Did you know that there was no awareness of cells more than 330 years ago? This is because they were too small to be seen by the naked eye. The invention of the microscope allowed scientists to examine and study cells in greater detail. Although scientist Robert Hooke invented the name "cell" after examining dead cells under his microscope, Anton van Leeuwenhoek was the first to observe active cells! Robert Brown identified the 'nucleus,' the engine that allows a cell to work, many years later.

 

Our surroundings are composed of living and non-living things. The question which always arises in our minds is: what is it that makes an organism living? What is it that differentiates living and non-living things? The vital answer to this question is the basic unit of life – the cell – in all living organisms.

 

Every living organism is composed of cells. Living organisms composed of single cells are called unicellular organisms, while organisms composed of many cells are called multicellular organisms.

Human bodies are made up of cells, which can also be called atoms of the body.  They are the foundation of all living things. Pre-existing cells can also give rise to new ones. They can be called the structural and functional units of life in simple words. Cytology is the scientific study of cells. The first person to describe a living cell was Anton Van Leeuwenhoek. Some species have only one cell, whereas others have large cooperating groups of cells. Cell biology is concerned with the form and function of a cell, from the most basic traits shared by all cells to the unique, highly complex tasks exclusive to specialized cells.

Matthias Schleiden, a German botanist, was the first to state that cells were the building elements of all plants in 1838. Theodor Schwann, a German botanist, asserted the next year that cells are the fundamental units of animals. These statements put an end to the idea that plants and animals have basic structural distinctions.

Their discoveries led to the creation of the 'Cell Theory.' Cells are the essential units of all living beings, according to the cell theory. However, the cell theory was unable to explain how new cells originate. In 1855, German scientist Rudolf Virchow declared that new cells develop from existing cells. As a result, the modified cell theory's three key points are as follows:

• • All living organisms have a basic functional and structural unit called the cell.

  • Cells are the building blocks of all living beings.

  • Pre-existing cells give rise to all cells.

Scientists found in the years after that the body units in charge of reproduction are also cells. Here are some interesting facts for you to consider. The female egg (ovum) is the human body's biggest cell. Sperm, on the other hand, is the smallest cell.

We can observe cells in everything; it could be an onion peel or human cheek under the microscope. The onion peel shows typical plant cells. Each cell has a distinct cell wall as its outer boundary, and the cell membrane is within it. The cells which are found in the human cheek have an outer membrane-bound structure called the nucleus. This nucleus carries the chromosomes, which in turn contain the genetic material, DNA. Cells with membrane-bound nuclei are known as eukaryotic, while cells that do not have membrane-bound nuclei are called prokaryotic. In both kinds of cells, i.e., eukaryotic and prokaryotic cells, a semi-fluid matrix known as cytoplasm holds the cell volume. The cytoplasm is the major arena of cellular tasks in plant and animal cells. Many chemical reactions take place inside the cytoplasm to keep the cell in its living state.

 

Apart from the nucleus, eukaryotic cells feature additional membrane-bound structures known as organelles. Such membrane-bound organelles do not exist in prokaryotic cells.

 

A ribosome is a complex molecular system that converts amino acids into proteins via protein synthesis, popularly known as translation. Protein synthesis is an important process that all living cells must perform.

 

Ribosomes are specialized cell components found in both bacterial and eukaryotic cells. Ribosomes are crucial for protein synthesis in all living cells. Another non-membrane-bound organelle present in animal cells that assists in cell division is the centrosome.

 

In terms of size and functionality, cells differ from each other. Mycoplasma, for example, is the smallest cell, measuring around 0.2 micrometers in length, whereas bacteria might measure 3–6 micrometers. The egg of an ostrich is the longest isolated single cell. Human red blood cells have a diameter of 7.6 micrometers when compared to other multicellular species. Nerve cells are the longest cells in a living body. The morphology of cells varies greatly as well. They might be disc-like, polygonal, tabular, cuboid, thread-like, or irregular in shape.

Cells in bacteria, blue-green algae, pleuropneumonia, and mycoplasma mainly represent prokaryotic cells. They are smaller and multiply quickly as compared to eukaryotic cells. Their shape and size may also vary. While they show a wide variety of functions and shapes, the organization of prokaryotic cells is quite similar.

