chemical composition of the body

Chapter 2

Chemical Composition of the Body

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Objectives

Explain how covalent bonds are formed and distinguish between nonpolar and polar covalent bonds.

Describe the nature of hydrogen bonds and explain their significance.

Describe the structure of DNA and RNA, and explain the law of complementary base pairing.


Chemical Bonds, Molecules, and Ionic Compounds

Chemical bonds: Interaction of valence electrons

between 2 or more atoms. Number of bonds determined by

number of electrons needed to complete outermost shell.


Covalent Bonds

Atoms share their valence electrons.

Nonpolar covalent bonds:

Electrons are equally distributed between the two identical atoms.

Strongest bond. H2 and 02.

Figure 2.2


Covalent Bonds (continued)

Polar bonds: Electrons are

shared between two different atoms.

Electrons may be pulled more toward one atom.

Has + and – poles. Oxygen, nitrogen,

phosphorous have tendency to pull electrons towards themselves.

Figure 2.4


Ionic Bonds

One or more valence electrons from one atom are completely transferred to a second atom.

Cation and anion attract, form ionic compound.

Weaker than polar covalent bonds.

Dissociate easily when dissolved in H20.

Form hydration spheres.

Make an ion/molecule more soluble.

NaCl Na++ Cl-

Figure 2.6

Figure 2.5


Ionic Bonds (continued)

Glucose, amino acids, are H20 soluble. Hydration spheres form around atoms of

oxygen, nitrogen, phosphorous. Hydrophilic molecules.

Molecules composed of nonpolar covalent bonds are not H20 soluble. Cannot form hydration spheres.

Hydrophobic molecules.


Hydrogen Bond

Hydrogen forms a polar bond with another atom, giving hydrogen has a slight + charge.

Weak attraction for a second electronegative atom.

Surface tension.

Insert fig. 2.7

Figure 2.7


Acid/Base

Acid: Molecule that can release protons

(H+). Proton donor.

Base: Negatively charged ion that can

combine with H+, and remove it from solution.

Proton acceptor.


pH

pH = log _1__ [H+] [H+] = molar concentration of H+. pH inversely related to [H+].

Because of logarithmic relationship, a solution with 10 times [H+] of H20 has a pH = 6; solution with 0.1 the [H+] has a pH = 8.


Organic MACROMolecules

Molecules that contain carbon and hydrogen.

Carbon has 4 electrons in outer shell and covalently bonds to fill its outer shell.

Functional groups: Inactive “backbone” to

which more reactive atoms are attached.

Carbonyl group: Aldehydes and ketones.

Carboxyl group: Organic acids (lactic and

acetic acids). Hydroxyl group:

Alcohol. Figure 2.10


Carbohydrates Organic molecules that

contain carbon, hydrogen and oxygen.

CnH2n0n. Monosaccharides:

Simple sugars. Glucose, fructose, galactose.

Disaccharide: 2 monosaccharides joined

covalently. Sucrose (glucose and

fructose), lactose (glucose and galactose), maltose (2 glucose).

Polysaccharide: Numerous monosaccharides

joined covalently. Starch (thousands of glucose

joined), glycogen (repeating glucose joined that are highly branched).

Mechanism for storing energy with less osmotic H20 movement.

Figure 2.13


Lipids

Diverse group of molecules. Differ greatly in chemical structure. Insoluble in polar solvents (H20). Consist primarily of hydrocarbon chains

and rings. Hydrophobic.


LIPIDS: Triglycerides (triacylglycerol)

Formed by condensation of glycerol and 3 fatty acids.

Fatty acids consist of nonpolar hydrocarbon chain with carboxyl end.

Saturated: Hydrocarbon chains

joined by single covalent bonds.

Unsaturated: Double covalent bonds

within hydrocarbon chain.

Figure 2.17

Figure 2.18


LIPIDS:Ketone Bodies

Hydrolysis of triglycerides in adipose tissue release free fatty acids.

Free fatty acids can be converted in the liver to ketone bodies.

Ketoacidosis: Increased ketone

bodies in the blood which lowers pH.

Figure 2.19


LIPIDS: Phospholipids

Phospholipids: Number of different

categories of lipids that contain phosphate group.

Nonpolar end is hydrophobic, polar end is hydrophilic.

Lecithin: Phosphate attached to

a nitrogen-containing choline molecule.

Figure 2.20


LIPIDS: Steroids

All have same basic structure; three 6-carbon rings joined to a 5-carbon ring. Nonpolar and

insoluble in H20. Cholesterol is

precursor for steroid hormones.

Figure 2.22


LIPIDS: Prostaglandins

Fatty acid with cyclic hydrocarbon group.

Derived from arachidonic acid.

Serve a variety of regulatory functions.

Blood vessel diameter, ovulation, uterine contractions, inflammation, blood clotting.

Figure 2.23


Proteins Large molecules

composed of long chains of amino acids.

20 different amino acids can be used in constructing a given protein.

Each amino acid contains an amino group (NH2) at one end and carboxyl group (COOH) at the other end.

Differences between amino acids are due to differences in functional groups (“R”).

Figure 2.24


Protein Structure Level

Primary structure: Sequence of the amino acids

in the protein is described. Secondary structure:

Weak hydrogen bonds form between hydrogen of 1 amino acid and the and oxygen of a different amino acid nearby.

a-helix or b-sheet. Tertiary structure:

Polypeptide chains bend and fold to produce 3 -dimensional shape.

Formed and stabilized by weak chemical bonds between functional groups.

Quaternary structure: Number of polypeptide

chains covalently linked together.

Figure 2.26


Nucleic Acids

Include DNA and RNA. Nucleotides:

Subunits of nucleic acids bonded together to form long polynucleotide chains.

Each composed of 3 smaller units:

5-carbon sugar. Phosphate group

attached to one end of sugar.

Nitrogenous base attached to other end of sugar.

Nitrogenous bases: Pyrimidines: single ring

of carbon and nitrogen. Purines: two rings of

carbon and nitrogen.Figure 2.29


NUCLEIC ACIDS: DNA and RNA

DNA: Basis of genetic code. Deoxyribose covalently

bonded to 1 of 4 bases: Purines: guanine and

adenine. Pyrimidines: cytosine and

thymine. Sugar-phosphate bonds

form the chain. Each base can form

hydrogen bonds with other bases.

Two strands are are produced by hydrogen bonding.

RNA: Consists of a single long

chain of nucleotides joined together by sugar-phosphate bonds.

Ribose covalently bonds to 4 bases.

Uracil replaces thymine.

Figure 2.32

chemical composition of the body

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