Transcript water and life - Parkway C-2

The Chemical
Basis of Life
TRACING LIFE DOWN TO THE CHEMICAL
LEVEL
• Biology includes the study of life at many levels
• In order to understand life, we will start at the
microscopic level of cells, and work our way
down to the macroscopic level, the ecosystem.
• Cells consist of enormous numbers of chemicals
that give the cell the properties we recognize as
life
Matter: Elements and Compounds
• Matter is anything that occupies space and has mass
• Matter is found on the Earth in three physical states
– Solid
– Liquid
– Gas
SOME BASIC CHEMISTRY
• Take any biological system apart and you eventually
end up at the chemical level
Matter
• Anything that fills space
• Element - cannot be broken down
• 92 natural elements
• Symbol - first letter or two
• 4 most common in humans 96%
– O -oxygen
C -carbon
– H -hydrogen
N -nitrogen
– Other 4 % trace elements such as iron
• Twenty-five elements are essential to life
– Four of these
make up about
96% of the
weight of the
human body
– Trace elements
occur in smaller
amounts
Figure 2.3
• Trace elements are essential for life
– An iodine deficiency causes goiter
Figure 2.4
Atomic components
• Compounds - 2 or more elements combined
• Atom - smallest unit of matter (indivisible)
• Basic structure of atoms (Sub atomic
Particles)
– Protons, neutrons, and electrons
• Protons
– positively charged
• Neutrons
– No charge
Atomic components
• Electrons
– Negative charge
– 1/2000th of a subatomic unit
• Nucleus contains
– Protons and neutrons
– Atomic core
• Electrons orbit nucleus
• # of electrons = # of protons in an uncharged
atom
• All the elements are listed in the periodic table
Atomic number
Element symbol
Mass number
Figure 2.2
Atoms
• Each element consists of one kind of atom
– An atom is the smallest unit of matter that still retains
the properties of an element
Nucleus
(a)
(b)
Cloud of negative
charge (2 electrons)
2
Protons
2
Neutrons
2
Electrons
Figure 2.5
Atoms
• Atomic number
– Equal to # of protons
– Unique to each element
– Identifies an element
• Atomic Mass
– # protons + # neutrons
• What about electrons?
• Isotope of an element
– Differ in number of neutrons
– May be radioactive
Isotopes
• Isotopes are alternate mass forms of an element
– They have the same number of protons and electrons
– But they have a different number of neutrons
Table 2.1
• Radioactive isotopes
– The nucleus decays, giving off particles and energy
• Radioactive isotopes have many uses in research
and medicine
– Example: PET scans
(a)
Hearing
words
Seeing
words
Speaking
words
Generating
words
(b)
Figure 2.6
• Uncontrolled exposure to radioactive isotopes can
harm living organisms by damaging DNA
– Example: the 1999 Tokaimura nuclear accident
Electron Arrangement and the Chemical Properties
of Atoms
• Electrons determine how an atom behaves when it
encounters other atoms
• Electrons orbit the nucleus of an atom in specific
electron shells
– The number of electrons in the outermost shell
determines the chemical properties of an atom
Why elements react (or don’t)
• Electrons fill ‘shells’
– 1st shell up to 2 electrons (innermost)
– 2nd shell up to 8 electrons
– 3rd shell up to 8 electrons
– 4th shell up to 8 electrons
– Etc
• Electrons fill innermost shells first
• Atoms interact to ‘fill’ outer shell
• Atoms with filled outer shells don’t react typically
• Atoms of the four elements
most abundant in life
First
electron shell
(can hold
2 electrons)
Outermost
electron shell
(can hold
8 electrons)
Electron
Hydrogen (H)
Atomic number = 1
Carbon (C)
Atomic number = 6
Nitrogen (N)
Atomic number = 7
Oxygen (O)
Atomic number = 8
Figure 2.7
Atom activity
• Atomic #
• Atomic mass
• Symbol
• Name of element
• Draw element
• Is the outermost electron shell filled?
• Will this element react with other elements?
• Elements can combine to form compounds
– These are substances that contain two or more
elements in a fixed ratio
– Example: NaCl (salt)
Chemical Bonding and Molecules
• Chemical reactions enable atoms to give up or
acquire electrons in order to complete their outer
shells
– These interactions usually result in atoms staying
close together
– The atoms are held together by chemical bonds
Ionic Bonds
• When an atom loses or
gains electrons, it
becomes electrically
charged
– Charged atoms are
called ions
– Ionic bonds are
formed between
oppositely charged
ions
Crystal of sodium chloride
Covalent Bonds
• A covalent bond
forms when two
atoms share one or
more pairs of outershell electrons
• Can form double
– 2 sets of electrons
• Triple bonds
– 3 pairs of electrons
Chemical Bonds
• The structure that results when atoms are joined
together by covalent bonds is called a molecule.