 

The cell wall in the prokaryotic cell surrounds the cell membrane, except in mycoplasma. There is no well-defined nucleus present in it. This means that genetic material is left unwrapped and not enclosed by a nucleus membrane. 

 

Cell envelope and its modification:

The cell envelope is the bacterial cell's outermost covering. Its general tasks include:

• • Cell protection.

  • Communication with the environment.

  • Cellular shape maintenance.

  • Cell stability and rigidity.

  • Permitting proper metabolism, development, and division.

 

The cell envelope protects the structural integrity of the cells. It defends prokaryotes against internal turgor pressure triggered by too much concentration of macromolecules in the cell. The cell envelope contains a three-layered structure. Each layer individually has different functions. However, they act together as a single protective unit. Mesosomes have a special membranous structure that forms a cell by extension of the plasma membrane. These extensions are available in the form of tubules, lamella, and vesicles. This is found helpful in the formation of cell walls.

 

Ribosomes and inclusion bodies:

 

Ribosomes in prokaryotic cells are formed with the plasma membrane of the cell. Bacterial ribosomes are approximately 20nm in diameter, whereas eukaryotic ribosomes approximately have a diameter of 25nm-30nm. Many ribosomes join a single mRNA (Messenger RNA in a single-stranded RNA molecule that complements one of a gene's DNA strands) to form a chain known as polyribosomes or polysomes.

 

Inclusion bodies

 

The inclusion bodies are microscopic particles that float freely inside the cytoplasmic matrix. As a result, they're also known as cytoplasmic inclusions. These cell inclusions occur when the pH drops and the cell's pool of soluble fusion proteins becomes depleted. They're the basic bodies that arise during infectious diseases or within virus-infected cells, such as rabies, herpes, and measles.

Eukaryotic cells are present in all animals, plants, and fungi. Eukaryotic cells have a well-organized nucleus with a nuclear envelope in it. Also, eukaryotic cells have a wide variety of complex locomotory structures. Not all eukaryotic cells are identical. Plants and animals have different cells. Plant cells possess cell walls and plastids, which are absent in animal cells. Animal cells have centrioles, which are lacking in almost all plant cells.

 

Cell membrane: The cell membrane consists of proteins and lipids. Lipids are arranged in the membrane having a polar head towards the outer sides and hydrophobic ends towards the inner part. This means hydrocarbons are protected from an aqueous environment. A membrane with fluid nature is essential for functions like cell growth, cell division, endocytosis, etc.

 

Cell wall: It is the outermost covering of the plasma membrane in plant cells. A cell wall gives shape to the cell and protects it from mechanical damage. It may also help in cell-to-cell interaction and create a barrier against undesirable macromolecules. The cell wall capable of growth in young plants is called the primary cell wall. The wall on the inner side, towards the cell's membrane side, is the secondary cell wall.

 

Endomembrane system: The eukaryotic cell's endomembrane system is a membranous component. A system of suspended membranous organelles exists in the cytoplasm of the cell. Even though they have various forms and functions, the organelles are a system since they are necessary for the cell to function. The cell membrane, vacuoles, the nuclear membrane, lysosomes, the Golgi complex, vesicles, and the endoplasmic reticulum are all organelles that operate together (ER).

1. Why is the cell known as the unit of life?

A. In a living organism, the cell is known as the structural and functional unit of life. It is because it is the most basic, smallest level in a living organism. The cellular level within an organism is where the metabolic processes occur that keep it alive.

 

2. What are the concepts included in Chapter 8 of NCERT Solutions for Class 11 Biology?

A. NCERT Solutions for Class 11 Biology Chapter 8 comprises concepts like cells and their discovery, structure, components, famous scientists, and their contribution in this field. It tells the students about the evolution of one organism to another organism (unicellular or multicellular). The structure of a cell and why it is called the functional unit of life are some of the chapter's most important topics. Brief descriptions of eukaryotic cells and prokaryotic cells have been discussed in the chapter as well.

 

3.Is Chapter 8 Biology Class 11 important for the board exams?

A. Yes. Cell: The Unit of Life Class 11 NCERT is an important chapter for the board exams. This chapter discusses some important cells' features, structure, functions, formation, and the two broad categories of cells and their respective functions. It helps you get a comprehensive picture of the definition and functions of a cell.