CHEMICAL BONDS
AND REACTIONS
• When you eat food, chemical reactions in
the digestive tract help break down that
food.
• Digestion takes place through the
interactions of stomach acid, hormones,
and other chemicals, along with a network
of nerves and muscles in the digestive
system.
Atoms and Elements
• Every physical thing you can think of, living or not, is
made up of very small particles called atoms
• Atoms are the smallest basic unit of matter.
• An element is a substance made up of one type of atom
and cannot be broken into simpler substances by ordinary
chemical means.
• The element in identified by its number of protons that in
specific for each element.
CHEMICAL BONDS
• The electrons of an atom are responsible for the chemical
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reactions.
The aim of a chemical bond is to have a full outermost
energy level of electrons. This aim is to achieve stability.
IONIC BONDS: atoms become stable by gaining or losing
electrons. When this happens these atoms become
charges and are called ions.
Atoms that gain electrons become negative ions (anions),
while atoms that lose electrons become positive ions
(cations).
Positive and negative
CHEMICAL BONDS
• COVALENT BONDS are the bonds that occur when the
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elements cannot transfer electrons, but share a pair or
more with each other.
Covalent bonds are generally weaker than ionic bonds.
A molecule is 2 or more atoms held together by covalent
bonds.
Covalent bonding is what makes it possible for atoms to
form very large molecules, often with very complex
shapes.
Many substances in living things are composed of large,
complex molecules.
COMPOUNDS
• COMPOUNDS are substances composed of atoms of 2 or
more different elements bonded together in specific ratios.
• Common compounds in living things include water H2O,
and carbon dioxide CO2
• The properties of the compound are often very different
from the properties of elements that make up the
compound.
PROPERTIES of WATER
Life depends on hydrogen bonds in water.
Water is a polar molecule, which means that they form
when atoms in a molecule have unequal pulls on the
electrons they share. In water the oxygen nucleus (8
protons) attracts the shared electrons more strongly than
do the hydrogen nuclei (1 proton each). So oxygen atom
gains a small negative charge, while hydrogen atoms gain
a small positive charge.
Opposite charges of polar molecules can interact to form
hydrogen bonds.
HYDROGEN BONDS
• When a hydrogen atom is a part of a polar molecule, the
hydrogen atom has a slight positive charge.
• The slightly positive hydrogen and a slightly negative
atom (often oxygen or nitrogen) are attracted, a hydrogen
bond is formed.
• Life depends on hydrogen bonds in water.
• Hydrogen bonds are part of the structures of proteins and
DNA molecules.
Properties of hydrogen bonds:
• Hydrogen bonds are about 20 times weaker than typical
covalent bonds, but they are relatively strong among
water molecules. As a result a large amount of energy is
needed to overcome the attractions among water
molecules.
Without hydrogen bonds water would boil at a much lower
temperature than it does, because less energy would be
needed to change liquid water into water vapor.
There are 3 important properties of water because of
hydrogen bond:
1) High specific heat: this means that water resists
changes in temperature. Water absorbs more heat energy
to increase its temperature. This is very important in cells
because it absorbs the heat produced from chemical
processes that occur inside the cell. So by this it regulates
the cell temperature and maintain homeostasis.
2) Cohesion: which is the attraction among molecules of a
substance. Hydrogen bonds make water molecules stick to
each other which produces surface tension, which makes a
kind of skin on water. Surface tension keeps small objects
from sinking (like a spider).
3) Adhesion: which is the attraction among water
molecules with molecules of different substances. Adhesion
is responsible for the upward curve on the surface of water
because its molecules are attracted to the glass of the test
tube.
Adhesion helps plants transport water from roots to leaves,
because water molecules stick to the sides of the tissues
through which water passes
If the water surface is curved down it means that adhesion
is greater than cohesion.
If the water surface is curved up it means that cohesion is
greater than adhesion.
WATER AS A SOLVENT
COMPOUNDS DISSOLVED in WATER:
Molecules and ions must be dissolved in
water to take part in chemical processes inside the cells.
Example: sugars and oxygen.
When one substance dissolves in another, it forms a
SOLUTION (mixture of substances that is same
throughout--- it is homogenous mixture).
The SOLVENT is the substance present in the greater
amount that dissolved the other substance.
The SOLUTE is the substance that dissolves in a solvent.
• Although water is known as the “UNIVERSAL SOLVENT”,
not all substances dissolve in water.
Water can dissolve polar substances because it is polar,
but non-polar substances dissolve in non-polar solvents.
• This phenomenon is known as “like dissolves like”.
Example: some vitamins are non polar, they do not dissolve
in water in the body, but they dissolve in lipids that make up
body fat. Then they are called fat-soluble vitamin.
Acids and bases
Compounds break up into ions when they dissolve in water.
ACID is the compound that releases a proton (hydrogen
ion H+) when dissolves in water. Acids increase the
concentration of H+ ions.
• BASES are the compounds that remove H+ ions from a
solution. Bases decrease the concentration of H+ ions.
The concentration of H+ is measured on pH scale, which is
between 0 and 14.
