Cellular Composition and Macromolecules

• Hydrogen, oxygen, nitrogen, carbon, phosphorus, and sulfur constitute over 99% of living cells' mass.
• These elements combine to form organic biomolecules, which synthesize building blocks for macromolecules.
• 99% of the molecules inside living cells are water molecules.
• Water accounts for about 70% of a cell's wet weight.
• The four general classes of macromolecules in living cells are nucleic acids, proteins, polysaccharides, and lipids.
• Macromolecules have molecular weights ranging from 1 x 103 to 1 x 106.
• They are created through polymerization of building blocks with molecular weights in the range of 50 to 150.
• Cells generally contain a greater variety of proteins than any other type of macromolecule.
• About 50% of the solid matter of the cell is protein (15% on a wet-weight basis).
• Cells typically have more protein molecules than DNA molecules.
• DNA is typically the largest biomolecule in the cell.
• Cells normally contain more protein than DNA.
• Data regarding the chemical composition of Escherichia coli (E. coli) are not greatly different for multicellular organisms, including mammals.
• Each E. coli contains a single chromosome, meaning it has only one unique DNA molecule.
• Mammals contain more chromosomes and thus have different DNA molecules in their nuclei.

Nucleic Acids

• Nucleic acids are nucleotide polymers that store and transmit genetic information.
• Only 4 different nucleotides are used in nucleic acid biosynthesis.
• Genetic information contained in nucleic acids is stored and replicated in chromosomes, which contain genes.
• A chromosome is a deoxyribonucleic acid (DNA) molecule, and genes are segments of intact DNA.
• The total number of genes in any given mammalian cell may total several thousand.
• When a cell replicates itself, identical copies of DNA molecules are produced.
• The hereditary line of descent is conserved, and the genetic information carried on DNA is available to direct the occurrence of virtually all chemical reactions within the cell.
• The bulk of genetic information carried on DNA provides instructions for the assembly of every protein molecule within the cell.
• The flow of information from nucleic acids to protein is commonly represented as DNA —> messenger ribonucleic acid (mRNA) —> transfer RNA (tRNA) —> ribosomal RNA (rRNA) —> protein
• The nucleotide sequence in a gene of DNA specifies the assembly of a nucleotide sequence in an mRNA molecule, which in turn directs the assembly of the amino acid sequence in protein through tRNA and rRNA molecules.

Proteins

• Proteins are amino acid polymers responsible for implementing instructions contained within the genetic code.
• Twenty different amino acids are used to synthesize proteins.
• About half are formed as metabolic intermediates, while the remainder must be provided through the diet; the latter group is referred to as "essential" amino acids
• Each protein formed in the body, unique in its own structure and function, participates in processes that characterize the individuality of cells, tissues, organs, and organ systems.
• A typical cell contains thousands of different proteins, each with a different function.
• Many proteins serve as enzymes that catalyze (or speed) reactions and virtually every reaction in a living cell requires an enzyme.
• Other proteins transport different compounds either outside or inside cells (e.g., lipoproteins and transferrin in plasma, or bilirubin-binding proteins in liver cells).
• Some act as storage proteins (e.g., myoglobin binds and stores O2 in muscle cells).
• Some are defense proteins in blood or on the surface of cells (e.g., clotting proteins and immunoglobulins).
• Some are contractile proteins (e.g., the actin, myosin and troponin of skeletal muscle fibers) and others are structural (e.g., collagen and elastin).
• Proteins, unlike glycogen and triglyceride, are usually not synthesized and stored as nonfunctional entities.

Polysaccharides

• Polysaccharides are polymers of simple sugars (i.e., monosaccharides).
• Some polysaccharides are homogenous polymers that contain only one kind of sugar (e.g., glycogen), while others are complex heterogeneous polymers that contain 8-10 types of sugar.
• Homogenous polymers are noninformational.
• Heterogenous polymers (e.g., proteins, nucleic acids, and some polysaccharides) are informational
• Polysaccharides can occur as functional and structural components of cells (e.g., glycoproteins and glycolipids), or merely as noninformational storage forms of energy (e.g., glycogen).
• The 8-10 monosaccharides that become the building blocks for heterogenous polysaccharides can be synthesized from glucose or formed from other metabolic intermediates.

Lipids

• Lipids are naturally occurring, nonpolar substances that are mostly insoluble in water (with the exceptions being the short-chain volatile fatty acids and ketone bodies).
• They are soluble in nonpolar solvents (like chloroform and ether).
• Lipids serve as membrane components (cholesterol, glycolipids and phospholipids), storage forms of energy (triglycerides), precursors to other important biomolecules (fatty acids), insulation barriers (neutral fat stores), protective coatings to prevent infection and excessive gain or loss of water.
• Lipids include some vitamins (A, D, E, and K) and hormones (steroid hormones).
• Major classes of lipids are the saturated and unsaturated fatty acids (short, medium, and long-chain), triglycerides, lipoproteins {i.e., chylomicrons (CMs), very low density (VLDL), low density (LDL), intermediate density (IDL), and high density lipoproteins (HDL)}, phospholipids and glycolipids, steroids (cholesterol, progesterone, etc.), and eicosanoids (prostaglandins, thromboxanes, and leukotrienes).
• All non-essential lipids can be generated from acetyl-CoA.
• Acetyl-CoA can be generated from carbohydrates, amino acids, short-chain volatile fatty acids , ketone bodies, and fatty acids.
• Simple lipids include only those that are esters of fatty acids and an alcohol (e.g., mono-, di- and triglycerides).
• Compound lipids include various materials that contain other substances in addition to an alcohol and fatty acid (e.g., phosphoacylglycerols, sphingomyelins, and cerebrosides).
• Derived lipids include those that cannot be neatly classified into either of the above (e.g., steroids, eicosanoids, and the fat-soluble vitamins).

Inorganic Elements

• Inorganic elements (macrominerals, trace elements, and ultra trace elements) are also important.
• Several are "essential" nutrients, and therefore like certain amino acids and unsaturated fatty acids, must be supplied in the diet.
• Like certain amino acids and unsaturated fatty acids, various inorganic elements are dietarily "essential".
• Inorganic elements are typically present in cells as ionic forms, existing as either free ions or complexed with organic molecules.
• Many "trace elements" are known to be essential for life, health, and reproduction, and have well-established actions (e.g., cofactors for enzymes, sites for binding of oxygen (in transport), and structural components of nonenzymatic macromolecules).

Physiological Chemistry

• Life depends upon chemical reactions.
• Most all diseases in animals are manifestations of abnormalities in biomolecules, chemical reactions, or biochemical pathways.
• Most all diseases in animals are manifestations of abnormalities in biomolecules, chemical reactions, or biochemical pathways.
• A fundamental understanding of physiological chemistry is needed not only to help illuminate the origin of disease, but also to help formulate appropriate therapies.