CHAPTER 2Life’s Chemical Basis

The Chemistry of LifeWe are made up of elements.

Atoms of one kind make up an element.

Atoms are the smallest unit of an element still maintaing the element’s properties.

25 of 92 naturally occurring elements are essential for life.

Carbon, oxygen, hydrogen and nitrogen make up 96% of living matter.

Human CompositionThe most common elements in the human body are oxygen, carbon and hydrogen.

Trace elements are required in minute quantities. ex. iodine essential for thyroid

Oxygen 61.0% Carbon 23.0 Hydrogen 10.0 Nitrogen 2.6 Calcium 1.4 Phosphorus 1.1 Potassium 0.2 Sulfur 0.2

Atoms

Made of three subatomic units:

protons- positive charge (p+), in nucleus

neutrons-no charge (n0), in nucleus

electrons- negative charge (e-), cloud around nucleus

Atomic number = # of protons ex. carbon’s number is 6

Mass number = protons + neutrons (isotopes vary)

Orbitals

Atoms are reactive if there are unpaired electrons in one or more orbitals.

Columns in periodic table behave similarly, same number of electrons in outer valence shell

First orbital can hold 2 electrons, second can hold 8

Different Representations

Three ways of showing the hydrogen atom

IsotopesIsotopes have the same number of protons but differ in the number of neutrons.

Carbon has three isotopes: carbon-12 (99% of carbon in nature), carbon-14 (most of remaining 1%), and carbon-13, which all behave identically in biological reactions.

Unstable, or radioactive isotopes decay spontaneously, giving off particles and energy and can transform to a different element if they give a proton. Ex. carbon-14 decays to nitrogen

Radioisotopes

Radioactive isotopes can be used in biology to date fossils, as tracers in metabolism, and to monitor cancer growth.

They release radiation which can be harmful to cells, but is thought to be safe in small amounts.

The type and amount absorbed matters.

TracersRadioisotopes can be substituted for stable molecules in biological pathways and then detected in the organism by radioactivity detecting devices.

Melvin Calvin used carbon-14 to trace photosynthesis.

The PET scanner is used in medicine to study metabolism--the rate at which cells take in glucose with a radioisotope attached, scanner forms image of body tissues.

Some radioisotopes used to kill cancer cells.

PET Scanner

BondingElectrons can move to outer orbitals when they gain energy such as plant pigments absorbing energy from the sun.

Electrons drop to inner orbitals when they emit energy.

Atoms with vacancies in outer shells will give up, acquire, or share electrons.

Oxygen, carbon, hydrogen, nitrogen, and calcium are

Terms

Molecule- 2 or more atoms joined in a chemical bond

Compound- a molecule with 2 or more different elements in proportions that never vary, ex. H2O, table salt NaCl

Mixture- 2 or more substances intermingle without bonding, proportions may vary, ex. sugar water

Major Bonds

Ionic bonds- ions (atoms that gain or lose electrons take on a negative or positive charge, respectively) of opposite charges attract and stay close to each other, ex. NaCl

Covalent bonds- atoms share electrons in hybrid orbital spanning both atomic nuclei if both atoms need an electron, this is stable and strong, and atoms can share one, two, or three electrons

More about covalent...

If both atoms share the electrons equally, the molecule is nonpolar, there is no difference in the charges at the “ends” of molecule, ex. H2, O2, N2

If electrons are not shared equally, the molecule is polar, and the atom of one “end” of the molecule has more protons in its nucleus so it has a greater pull on the electrons, giving that end a slight negative charge, ex. H2O

Types of Bonds

Hydrogen bonds- this is a weak attraction between covalently bound hydrogen atoms and an electronegative (polar) atom in a different molecule or in a different region of the same molecule in large molecules that fold back on themselves, ex. DNA

the bonds form and break easily, but together can stabilize a molecule ex. liquid water

Ionic Bond, NaCl

sodiumatom

chlorineatom

electrontransfer

sodiumion

chlorine ion

Covalent Bond, Nonpolar

HydrogenH2

Polar Covalent Bond

water (H2O) H—O—H

slight positive charge

slight positive charge

slight negative charge

-

++

Hydrogen Bond

water molecules, DNA

Properties of Water

POLARITY

oxygen end slight neg. charge, hydrogen ends slight positive

attracts ions and other polar molecules, creating hydrogen bonds

makes water cohesive, a universal solvent, and have a high specific heat and heat of vaporization

Universal Solvent

hydrophilic: molecules attracted to water, ex. sugar, ionic and polar molecules

hydrophobic: molecules repelled by water, ex. oil, lipids in cell membrane, nonpolar molecules

Solvent Properties

ions, polar molecules easily dissolve in water

water clusters around solute forming sphere of hydration

important in fluids for life (blood, sap)

spheres of hydration

Temperature Stabilizer-High Specific Heat

cells release a lot of metabolic heat

heat makes hydrogen bonds vibrate more, disrupting bonds, and some bonds break and water molecules escape

water can absorb a lot of heat energy before it rises in temperature and vice versa

High Heat of Vaporization

as water heats, molecular motion increases, some water molecules escape into the air, cooling the surface = evaporation

prevents overheating, moderates Earth’s climate

Aquatic Ecosystems

hydrogen bonds are locked into position in ice, making ice less dense than liquid water

ice can float on a lake, insulating the water and life below

liquid

Text

ice

Cohesion Property

water resists rupturing when stretched or under tension

hydrogen bonds create surface tension

as plants transpire, cohesion of hydrogen bonds pulls replacement water molecules into cells

animation

Acids and Bases

water molecules split into H+ and OH- ions

pH scale measures the concentration of hydrogen ions in solutions

higher H+ = lower pH (less OH-)

lower H+ = higher pH (more OH-)

Acids and Bases

one unit increase on pH scale= 10 times less H+

one unit decrease on pH scale = 10 times more H+

lower pH= acids, donate or release H+ when dissolved in water, ex. lemon juice, gastric juice

higher pH= bases, accept or combine with H+ when dissolved in water, ex. sea water, egg white

Acids and Basesweak acids do not really want to donate, ex. carbonic acid, H2CO3

strong acids do

Salts

any compound that dissolves easily in water and releases ions other than H+ and OH-

acid + base = salt: HCl + NaOH NaCl + H2O

Na, K, Ca ions important for life

Buffers

salt and weak acid working together to maintain homeostasis

system to respond to slight changes in pH so cell processes are not disrupted

blood has carbonic acid and bicarbonate