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Chapter 1
Chemical Composition of Living Cells
2
All living organisms, from microbes to mam- mals, are composed of chemical substances from both the inorganic and organic world, that appear in roughly the same proportions, and perform the same general tasks. Hydrogen, oxygen, nitrogen, carbon, phosphorus, and sulfur normally make up more than 99% of the mass of living cells, and when combined in various ways, form virtually all known organic biomolecules. They are initially utilized in the synthesis of a small number of building blocks that are, in turn, used in the construction of a vast array of vital macromolecules ( Fig 1-1 ). There are four general classes of macromol- ecules within living cells: nucleic acids, proteins, polysaccharides, and lipids. These compounds, which have molecular weights ranging from 1 x 10^3 to 1 x 10^6 , are created through polymerization of building blocks that have molecular weights in the range of 50 to
- Although subtle differences do exist
between cells (e.g., erythrocyte, liver, muscle or fat cell), they all generally contain a greater variety of proteins than any other type of macromolecule, with about 50% of the solid matter of the cell being protein (15% on a wet- weight basis). Cells generally contain many more protein molecules than DNA molecules, yet DNA is typically the largest biomolecule in the cell. About 99% of cellular molecules are water molecules, with water normally accounting for approximately 70% of the total wet-weight of the cell. Although water is obviously important to the vitality of all living cells, the bulk of our attention is usually focused on the other 1% of biomolecules. Data in Table 1-1 regarding the chemical composition of the unicellular Escherichia coli (E. coli), are not greatly different for multicel- lular organisms, including mammals. Each E. coli, and similar bacterium, contains a single chromosome, therefore, it has only one unique
Overview
- Hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus normally makeup more **than 99% of the mass of living cells.
- Ninety-nine percent of the molecules inside living cells are water molecules.
- Cells normally contain more protein than DNA.
- Homogenous polymers are noninformational.
- All non-essential lipids can be generated from acetyl-CoA.
- Like certain amino acids and unsaturated fatty acids, various inorganic elements** **are dietarily "essential".
- Most all diseases in animals are manifestations of abnormalities in biomolecules,** chemical reactions, or biochemical pathways.
14 Amino Acid and Protein Metabolism 3
DNA molecule. Mammals, however, contain more chromosomes, and thus have different DNA molecules in the nucleus.
Nucleic Acids
Nucleic acids are nucleotide polymers (from the Greek word poly , meaning "several", and mer , meaning "unit"), 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 (from the Greek word gennan , meaning "to produce"). 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 repli- cates itself, identical copies of DNA molecules are produced, therefore the hereditary line of descent is conserved, and the genetic informa- tion 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
O
OH
O-^ CH 2
O O
HO OH OH OH
CH 2 OH
CH 3 (CH 2 ) 14 COOH (Palmitic Acid)
Nucleotides
H 2 N CH R Amino acids
C OH
H C
CH 3
CH 3
CH 3
HO
CH 2 CH 2 CH 2 C CH (^3) CH (^3)
H
O
C Acetyl CoA
H 3 C S CoA
O
Simple Sugars (e.g. Glucose)
B 1 B 2 B 3 B 4 R 1 R 2 R 3 R 4 NUCLEIC ACIDS PROTEINS
POLYSACCHARIDES
(Cholesterol)
B = N-Containing Base R = Side Chain
B
LIPIDS
Formation of Macromolecules Within Cells
Hydrogen, Oxygen, Nitrogen, Carbon, Phosphorus, Sulfur
Figure 1-
14 Amino Acid and Protein Metabolism 5
stores O 2 in muscle cells); others as defense proteins in blood or on the surface of cells (e.g., clotting proteins and immunoglobulins); others as contractile proteins (e.g., the actin, myosin and troponin of skeletal muscle fibers); and others are merely structural in nature (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). (The term saccharide is derived from the Greek word sakchar , meaning "sugar or sweetness".) Some polysaccharides are homogeneous polymers that contain only one kind of sugar (e.g., glycogen), while others are complex heteroge- nous polymers that contain 8-10 types of sugars. In contrast to heterogenous polymers (e.g., proteins, nucleic acids, and some poly- saccharides), homogenous polymers are considered to be " noninformational ". Polysac- charides, therefore, can occur as functional and structural components of cells (e.g., glyco- proteins and glycolipids), or merely as nonin- formational 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 inter- mediates (see Chapter 20).
Lipids
Lipids (from the Greek word lipos , meaning "fat") are naturally occurring, nonpolar substances that are mostly insoluble in water (with the exceptions being the short-chain volatile fatty acids and ketone bodies), yet soluble in nonpolar solvents (like chloroform and ether). They serve as membrane compo- nents (cholesterol, glycolipids and phospho- lipids), 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, and 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 lipids can be synthesized from acetyl-CoA , which in turn can be generated from numerous different sources, including carbohydrates, amino acids, short-chain volatile fatty acids (e.g., acetate), 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, sphin- gomyelins, and cerebrosides), and derived lipids include those that cannot be neatly classi- fied into either of the above (e.g., steroids, eicosanoids, and the fat-soluble vitamins). Although the study of physiological chemistry emphasizes organic molecules, the inorganic elements (sometimes subdivided into macro- minerals, trace elements, and ultra trace elements), are also important (see Chapter 48). Several are " essential " nutrients, and therefore like certain amino acids and unsaturated fatty acids , must be supplied in the diet. 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 macromole- cules; see Chapters 48-52). Some investigators have speculated that perhaps all of the elements on the periodic chart will someday by shown to exhibit physiologic roles in mam- malian life. Because life depends upon chemical reactions, and because most all diseases in animals are manifestations of abnormalities in biomolecules, chemical reactions, or biochem- ical pathways, physiological chemistry has become the language of all basic medical sciences. A fundamental understanding of this science is therefore needed not only to help illu- minate the origin of disease, but also to help formulate appropriate therapies. The chapters which follow were designed, therefore, to assist the reader in developing a basic rational approach to the practice of veterinary medicine.
Questions
- The most prevalent compound in a living cell is normally: a. Protein b. Nucleic acid c. Water d. Lipid e. Polysaccharide
- The basic building block for all lipids is: a. Water b. Acetyl-CoA c. Phosphorus d. Nucleic acid e. Arginine 3. The largest biomolecule in a living cell is usually: a. Glycogen b. Protein c. Cholesterol d. Deoxyribonucleic acid e. Triglyceride 4. Which one of the following is a largely homogenous polymer, and therefore "noninformational"? a. mRNA b. Phospholipid c. Protein d. Hydrogen e. Glycogen 5. Select the FALSE statement below: a. Some inorganic elements are consid- ered to be "essential" nutrients. b. Triglycerides are considered to be "simple" lipids. c. Some polysaccharides are complex polymers in that they contain several different types of sugars. d. Virtually every reaction in a living cell requires an enzyme. e. Only 10 "essential" amino acids are used in the synthesis of proteins. 6. About 50% of the solid matter in a cell is normally composed of: a. Nucleic acids b. Protein c. Carbohydrate d. Lipid e. Inorganic ions
Answers
- c
- b
- d
- e
- e
- b
6 Chemical Composition of Living Cells
