Structure of Cell: Definition, Structure, Types and Functions

Structure of Cell: Cell is the basic functional unit that makes up all living organisms. All organisms, including ourselves, start life as a single cell called the egg. Cells are small microscopic units that perform all essential functions of life and are capable of independent existence.

With the invention of microscopes, many unknown facts about cells were discovered. In this article, we have brought detailed information about the cell, its structure, types, functions and a lot more interesting facts about cells.

Cell Definition

Cells are the fundamental structural and functional unit of all living beings including plants, animals and microorganisms.

All living organisms in this universe are made up of cells. We cannot see cells with naked eyes as they are only \(10\) microns in size whereas human eyes cannot see objects less than \(100\) microns.

A body of a plant or animal is made up of thousands of cells; each of these cells has its own life. The cells in plants differ from the cells present in animals.

Discovery of Cells

Cells in Latin means ‘little rooms’. This name was given by Scientist Robert Hooke in the year \(1665\) who discovered the cell using a self-designed microscope. While studying a thin slice of cork (a substance that is obtained from the bark of a tree), Robert Hooke saw that the cork resembled the structure of a honeycomb consisting of many little compartments. Hooke called these little boxes as ‘cells’.

Robert Hooke’s discovery was important because it indicated that for the first time living organisms consisted of a number of small structures or units. The ordinary compound microscope of today is a greatly improved design of the original Hooke’s microscope. However, the cells which Hooke observed had no information about the organelles which are to be present inside the cell in most living organisms.

In \(1674\), Antony Van Leeuwenhoek, a Dutch microscopist, made an important contribution to the cell theory. He was the first to discover algae and bacteria after observing the movements of tiny organisms in the water.

In 1881, Robert Brown, a Scottish botanist, added to the cell theory through his discovery of nuclei in plant cells. His findings helped in understanding the cell theory a lot better.

Cell Theory

Cell Theory was proposed by three German Scientists: Theodor Schwann, Matthias Schleiden and Rudolf Virchow. The theory they presented included the following:

The cell theory included three major points:

1. All organisms are composed of cell and cell products.
2. The cells are the unit of function for all living organisms.
3. All cells arise from pre-existing cells.

The cell theory was refined further in \(1855\) when another German biologist, \({\rm{R}}\) Virchow presented the idea that all cells arise from pre-existing cells. Hence, the cell theory comprises of the following postulates:

1. All organisms are composed of cells and cell products.
2. All metabolic reactions take place in cells. Thus, cells are structural and functional units of life. 
3. All cells arise from pre-existing cells only; no cell can originate spontaneously.
4. Every organism starts its life as a single cell. Viruses are an exception to the cell theory.

Types of Cells

Different types of cells are responsible for various functions. Cells are classified into two types based on their structure:

1. Prokaryotic Cell
2. Eukaryotic Cell

Differences between prokaryotic and eukaryotic cells are mentioned in the table below:

Structure of Cell

A cell structure is composed of many components which are present inside the cell. These components carry out the various important functions which are important in the main functioning of the cell. Various kinds of cells show special differences, yet they all have some basic structural plan consisting of three essential parts: (i) cell membrane (plasma membrane), (ii) cytoplasm and (iii) nucleus.

Apart from these three components, cells have some living parts that are called cell organelles. These components have definite shapes, structures and functions.

Let’s know these components in a little more detail:

Cell Membrane (Plasma Membrane)

Have you ever wondered how the movement of substances takes place in the cell? How do substances move out of the cell?

It is the outermost covering of the cell that allows or permits the entry and exit of some materials in and out of the cell, this layer is called the plasma membrane or cell membrane. Below are some characteristics features of the cell membrane:

1. The cell membrane is found in cells of all living organisms.
2. The cell membrane separates the contents of the cell from its external environment.
3. Apart from in and out movement of substances, the cell membrane also prevents the movement of some other materials. Hence, it is also called a selectively permeable membrane.
4. The cell membrane also protects components of the cell from damage or leakage by wall-like structures around the cell.
5. It is known to perform certain physical activities such as diffusion and osmosis for the intake of some substances.
6. The cell membrane is flexible and is made up of organic molecules called lipids and proteins.
7. The flexibility of the cell membrane also enables the cell to immerse in food and other material from its external environment. Such processes are known as endocytosis.

Cell Wall

In addition to the plasma membrane, plant cells have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane.

