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COMMON SUBSTANCES IN
LIVING SYSTEMS
The common substances found in living systems are water,
carbon dioxide, oxygen, ammonia, mineral salts,
carbohydrates, lipids, proteins, nucleic acids, and ad-
enosine triphosphate.
Water
- Approximately, 60% to 80% of a cell is water (H 2 O).
Water is a slightly polar molecule: the two hydrogen
atoms have a partial positive charge and the oxygen
atom a partial negative charge. This explains why
ionically bonded molecules dissociate when placed in
water. - Water has many important roles in cells: it takes part
in some reactions; it serves as a medium or solvent for
other reactions to occur in; it serves as a basis for the
transportation of materials; it absorbs and releases
heat, maintaining body temperature; it protects; and it
is the base for all body lubricants.
Carbon Dioxide
- Carbon dioxide (CO 2 ) gas is produced as a waste
product of cellular respiration.^
- It is necessary for plants to produce oxygen gas in
the photosynthetic reaction, which converts the
sun’s radiant energy into usable chemical energy
like glucose for plant and animal survival.^
- All of the carbon in the carbon-containing mol-ecules
of life comes either directly or indirectly from carbon
dioxide gas.
Oxygen
- Molecular oxygen (O 2 ) is required by all organisms
that breathe air.^
- It is necessary for cellular respiration to occur, con-
verting glucose into ATP molecules: C 6 H 12 O 6 1 6O 2
S ATP (energy)^1 6CO^2 1 6H^2 O.^
- Oxygen comes from plants in the photosynthesis
process: (6CO 2 1 12H 2 O S C 6 H 12 O 6 (glucose) 1 6O 2
1 6H 2 O).
Ammonia
- Ammonia (NH 3 ) is produced as a by-product of the
breakdown of amino acids.^
- Amino acids contain nitrogen and are the building
blocks of proteins.
Chapter 2- Ammonia, which is toxic, is converted to harmless
urea by enzymes in our liver.
Mineral Salts- The mineral salts are calcium (Ca^1 ), phosphate
(PO 42 ), chloride (Cl^2 ), sodium (Na^1 ), and
potassium (K^1 ).^ - Calcium is needed for muscle contraction and
strong bones.^
3.^ Phosphate is needed to make ATP.^ - Sodium, potassium, and chloride are necessary for
muscle contraction and nervous transmission.
Carbohydrates- Carbohydrates are composed of carbon, hydrogen,
and oxygen in a 1:2:1 ratio. The simplest carbohy-
drates are five-^ and six-carbon sugars.^ - Important five-carbon sugars are deoxyribose and
ribose; important six-carbon sugars are glucose and
fructose.^ - Carbohydrates have two important functions:
energy- storage and structural strengthening of
the cell.
Lipids- Fat is a major type of lipid; 95% of fats in the human
body are triacylglycerols, which are composed of
glycerol and fatty acids.^ - A fat is called saturated if the fatty acids contain
single covalent bonds. These can contribute to
cardiovascular- disease. A fat is called unsaturated if
the fatty acids have one or more double covalent
bonds. These are good for you.^ - Fats are a source of energy, act as insulators for the
body, and protect organs.
Proteins- Proteins contain carbon, oxygen, hydrogen,
nitrogen,- and sulfur.^
2.^ Amino acids are the building blocks of protein.^ - Proteins are a source of energy. There are structural
proteins like actin and myosin in muscle cells, and
proteins are an essential part of a cell’s membra-nous
structures.^ - Enzymes are protein catalysts that make chemical-
reactions occur in cells. The functioning of our
immune- system is based on proteins.