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| Summary |
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 The macromolecules found in a cell are large molecules with a complex structure and very high molecular weight. They are represented by polysaccharides, proteins and nucleic acids. |
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 Polysaccharides are polymers of monosaccharides. They are linear, branched or unbranched molecular chains. They are mainly of two types: homopolysaccharides that are composed of monomers of the same kind and heteropolysaccharides that are composed of modified monosaccharide units. |
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 Starch and glycogen are homopolysaccharides that serve as reserve carbohydrates in plant cells and animals respectively. |
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 Cellulose is a structural homopolysaccharide found in the cell wall of plant cells. Pectin, hemicellulose and lignin are also similar polysaccharies that occur in the cell wall of some specific plant cells. |
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 Monopolysaccharides and heteropolysaccharides occur in certain types of animal cells. |
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 Chitin is a hard impermeable polysaccharide that forms the exoskeleton in arthropods. |
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 Peptiglycons are found in the cell walls of prokaryotic cells. |
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 Proteins are polymers of aminoacids arranged in a specific sequence which is decided by the genetic code present in the DNA molecule. |
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 The polypeptide chain in a protein may be straight (primary structure) or may form helical coils (secondary structure) or a 3-dimensional structure (tertiary structure). Sometimes two or more polypeptides may function together as a protein molecule (quaternary structure). |
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 Protein serve several functions in the cell, the most significant being their role as enzymes. |
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 Proteins also form structural components or may form components responsible for electron transport or transport of respiratory gases. |
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 Nucleic acids are polymers of nucleotides, joined together by phospho diester linkages. |
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 There are two types of nucleic acids deoxy ribose nucleic acid (DNA) and ribose nucleic acid (RNA). |
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 DNA functions as the genetic material except in some viruses. |
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 DNA molecule is composed of two polynucleotide chains that are coiled around each other. Each chain has nucleotides of adenine, guanine, cytosine and thymine. |
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 The structural details of the DNA molecule are very well explained by the double helix model proposed by Watson and Crick. |
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 The most significant aspect of DNA molecule is the highly specific nitrogen base pairing between the two opposite chains. A purine pairs only with a pyrimidine. Among purines adenine pairs only with thymine and guanine pairs only with cytosine. |
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 DNA molecule is capable of producing its own identical copies by a process called replication, which occurs regularly prior to cell division. |
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 RNA is a nucleic acid with a single polynucleotide chain consisting of nucleotides of adenine, guanine, cytosine and uracil. |
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 There are 3 types of RNA messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA) all of which have a specific role in the biosynthesis of proteins. |
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 RNA cannot undergo replication and have to be synthesised only from DNA. |
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 Enzymes are basically proteins that function as biological catalysts. |
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 Enzymes carry on their catalytic action by lowering the activation energy required for a reaction to proceed. |
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 Sometimes an enzyme may have non-protein prosthetic groups. It is called co-factor if it is a mineral ion and a coenzyme if it is an organic compound. Such an enzyme is called apoenzyme. The apoenzyme alongwith its prosthetic group is called holoenzyme. |
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 Enzymes are highly specific about their substrates. Enzyme actively depends on temperature and pH. |
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 The active site of an enzyme binds with the reaction site of the substrate forming an enzyme substrate complex. It is followed by the formation and release of products, along with the enzyme. |
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 A substance closely similar to a substrate in its molecular structure may block the active site and inhibit the enzyme (competitive inhibition). |
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 A substance which is not similar in structure to a substrate can also inhibit an enzyme by binding to a site other than the active site (non-competitive inhibition). |
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 A substance can bind with an enzyme; modify the active site so that the substrate cannot bind (allosteric inhibition). |
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 Enzymes are classified into 6 categories oxidoreductases, hydrolases, transferases, isomerases, lyases and ligases based on the reaction they catalyse. |
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 Enzymes are generally named by adding a suffix "ase" to the name of the substrate. |
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