Chemicals of Life

Chemistry• chemicals give cells properties of life

–must know principles of chemistry to understand biology

–organisms-bags of chemicals

–structure determines function

• Hierarchy

• chemicalsorganellescellstissuesorgansorganisms

Matter• living things are made of

matter

–anything that occupies space & has mass

• composed of elements

Elements• substances which cannot be decomposed

into simpler substances by chemical means

• 92 naturally occurring ones

• 25 found in living things

• 96%-H, O, C & N• 4%-Ca, P, K & S• trace elements-Fe, Mg, Mn (manganese) & I

– called essential– cannot live without them

Periodic Table

Atoms• elements are composed of atoms

• only one kind of atom for each element

– smallest units of matter that retain the properties of an element

Sub-atomic Particles• Protons• Electrons• Neutrons

–different elements have different numbers of sub-atomic particles

–chemical & physical differences of elements are due to these differences

Sub-Atomic Particles• Protons

– one positive charge– found in nucleus of

atom

• Neutrons– no charge– found in nucleus of

atom

• Electrons– one negative charge– orbit nucleus

Periodic Table• atom’s structure

determines how element it comprises forms compounds & molecules

• key to this can be found by knowing number of sub-atomic particles an element possess

• found in Periodic Table of the Elements

atomic number-number in upper left corneratomic number-number in upper left corneratomic weight or mass number-number atomic weight or mass number-number located on bottomlocated on bottom

Atomic Number• gives number of

protons in an atom

• Helium

• Atomic Number = 2

–has 2 protons

Mass Number• found by adding number

of neutrons & number of protons

• He = 4• some atoms of an element

may have different mass numbers

• these are isotopes– same number of

protons & electrons but different number of neutrons

Number of Electrons• net charge of an atom is zero• each proton has one positive

charge• each electron has one

negative charge• neutrons have no charge• to be neural atom must have

the same number of electrons and protons

• atom of helium with 2 protons • has 2 electrons

Atomic Number• How many protons does Sulfur have?

• How many electrons?

• Why?

Chemical Properties• arrangement of electrons

determines chemical properties of an atom

• electrons orbit around nucleus• found at different energy levels-

shells• each shell accommodates a

specific number of electrons

Electron Shells• innermost shell-2 • 2nd & 3rd-8• number of electrons in

outermost shell determines chemical properties of atom

• those with shells that are not full will interact with other atoms & participate in chemical reactions

• those with full shells do not interact-inert

Electron Shells• H-only one electron in outer most shell

– very reactive

• C, N, & O• also highly reactive since outer shells are incomplete• He-inert or nonreactive because outer shell is full

Reactivity of Chlorine

• Is Chlorine reactive or not reactive?

• How can you tell?

Atom Interactions-Chemical Bonds• when an atom with an incomplete outer

shell reacts with another atom with an incomplete outer shell they can interact or form a bond

• Share

• Donate

• Receive electrons

• in this way both atoms can have a completed outer shell

Types of Chemical Bonds

• Covalent Bonds– share electrons

• Ionic– give or receive

electrons

Sodium & Chloride

Ionic Bonding

Covalent Bonds• 2 atoms with incomplete shells

• each share a couple electrons

• so at any one time one atom has a completed outer shell

• forms molecules

In Class Exercise• Pretend you are an atom

• Take the number of your birth month as your atomic number

• Determine the configuration of electrons in your valence shell

• Find another atom that you might interact with to form ions or new molecules

• Demonstrate-draw on the board

Chemical Reactions• elements combine to make molecules &

compounds• 2 H2 + O2 2H2O• 2 molecules of H react with one

molecule of O (reactants) to form 2 molecules of water (product)

• arrow indicates direction of reaction• two sides of equation on either side of

arrow must balance

Water• single most important

constituent of body

• life on Earth depends on unusual structure & nature of water

Importance of Water• Organisms consist mostly of water

– 2/3rds total body weight of humans

• Biochemistry is a wet chemistry– biological molecules do not

react chemically unless in solution

• Water is an important reactant– nearly all chemical reactions in

the body occur in water• Foods are digested to their

building blocks by decomposition reactions called hydrolysis– involves addition of water

• When large molecules form from smaller ones, water is removed in dehydration synthesis or condensation reactions

Water Structure• 2 H atoms attached covalently

to one O2 molecule• sharing of electrons is not equal• oxygen pulls more on

electrons than does hydrogen • electrons spend more time near

oxygen than hydrogen• unequal sharing produces-

polar bond• Nonpolar bond

– bonds in which two atoms involved have an equal pull on electrons

Water Structure• O2 attracts electrons more strongly than H-

giving water an asymmetrical distribution of charge

• result-V-shape

• H end has positive charge

• Oxygen end has negative charge

• water has 2 poles-polar

• polarity results in weak electrical attractions between neighboring water molecules

Polarity of Water• Polarity results in weak

electrical attraction between neighboring water molecules

• slightly positive Hs in one water molecule attract slightly negatively charged O2 in another water molecule

• opposite electrical charges attract

• water molecules attract each othermaking water kind of sticky

• these weak attractions-hydrogen bonds– much weaker than covalent

or ionic bonds

Polarity, H Bonding & Water’s Properties

• cohesive nature

• ability to moderate temperature

• ice floating

• universal solvent properties

Dissociation• compounds formed by ionic

bonds will ionize or dissociate in water

• dissociation produces cations (+) & anions (-)

• aqueous solution containing anions & cations will conduct electrical currents

• cations move to negative side having a + change & anions move to positive side having a – charge

• soluble inorganic molecules whose ions will conduct an electrical current in solution are called electrolytes

• NaCl is an electrolyte• NaCl + H2ONa+ + Cl-

Acids• H+ & OH- are in solutions at all times• H2O <-------> H+ + OH- -reversible

reaction• some compounds add more hydrogen ions• others remove them• compound that donates hydrogen ion-

acid• HClH+ + Cl-

• acidic solution is one that has more H+ than OH-

Bases• compound that accepts or

removes hydrogen ions is a base

• NaOHNa+ +OH-

• basic solution- has more OH- than H+

pH scale• pH = potential hydrogen• scale developed in

1909 by Dane Soren Sorensen– beer brewer looking

for a way to check acidity of beer

• describes how acidic or basic a solution is

• scale ranges from 0-14• 0 = most acidic• 14 = most basic• 7=neutral

pH• at neutral pH• H+ = OH-

• pH < 7 is acidic• pH > than 7 is more basic or alkaline

Buffers• pH of blood ranges

between 7.35-7.45• value must be maintained

in narrow range• even small change can

lead to severe metabolic consequences

• biological fluids contain buffers– substances that resist

changes in pH by accepting H+ when in excess & donating H+

when depleted