Transcript The Chemistry of Life

The Chemistry of Life
Learning Objectives
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Describe the basic structure of an atom.
Understand how electrons determine atom
interaction.
Define the term isotope and understand
how they are used in science.
Describe ionic, covalent and hydrogen
bonds.
Learning Objectives
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Understand the biologically important
characteristics of water.
Understand the four major classes of
organic molecules, their functions, and
their building blocks.
Atoms
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All matter is composed of atoms; the
smallest particles that can be divided and
still maintain its chemical properties.
Atom:
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Protons (+)
Electrons (-)
Neutrons ( )
3.1 Atoms
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An atom can be characterized by the number of
protons it has or by its overall mass
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atomic number
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the number of protons in the nucleus
atoms with the same atomic number exhibit the same
chemical properties and are considered to belong to the
same element
mass number
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the number of protons plus neutrons in the nucleus
electrons have negligible mass
3.1 Atoms
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Electrons determine the chemical behavior
of atoms
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these subatomic components are the parts of
the atom that come close enough to each
other in nature to interact
Ions…What Are They?
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In an electronically neutral atom, there are
an equal number of protons and electrons.
Ions form when atoms do not have an
equal number.
Therefore, all ions are electrically charged.
Isotopes
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Isotopes – atoms that have the same
number of protons but different numbers
of neutrons
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most elements in nature exist as mixtures of
different isotopes
What’s an Isotope?
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The number of protons of an atom is its
atomic number.
Atomic mass of an atom includes the
number of protons and neutrons.
The number of protons never varies, but
the number of neutrons may
change…giving rise to an isotope.
Ions and Isotopes
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Short-lived isotopes
decay rapidly and do
not harm the body
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Can be used as
tracers to study how
the body functions
Figure 3.7 Using a
radioactive tracer to
identify cancer
3.2 Ions and Isotopes
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Some isotopes are unstable and break up
into particles with lower atomic numbers
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this process is known as radioactive decay
Radioactive isotopes have multiple uses
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Medicine: detection and treatment of
disorders
dating fossils
3.2 Ions and Isotopes
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Dating fossils
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the rate of decay of a radioactive element is constant
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by measuring the fraction of radioactive elements that
have decayed, scientists can date fossils
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the older the fossil, the greater the fraction of its
radioactive atoms that have decayed
Figure 3.8 Radioactive isotope dating
How Electrons Determine Actions
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Electrons carry energy (potential energy)
Energy levels surrounding the nucleus
reflects the amount of energy possessed
by the electron existing there.
Less energy is present in electrons close
to the nucleus.
Oxidation and Reduction
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Oxidation occurs when there is an
interaction between an electron and two
atoms..the loss of an electron is oxidation.
Reduction occurs when one atom gains an
electron from another atom.
Electron Orbitals
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The path that an electron follows around a
nucleus is called an orbital.
Each orbital holds 2 electrons.
The first energy level has one orbital..2
electrons.
The second and third energy level has 4
orbitals..8 electrons each.
When orbitals are not filled with electrons, the
atoms are ready to react to fill those orbitals.
Molecules
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A molecule is made up of two or more
atoms held together by a chemical bond.
Chemical bonds firm between atoms
through the interaction of electrons.
What types of chemical bonds do you
know about?
Ionic Bonds
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Ionic bonds form when ions are electrically
attracted to each other by opposite
charges.
NaCl is an example of ionic bonding.
Na gives up an electron to Na; Na+ClIonic bonds are strong and not directional,
supports the formation of crystal.
Covalent Bonds
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Covalent bonds form when electrons are
shared between atoms.
Most organic molecules are formed from
covalent bonds.
Two key properties make covalent bonds
ideal for use in biological molecules:
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They are strong
They are very directional
Hydrogen Bonds
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Hydrogen bonds result from weak
attractions between hydrogen atoms and
the larger atoms of polar molecules.
Hydrogen bonds are weak and highly
directional.
They play an important role in maintaining
the conformation of large, biologically
important molecules.
Hydrogen Bonds and Water
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All organisms are made up of a large amount of
water.
Water is biologically important because it is a
polar molecule and forms hydrogen bonds
between its own molecules.
Unique properties given to water by hydrogen
include:
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Cohesion
Heat storage
Ice formation
High polarity
High heat of vaporization
Heat Storage
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Water has the capacity for heat storage
because of its hydrogen bonds.
Water changes temperature slowly…good
for living organisms.
Assists in regulating homeostasis.
Ice Formation
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When water freezes, the hydrogen bonds
space water molecules apart.
Makes ice less dense than water
High Heat of Vaporization
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A large amount of energy is needed to
break hydrogen bonds in water and turn
the water into vapor.
This mechanism of energy use is why
evaporative cooling removes heat from the
body.
Cohesion
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The attraction of water molecules to other
water molecules.
The surface tension of water is a result of
cohesion.
When other polar molecules are attracted
to water molecules it is called adhesion.
High Polarity
Hydrophilic (water-loving) molecules
attach to water molecules making them
water soluble. (Polar molecules)
 Hydrophobic (water-fearing) molecules are
not readily soluble in water (non-polar).
Why is this important?
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Water Chemistry
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Water ionizes spontaneously, forming
hydrogen ions (H+) and hydroxyl ions
(OH-).
This process is called ionization:
H20
 Hydrogen ion concentration in solution can be
described using the pH scale.
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pH Scale
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Acids are solutions with an increased
concentration of hydrogen ions.
Bases are substances that combines the
hydrogen ion concentration in solution.
The pH inside the cells of most living organisms
is close to 7.
A buffer is a substance that acts as a reservoir
for hydrogen ions. It resists change to pH.
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Blood has the acid-base pair of carbonic acid and
bicarbonate as a buffer (pH of blood is 7.4)
Figure 3.15 The pH scale
Examples of pH in nature
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Hydrangea macrophylla
blossoms are either pink or
blue, depending on a pHdependent mobilization and
uptake of soil aluminium
into the plants.
Examples of pH in living systems
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The pH of different cellular compartments,
body fluids, and organs is usually tightly
regulated in a process called acid-base
homeostasis.
The pH of blood is usually slightly basic
with a value of pH 7.4. This value is often
referred to as physiological pH in biology
and medicine.
Examples of pH in living systems
Compartment
pH
Gastric acid
0.7
Lysosomes
4.5
Granules of chromaffin cells
5.5
Urine
6.0
Neutral H2O at 37 °C
6.81
Cytosol
7.2
Cerebrospinal fluid (CSF)
7.3
Blood
7.34 – 7.45
Mitochondrial matrix
7.5
Pancreas secretions
8.1
3.5 Water Ionizes
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The pH in most living cells and their
environments is fairly close to 7
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Acids and bases are routinely encountered by
living organisms
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proteins involved in metabolism are sensitive to any
pH changes
from metabolic activities (i.e., chemical reactions)
from dietary intake and processing
Organisms use buffers to minimize pH
disturbances
3.5 Water Ionizes
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Buffer – a chemical substance that takes
up or releases hydrogen ions
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buffers don’t remove the acid or the base
affecting pH but minimize their effect on it
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most buffers are pairs of substances, one an
acid and one a base