• The stronger the acid, the higher the concentration of H ions
H+.
• The stronger the base, the lower the concentration of hydrogen
ions H+.
• In order to maintain homeostasis, most organisms,
including humans, need to keep pH within a very narrow
range around neutral (pH 7).
Keeping pH regulated occurs by substances called buffers,
which is a compound that can bind to H+ ions when the
concentration increases, and can release H+ ions when the
concentration decreases.
CHEMICAL REACTIONS AND
ENZYMES
CHEMICAL REACTIONS change substances into different
substances by breaking and forming new chemical
compounds, rearranging atoms in the process.
REACTANTS are the substances changed during a
chemical reaction.
PRODUCTS are the substances made by a chemical
reaction.
Example :
Cellular respiration:
Glucose + oxygen
Reactants
carbon dioxide + water
Products
Chemical equations are used to show what happens during
a reaction.
C6H12O6 + 6O2
6CO2 + 6H2O
Reactants are on the left side of the equation and the
products are on the right side of the equation. The arrow
shows the direction of the reaction.
It is very important to understand that atoms are not
created nor destroyed, only rearranged.
CHEMICAL EQUILIBRIUM
• Some chemical reactions go from reactants to products
until all the reactants are consumed. The reactions can be
in one direction (irreversible) or 2 way directions
(reversible).
• In an irreversible chemical reaction, the reaction proceeds
in one direction until at least one reactant is completely
consumed.
• In a reversible chemical reaction, the reaction proceeds to
an equilibrium point. At this point, both reactants and
products are present. The chemical reaction does not stop
but continues in both directions at equal rates, so the net
concentrations of each reactant and product do not
change. If some of the products of one reaction are
removed, the chemical reaction proceeds in the direction
required to restore the reactants and products to
equilibrium again. A reversible reaction will always
maintain an equilibrium as long as there are reactants and
products.
Example:
CO2 concentration is high around the cells, the reaction
moves toward the right and carbonic acid forms. In lungs,
the CO2 concentration is low. The reaction goes in the other
direction, and carbonic acid breaks down.
CO2 + H2O
H2CO3
Chemical reaction releases or absorbs energy:
Some energy must be absorbed by the reactants in any
chemical reactions to break their chemical bonds.
ACTIVATION ENERGY is the amount of energy that needs
to be absorbed for a chemical reaction to start.
EXOTHERMIC CHEMICAL REACTIONS releases more
energy than it absorbed. This occurs when the products
have a lower bond energy than the reactants. The excess
energy is often given off as heat or light.
Example: cellular respiration
ENDOTHERMIC CHEMICAL REACTION absorbs more
energy that it releases. This occurs when the products have
a higher bond energy than reactants.
Example: photosynthesis.
CATALYSTS
• Chemical reactions in living things often need to happen
quickly, but some have a high activation energy that
makes this not possible.
• Often the activation energy comes from an increase in
temperature. Once the reaction starts, however, it might
proceed slowly.
• However, the activation energy, and thereby the rate of
the chemical reaction, can be changed with a catalyst.
• A catalyst is a substance that increases the rate of the
reaction. Catalysts are neither changed nor consumed
during a reaction, so they are not a part of the equation.
Catalysts provide an alternate way for the reaction to
occur that requires less activation energy.
ENZYMES
One way to provide the necessary activation
energy for a reaction is to increase the
temperature of the system. However, chemical
reactions in organisms must take place at the
organism’s body temperature, which must remain
within narrow range. In addition, the reactants are
often present in low concentrations. To lower the
activation energy and help molecular collisions be
more efficient, cells use biological catalysts.
• The catalysts used in living organisms are called
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ENZYMES.
They lower the activation energy and increase the rate of
chemical reactions. This occurs in both reversible and
irreversible reactions.
Example:
There is an enzyme in the saliva (amylase) in
the mouth for the breaking down of starches in food.
There is another enzyme in the intestines
(maltase) which breaks down sugar maltose into 2
molecule of glucose.
• Enzymes are specific. This means that each enzyme has
a certain structure which is compatible with the substance
that it acts on. They fit like key and lock.
• The specific reactants that the enzymes work on are
called SUBSTRATES.
• The substrates fit into the active site of the enzyme, so if
the enzyme structure changes it will never work with the
substrate.
• Enzymes bend slightly when they are about to bound to
their substrate. This helps in weakening the bonds of the
substrate to be easily broken.
• Almost all enzymes are proteins. Interactions between
different parts of the protein cause it to function properly
as a catalyst.
• Changes in temperature and pH could cause these
enzymes to denature, because the hydrogen bonds begin
to break apart.
• There are some cases where denatured proteins become
renatured or regain their normal shape.
• A change in pH can also affect the hydrogen bonds in
enzymes and so can cause denaturation.
• Most of the enzymes work best at neutral pH. Otherwise
may stop working.
• Some enzymes, those in the stomach, work best in acidic
pH, while some other enzymes, those in small intestine,
work best in slightly basic conditions.
• So changes in the temperature or the pH causes the
denaturing of the proteins in the enzymes and so the
shape changes which makes it incompatible with the
structure of the substrate.
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