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# CH 2. MOLECULAR COMPONENTS OF CELLS ## 1. Introduction This chapter describes the chemical basis of cells and the properties of chemical molecules found within. Analysis of cells at the atomic level reveals that only 27 chemical elements (among more than 100 found on Earth) occur in living organ...

# CH 2. MOLECULAR COMPONENTS OF CELLS ## 1. Introduction This chapter describes the chemical basis of cells and the properties of chemical molecules found within. Analysis of cells at the atomic level reveals that only 27 chemical elements (among more than 100 found on Earth) occur in living organisms. Four elements, C, H, O and N, are the most abundant, representing about 96% of the atoms. Other elements such as P, Ca, S, Cl, Na, K, and Mg represent about 4%. Trace elements like Cu, Zn, Se, I, and Mo exist in very low concentrations. | Molecule Type or Class | % of Cell Weight | Number or Types | |---|---|---| | H₂O | ~70% | ~28 | | Inorganic Ions | ~0.4%| ~200+ | | Carbohydrates and precursors| ~0.3% | ~200+ | | Lipids and precursors | ~0.4% | ~200+ | | Nucleotides and precursors | ~0.2% | ~300+ | | Other small molecules | ~0.2% | ~200+ | At the molecular level, all cells contain carbohydrates, lipids, nucleic acids and small quantities of other diverse organic compounds like vitamins and soluble salts. Proteins, carbohydrates, lipids and nucleic acids are organic molecules. In this class, certain molecules may be very large and are termed macromolecules. ## II. Inorganic Compounds ### 1. Water Water is the most abundant constituent of cells, accounting for 65% (average for animal cells) and 80% (average for plant cells). The abundance differs between cell types and is the lowest in bone cells. Water is extremely polar due to partial charges on oxygen and hydrogen atoms. This polarity allows water molecules to form hydrogen bonds with each other and other cell components. Water's properties are essential for many biological processes, such as solute solubility, hydrolysis, photosynthesis, thermal regulation, and diffusion. ### 2. Mineral Salts Salts like sodium chloride, potassium chloride, and magnesium sulfate, when dissolved in water, become ionized. Each ion is surrounded by water molecules. The balance of these ions, as well as the acid-base balance (pH) and osmotic pressure, are crucial for physiological processes like membrane permeability, cell division, muscle contraction and enzymatic activities. The ion concentration inside a cell is different from the liquid (e.g., of the intercellular space), so the passage of ions through the plasma membrane is selective and controlled. Certain mineral compounds can exists as insoluble solids such as calcium carbonate and calcium phosphates, forming structures like skeletons, etc ## III. Organic Compounds Organic compounds, such as monosaccharides, fatty acids, nucleotides, amino acids occur as monomers, or as large macromolecules such as proteins, polysaccharides (starch, cellulose and glycogen), DNA and RNA. These macromolecules are essential for various cellular processes. ## IV. PROTEINS ### 1. Introduction Proteins are the most abundant and diverse class of macromolecules, comprising 55-85% of the cell's dry weight. They are encoded by genes and perform a vast array of functions, including catalysis, structure, and transport. Proteins are unbranched polymers of amino acids. Their sequences are determined by genetic information. Though DNA carries genetic instructions, proteins carry out the actions. Proteins like hemoglobin are crucial in functions such as oxygen/carbon dioxide transport. ### 2. Functions of Proteins Proteins determine cell structure, shape, and perform many physiological functions. Proteins also participate in storing and transporting various particles and play important roles in cell communication, immune response, muscle contraction (using chemical energy, ATP) and sensory functions (e.g., sight, hearing). ### 3. Diverse Chemical Composition Proteins are relatively homogeneous in terms of their function, structure and sequences. They're long polymer chains, composed of linear sequences of 20 different types of amino acids. The sequence determines protein types and their diversity, 3D structures and functions. ### 4. The Amino Acids Amino acids are amphoteric molecules, composed of carbon, oxygen, nitrogen, hydrogen, and sulfur. 20 amino acid types are found in proteins. 19 have a similar chemical formula but differ only by the structure of their side chains (R groups). Among them, 11 are classified as essential and must be obtained from the diet. Amino acids can be grouped by polarity and charge of their side chains (e.g., nonpolar/hydrophobic, polar/hydrophilic, acidic or basic). ### 5. Polypeptide (peptide) A polypeptide is an unbranched chain of amino acids linked by covalent peptide bonds, formed via a dehydration reaction. The resulting protein shape (3D structure) is key component of function.