Chemical Basis of Life

Chapter 2

Bio 160

Atoms

• Atoms – smallest complete unit of an element

Neutrons (neutral)

Electrons (-)

Protons (+)

Atoms– Nucleus of an atom is comprised of neutrons

and protons and therefore has a + charge; electrons (-) are attracted to the nucleus and move around the nucleus in specific areas of space called shells

1st shell – holds 2 e-

2nd shell – holds 8 e-

3rd shell – holds 8 e-

Elements

• Elements – consist of only one kind of atom

arranged in periodic table by symbols, along with atomic number and weight

– Atomic number = number of protons = number of electrons (in a neutral atom)

atomic number determines which element is in question

Elements

– Atomic weight = number of protons + number of neutrons

(mass (weight) of protons or neutrons is 1840 x larger than that of e-)

– Isotopes - elements with the same # of protons but a varying # of neutrons, ie, element with the same atomic number but a different atomic weight

Elementsatomic weights are not whole numbers, but

merely an average of all isotopes of an element

unstable isotopes emit energetic particles and are known as radioisotopes

– All elements with atomic number > 84 are radioactive but these elements are not normally present in biological material

Molecules & Compounds

• Molecules - chemical combination of 2 or more atoms of one or more elements bonded together by chemical bonds (atoms don't have to be different) (H2 is a molecule as is H2O)

• Compounds - 2 or more different elements joined together

Chemical Bonding

• Electrons are the part of an atom that actively participates in a chemical reaction (never the nucleus)

• Octet rule: atoms react with one another to achieve 8 e- in outer shell (except 1st shell)

• Chemical behavior of atoms can be explained by behavior of electrons

Chemical Bonding

• Atoms with fewer than 8 electrons in outer shell may transfer or share electrons to complete outer shell

• Ions - atoms that are electrically charged either by gaining or losing electrons

• When atoms react with one another they are attracted to each other by chemical bonds

Types of Chemical Bonding

• Ionic bond – formed when electrons are transferred from one atom to another; the bond is therefore the electrical attraction between 2 oppositely charged ions

compounds containing ionic bonds readily separate (dissociate) into ions in water and are called electrolytes because as charged particles they can conduct an electric current

examples of electrolytes are acids, bases, and salts

Types of Chemical Bonding

if outer shell has 1,2, or 3 e-, the atoms tend to lose the electron and become + ions = cations

if outer shell has 6 or 7 e-, the atoms tend to gain electrons and become - ions = anions

if outer shell has 4 or 5 e-, they don't tend to form ions; they tend to share electrons and form covalent bonds

Types of Chemical Bonding

• Covalent bond – results from sharing electrons; atoms with 4 or 5 electrons in outer shell tend to share electrons and will not form ions; therefore covalent compounds tend to be nonelectrolytes

Carbon has 4 electrons in outer shell and needs 4 more electrons

More compounds of carbon than any other element

Types of Chemical Bonding

– Nonpolar covalent - equal sharing of electrons; no charged regions

water is a good example of a polar covalent compound

– Polar Covalent - electrons are not equally shared; results in charged regions

Types of Chemical Bonding

• Hydrogen bonds - weak attraction between a slightly + hydrogen and a slightly -oxygen or nitrogen

H bonds can be between molecules (intermolecular; as in water) or within molecules (intramolecular; as in DNA and protein)

H bonds are significantly weaker than covalent or ionic bonds

Chemical Reactions

• Chemical Reactions – the making and breaking of chemical bonds

– Reactants – starting materials in a reaction

– Products – ending materials in a reaction

Types of Chemical Reactions

• Synthesis - reactions that consume energy to build complicated molecules from simpler ones

• Decomposition - reactions that break down complex molecules into simpler compounds

• Exchange reactions

• Reversible reactions

Acids and Bases• Acids - substances that release H+ in water

• Bases - release OH- or accept H+

• pH - measure of H+ concentration

• Salts - strong acid + strong base→ salt and H2O

Organic Compounds

• Organic compounds - any substance that contains both carbon and hydrogen

monomer + monomer → polymer + H2O (dehydration synthesis = condensation)

polymer + H2O → monomer + monomer (digestion reaction = hydrolysis)

– Polymer - long chain of a repeating molecular unit

Carbohydrates

• Carbohydrates (contain C, H, O) = sugars and starches

most abundant group of organic compounds

provides most readily available source of energy

short term storage of energy

Carbohydrates

Hexose - 6 carbon sugar

Pentose - 5 carbon sugar - ribose/deoxyribose of RNA/DNA

– Monosaccharides

glucose - most abundant hexosegalactose fructose

Carbohydrates

– Disaccharides - Monomer + Monomer = disaccharide

Sucrose – Table sugar

Lactose - Milk

Carbohydrates– Polysaccharides

Starches - principal storage polysaccharide in plants; ex) potatoes, rice, grain

Glycogen - principal storage polysaccharide in animals

Lipids - all tend to be insoluble in water

Lipids

– glycerol (monomer) + 3 fatty acids(monomers)→triglyceride

saturated fats – all bonding sites of the fatty acid are filled with hydrogen

unsaturated fats - some double bonds present so not all bonding sites are filled with hydrogen and the fatty acid has a kink in its shape, preventing solidification

• Triglycerides (fats) - largest of class of lipids

Lipids• Phospholipids - in cell membranes and membranes of

organelles– consists of glycerol + 2 fatty acids + phosphate group

(- charge)

fatty acid tail - hydrophobic = insoluble in water

phosphate head - hydrophilic = soluble in water

– phospholipids are arranged as a bilayer with phosphate heads toward water (outside of bilayer in contact with aqueous solutions inside and outside the cell) and the fatty acid tails toward the interior of the membrane

Lipids

• Steroids

cholesterol (precursor from which most other steroids are synthesized)

sex hormones - estrogen, progesterone, testosterone

cortisol and aldosterone

Vitamin D and bile salts

Proteins

• Protein - most abundant organic molecule in organism (makes up 50% of dry weight)

Monomer = amino acid; polymer = protein (polypeptide chain)

Proteins - contain C, H, O, N and some S

Amino acids are linked by peptide bonds (covalent bonds)

Proteins

• Proteins have four levels of structure which contribute to their unique three-dimensional shape, or conformation

• Denaturation - Proteins lose their conformation, hence their function, due to exposure to heat, radiation, electricity, pH changes, or various chemicals

Nucleic Acids

• DNA and RNA

– pentose sugar (5-carbon monosaccharide) = ribose/deoxyribose

– phosphate group

monomer = nucleotide

– nitrogenous base