Chapter 2 – Chemical Composition of the Body

“Because living things, including humans, are composed only of

chemicals, it is absolutely essential for a (physiology) student to have a basic understanding of chemistry.”

Sylvia Mader..

• Atoms – smallest unit of an element that can undergo a chemical change.

• Composed of subatomic particles:Protons = (+ charge) found in the nucleusNeutrons = (no charge) found in the nucleusElectrons = (e-,- charge) found orbiting the

nucleus in the electron cloud

• The number of protons determines the atom’s identity, e.g. 6 P = carbon..

• The number of N and/or e- can change.Isotope = change the number of NIon = change the number of e-

cation = positive ion (how does it become positive?)

anion = negative ion (how does it become

negative?)..

• Atomic number = sum of P• Atomic mass = sum of P and N

• The position of an atom’s e- is the electron configuration. We will not study configurations except for the outermost level of e- = valence shell

• The e- found there are the valence e-.• The valence e- are the ones involved

in ordinary chemical reactions..

• Bonding is accomplished by interactions between two atom’s valence e-.– If e- are shared between two atoms that

forms a covalent bond.• Single bonds = one shared pair• Double bonds = two shared pairs• Triple bonds = three shared pairs

– If e- are transferred from one atom (ion) to another that forms an ionic bond.

• Hydrogen bonds are special (polar) covalent bonds that are very important to physiology.

Covalent AnimationIonic Animation..

• Electronegativity is the property that describes an atom’s attraction for a shared pair of e-.

• If two atoms with different electro-negativity values share e-, i.e. form a covalent bond, one of the atoms will have a “larger share” of the e-.

• This produces a molecule with differently charged ends (poles). This type of molecule is called polar..

• Bonds formed between the hydrogen end (+ charged) of a polar molecule and the – end of any other polar molecule or highly electronegative atom (e.g. P, N, O) are called hydrogen bonds.

• These hydrogen bonds are very important because they alter the physical and chemical properties of many molecules (especially water)..

• Molecules that are formed by polar covalent bonds have a tendency to break apart when the electron from the hydrogen is transferred to the more electronegative atom. This is called dissociation or ionization.

• Water ionizes to form equal amounts of hydroxyl (OH-) and hydrogen (hydro-nium, H+) ions..

• An acid is a molecule that can release protons (H+).

–Proton donor.

• A base is a molecule that can combine with H+ and remove it from solution. Bases are also defined as releasing OH-.

–Proton acceptor..

• 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.

• Hydrogen plus hydroxide molar concentrations will always equal 10-

14..

• A buffer is a system of molecules and ions that act to prevent changes in [H+] and stabilizes pH of a solution.

• In blood:* H20 + C02 H2C03 H+ +

HC03-

–Reaction can proceed in either direction (depending upon the concentration of molecules and ions)..

• Arterial blood pH remains remarkably stable at 7.35 – 7.45 because of buffering systems especially the bicarbonate/carbonic acid system. HC03

- and H2C03 act as buffer pair.

•Acidosis = pH < 7.35.•Alkalosis = pH > 7.45..

Organic Molecules• Composed primarily of H, C, O, & N.• Look at the common functional groups.

Carbohydrates

• Organic molecules that contain carbon, hydrogen and oxygen.– CnH2n0n.

• Monosaccharides = simple sugars.– Structural isomers:

•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)..

• Organic molecules are built by dehydration synthesis:

C6H12O6 + C6H12O6 C12H22O11 + H2O

• Organic Molecules are broken down by hydrolysis C12H22O11 + H2O C6H12O6 + C6H12O6

Lipids• Insoluble in water because of nonpolar

molecules– Triglycerides = 3 fatty acids + glycerol

• Saturated = joined by only single bonds• Unsaturated = joined by at least one double

bond

• Hydrolysis of triglycerides in adipose tissue releases free fatty acids.– Free fatty acids can be converted in the

liver to ketone bodies.– Excess ketone bodies can lower blood pH..

– Phospholipids = phosphate + fatty acid• phosphate end is polar = hydrophilic• fatty acid end is nonpolar = hydrophobic

– Steroids = aromatic rings = three 6-carbon rings joined to a 5-carbon ring• Steroid hormones are cholesterol derivitaves..

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”).

• Amino acids are joined by peptide bonds..

Protein Structure Levels

• Primary structure is the sequence of the amino acids in the protein.

• Secondary structure is produced by weak hydrogen bonds between hydrogen of one amino acid and the and oxygen of a different amino acid nearby.

-helix or -sheet..

• Tertiary structure is formed when polypeptide chains bend and fold to produce 3 -dimensional shape.– Formed and stabilized by weak

chemical bonds between functional groups.• Each type of protein has its own own

characteristic tertiary structure.

• Quaternary structure is produced when a number of polypeptide chains covalently linked together..

Nucleic Acids

• Include DNA and RNA.• Nucleic acids are composed of

nucleotides to form long polynucleotide chains.– Each nucleotide is composed of 3

smaller units:• 5-carbon sugar (deoxyribose or ribose).• Phosphate group attached to one end of

sugar.• Nitrogenous base attached to other end

of sugar..

• The “backbone” of the nucleic acid is formed by the sugar and phosphate pairs.

• The “rungs” are formed by paired nitrogenous bases.– Nitrogenous bases complementary pair

• A + T (U)• C + G..