The cell – definition, structure, and components

The cell – definition, structure, and components

The cell is the basic and functional unit of life; it is that part of an organism that makes up the organism and cannot be further divided into simpler units.

It was discovered in 1665 by an English scientist named Robert Hooke. Upon examination of a cork under a microscope, Hooke discovered that there were small “spaces” which were enclosed by a wall; he called them cells.

Many years later, other scientists discovered that both plants and animals are made up of cells.

Can we see cells?

The simple answer would be “NO” because, before the invention of the microscope, it was impossible to determine or tell what a cell looked like. In fact, with the naked eyes, it is still impossible to see a cell.

However, with the invention of both the light and electron microscope, we can now see very detailed structures of the cell from both plants and animals; Although the electron microscope gives a more detailed view as compared to the light microscope, the light microscope is more handy and less expensive to come by, hence is more commonly used.

Where do we find cells?

Both plants and animals are made up of cells; though their cells differ when viewed under the microscope. The major feature of the plant cell is the cell wall, which is absent in an animal cell.

What do cells contain?

When the English scientist examined the cork under the microscope, they seemed to be empty and surrounded by walls; with time, it was discovered that Hooke had seen the structure of a dead cell, and living cells had certain structures in them. Some of the components of a cell include

  • Cytoplasm
  • Nucleus
  • Ribosome
  • Endoplasmic reticulum
  • Mitochondria
  • Golgi apparatus
  • Lysosome
  • Centrioles
  • Chloroplast
  • Cell membrane etc

The Cytoplasm

The cytoplasm or cytosol is that area of the cell which harbours every other organelle seen in the cell and is enclosed by the cell membrane. It contains various enzymes and as a result, many reactions can take place here. For instance; some steps in the synthesis of haemoglobin take place in the cytoplasm and the others occur in the mitochondria.


The nucleus is the largest organelle in a cell and it has a smaller structure called the nucleolus. It is enclosed by the nuclear envelope; a membrane with nuclear pores that allow for the movement of substances into and out of the nucleus.

The nucleus contains structures called chromosomes; chromosomes, in turn, contain DNA (deoxyribonucleic acid) which is responsible for the genetic makeup of an individual. This means that the nucleus controls every activity that goes on in the cell.


Ribosomes are very small organelles located in the cytoplasm; they are only about 25 nanometers in diameter. Each ribosome has a small and a large subunit and they play a role in the synthesis of proteins. Some ribosome is found freely in the cytoplasm, while others may be attached to the endoplasmic reticulum.

Endoplasmic reticulum (ER)

This is seen as a network tube-like structures when viewed with the electron microscope. The endoplasmic reticulum is further divided into smooth and rough ER depending on the presence or absence of ribosome on its surface.

The smooth ER lacks ribosome, and the reverse is the case for the rough ER. The rough ER takes part in the synthesis of proteins, while the smooth ER is responsible for the synthesis of lipids, and certain hormones.


The mitochondrion is commonly referred to as the powerhouse of a cell. It is responsible for the production of energy in the form of ATP required for cellular activities and function.

Golgi Apparatus and lysosomes

Golgi apparatus or Golgi complex in the cell is responsible for modification of molecules (especially those ones that have been synthesized by the cell). For example, methionine is removed in the Golgi body after proteins have been synthesized to give a functional protein.

Golgi bodies are also responsible for the formation of lysosomes which contain hydrolytic enzymes responsible for the breakdown of substances.


These are organelles in the cell that play a major role in cell division.


Chloroplast is seen in only plant cells, and it is necessary for the production of chlorophyll used by cells to make food.

Cell membrane

The cell membrane majorly functions to protect the cell from external attack as well as to regulate what goes in or out of the cell.

Other organelles which may be seen are microvilli and microtubules. Many cells together can make up a tissue, tissues make up organs, and organs make up systems. This organizational structure is peculiar to humans and enables the proper functioning of various parts of the body.

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