Cells

Cells are the basic, fundamental unit of life. So, if we were to break apart an organism to the cellular level, the smallest independent component that we would find would be the cell.

Table Of Contents

Cells

Cells are the fundamental unit of life. They range in size from 0.0001 mm to nearly 150 mm across

What is a Cell?

Cell is the structural and fundamental unit of life. The study of cells from its basic structure to the functions of every cell organelle is called Cell Biology. Robert Hooke was the first Biologist who discovered cells.

All organisms are made up of cells. They may be made up of a single cell (unicellular), or many cells (multicellular).  Mycoplasmas are the smallest known cells. Cells are the building blocks of all living beings. They provide structure to the body and convert the nutrients taken from the food into energy.

Cells are complex and their components perform every function in a living being. They are of different shapes and sizes, pretty much like bricks of the buildings. Our body is made up of cells of different shapes and sizes.

Cells are the lowest level of organization in every life form. From organism to organism, the count of cells may vary. Humans have the number of cells compared to that of bacteria.

Cells comprise several cell organelles that perform specialized functions to carry out life processes. Every organelle has a specific structure. The hereditary material of the organisms is also present in the cells.

Cell Definition

A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes.

Cells are the structural, functional and biological units of all living beings. A cell can replicate itself independently. Hence, they are referred to as the building blocks of lifeEach cell contains a cytoplasm which is enclosed by a membrane and contains several biomolecules like proteins, nucleic acids, etc. Moreover, cellular structures called cell organelles are suspended in the cytoplasm.

Characteristics of Cells

Following are the various important characteristics of cells:

  • Cells provide structure and support to the body of an organism.

  • The cell interior is organized into different special compartments or organelles surrounded by a separate membrane.

  • The nucleus(major organelle) holds genetic information necessary for reproduction and cell growth.

  • Every cell has one nucleus and other types of organelles exist in multiple copies in the cytoplasm.

  • Mitochondria, a double membrane-bound organelle is mainly responsible for the energy transactions vital for the survival of the cell.

  • Lysosomes digest unwanted materials in the cell.

  • Endoplasmic reticulum plays a significant role in the internal organization of the cell by synthesizing selective molecules and processing, directing and sorting them to their appropriate locations.

Also Read: Nucleus

Cell Structure

The cell structure comprises of certain components with specific functions essential to carry out life’s processes. These components include- cell wall, cell membrane, cytoplasm, nucleus, and cell organelles.

Cell Membrane

  • The cell membrane supports and protects the cell. It controls the movement of substances in and out of the cells. It separates the cell from the external environment. The cell membrane is present in all the cells.
  • The cell membrane is the outer covering of a cell within which all other organelles including cytoplasm and nucleus are enclosed. It is also referred to as the plasma membrane.
  • By structure, it is a porous membrane (with pores) which permit the movement of selective substances in and out of the cell.  Besides this, the cell membrane also protects the cellular component from damage and leakage.
  • It forms the wall like structure between two cells as well as between the cell and its surroundings.
  • Plants are special, so are their cell structures. Since plants are immobile and cannot hide or run away as animals do during the atmospheric changes. Plants have an advanced cell structure that protects them and makes them rigid and is called a cell wall.

Cell Wall

  • The cell wall is a thick outer layer and is present only in plants cell which protects the plasma membrane and other cellular components. The cell wall is the outermost layer of plant cells.
  • It is a rigid and stiff structure surrounding the cell membrane.
  • It provides shape and support to the cell and protects it from mechanical shocks and injuries.
  • The cell wall of the plant cell is made up of cellulose which provides shape to the cells.

Cytoplasm

  • The cytoplasm is a thick, clear, jelly-like substance present inside the cell membrane.
  • Most of the chemical reactions within a cell take place in this cytoplasm.
  • The cell organelles including endoplasmic reticulum, vacuoles, mitochondria, ribosomes, are all suspended in this cytoplasm.

Nucleus

  • The nucleus contains the hereditary material of the cell, the DNA.
  • It sends signals to the cells to grow, mature, divide and die.
  • The nucleus is surrounded by the nuclear envelope that separates the DNA from the rest of the cell and protects the DNA.

Cell Organelles

Cells are composed of various cell organelles that perform certain specific functions to carry out life’s processes. The different cell organelles along with its important functions are mentioned in the table provided below:

Cell Organelles

Functions

Nucleolus

It is the site of ribosome synthesis. Also, it is involved in controlling cellular activities and cellular reproduction.

Nuclear membrane

Protects the nucleus by forming a boundary between the nucleus and other cell organelles.
Chromosomes

They play an important role in determining the sex of an individual. Each human cells contain 23 pairs of chromosomes.

Endoplasmic reticulum

Involved in the transportation of substances throughout the cell. It plays a primary role in the metabolism of carbohydrates, synthesis of lipids, steroids, and proteins.

Golgi Bodies

It is called the cell’s post office as it is involved in the transportation of materials within the cell.

Ribosome

They are the protein synthesizers of the cell.

Mitochondria

It is called the cell’s powerhouse as it is involved in producing and transforming the energy (ATP).

Lysosomes

It protects the cell by engulfing the foreign bodies entering the cell and helps in cell renewal. Therefore, it is the cell’s suicidal bags.

Chloroplast

They are mainly involved in photosynthesis. As it contains the green coloured pigment called chlorophyll.

