is disrupted, either by mechanical means or by lysing the membrane by Tween-20 (a lipid solvent), the organized particles inside the cell are homogenized. This is usually carried out in 0.25 M sucrose at pH 7.4. The organelles could then be separated by applying differential centrifugal forces. Albert Claude got Nobel Prize in 1974 for fractionating subcellular organelles.
Some enzymes are present in certain organelles only; such specific enzymes are called as marker enzymes. After centrifugation, the separated organelles are identified by detection of marker enzymes in the sample.
It is the most prominent organelle of the cell. All cells in the body contain nucleus, except mature RBCs in circulation. The uppermost layer of skin also may not possess a readily identifiable nucleus. In some cells, nucleus occupies most of the available space, e.g. small lymphocytes and spermatozoa. Nucleus is surrounded by two membranes: the inner one is called perinuclear membrane with numerous pores. The outer membrane is continuous with membrane of endoplasmic reticulum.
Nucleus contains the DNA, the chemical basis of genes which governs all the functions of the cell. The very long DNA molecules are complexed with proteins to form chromatin and are further organized into chromosomes. DNA replication and RNA synthesis (transcription) are taking place inside the nucleus. In some cells, a portion of the nucleus may be seen as lighter shaded area; this is called nucleolus. This is the area for RNA processing and ribosome synthesis. The nucleolus is very prominent in cells actively synthesizing proteins. Gabriel Valentine in 1836 described the nucleolus.
It is a network of interconnecting membranes enclosing channels or cisternae that are continuous from outer nuclear envelope to outer plasma membrane. Under electron microscope, the reticular arrangements will have railway track appearance. George Palade was awarded Nobel Prize in 1974, who identified the ER. This will be very prominent in cells actively synthesizing proteins, e.g. immunoglobulin secreting plasma cells. The proteins, glycoproteins and lipoproteins are synthesized in the ER.
Detoxification of various drugs is an important function of ER. Microsomal cytochrome P-450 hydroxylates drugs such as benzpyrine, aminopyrine, aniline, morphine, phenobarbitone, etc. According to the electron microscopic appearance, the ER is generally classified into rough and smooth varieties. The rough appearance is due to ribosomes attached to cytoplasmic side of membrane where the proteins are being synthesized. When cells are fractionated, the complex ER is disrupted in many places. They are automatically re-assembled to form microsomes.
1. Camillo Golgi described the structure in 1898. The Golgi organelle is a network of flattened smooth membranes and vesicles. It may be considered as the converging area of endoplasmic reticulum. While moving through ER, carbohydrate groups are successively added to the nascent proteins. These glycoproteins reach the Golgi area. Golgi apparatus is composed of cis, medial and Trans cisternae. Glycoproteins are generally transported from ER to cis Golgi (proximal cisterna), then to medial Golgi (intermediate cisterna) and finally to trans Golgi (distal cisterna) for temporary storage.
Trans Golgi are particularly abundant with vesicles containing glycoproteins. Newly synthesized proteins are sorted first according to the sorting signals available in the proteins. Then they are packed into transport vesicles having different types of coat proteins. Finally, they are transported into various destinations; this is an energy dependent process. Main function of Golgi apparatus is protein sorting, packaging and secretion. The finished products may have any one of the following destinations:
- they may pass through plasma membrane to the surrounding medium. This forms continuous secretion, e.g. secretion of immunoglobulins by plasma cells.
- They reach plasma membrane and form an integral part of it, but not secreted.
- They are formed into a secretory vesicle, where these products are stored for a longer time. Under appropriate stimuli, the contents are secreted. Release of trypsinogen by pancreaticacinar cells and release of insulin by beta cells of Langerhans are cited as examples.
- The synthesized materials may be collected into lysosome packets.
Discovered in 1950 by Rene de Duve (Nobel Prize 1974), lysosomes are tiny organelles. Solid wastes of a township are usually decomposed in incinerators. Inside a cell, such a process is taking place within the lysosomes. They are bags of enzymes. Endocytic vesicles and phagosomes are fused with lysosome (primary) to form the secondary lysosome or digestive vacuole. Foreign particles are progressively digested inside these vacuoles.
Completely hydrolyzed products are utilized by the cell. As long as the lysosomal membrane is intact, the encapsulated enzymes can act only locally. But when the membrane is disrupted, the released enzymes can hydrolyze external substrates, leading to tissue damage.The lysosomal enzymes have an optimum pH around 5.
These enzymes are a) Polysaccharide hydrolysing enzymes (alpha-glucosidase, alpha-fucosidase, beta-galactosidase, alphamannosidase, beta-glucuronidase, hyaluronidase, aryl sulfatase, and lysozyme) b Protein hydrolysing enzymes (cathepsins, collagenase, elastase, peptidases) c. Nucleic acid hydrolyzing enzymes (ribonuclease, deoxyribonuclease)d. Lipid hydrolyzing enzymes (fatty acyl esterase, phospholipases)
The peroxisomes have a granular matrix. They are of 0.3–1.5 μm in diameter. They contain peroxidases and catalase. They are prominent in leukocytes and platelets. Peroxidation of polyunsaturated fatty acids in vivo may lead to hydroperoxide formation, ROOH → R- OO• . The free radicals damage molecules, cell membranes, tissues and genes. Catalase and peroxidase are the enzymes present in peroxisomes which will destroy the unwanted peroxides and other free radicals.
They are spherical, oval or rod-like bodies, about 0.5–1 μm in diameter and up to 7 μm in length. Erythrocytes do not contain mitochondria. The tail of spermatozoa is fully packed with mitochondria. Mitochondria are the powerhouse of the cell, where energy released from oxidation of food stuffs is trapped as chemical energy in the form of ATP.
Mitochondria have two membranes. The inner membrane convolutes into folds or cristae. The inner mitochondrial membrane contains the enzymes of electron transport chain. The fluid matrix contains the enzymes of citric acid cycle, urea cycle and hem synthesis. Cytochrome P-450 system present in mitochondrial inner membrane is involved in steroidogenesis. Superoxide dismutase is present in cytosol and mitochondria
Mitochondria also contain specific DNA. The integral inner membrane proteins, are made by mitochondrial protein synthesizing machinery. However the majority of proteins, especially of outer membrane are synthesized under the control of cellular DNA. The division of mitochondria is under the command of mitochondrial DNA. Mitochondrial ribosomes are different from cellular ribosomes. Antibiotics inhibiting bacterial protein synthesis do not affect cellular processes, but do inhibit mitochondrial protein biosynthesis.
Taking into consideration such evidences, it is assumed that mitochondria are parasites which entered into cells at a time when multicellular organisms were being evolved. These parasites provided energy in large quantities giving an evolutionary advantage to the cell; the cell gave protection to these parasites. This perfect symbiosis, in turn, evolved into a cellular organelle of mitochondria
The plasma membrane separates the cell from the external environment. It has highly selective permeability properties so that the entry and exit of compounds are regulated. The cellular metabolism is in turn influenced and probably regulated by the membrane. The membrane is metabolically very active.The enzyme, nucleotide phosphatase (5′ nucleotidase) and alkaline phosphatase are seen on the outer part of cell membrane; they are therefore called ecto-enzymes.
Membranes are mainly made up of lipids, proteins and small amount of carbohydrates. The contents of these compounds vary according to the nature of the membrane. The carbohydrates are present as glycoproteins and glycolipids. Phospholipids are the most common lipids present and they are amphipathic in nature. Cell membranes also contain cholesterol.