CHEMICAL BASIS OF LIFE
(Chapters 2-3)
1. The characteristics of all living things are determined by their similar chemical
makeup. 90 chemical elements occur in nature & 25 of these are essential to
life. Living things are 96% carbon, hydrogen, oxygen, & nitrogen.
2. Elements are composed of atoms:
a) the center is called the nucleus & contains positively charged particles
called protons & particles that have no electrical charge called neutrons
b) electrons are negatively charged particles – they are located on energy
levels outside the nucleus
c) atoms contain equal numbers of electrons & protons – this gives them no
electrical charge
3. A compound is a substance composed of atoms of 2 or more different elements.
(H2O, CH4, NaCl, CaCl2) This happens because atoms become more stable when
their outermost energy level is full. (2  8  18)
4. Atoms combine with other atoms & are held together by chemical bonds.
a) covalent bond - sharing of electrons (water H2O & sugar C6H12O6 &
methane CH4 & ammonia NH4) – this is a very strong bond (handcuff
chains) & is common in living things (sugars, fats, proteins, & water)
b) ionic bond - gaining or losing electrons – these atoms have electrical
charge & are called ions (salt NaCl & sodium fluoride NaFl) – this is a weak
bond (magnets) – ions are most often part of biological processes
(transmission of nerve impulses, muscle contraction, plant roots absorbing
minerals)
5. Water is essential to life - it makes up 70% - 95% of most living things. It
transports materials within living things & most life processes occur only when
molecules are dissolved in water.
6. Water is an inorganic compound because it contains no carbon. A molecule of
water consists of 1 atom of oxygen & 2 atoms of hydrogen held together at
an angle by a covalent bond.
7. The hydrogen side of a water molecule is positively charged & the oxygen side
is negatively charged. This uneven distribution of electrical charge makes
water a polar compound.
8. Water molecules are attracted to each other. The positively charged
hydrogen atoms of 1 water molecule attract the negatively charged oxygen
atoms of another water molecule. This attraction of opposite charges forms a
weak bond called a hydrogen bond.
9. The properties of water are a result of the polarity of water molecules & the
weak hydrogen bonding with other water molecules.
a) easily dissolves other compounds (salt, sugar, oxygen)
b) moves upward by capillarity (plants, blood, meniscus, capillary tube)
because molecules stick to other substances by adhesion
c) expands when it freezes (pond ice floats, cracks rocks to form soil)
d) resists change in temperature & helps maintain environmental
conditions (protection of aquatic species, our body temp. & air temp.)
e) droplets hold together like a magnet – this is called cohesion – it
maintains surface tension (insects/pond) & buoyancy (jellyfish)
10. Organic compounds contain carbon & are made by living things. They are
covalently bonded to other carbon atoms & to hydrogen, oxygen, & nitrogen
to form these 4 compounds: carbohydrates, lipids, proteins, & nucleic acids.
11. Carbohydrates:
a) sugars & starches are used by cells to store & release energy
b) carbohydrates are composed of carbon, hydrogen, & oxygen molecules
that form a ring-like structure (CHO = 1:2:1 ratio)
c) monosaccharides are simple sugars - glucose & fructose
d) disaccharides are 2 sugars linked together - glucose + fructose = sucrose
e) polysaccharides are many monosaccharide subunits - these complex
carbohydrates include: starch (plant food storage in seeds & roots),
glycogen (mammals store in liver), chitin (exoskeletons of insects, spiders,
shrimp, etc.) & cellulose (plant cell walls)
12. Lipids:
a) store energy & are a major part of membranes surrounding all living cells
b) lipids are made up of fatty acids attached to glycerol molecules
c) these large non-polar molecules do not dissolve in water – these long
molecules contain only carbon, hydrogen, & oxygen - there are always
twice as many hydrogen atoms as oxygen atoms (H:O = 2:1)
d) commonly called fats (animal body fat, waterproof wax on leaves) & oils
e) other lipids include steroids found in hormones, cholesterol, &
animal/plant venom
13. Proteins:
a) many small molecules bond together to form large complex proteins
called polymers - composed of carbon, hydrogen, oxygen, & nitrogen
b) examples in nature include:
1) make up tissues & organs – muscles
2) enzymes carry out the chemical reactions in cells – all of these
reactions are referred to as metabolism
3) antibodies destroy germs
4) pigments that give hair, eyes, & flowers their colors
c) 20 amino acids are the basic building blocks of 1000’s of different types
of protein molecules - these amino acids are held together by peptide
bonds & form a chain called a polypeptide
d) the structure of an amino acid can be recognized by the presence of a
NH2 group & a COOH group
14. nucleic acids:
a) very large complex molecules that store important information for cell
characteristics & processes
b) examples are DNA – deoxyribonucleic acid & RNA – ribonucleic acid
c) nucleic acids are composed of 1000’s of subunits called nucleotides
d) 1 nucleotide = a phosphate group + a 5-carbon sugar + a nitrogen
base
15. pH is the acidity or alkalinity of a solution. This results from dissociation
when a water molecule breaks apart into 2 ions of opposite charge:
H2O  H+ (hydrogen ions) = OH- (hydroxide ions)
a) more H+ result in acids forming – this creates an acidic solution
b) more OH- result in bases forming – this creates an alkaline/basic solution
16. Acids & bases are measured with a pH scale ranging from 0-14:
0---1---2---3---4---5---6---7---8---9---10---11---12---13--14
acidic
neutral
basic
a) acid solutions have a pH of 0 to 6.9 – sour taste & are highly corrosive
b) base solutions have a pH of 7.1 to 14 – bitter taste & feel slippery
17. Enzymes (also called catalysts) are protein molecules that change the rate of
chemical reactions in cells to breakdown or create molecules. This process is
similar to a lock & key mechanism:
a) an area on the enzyme’s surface called the active sites attaches to a
specific molecule called the substrate (hydrogen peroxide or sucrose)
b) the enzyme weakens the chemical bonds – this results in the hydrolysis
or breakdown of the substrate into smaller molecules called products
(water & oxygen or glucose & fructose)
c) after the reaction, enzymes are released unchanged & they can repeat
the reaction again & again
d) enzymes may fail to work if their environment is changed in some
way – for example, a change in temperature or pH can cause a change
in the shape of the enzyme or substrate & the reaction can’t occur