chapter 2 ap bio - mrs. cousineau's...
TRANSCRIPT
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CHAPTER 2Life’s Chemical Basis
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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.
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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
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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)
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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
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Different Representations
Three ways of showing the hydrogen atom
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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
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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.
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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.
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PET Scanner
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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
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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
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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
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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
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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
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Ionic Bond, NaCl
sodiumatom
chlorineatom
electrontransfer
sodiumion
chlorine ion
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Covalent Bond, Nonpolar
HydrogenH2
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Polar Covalent Bond
water (H2O) H—O—H
slight positive charge
slight positive charge
slight negative charge
-
++
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Hydrogen Bond
water molecules, DNA
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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
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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
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Solvent Properties
ions, polar molecules easily dissolve in water
water clusters around solute forming sphere of hydration
important in fluids for life (blood, sap)
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spheres of hydration
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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
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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
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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
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liquid
Text
ice
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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
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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-)
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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
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Acids and Basesweak acids do not really want to donate, ex. carbonic acid, H2CO3
strong acids do
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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
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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