• A molecule is the smallest unit of most compounds.
• ***************************************
Chemical Bonds
• In a water molecule,
each hydrogen atom
forms a single covalent
bond with the oxygen
atom.
Water Molecule
WATER AND LIFE
• Life on Earth began in water and evolved there for 3
billion years
– Modern life still remains tied to water
– Your cells are composed of 70%–95% water
• The abundance of water is a major reason Earth is
habitable
Figure 2.10
Figure 2.10x
The Structure of Water
• Studied in isolation, the water molecule is
deceptively simple
– Its two hydrogen atoms are joined to one oxygen
atom by single covalent bonds
H
H
O
Unnumbered Figure 2.2
• But the electrons of the covalent bonds are not
shared equally between oxygen and hydrogen
– This unequal sharing makes water a polar molecule
– Electronegativity – how strongly it pulls electrons to
nucleus (oxygen very electronegative)
()
()
()
()
• The polarity of
water results in
weak electrical
attractions between
neighboring water
molecules
– These
interactions
are called
hydrogen
bonds
()
Hydrogen bond
()
()
()
()
()
()
()
(b)
Figure 2.11b
Water’s Life-Supporting Properties
• The polarity of water molecules and the hydrogen
bonding that results explain most of water’s lifesupporting properties
– Water’s cohesive nature
– Water’s ability to moderate temperature
– Floating ice
– Versatility of water as a solvent (to break up other
molecules into ions which are used in biological
reactions)
The Cohesion of Water
• Water molecules
stick together as a
result of hydrogen
bonding
– This is called
cohesion
Microscopic tubes
– Cohesion is
vital for water
transport in
plants
Figure 2.12
• Surface tension is the measure of how difficult it is
to stretch or break the surface of a liquid
– Hydrogen bonds
give water an
unusually high
surface tension
How Water Moderates Temperature
• Because of hydrogen bonding, water has a strong
resistance to temperature change
– Because of hydrogen bonds
• Heat and temperature are related, but different
– Heat is the amount of energy associated with the
movement of the atoms and molecules in a body of
matter
– Temperature measures the intensity of heat
• Average speed of molecules
• Not total amount of energy
• Water can absorb and store large amounts of heat
while only changing a few degrees in temperature
Water can moderate temperatures
– Earth’s giant water
supply causes
temperatures to stay
within limits that
permit life
– Evaporative cooling
removes heat from the
Earth and from
organisms
The Biological Significance of Ice Floating
• When water molecules get cold, they move apart,
forming ice
– A chunk of ice has fewer molecules than an equal
volume of liquid water
Which picture is liquid
water and which one is
ice?
• The density of ice is lower than liquid water
– This is why ice floats
Hydrogen bond
Ice
Liquid water
Stable hydrogen bonds
Hydrogen bonds
constantly break and re-form
• Since ice floats, ponds, lakes, and even the oceans
do not freeze solid
– Marine life could not survive if bodies of water froze
solid
Water as the Solvent of Life
• A solution is a liquid consisting of two or more
substances evenly mixed
– The dissolving agent is called the solvent
– The dissolved substance is called the solute
Ion in solution
Salt crystal
• When water is the solvent, the result is called an
aqueous solution
• Water acts as a solvent to break up other molecules
into ions which are used for biological reactions.
Acids, Bases, and pH
• Acid
– A chemical compound that donates H+ ions to
solutions
• Base
– A compound that accepts H+ ions and removes them
from solution
pH scale describes acidity
Buffers are substances that resist pH change
– They accept H+ ions when they are in excess
– They donate H+ ions when they are depleted
• Buffering is not
foolproof
– Example: acid
precipitation
Figure 2.18x1
Chemical Reactions
• Cells constantly rearrange molecules by breaking
existing chemical bonds and forming new ones
– Such changes in the chemical composition of matter
are called chemical reactions
Hydrogen gas
Oxygen gas
Reactants
Water
Products
Unnumbered Figure 2.1
• Chemical reactions can be symbolized with
equations
– On the left side of the equation are the reactants, the
starting materials
– On the right side of the equation are the products, the
end materials
• Chemical reactions cannot create or destroy matter
– They only rearrange it