Chemistry and Organic Compounds for Nursing School Entrance Exam Study Guide (page 3)

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Chemistry and Organic Compounds for Nursing School Entrance Exam Practice Problems

Organic Compounds

1. Definition

Organic compounds are compounds made of carbon and hydrogen (hydrocarbon) and heteroatoms such as oxygen, nitrogen, the halogens, phosphorus, sulfur, and others.

2. Stereoisomers

Stereoisomers are two molecules having the same molecular formula and structure but different spatial orientation with respect to the median axis or plane of the molecule. Their three-dimensional shapes are, therefore, very different.

3. Carbohydrates
   1. Function

   Carbohydrates (or sugars) serve as the main source of energy for living organisms. They are made of one, two, or more rings of carbon, hydrogen, and oxygen. The names of carbohydrates end with the suffix, -ose (for example, glucose and fructose).

   2. Monosaccharides

   Monosaccharides are the simplest carbohydrate structures made of one ring that can contain five C atoms, called a pentose, or six C atoms, called a hexose. An example of a pentose is ribose which is a constituent of RNA. One example of a hexose is galactose, that is derived from milk-sugar lactose.

   3. Disaccharides

   Disaccharides are dimeric sugars made of two monosaccharides joined together in a reaction that releases a molecule of water (dehydration). The bond between the two sugar molecules is called glycosidic linkage and can have either an axial (β-glycoside) or an equatorial (α-glycoside) orientation with respect to the ring conformation.

   Examples:
   Maltose is two glucose molecules joined together, found in starch.
   Lactose is one galactose joined to one glucose, found in milk.
   Sucrose is one fructose joined to one glucose, found in table sugar.
   4. Polysaccharides

   Polysaccharides are polymers or a long chain of repeating monosaccharide units.

   • Starch is a mixture of two kinds of polymers of α glucose (linear amylose and amylopectin). Amylose contains glucose molecules joined together by α-glycosidic linkages while amylopectin has an addition of branching at C-6. They are storage polysaccharides in plants.
   • Glycogen consists of glucose molecules linked by α-glycosidic linkage (C-1 and C-4) and branched (C-6) by α-glycosidic linkage. Glycogen is the storage form of glucose in animals (in liver and skeletal muscle).
   • Cellulose consists of glucose molecules joined together by β-glycosidic linkage. Cellulose is found in plants and is not digested by humans (since they lack the necessary enzyme).
   5. Condensation and Hydrolysis

   Condensation is the process of bonding together separate monosaccharide subunits into a disaccharide and/or a polysaccharide. It is also called dehydration synthesis, as one molecule of water is lost in the process. It is carried out by specific enzymes.

   Hydrolysis is the reverse process of condensation as a water molecule and specific enzymes break all the glycosidic linkages in disaccharides and polysaccharides into their constituting monosaccharides.

4. Lipids
   1. Function

   Lipids are a diverse group of compounds that are insoluble in water and polar solvents but soluble in nonpolar solvents. Lipids are stored in the body as a source of energy (twice the energy provided by equal amounts of carbohydrates).

   2. Triglycerides

   Triglycerides are lipids formed by condensation of glycerol (one molecule) with fatty acids (three molecules). They can be saturated (from fatty acid containing only C-C single bonds) or unsaturated (presence of one or more C=C double bonds). Triglycerides are found in the adipose cells of the body (neutral fat) and are metabolized by the enzyme lipase (an esterase) during hydrolysis, producing fatty acids and glycerol.

   3. Ketone Bodies

   There are three ketone bodies formed during the breakdown (metabolism) of fats: acetoacetate, β-hydroxybutyrate, and acetone. They are produced to meet the energy requirements of other tissues.Fatty acids—produced by hydrolysis of triglycerides—are converted to ketone bodies in the liver. They are removed by the kidneys (ketosuria), but if they are found in excess in the blood (ketonemia), ketone bodies can cause a decrease of the blood pH and ketoacidosis may result. In ketouria, acetone is exhaled via the lungs. The whole process is called ketosis. Ketosuria and ketonemia are common in patients with diabetes mellitus and cases of prolonged starvation.

   4. Phospholipids

   Phospholipids are lipids containing a phosphate group. They are the main constituents of cellular membranes.

   5. Steroids

   Steroids are organic compounds characterized by a core structure known as gonane (three cyclohexane—six carbon rings and one cyclopentane—or five C rings fused together). Steroids differ by the functional groups attached to the gonane core. Cholesterol is an example of a steroid and is a precursor for the steroid hormones such as the sex hormones (androgens and estrogens) and the corticosteroids (hormones of the adrenal cortex).

5. Proteins
   1. Functions

   Every organism contains thousands of different proteins with a variety of functions: structure (collagen, histones), transport (hemoglobin, serum albumin), defense (antibodies, fibrinogen for blood coagulation), control and regulation (insulin), catalysis (enzymes), and storage.

   2. Structure

   Proteins (also called polypeptides) are long chains of amino acids joined together by covalent bonds of the same type (peptide or amide bonds). There are 20 naturally occurring amino acids, each characterized by an amino group at one end and a carboxylic acid group at the other end. Different proteins have different numbers and kinds of additional functional groups.

   The sequence of amino acids in the long chain defines the primary structure of a protein.

   A secondary structure is determined when several residues, linked by hydrogen bonds, conform to a given combination (for example, the α-helix or β-turns).

   Tertiary structure refers to the three-dimensionally folded conformation of a protein. This is the biologically active conformation (crystal structure).

   A quaternary structure can result when two or more individual proteins assemble into two or more polypeptide chains.

   Conjugated proteins are complexes of proteins with other biomolecules (for example, glycoproteins, also called sugar proteins).

   3. Enzymes

   Enzymes are biological catalysts whose role is to increase the rate of chemical (metabolic) reactions without being consumed in the reaction. They do so by lowering the activation energy of a reaction by binding specifically (in the active site) to their substrates in a "lock and key" or "induced-fit" mechanism. They do not change the nature of the reaction (in fact, any change is associated with a malfunctioning enzyme, the onset of a disease) or its outcome. (See the following.)

   Enzyme activity is influenced by:

   • temperature; proteins can be destroyed at high temperatures and their action is slowed at low temperature.
   • pH; enzymes are active in a certain range of the pH.
   • concentration of cofactors and coenzymes (vitamins)
   • concentration of enzymes and substrates
   • feedback reactions

   

   Enzyme names are derived from their substrate names with the addition of the suffix, -ase. An example is sucrase (substrate is sucrose). There are categories of enzymes according to the reactions they catalyze (for example, the kinases, or phosphorylation).

   Enzymes are often found in multienzyme systems that operate by simple negative feedback.

   

   4. Protein Denaturation

   Protein denaturation occurs when the protein configuration is changed by the destruction of the secondary and tertiary structures (reduced to the primary structure).Common denaturing agents are alcohol, heat, and heavy metal salts.