Download What is life composed of?

 What is life’s chemical basis?
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Atoms
Fundamental building blocks of matter
Nucleus
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Positively charged protons
Uncharged neutrons (except for hydrogen)
Electrons move around the nucleus
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Negatively charged
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What are elements?
Element
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pure substance consisting of atoms
same number of protons (atomic number)
Isotopes
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atoms of same element
differ in number of neutrons (atomic weight)
Living cells have ~
H- 62%
O- 24%%
C – 12%
N – 2%
Isotopes emit radiation!
Radioisotopes
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are not stable
emit particles and energy as they decay spontaneously into other elements
Example: 14C → 14N
Tracer
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Molecule with a detectable substance attached
PET scans
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Electrons occupy orbitals
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space around nucleus
two electrons
Shell model
How electrons are arranged?
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energy levels as successively larger circles, or shells
view an atom’s electron structure
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Atoms are neutral!
atom with equal numbers of protons and electrons has no net charge
Ions (positive or negative)
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Atoms that have gained or lost electrons
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Difference between molecules and mixtures
Chemical bond
Attractive force that unites atoms into a molecule
Compounds
Molecules consisting of two or more elements
Mixture
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Substances intermingle but don’t bond
2.4 What Happens When Atoms Interact?
Whether one atom will bond with others
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depends on number and arrangement of its electrons
Common interactions in biological molecules:
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Ionic bond
Covalent bond
Hydrogen bond
What are ionic bonds?
Strong association between a positive ion and a negative ion (attraction of opposite
charges)
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What are covalent bonds?
Two atoms share a pair of electrons
Nonpolar covalent bond
Atoms share electrons equally
Polar covalent bond
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Electrons are shared unequally
One end slightly negative, other slightly positive
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Polar molecule has a separation of charge
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What are hydrogen bonds?
Form between a hydrogen atom and an electronegative atom
Each with separate polar covalent bonds
Are not chemical bonds
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Do not make atoms into molecules
Individually weak
Collectively stabilize structures of large molecules
How is water molecule formed?
Water molecules are polar
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Form hydrogen bonds with other polar molecules
Hydrophilic substances (water-loving)
Hydrophobic substances (water-dreading)
Why is water important for life?
Polarity gives liquid water unique properties that make life possible:
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Resistance to temperature changes
Internal cohesion
Dissolves polar and ionic substances
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What are acids and bases?
pH scale
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hydrogen ion (H+) concentration
0 (most acidic) to 14 (most basic or alkaline)
pH 7 (neutral)
H+ & OH– concentrations equal
Acids donate H+ in water
More H+ than OH-
Bases accept H+ in water
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More OH- than H+
How are salts formed?
Compounds that dissolve easily in water, and release ions other than H+ and OH-
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Form when an acid interacts with a base
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Example: NaCl
HCl + NaOH ↔ NaCl + H2O
What are buffer systems?
A set of chemicals (a weak acid or base and its salt) that keeps the pH of a solution
stable
buffers help maintain homeostasis
processes proceed only within a narrow pH range, usually near neutrality
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Acidosis
Alkalosis
One donates ions, the other accepts them