Vacuoles

Stores food, water, and other waste materials.

Types of Cells

Cells are similar to small factories with different labourers and departments that work all the time to make life possible. Various types of cells perform different functions.

There are two major kinds of living organisms based on their cellular structure namely:

  • Prokaryotes
  • Eukaryotes

Main Article: Difference Between Prokaryotic and Eukaryotic Cells

Prokaryotes

  1. Prokaryotes are made up of cells with no nucleus.

  2. They all are single-celled microorganisms including archaea, bacteria and photosynthetic blue-green algae known as cyanobacteria.

  3. The cell size ranges from 0.1 to 0.5 µm in diameter.

  4. The hereditary material DNA is found in the nucleoid present in the central part of the cell.

  5. They reproduce by binary fission.

Detailed Information: Prokaryotic Cells

Eukaryotes

  1. Eukaryotes are made up of cells consisting of a true nucleus.

  2. This large category involves all plants, fungi (such as moulds, yeast, and mushrooms), protozoa (Plasmodium falciparum and parasite that cause malaria) and animals.

  3. The plasma membrane is responsible for monitoring the transport of nutrients and electrolytes in and out of the cell and also responsible for cell to cell communication.

  4. Cellular life is entirely dependent on the various chemical process for survival. These chemical reactions mainly occur in a watery solution within the cell known as cytoplasm.

  5. They reproduce sexually as well as asexually.

  6. There are some contrasting features between plant and animal cell. For eg., the plant cell contains chloroplast, central vacuoles, and other plastids, whereas the animal cell does not.

In-depth Reading: Eukaryotic Cells

Cell Theory 

Cell Theory was proposed by the German scientists,  Theodor Schwann, Matthias Schleiden, and Rudolf Virchow. The cell theory states that:

  1. All living species on Earth are composed of cells.

  2. A cell is the basic unit of life.

  3. All cells arise from pre-existing cells.

A modern version of the cell theory was eventually formulated and it contains the following postulates:

  1. Energy flows within the cells.

  2. Hereditary information is passed on from one cell to the other.

  3. The chemical composition of all the cells is the same.

Functions of a Cell

A cell performs six major functions essential for the growth and development of an organism. The functions of a cell include:

Provides Support and Structure

All the organisms are made up of cells. They form the structural basis of all the organisms. The cell wall and the cell membrane are the main components that function to provide support and structure to the organism. For eg., the skin is made up of a large number of cells. Xylem present in the vascular plants is made of cells that provide structural support to the plants.

Facilitate Growth Mitosis

In the process of mitosis, the parent cell divides into the daughter cells. Thus, the cells multiply and facilitate the growth in an organism.

Allows Transport of Substances

Various nutrients are imported by the cells to carry out various chemical processes going on inside the cells. The waste produced by the chemical processes is eliminated from the cells by active and passive transport.

Small molecules such as oxygen, carbon dioxide, and ethanol diffuse across the cell membrane along the concentration gradient. This is known as passive transport.

The larger molecules diffuse across the cell membrane through active transport where the cells require a lot of energy to transport the substances.

Energy Production

Cells require energy to carry out various chemical processes. This energy is produced by the cells through a process called photosynthesis in plants and respiration in animals.

Aids in Reproduction

A cell aids in reproduction through the processes called mitosis and meiosis. Mitosis is termed as the asexual reproduction where the parent cell divides to form daughter cells. Meiosis causes the daughter cells to be genetically different from the parent cells.

Thus, we see, why cells are known as the structural and functional unit of life. This is because they are responsible for providing structure to the organisms and performs several functions necessary for carrying out life’s processes.

Important Questions about Cells

1. What is a Cell?
A cell is defined as the basic, structural and functional unit of all life.

2. State the characteristics of cells

  • Cells provide the necessary structural support for an organism.
  • The genetic information necessary for reproduction is present within the nucleus.
  • Structurally, the cell has cell organelles which are suspended in the cytoplasm.
  • Mitochondria is the organelle responsible for fulfilling the cell’s energy requirements
  • Lysosomes digest metabolic wastes and foreign particles in the cell.
  • Endoplasmic reticulum synthesizes selective molecules and processes them, eventually directing them to their appropriate locations.

3. State the cellular components.

  • Cell membrane
  • Cell wall
  • Cell organelles
    • Nucleolus
    • Nuclear membrane
    • Endoplasmic reticulum
    • Golgi Bodies
    • Ribosome
    • Mitochondria
    • Lysosomes
    • Chloroplast
    • Vacuoles

4. State the types of cells

Cells are primarily classified into two types, namely

  • Prokaryotic cells
  • Eukaryotic cells

5.  Elaborate on Cell Theory 
Cell Theory was proposed by  Matthias Schleiden, Theodor Schwann, and Rudolf Virchow, who were German scientists. The cell theory states that:

  • All living species on Earth are composed of cells.
  • A cell is the basic unit of life.
  • All cells arise from pre-existing cells.

Also Read: Difference Between Plant Cell and Animal Cell

For more information on cells, cell structure, cell types or any other related topics, explore BYJU’S Biology. Alternatively, download BYJU’S app for a personalized learning experience.


Practise This Question

A wire of resistance 0.5Ωm1 is bent into a circle of radius 1 m. An identical wire is connected across a diameter AB as shown in fig. The equivalent resistance is