Basic Cellular Biology

Living organisms are complicated, albeit organized, systems. Organization manifests itself as a molecular hierarchy of systems and pathways, occurring as organs (in larger creatures) made up of tissues, which themselves are made up of cells. The chemistry of living cells resembles the chemistry of organic reactions. The constituents of living cells are called biomolecules. Cells contain structures called organelles, which are complex assemblies of large molecules (macromolecules). Both of these components serve functional purposes. Also, living organisms can self-replicate.

Some reactions in the cell require energy to occur. Living systems harness energy from the environments in which they occur. Organisms acquire energy either through photosynthesis (like plants) or through the metabolism of food (like animals), by forming special biomolecules, such as adenosine triphosphate (ATP) or NADPH2, that can serve as chemical sources of stored energy for thermodynamically unfavorable reactions.

All biomolecules contain carbon. The major precursors of biomolecules are water, carbon dioxide, ammonium, nitrate, and nitrogen. Precursors form metabolites. Metabolites are simple, organic compounds that are intermediates in cellular reactions, such as the biosynthesis of amino acids, sugars, nucleotides, and fatty acids. Metabolites are converted into larger macromolecules, such as proteins, polysaccharides, polynucleotides, and lipids. Macromolecules interact to form supramolecular complexes, such as enzyme complexes, ribosomes, chromosomes, and cytoskeletal elements. Organelles are membrane-bound sites of supramolecular complexes, such as the nucleus, mitochondria, and chloroplasts. Prokaryotic cells do not contain organelles. Eukaryotic cells are believed to have originally formed organelles from engulfed bacteria (symbiotic theory).

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