CELL
Cytoplasm: It is the watery environment inside the cell. Cytoplasm includes salts, an assortment of organic molecules, including many enzymes that catalyze reactions, as well as water. Cytoplasm contains mostly water, from 80 to 97% in different cells, except for spores and other inactive forms of living material, in which water may be present in lesser amounts. The cytoplasm is seperated from the wattery extracellular fluid, which is outside, by the plasma membrane. It contains discrete membrane-enclosed structures called organelles. Each of the organelles performs a specific cellular function. They also have specialized proteins that provide an intracellular support system.
Mitochondrion: It is a sausage-shaped structure within cells that has the specific function of generating energy by using oxygen to break down foodstuffs. They contain coloured chemicals (cytochromes) and enzymes needed for aerobic respiration. When mitochondria are tightly packed together, the cytochromes impart a reddish-brown colour to cells. Mitochondria are particularly dense in slow twitch muscle fibres used for activities of long duration. Regular endurance training increases the density of mitochondria within muscle. In addition, training seems to increase the ability of each mitochondrion to generate energy, further improving endurance capacity.
Ribosomes: Small particles, present in large numbers in every living cell, whose function is to convert stored genetic information into protein molecules. In this synthesis process, a molecule ofmessenger ribonucleic acid is fed through the ribosome, and each successive trinucleotide codon on the messenger is recognized by complementary base-pairing to the anticodon of an appropriate transfer RNA (tRNA) molecule, which is in turn covalently bound to a specific amino acid. The successive amino acids become linked together on the ribosome, forming a polypeptide chain whose amino acid sequence has thus been determined by the nucleic acid sequence of the mRNA. The polypeptide is subsequently folded into an active protein molecule. Ribosomes are themselves complex arrays of protein and RNA molecules.
Ribosomes are composed of two subunits, one approximately twice the size of the other. In the bacterium Escherichia coli, whose ribosomes have been the most extensively studied, the smaller subunit (30S) contains 21 proteins and a single 16S RNA molecule. The larger (50S) subunit contains 32 proteins, and two RNA molecules (23S and 5S). The overall mass ratio of RNA to protein is about 2:1. Cations, in particular magnesium and polyamines, play an important role in maintaining the integrity of the ribosomal structures. The ribosomes are considerably larger in the cytoplasm of higher organisms (eukaryotes). Nevertheless, all ribosomal RNA molecules have a central core of conserved structure, which presumably reflects the universality of the ribosomal function.
Endoplasmic Reticulum: Membrane system within the cytoplasm of a eukaryotic cell, important in the synthesis of proteins and lipids. The ER usually makes up more than half the membrane of the cell and is continuous with the outer membrane of the nuclear envelope. There are two distinct regions of ER: the rough ER, or RER (so called because of the protein-synthesizing ribosomes attached to it), and the smooth ER (SER), which is not associated with ribosomes and is involved in the synthesis of lipids and the detoxification of some toxic chemicals.
Lisosomes: Membrane-enclosed organelle found in all eukaryotic cells that is responsible for the cell's digestion of macromolecules, old cell parts, and microorganisms. Lysosomes contain a wide variety of enzymes that break down macromolecules such as nucleic acids, proteins, and polysaccharides. Many of the products of lysosomal digestion, including amino acids and nucleotides, are recycled back to the cell for use in synthesizing new cellular components.
Centriole: A morphologically complex cellular organelle at the focus of centrosomes in animal cells and some lower plant cells. Centrioles typically are not found singly; the centrosome of higher animal cells contains a pair of centrioles (together called the diplosome). Centrioles are typically 300–700 nm in length and 250 nm in diameter. Although they can be detected by the light microscope, an electron microscope is required to resolve their substructure. At the electron microscopic level, a centriole consists of a hollow cylinder of nine triplet microtubules in a pinwheel arrangement. The only clearly demonstrated role for the centriole is to organize the axoneme (central microtubular complex) of the primary cilium in cells having this structure, and the flagellar axoneme in sperm cells. Some authorities assert that when present in the centrosome, centrioles contain activities that serve to organize the centrosome, determine the number of centrosomes in a cell, and control the doubling of the centrosome as a whole before mitosis. Others believe that centrioles have no role in the formation and doubling of the centrosomes but are associated with the centrosomes only to ensure the equal distribution of basal bodies during cell division.
Cytoskeleton: System of microscopic filaments or fibres, present in the cytoplasm of eukaryotic cells, that organizes other cell components, maintains cell shape, and is responsible for cell locomotion and for movement of the organelles within it. Three major types of filaments make up the cytoskeleton: actin filaments, microtubules, and intermediate filaments. Actin filaments occur as constantly changing bundles of parallel fibres; they help determine cell shape, help the cell adhere to surfaces, help the cell move, and assist in cell division during mitosis. Intermediate filaments are very stable structures that form the cell's true skeleton; they anchor the nucleus within the cell and give the cell its elastic properties.
Golgi Complex: The Golgi apparatus (GA), also called Golgi body or Golgi complex and found universally in both plant and animal cells, is typically comprised of a series of five to eight cup-shaped, membrane-covered sacs called cisternae that look something like a stack of deflated balloons. Similarly, the number of Golgi bodies in a cell varies according to its function. Animal cells generally contain between ten and twenty Golgi stacks per cell, which are linked into a single complex by tubular connections between cisternae. This complex is usually located close to the cell nucleus. The Golgi apparatus is often considered the distribution and shipping department for the cell's chemical products. It modifies proteins and lipids (fats) that have been built in the endoplasmic reticulum and prepares them for export outside of the cell or for transport to other locations in the cell. Proteins and lipids built in the smooth and rough endoplasmic reticulum bud off in tiny bubble-like vesicles that move through the cytoplasm until they reach the Golgi complex. The vesicles fuse with the Golgi membranes and release their internally stored molecules into the organelle. Once inside, the compounds are further processed by the Golgi apparatus, which adds molecules or chops tiny pieces off the ends. When completed, the product is extruded from the GA in a vesicle and directed to its final destination inside or outside the cell. The exported products are secretions of proteins or glycoproteins that are part of the cell's function in the organism. Other products are returned to the endoplasmic reticulum or may undergo maturation to become lysosomes.
