Table of contents
Learning Objectives
By the end of this section, you should be able to:
- Describe a cell, how it looks, its macro structures.
- Explain cell theories.
- Explain the categories of cell.
Introduction
A cell is the simplest unit of every living organism. Sometimes, cell of a living organism is related to an atom of a chemical substance (although atoms are far smaller than cells); just as an atom is the smallest unit of a chemical element, so is cell the smallest unit of every living organism.
 |
| A Typical Animal Cell |
Cells are so small that you will need a microscope to see them, just as you will need a microscope to see an atom of a chemical element, or can you see an atom with your bare eyes? If you can, let me know via comment box so I can come and start using your eyes as my microscope.
Basically, cells are so small you cannot see them, but there are a few exceptions, some cells are very large, an example is an ostrich egg.
How small is a cell?
A normal cell is about 0.02 millimetres (0.02 mm) in diameter.
Do You know how small that is?
Take out a ruler, a ruler is graduated in Centimetres (that is, it measures in Centimetres). The 1, 2, 3 that you see are in Centimetres while the small vertical lines are millimetre values.
Now, we want to measure 0.02 out of two consecutive vertical lines, that is, dividing that small space by 50.
Can you, now, imagine how small a cell would be? I believe your answer is yes!
Usually, because the cell is smaller than 1 millimetre, we use micrometres (also called microns, and is 1000 times smaller than a mm, that is, 1000 micrometres will make 1 millimetre) μm.
Therefore, a normal cell's size is about (0.02 × 1000)μm = 20μm.
To clearly view a cell, we use an electron microscope, because with a light microscope, you can view only the macro structures of a cell which are the:
- The outer cell lining or membrane called the plasma membrane.
 |
| Plasma/Cell Membrane |
- The inner region which is called cytoplasm.
- The Nucleus which is the giant round ball you see inside a cell.
However, when we an electron microscope to increase magnification by thousands of times, we see these blurry structures clearly and we see that they are very complex. Each of these structures has its own specialised function and they are called Organelles (meaning little organs).
Organisms such as Bacteria have only one cell, such organisms are referred to as UNICELLULAR.
Other organisms such as humans and goats have billions of cells and are referred to as MULTICELLULAR.
Each cell is an amazing world unto itself: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary.
Even more amazing is that each cell stores its own set of instructions (commands) for carrying out each of these activities.
Cell theories
Cell was discovered by a scientist named Robert Hooke, and he found it out by studying the bark of a cork tree. When he looked at cork with a simple microscope, he found materials that appeared like boxes, making up the cork.
He therefore named them Cells ('spaces' was the literal meaning in his language).
Further experiments were carried out in subsequent years and it was discovered the simplest form of every living organism is cell. Robert Hooke is therefore referred to as being the Father of Cell.
Other people have contributed significantly to the science of cells and therefore, scientists have published 7 important cell theories.
They are simply basic things you should know about cells. The cell theories are:
- A cell is the simplest and basic unit of all life.
- There is no life apart from the life of a cell.
- All cells come from previously existing cells.
- Living organisms can exist as just one cell, or numerous cells.
- The activity of an organism depends on the total activity of independent cells.
- Cells contain DNA which is found in the chromosome, and RNA which is found in a Cell's nucleus and cytoplasm.
- All cells have basically the same chemical composition in different organisms of similar species.
Categories of Cell
Cell is split into 2 categories:
- Prokaryotic Cell.
- Eukaryotic Cell.
Prokaryotic cell
Prokaryotic cells are the simplest types of cells, and they are the first kinds of cells to evolve.
 |
| A prokaryotic cell |
A prokaryotic cell has no Nuclear membrane.
Nuclear membrane is the membrane that surrounds the nucleus of a cell—Just like my skin surrounds me.
A prokaryotic cell—because it does not have a nuclear membrane, it therefore does not possess a typical nucleus. Instead, its DNA is scattered within the cell.
Prokaryotes are organisms that have a prokaryotic cell and they are generally unicellular.
They do not develop.
Bacteria are the best known and most studied prokaryotic organisms, and there is a second group called Archaea.
Prokaryotes can either exist as free living, colony or filaments.
Each cell in a bacterial colony is identical to the others, but independent.
Although the cells may appear very close to one another (because they do not separate after Division or they are enclosed in a common sheath or slime secreted by the cells), there is no communication between the cells.
Prokaryotes are capable of inhabiting almost every place on earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies.
Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane.
Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells.
Most of the functions of organelles, such as mitochondria, Golgi apparatus, are taken over by the prokaryotic plasma membrane.
Prokaryotic cells have three architectural regions:
- Appendages / legs called flagella and pili. These are simply proteins that are attached to a cell's surface and are used for movement.
- A cell envelope (like skin or flesh in human) consisting of a capsule, a cell wall, and a plasma membrane.
- A cytoplasm that contains the cell genome (DNA) and ribosomes and various other structures. The DNA of prokaryotes is arranged in circular structures called a Plasmids.
Eukaryotic cell
Eukaryotic cells are about 10 times bigger than the size of a prokaryotic cell.
The major difference between a prokaryotic cell and an eukaryotic cell is that eukaryotic cells contain membrane-bounded components in its structure. That is, components of an eukaryotic cell are covered with membranes.
Most important among these, is the presence of a nucleus—A membrane that houses the eukaryotic cell’s DNA.
It is this nucleus that gives the eukaryote (which means true nucleus)—its name.
 |
| Eukaryotic Cell |
Eukaryotic organisms also have other specialized structures, called organelles, which are small structures within cells that perform dedicated functions.
As the name implies, you can think of organelles as small organs. There are a dozen different types of organelles commonly found in eukaryotic cells. You can find out these cells Organelles here.
 |
Eukaryotic organisms are majorly multicellular, while some are unicellular like protists and a few Fungi.
Example of organisms that have eukaryotic cells are animals, human, insects, protists, Fungi.
Summary
📌 A cell is the simplest and basic unit of life.
📌 A cell is microscopic, typically 20 micrometres, excluding some cells which are clearly visible to the eyes.
📌 A cell has three macro structures:
- Plasma membrane.
- Cytoplasm.
- Nucleus.
📌 Many organisms exist as one cell and they are referred to as unicellular. Other organisms have many cells and are referred to as Multicellular.
📌 Each cell is independent.
📌 The Father of cell is Roberk Hooke and he found out cells when he studied the bark of a cork tree.
📌There are 7 cell theories which are:
- A cell is the simplest and basic unit of all life.
- There is no life apart from the life of a cell.
- All cells come from previously existing cells.
- Living organisms can exist as just one cell, or numerous cells.
- The activity of an organism depends on the total activity of independent cells.
- Cells contain DNA which is found specifically in the chromosome, and RNA which is found in the Celll nucleus and cytoplasm.
- All cells are basically the same in chemical composition in organisms of similar species.
- Prokaryotic cell.
- Eukaryotic cell.
📌 A prokaryotic cell is simple and lacks nuclear membrane.
📌 An eukaryotic cell is complex and possesses a nuclear membrane.
📌 An organism that possesses prokaryotic cell is called a prokaryote, example are Bacteria.
📌 An organism that has an eukaryotic cell is called an eukaryote, example is mammals.
Recommended Video
Test Questions
1.
Which of the following is found in all Prokaryotic cells but only some Eukaryotic cells?
This quiz has been created using the tool FunaabExplains Quiz Generator
