Transcript BIOL_1010_Chpt2

The Chemical Basis of Life
Chapter 2
Matter
• Substance that has mass and takes up space
– Compose all living things
• Generally found in 1 of 3 states
• Composed of 1 or more elements
Elements
• 92 occur naturally on Earth
• 4 make up 96% of the human body (CHON)
• Composed of atoms
Atoms
• Smallest particles that retains properties of an element
• Made up of subatomic particles:
– Protons (+) in nucleus
– Electrons (-) orbits nucleus
– Neutrons (no charge) in nucleus
• Protons and neutrons
– Mass of about 1
• Electrons
– Mass is negligible (1/2000)
Reading A Periodic Table
• Elements differ depending on the number of
subatomic particles
• Atomic symbol
– 1st letter or 2 (usually)
• Atomic number
– Determined by number of protons
– Element specific
• Mass number
– Determined by number of protons + neutrons
Isotopes
• Atoms with different number of neutrons
– Effects mass number how?
– Effects atomic number how?
• Can be stable or unstable (radioactive)
• Behave the same as respective element (electrons are key)
• Occur naturally as a mix in elements
– Living cells can’t distinguish between them
– Applications
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Dating fossils
Biological tracers
Brain scanning
Cancer treatments
• Dangers
– Radioactive atoms give off energy that destroys chemical bonds when
they collide
Chemical Properties of Atoms
• Electrons are key
– Move in orbitals called shells
– Repel one another, but attracted to protons
• Electron shells
– Outermost determines chemical properties
• Closer to the nucleus = lower energy and are filled first
– Holds up to 2 or 8 electrons
• Filled are unreactive
• Unfilled are reactive
– Number differs between atoms
Electron Shell Models
SODIUM
11p+ , 11e-
CHLORINE
17p+ , 17e-
electron
proton
neutron
CARBON
6p+ , 6e-
OXYGEN
8p+ , 8e-
HYDROGEN
1p+ , 1e-
HELIUM
2p+ , 2e-
NEON
10p+ , 10e-
Chemical Bonds
• Hold 2 or more atoms together
– Complete outer shells
– By sharing, donating, or receiving electrons
• Form molecules (H2, I2, and O2) or compounds
(H20, NaCl, C6H12O6)
– Demonstrates emergent properties
• 2 H+ (gas) + O- (gas) = H2O (water)
• Na+ (metal) + Cl- (poisonous gas) = NaCl (table salt)
Ionic Bond
• One atom loses electrons  cation (charge?)
• Another atom gains these electrons  anion
(charge?)
• Charge difference attracts the two
– Very weak bond
– Table salt (NaCl)
cation
anion
Covalent Bond
• Atoms share outer pair or pairs of valence
electrons
– Single, double, or triple covalent bond
• Non-polar covalent
– Electrons shared equally
– E.g.: hydrogen gas/ H2/ H-H
• Polar covalent
– Electrons spend more time near most electronegative
nucleus
– E.g.: water/ H20
• electrons more attracted to O nucleus than to either H nuclei
Hydrogen Bond
• Positive charge on H attracts
negative charge on another atom
• Individually weak, but often
numerous = strong
• Important to many biological
compounds
– E.g. water
• Makes up 70 – 90% of all living things
• Bonds create unique properties
Properties of Water
• Solubility
– Solvent: dissolving agent
– Solute: substance that is dissolved
– Solution: liquid mix of 2+ substances
• Aqueous solution when water is solvent
• Polarity
– ‘Like dissolves like’
– Hydrophilic
• Sugar or salt and water
– Hydrophobic
• Oil and water
Properties of Water (cont.)
• Water movement
– Cohesion
• Creates surface tension
– Overfilling a glass, a ‘belly flop’, or beading of water
– Adhesion
• Water moves up plants against gravity or paper towels
• Temperature stabilization
– Molecule mov’t affects temp
– Hard to change H-bonds
• Increase  ice formation
• Reduce  evaporation
• Vary  internal temp.
pH Scale
• Measures [H+] of a solution (acidity)
• Change of 1 on scale means 10X change in
[H+] (logarithmic scale)
Highest H+
Lowest H+
0---------------------7-------------------14
Acidic
Neutral
Basic
pH Scale (cont.)
• Acids
– Increase [H+]
– pH less than 7
• Bases
– Decrease [OH-]
– pH greater than 7
• Neutrals
– Equal [H+] and [OH-]
– pH of 7.0
Buffer Systems
• Minimize shifts in pH
• Can accept H+ when in excess and donate H+
when depleted
– Form water
• Common in biological fluids
– Human blood at 7.4, slight deviations can be
deadly
Making and Breaking Bonds
• Chemical reactions are often reversible
– Chemical equilibrium: forward and reverse reactions
occurring at the same rate (no net change)
• Starting materials are reactants, ending are
products
• # of atoms are conserved
– 1st law of thermodynamics