Cell as the Basic Unit of Life

Introduction

As we know, every living organism is made up of cells. Thus, the cell acts as both structural and functional units in them. A cell contains several small organelles embedded in them. The cell is microscopic and is easily visible by light microscopes. However, to study the detailed structure of the cell and its different organelles, an electron microscope is a must. Anton Von Leeuwenhoek was the first person to see and describe a cell. The nucleus of the cell was later discovered by Robert Brown.

Moreover, Robert Hooke was the first scientist to observe a cell under a microscope. After the discovery of the microscope, Robert Hook observed tiny box-like cavities while observing a cork under the microscope. Thus, he discovered plant cells in 1665. These cells vary significantly in their shapes and sizes from species to species. Even the same individual has great variance in their cell shapes, according to the function. For instance, a neuron is completely different in both structure and function from the sperm cell. Study this cell – the unit of life study material to understand the concepts thoroughly.

Types of Cells

Cells can be of the following two types, based on the presence and absence of a well-defined nucleus:

  • Prokaryotic cells: Prokaryotic cells are primitive cells that lack a well-defined nucleus. These types of cells are mostly present in bacteria and archaea. The characteristics of these cells include:
    • They have a simpler organisation
    • The DNA is not bound and is in direct contact with the cytoplasm
    • They have only a single circular chromosome
    • These cells are approximately 0.1 to 5.0 μm in diameter
  • Eukaryotic cells: All the plants, animals, fungi, slime moulds, protozoa, and algae are eukaryotes. The characteristics of these cells include:
    • They have a well-defined and membrane-bound nucleus
    • They have a typical structural organisation, unlike the prokaryotes
    • Their DNA is present inside the well-developed nuclei
    • These cells are approximately 10 to 100 μm in diameter

The Cell Theory

Matthias J. Schleiden (a German botanist) and Theodor Schwann (a British zoologist) proposed the cell theory in 1839. The basic postulates of this theory are as follows:

  • One or more cells comprise all the living organisms of the planet
  • The cell is the unit of life
  • Energy flows among different cells
  • New cells only originate from pre-existing ones
  • Every cell in the living organism has the same chemical composition
  • The cell consists of genetic information which is transferred from one generation to another

Cellular Components

  • Cell wall: The outermost covering of the plant cell is the cell wall. It is absent in animal cells. Cellulose makes up the cell wall and gives an adequate shape to the cell wall. The cell wall of plants has three parts: primary wall, middle lamella, and secondary wall. Plasmodesmata forms the connection between other cells and the middle lamella. The detailed study of the cell wall is only possible by an electron microscope
  • Cell membrane: The cell membrane is present in both plant and animal cells. It provides a framework for the animal cells as they lack a cell wall. It is a selectively permeable bilayer that only allows specific substances to enter and leave the cell. Thus, it acts as a protective barrier for the entry of pathogens. The membrane comprises three types of lipids and two types of proteins. It may contain phospholipids, glycolipids, and sterol. The protein components may include peripheral or integral proteins
  • Nucleus: The nucleus is a double-membraned organelle, i.e., the ‘control centre’ of the cell. It is responsible for regulating all metabolic activities that occur in the cell. The nucleus contains DNA, the hereditary material of the organism. The nuclear pores on the nuclear membrane connect the interior of the nucleus to the cytoplasm
  • Plastids: Plastids are the characteristic feature of only plant and algal cells. Based on their colour pigments, they consist of three types – chloroplast (green), chromoplast (different colours, like in fruits and flowers), and leucoplast (colourless)
  • Ribosomes: Ribosomes are called ‘protein factories of the cell, as they translate the RNA into protein units. They synthesise proteins by encoding the mRNA. They are either scattered in the cytoplasm or attached to the endoplasmic reticulum. The eukaryotes have two types of ribosomes: cytosolic and organellar ribosomes
  • Endoplasmic reticulum: The study by electron microscope revealed a network of tiny, tubular structures scattered in the cytoplasm. These structures are the endoplasmic reticulum (ER). They can be of two types: smooth ER and rough ER. The difference between them is that rough ER has granular ribosomes attached, whereas smooth ER lacks these granular ribosomes on their surfaces
  • Mitochondria: Mitochondria is the site of adenosine triphosphate (ATP) production. ATP is called the energy currency of the cell. As mitochondria produce ATP, they are called the ‘powerhouse of the cell’
  • Vacuoles: Vacuoles are vesicles filled with water, dissolved inorganic ions, sugars, enzymes, etc. Plants usually have one large vacuole. Animals either have smaller and many vacuoles or not at all. They act as storage organelles and maintain the turgor pressure
  • Peroxisomes: Peroxisomes are responsible for the breakdown of fats. They are small membrane-bound structures that are the derivatives of smooth ER
  • Golgi apparatus: The Golgi apparatus is responsible for storing and transporting proteins that ribosomes synthesise. They are single membrane-bound organelles. They contain flattened membrane sacs called cisternae
  • Lysosomes: Lysosomes, ‘the suicidal bags of the cell,’ are sac-like structures. They contain digestive enzymes. They perform phagocytosis, i.e., they engulf the pathogens and other dead cells. In certain circumstances, the lysosomes start digesting healthy cells. Due to this, they are called the suicidal bags of the cell
  • Centrosome and centrioles: Centrosomes are made up of a pair of cylindrical structures, i.e., centrioles. They are composed of tubulin proteins. They help in providing the proper shape to the cell
  • Cilia/flagella: Cilia and flagella are hair-like protrusions that provide locomotion to the cell. However, cilia are smaller than flagella. Cilia are present more in number than flagella. They are mostly seen in lower organisms

Conclusion 

Cell – the unit of life is responsible for every metabolic function in a living organism. The first-ever discovery of cells was made by Anton Von Leeuwenhoek. However, Robert Hooke coined these tiny substances as ‘cells.’ The cells contain several electron-microscopic cell organelles. The cell theory was proposed by Matthias J. Schleiden (a German botanist) and Theodor Schwann (a British zoologist) in 1839.

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