1. The plant cell wall is the outermost layer of a plant cell.
2. It is mainly composed of cellulose which is a complex substance and provides structural strength to plants or cells.
3. Cell walls permit the cells of plants to withstand very dilute (hypotonic) external media without bursting.
4. Cell walls being rigid resist the expansion of the cell and establish turgidity due to endosmosis.
5. Cell walls being rigid resist the expansion of the cell and establish turgidity due to endosmosis that maintains an erect posture of the young shoots. The cell walls prevent the bursting of the cells due to excessive endosmosis.

Nucleus

The nucleus is the largest and most distinct organelle containing the genetic material that controls the metabolic activities of the cell. The nucleus was discovered by Robert Brown in orchid cells in \(1831.\)

1. The nucleus is chemically composed of proteins, DNA, RNA and lipids.
2. The shape of the nucleus may be spherical, oval or discoidal. The nucleus in plant cells is lens-shaped and peripheral in position due to a large central vacuole. It is spherical in animal cells and is located at the centre.
3. The nucleus has a nuclear envelope, nuclear sap, nuclear matrix, chromatin and nucleolus.
4. The nuclear envelope separates the nucleus from the cytoplasm, it has many pores (the nuclear pores) and encloses the liquid ground substance, the nucleoplasm.
5. The nucleolus and chromatin are found within the nucleoplasm.
6. The nucleolus is rich in protein and RNA (ribonucleic acid) molecules and acts as the site for ribosome formation. It is known as the factory of ribosomes.
7. Ribosomes are helpful in protein synthesis in the cytoplasm.

Cytoplasm

The part of the cell which occurs between the plasma membrane and nuclear envelope is called the cytoplasm.

1. The Inner granular mass of the cytoplasm is called endoplasm whereas the outer clear or glassy layer is called cell cortex or ectoplasm.
2. The cytoplasm consists of an aqueous ground substance called the cytosol.
3. The cytosol contains a variety of cell organelles and other inclusions such as insoluble waste and storage products, starch, glycogen, lipid, etc.
4. Cell organelles include nucleolus, nuclear membrane, chromosomes, endoplasmic reticulum, Golgi bodies, ribosome, mitochondria, lysosomes, chloroplast, vacuoles, etc.

Cell Organelles of a Cell

Eukaryotic cells need a lot of chemical activities to support their complicated structure and function. To keep these activities separate from each other, these cells use membrane-bound little structures also called ‘organelles’ within themselves. These are important because they carry out some very crucial functions in cells.

We have talked about the nucleus in a previous section. Some other important cell organelles are discussed below:

Functions of Cells

Cells are the fundamental unit of all living organisms. Different types of seeds perform different functions. Some of the important functions are mentioned below:

1. Facilitate growth through mitosis.
2. Provide structure and support.
3. Allows passive and active transport of materials.
4. Production of energy.
5. Helps in the reproduction process.

Summary

All organisms irrespective of unicellular or multicellular are made of cells. Hence, cells are the building blocks of life. They have the ability to carry all essential functions on their own. The structure of a cell includes various organelles that perform all crucial functions such as the production of energy, replication, transport of materials, etc.

Cells of prokaryotes differ from eukaryotes based on their structure. Eukaryotic cells are much more evolved than prokaryotes. They contain a well-defined nucleus and other important cell organelles to perform various metabolic activities.

Frequently Asked Questions (FAQs) on Structure of Cell

Q.1. What is a cell?
Ans: The cell is the smallest, fundamental and functional unit that makes up all living beings including microorganisms, plants, animals and humans.

Q.2. What are the five cell structures?
Ans: A cell consists of many different structures that have definite shapes, structures, and functions of their own. Some of these structures are (1) Cell Wall (2) Mitochondria (3) Chloroplast (4) Cell Membrane and (5) Nucleus 

Q3. What is the structure of a human cell?
Ans: A cell structure is composed of many components which are present inside the cell. The basic structure of a cell consists of three essential parts that are cell membrane (plasma membrane), cytoplasm, and nucleus.

Q4. How many types of cells are there? What are they?
Ans: There are two types of cells. They are Prokaryotic cells and Eukaryotic cells.

Q5. Describe Cell Theory.
Ans: Cell Theory was proposed by three German Scientists: Theodor Schwann, Matthias Schleiden and Rudolf Virchow. The cell theory included three major points:

1. All organisms are composed of cell and cell products.
2. The cells are the unit of function for all living organisms.
3. All cells arise from pre-existing cells.

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