A cell is the smallest unit of life that can carry out all the functions of a living thing. Living things can be
either unicellular (made up entirely of only one cell) or multicellular (made up of many cells). Most of
the organisms you are familiar with are multicellular, including humans. Sizes of cells can vary greatly,
from the smallest microscopic bacteria to the huge ostrich egg, which can contain as much as one liter of
contents. Shape of cells can vary greatly as well; the shape of cells can have a lot to do with their
The study of cells is relatively new in our history. We didn’t have any knowledge about cells because
their small size wasn’t known…we couldn’t see them. Sometime around the start of the 1600s a group of
Dutch eyeglass makers invented the microscope. It was a crude tube that contained a couple of lenses that
could magnify things about 9-10 times their normal size. Over the next 50-60 years different inventors
developed newer and more powerful microscopes that were capable of magnifying things even larger. In
1665 British scientist Robert Hooke published a set of drawings he made from observing a magnified
piece of cork (which is from the bark of a tree). His drawings look like little rectangular rooms, which
reminded him of the small rooms of a monastery, so he named them cells. These plant cells were the first
cells seen by a human. In the 1670s, a Dutch fabric-store owner named Anton van Leeuwenhoek made
his own microscope (it was a hobby of his to grind lenses) and observed some pond water. In this pond
water he observed a world of living microscopic organisms and living cells. These were the first animal
cells and the first living cells observed by a human.
Scientists began using the microscope as their primary tool of scientific study. They also kept producing
better and more powerful microscopes every few years. It wasn’t until 1838 that German botanist
Matthias Schleiden determined that all plants are made of cells. A year later, in 1839, German zoologist
Theodor Schwann discovered that all living things are made of cells, not just plants. At this time, people
still believed in the theory of spontaneous generation, which meant that living things could spontaneously
appear from nonliving matter. In 1858, yet another German doctor named Rudolf Virchow proved that
new cells only came from other existing cells. (This was almost 200 years after Francisco Redi performed
his experiment with rotting meat, proving that maggots did not spontaneously appear from rotted meat!)
These three men (Schleiden, Schwann, and Virchow) are credited with developing what is known as the
cell theory. The cell theory states three things:
1) cells are the basic units of all life
2) all organisms are made of one or more cells
3) all cells arise from other pre-existing cells.
Up until the 1950s all microscopes were light microscopes, meaning that they allowed light to pass
through the object being magnified. It was easy to use light microscopes to view living specimens. Light
microscopes can magnify specimens up to as much as 1000 times their actual size. Scientists developed
even more powerful microscopes in the 1950s that could magnify things several hundred thousand times
their actual size. Transmission electron microscopes (TEM) can transmit a ray of electrons through a
preserved specimen to produce images from things that have been sliced extremely thin. Scanning
electron microscopes (SEM) can scan the outside of an object to produce an image of a specimen that
almost looks 3-dimensional.
The cell membrane – the cells of all living organisms are surrounded by a thin layer of lipid and protein
called the cell membrane. The cell membrane is sometimes referred to as the plasma membrane. The cell
membrane separates the cell’s contents from its environment. The cell membrane functions much like a
fence with gates, controlling what can enter and leave the cell. The structure of a cell membrane will
consist of a lipid bilayer (two layers) with larger proteins embedded in the lipids. The cell membrane is a
fluid structure, meaning it is always in motion like the surface of a lake on a windy day. The proteins
“drift” along the lipid bilayer, constantly changing positions like a boat on that lake’s surface. The
“heads” of the phospholipids are polar and hydrophilic (attracted to water/aqueous solutions), while the
“tails” of the phospholipids are nonpolar and hydrophobic (repelled by water/aqueous solutions).
Inside the cell membrane is the cytoplasm. Cytoplasm is a semi-fluid substance made mostly of water
and other organic compounds. Cytoplasm provides a medium for all the cell parts (called organelles) to
exist in.
The nucleus is the control center of the cell, much like the brain is the control center of humans. It is
enclosed by a membrane called the nuclear envelope. The nucleus contains the genetic material (DNA)
that determines what an organism will look like. This genetic DNA is found on structures called
chromosomes. Humans have 46 of these chromosomes, found in 23 pairs. Chromosomes will usually
not be visible under a microscope; they will appear as a jumbled mass called chromatin in most cell
stages of life. Also in the nucleus is a structure called the nucleolus, which is involved in making
organelles called ribosomes. Ribosomes are used to help make proteins during the process of protein
Living things are classified into two large groups, based on whether or not they have a nucleus in their
cells. Prokaryotes are organisms that do not contain a nucleus in their cells, and they lack most other
membrane-bound organelles. Prokaryotes include organisms like bacteria and some other unicellular
algae life forms. All prokaryotes are unicellular. Eukaryotes are organisms that do contain a nucleus in
their cells and they do have all other membrane-bound organelles. All other living things, including
plants and animals, are eukaryotes. Prokaryotic cells will contain some DNA and are generally much
smaller than eukaryotic cells. Even though prokaryotic cells contain fewer internal organelles, they still
perform all features of living things such as reproduction and response to the environment.
Cells are complex structures with many jobs to perform. Eukaryotic cells have the ability to divide all
those jobs up among many different organelles. Below is a partial list of organelles and their functions:
1) ribosomes – found in the nucleus and outside the nucleus in the cytoplasm. It is on the ribosomes
that proteins are made.
2) Endoplasmic reticulum – found in the cytoplasm, and also known as “ER.” ER is an extensive
network of membranes that produces materials for the cell. ER can be rough, meaning that is has
ribosomes on it. ER can also be smooth, meaning that is does not have ribosomes on it.
3) Golgi Apparatus – Golgi is a series of flat, membrane-bound sacs where molecules are modified,
packages, and distributed to their destinations. Golgi is like a conveyor belt in a cell, or a
transportation system. Golgi is found in the cytoplasm.
