BIOLOGY MADE SIMPLE
“If you can’t explain it simply enough, you didn’t understand it well enough” (Albert Einstein)
What is Biology?
Biology is the science that studies everything that is alive, or put more academically, Biology is the science that
studies living organisms.
Living organisms are those that can do the following 3 things:
Ability to generate other living organisms similar to themselves. This function is important for the perpetuation of
the species since the lifespan of living organisms is limited. The ability to reproduce resides in the nuclei of the
cells, in the DNA.
Ability to metabolize - that is, to extract, transform and use - the energy from nutrients in the environment in
order to grow and perform vital functions. The process of metabolism involves the creation (anabolism) and
destruction (catabolism) of substances. During anabolism the living being obtains substances with which it builds
its own molecules to grow. Anabolism consumes energy. During catabolism the products obtained in the previous
stage are destroyed releasing energy to maintain vital functions.
3) Relate to the environment:
Ability to receive stimuli and react to them.
The Organization of Life
(or levels of organization)
Living organisms (or beings) are like buildings. If
you want to construct a building you need bricks.
Many bricks put together with mortar form a wall,
several walls will form a room, several rooms will
form an apartment and several apartments will form
Likewise, many buildings form a district, many
districts form a city, many cities form a country,
many countries form continents, and they all form
Now back to living organisms: The bricks are the
atoms. Several bricks with mortar are a
molecule, a whole wall is a tissue, the room is an
organ, the apartment is a system (or group of
organs) and the building is the living organism
(that is, you!).
You and others form a population, the population
in their environment (or ecosystem) is called a
community, and everything put together is the
Earth and the atmosphere or biosphere. Nice.
Source: www. worldpress.com
From an Atom to a Tom
If you chop something many, many, maaany times, there will come a time when you cannot make it any smaller
and this is called “atom”. Atom means “not divisible” [or “can’t be made any smaller”]. That was true for a long
time until someone spoiled it with the development of the electron microscope that showed that atoms are made
of 3 particles: a nucleus made of protons and neutrons, and orbiting particles called electrons. It’s pretty much like
the Earth and the Moon but often with several Moons.
Imagine you are chopping off a can of soft drink and a ring made of gold. The can is made of aluminium and the
ring, well..., of gold. Aluminium and gold are substances (more fancily known as “elements”). The smallest you
could make the can would be an atom of aluminium and the smallest you could make the ring would be an atom of
gold. Put differently, an atom is the smallest possible piece of an element that retains all the properties of that
There are a bit over 100 elements but only 6 of them make up most of what exists on Earth. The acronym
CHNOPS stands for Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur. The acronym goes from
most to least common of the most common elements (hope you got that). Put several elements (atoms of one
type) together and you’ll have a compound (molecule). Examples of compounds (or molecules) are water (2
hydrogens + 1 oxygen) and salt (1 sodium + 1 chloride).
Cells are made of many different molecules. Cells of the same kind form tissues. For example cells with stretchy
fibres inside form a tissue called the muscle. Cells with lots of calcium form a tissue called the bone. An organ like
the stomach is made of several tissues (muscle, epithelial and nerve tissues). A system is made of several organs
working towards the same end. For example the heart, arteries, veins, the thorax, and specific nerves form the
cardio-vascular system. Put all the systems together (cardio-vascular, respiratory, gastro-intestinal, etc.) together
and you’ve got an organism (or a being, and they often have names like Mary or Tom. This explains the silly
header of this section: From an atom to a Tom...).
Classification of Cells
We already know what it means to be alive: To be able to reproduce, feed, and relate to the environment.
Knowing this, we can say that the cell is “the smallest living thing” or that the cell is ” the smallest unit of
There are 2 types of cells based on whether their nucleus has a membrane (eukaryotic cells) or not
(prokaryotic cells). Animal cells (that is, human cells) are eukaryotic. Below are 2 images of an animal
All living organisms are made of cells. One kind of cell makes one kind of tissue. Only bone cells can make
bone tissue, only muscle cells can make muscle tissue, etc. Our bodies have many different kinds of cells
because we have many different kinds of tissues. Tissues are groups of cells all doing the same job.
Animal cell for beginners Animal cell for pros
Classification of Tissues
There are 4 major kinds of tissues: epithelial, connective, nerve, and muscle tissue.
The job of the epithelial tissue is to shield the body and protect it from injury and infection. The skin
and the lining of hollow organs like the stomach, intestines, blood vessels, bronchi, etc. are made of
Connective tissue supports our bodies like an internal scaffold. Bone, cartilage, and fatty tissues are
types of connective tissues. The blood is a special type of connective tissue. It is a liquid tissue.
Nerve tissue is the information network of our bodies. Nerves send information from one part of our
body to another. Nerve cells, or neurons, have many short thin branches (dendrites) and one long fat
branch (axon) that connect one cell to another.
Muscle tissue can contract and relax. There are 3 types of muscle tissue: Striated, smooth, and cardiac.
The striated muscle tissue is attached to the bones and allows us to move whenever we want to, that’s
why it is also called voluntary muscle tissue. Smooth muscle tissue makes involuntary muscles move.
Involuntary muscles move on their own and we could not make them move if we wanted to. These
muscles are inside hollow organs - like the intestine- behind the epithelium. Cardiac muscle tissue is the
type of tissue that makes our heart pump.