Elements of Life
There are billions of living organisms on Earth. Some living organisms are microscopic and contain only a single cell, such
as a bacterium. However, plants, insects, reptiles, mammals, and humans are larger, more complex living organisms.
Because living organisms are so different from each other, one might think that many different types of matter make up
the different living things. In fact, only a few elements make up most of the living organisms in the world.
Carbon (C) plays a central role in living organisms because carbon atoms easily bond with other atoms to form
molecules. When carbon atoms form bonds, they share electrons in their outer energy level with other atoms. This type
of bond is called a covalent bond.
Each carbon atom has four electrons in its outer energy level. This means that it can bond with as many as four other
atoms. For example, a methane molecule (CH4) has one carbon atom at the center. Each of the four electrons in the
carbon atom’s outer energy level is in a single bond with a hydrogen atom.
Most drawings one-dimensional model, however, a methane molecule has a tetrahedral structure. It looks like a
pyramid with four faces (three sides and a base).
Sometimes carbon forms a double bond by sharing two electrons with another atom. For example, a carbon
dioxide (CO2) contains one carbon atom that has a linear structure. It forms a straight line. Formaldehyde molecule
(CH2O) has one carbon atom with one double bond to an oxygen atom as well as two single bonds to hydrogen atoms.
This type of molecule has a planar structure. The connection of the atoms creates a flat shape.
In a few molecules, carbon has a triple bond. For example, when fossil fuels burn, they usually release carbon
monoxide (CO). A carbon monoxide molecule forms from a triple bond between one carbon atom and one oxygen atom.
We call carbon “the building block of life” because carbon atoms are in all living things. Carbon atoms often
bond with each other to form long chains. These carbon chains may be linear. However, sometimes the two ends of a
chain connect to form a ring. Other types of atoms bond at the unbounded electrons of the atoms in the carbon chain.
When this happens, large organic compounds form. Every organic compound has a carbon as its base. Examples of
organic compounds include carbohydrates, lipids, proteins, and nucleic acids. Inorganic compounds do not contain
carbon. Examples of inorganic compounds include table salt, sulfuric acid, and zinc sulfate. A few simple compounds that
contain carbon, such as carbon dioxide and calcium carbonate are also classified as inorganic compounds.
Two other elements that are required for life are hydrogen (H) and oxygen (O). Both of these elements are
required to form water (H2O), which is the most abundant substance in living organisms. Hydrogen and oxygen can also
bond with carbon to form organic compounds. Oxygen gas (O2) is critical to life; humans and animals need to breathe it.
Oxygen is also found in the carbon dioxide (CO2) that plants absorb through their leaves.
Nitrogen (N), sulfur (S) and phosphorus (P) are not as abundant as carbon, hydrogen, and oxygen in living
organisms. However, these three elements are found in many important organic compounds. Nitrogen and sulfur bond
with carbon, hydrogen and oxygen to form proteins that organisms need to live. Along with carbon, hydrogen, oxygen
and nitrogen, phosphorus is part of every cell’s DNA. DNA, or deoxyribonucleic acid, is a chemical compound that passes
genetic information from one cell to another.
Carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur can combine in many different kinds of bonds. The
variety of bonds allows carbon-based molecules to have a wide range of shapes. The smaller molecules with linear,
planar, and tetrahedral shapes can bond to make an almost unlimited number of shapes of larger molecules. The
functions and chemical properties of molecules depend on their shapes. Therefore, carbon’s ability to bond in so many
ways allows for a vast diversity of life.
Molecules of Living Things
Living organisms contain different molecules and chemical compounds. Some of the molecules found in living
organisms are very small, inorganic compounds. For example, living organisms consist mostly of water. Salt is another
small, inorganic compound. It is a common mineral that helps maintain fluid balance in many living organisms.
Living organisms also contain thousands of different organic compounds. Organic compounds are large
molecules that make up living things and that are made by living things. All organic compounds contain carbon. There
are four main groups of organic compounds: carbohydrates, proteins, lipids and nucleic acids.
Carbohydrate molecules contain carbon, hydrogen, and oxygen. Some carbohydrates, such as fructose and
glucose, are simple sugar molecules. Other carbohydrates, such as starch, are large, complex molecules. Complex
carbohydrates consist of dozens, and sometimes hundreds, of simple sugar molecules.
Green plants make their own carbohydrates during photosynthesis. Animals, including humans, must eat to get
carbohydrates in order to get the energy or to use the sugars to make structures such as insect shells.
Proteins are the most diverse group of organic compounds found in living organisms. They are important
because they determine the structure and function of an organism. Each specific type of protein molecule is formed by a
unique combination of amino acids. Proteins can be grouped into several general types based on their functions. See
chart.
Some Types of Proteins
Function
Structural proteins
Compose living organisms’ hair, tendons, ligaments, bones and other things
Contractile proteins
Compose muscle and allow them to move.
Transport proteins
Move things from one place to another, for example, into and out of cells.
Defensive proteins
Control damage; as antibodies, defend organisms from diseases.
Regulatory proteins
Control internal functions; as hormones, carry messages.
Enzymes
Start or speed up chemical reactions, such as those involved in digestion.
Lipids are organic compounds that contain mostly carbon and hydrogen. They do not dissolve in water. Fats, oils
and waxes are common examples of lipids. Many organisms store lipids as a reserve, or backup energy supply. When an
organism begins to run out of the carbohydrates, it can break down lipids. Lipids are also essential parts of cell
membranes and the myelin coats nerve cells. Lipids also store some vitamins and cushion internal organs.
Nucleic acids are another type of organic compound in living organisms. Nucleic acids are composed of carbon,
hydrogen, oxygen, nitrogen, and phosphorus. There are two types of nucleic acids: DNA and RNA. DNA
(deoxyribonucleic acid) contains the genetic information that passes from one cell to another. It provides instructions to
a cell on how to build the proteins the organism needs. RNA (ribonucleic acid) actually builds the proteins within each
cell.
Guiding Questions:
1. Elements of Life
a. What is a difference between a bacterium and a plant?
b. How many electrons are in a carbon atom’s outer energy level?
c. What are three structures of carbon-based molecules?
d. Why do we call carbon the building block of life?
e. What is the most abundant substance in living organisms?
f. What two elements bond with carbon, hydrogen and oxygen to form proteins?
2. Molecules of Living Things
a. What do living organisms consist of?
b. What elements make up carbohydrates?
c. Which molecules combine in unique ways to form protein molecules?
d. What are some common examples of lipids?
e. What are the two types of nucleic acids?