4) Mitochondria – the mitochondria is the place where our food (glucose) is converted into a usable
form of energy called ATP during the process of cellular respiration. Mitochondria are found in
the cytoplasm of cells.
5) Lysosomes – also found in the cytoplasm, lysosomes are involved in the breakdown of large
molecules such as carbohydrates, proteins, and lipids. Lysosomes also will digest worn out cell
parts that are no longer useful to the cell. Lysosomes contain powerful digestive enzymes.
6) Cilia – short, hair-like projections that are found in large numbers on the surface of certain cells.
Cilia are used for movement, waving through a liquid environment. Cilia can be used for
capturing food in some small organisms that live in freshwater.
7) Flagella – long, whip-like projections found either alone or in pairs on the surface of certain cells.
Flagella are used for movement; some flagella spin like propellers, while others will move much
like the snapping of a whip.
8) Cytoskeleton – a network of support structures (like a scaffold, or like the two-by-fours found in
the wall of a house) made of protein fibers and tubes. Include parts called microfilaments and
9) Cell wall – an extra protective covering that is found outside the cell membrane in plants, fungi,
and bacteria. Cell walls provide extra support and protection.
10) Chloroplasts – found in the cytoplasm of plant cells. Chloroplasts contain chlorophyll, and it is
in the chloroplast that photosynthesis occurs.
11) Chlorophyll – this is the green liquid found inside a chloroplast. Chlorophyll has the ability to
trap the energy found in sunlight so that it can be converted and stored in glucose during
12) Vacuoles – vacuoles are found in the cytoplasm of both plants and animals. Vacuoles are
basically storage facilities within a cell. Plant cells have large vacuoles (mostly for storing water)
and animal cells will have several smaller vacuoles.
13) Centrioles-found in pairs in the cytoplasm of animal cells and assist in cellular reproduction.
14) Vesicles-membrane-bound sacs in which materials are stored or transported; found in the
cytoplasm of both plant and animal cells.
The cell membrane is considered to be semi-permeable; this means that it will allow some things to
pass in and out of the cell, but it will not allow other things to pass in or out. Water can pass across
easily, but larger things like proteins and carbohydrates cannot. Ions cannot pass in or out freely due
to their electrical charge. This movement of particles in and out of the cell can either happen easily
and without energy, or it can require the addition of cellular energy. Passive transport is the
movement of a substance across a cell membrane without the input of the cell’s energy. Active
transport is the movement of a substance across a cell membrane that requires the input of a cell’s
Passive transport – the most common type of passive transport is diffusion. Diffusion is the random
movement of particles from an area of higher concentration to an area of lower concentration. It is
through diffusion that oxygen and carbon dioxide cross the cell membrane. The rate of diffusion can
depend on temperature and size of the molecules involved. Molecules diffuse faster at high
temperatures than at low temperatures, and small molecules diffuse faster than large ones. Diffusion
always occurs down a concentration gradient or a pressure gradient. Diffusion will happen along that
concentration gradient until an equilibrium has been reached….that means that a particular substance
will be entering the cell at the same rate that it is leaving the cell. Facilitated diffusion involves the
use of carrier molecules in assisting some materials to cross the cell membrane, such as large
molecules or ions. It is through facilitated diffusion that glucose travels from the blood into body
cells. Osmosis is a special term given to describe the diffusion of water. Water is such a plentiful
substance and is so important that we give the diffusion of water its own name.
Active transport - sometimes we have to move molecules against the concentration gradient, or from
the area of lower concentration to the area of higher concentration. This requires extra energy since it
is going against the natural pressure of the concentration. We will use carrier molecules (just like in
facilitated diffusion) to pump ions and molecules across the cell membrane. Examples of this might
be the way that plant roots absorb nutrients from the soil. The concentration inside the roots is
greater, yet a plant doesn’t want the nutrients to diffuse out of the roots, so active transport keeps more
nutrients moving into the roots, against the concentration gradient. Small molecules can be brought
into and out of the cell by the processes of endocytosis and exocytosis. Endocytosis involves the cell
membrane “wrapping” around a desirable tiny molecule and bringing it into the cell. The cell
membrane will begin to surround a small molecule until the cell membrane can “pinch” off a saclike
portion to form a vesicle. Pinocytosis is a form of endocytosis in which liquids are brought into the
cell. Phagocytosis is a form of endocytosis in which food particles or other solids are brought into the
cell. Exocytosis is just the opposite process…..a cell will expel small particles by having vesicles fuse
with the cell membrane, and then pushes the particles out of the cell.
A vesicle is formed to bring in or to take out the particles. Remember, this active transport is an energyrequiring process. The cell membrane is involved in taking in or expelling the particles.
Solute – the substance that dissolves in another substance. *present in the lesser amount*
Solvent – the more plentiful substance that causes a solute to dissolve. *present in the greater amount*
Solution – the mixture of the solvent and solute.
EX: Kool-Aid. The water is the solvent, the powdered kool-aid is the solute, and the result after it is
combined is the drinkable solution called Kool-Aid.
Cellular solutions can be hypotonic, hypertonic, or isotonic.
Hypotonic – a cell placed in a hypotonic solution (like pure water) will swell up and burst because the
concentration of solutes is greater inside the cell. Water will rush into the cell and cause it to burst.
Hypertonic – a cell placed in a hypertonic solution (like salt water) will shrivel because the concentration
of solutes is greater outside the cell. Water will rush out of the cell and cause it to shrivel up.
Isotonic – a cell placed in an isotonic solution will stay its normal size because the concentration of
solutes is equal inside and outside the cell. An equilibrium is reached, with water moving in and out of
the cell at the same rate.