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Organization of Life and Cell Theory

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As it was stated earlier, all organisms are made of cells, which are life’s basic units. Despite having huge differences in sizes and shapes of all the living organisms from the tiniest living organisms of a single-celled organisms to the largest 10 kilometers-squared size fungi, Armillaria Ostoyae, are consist of cells.


This conclusion forms the basis of what has come to be known as the cell theory. The cell theory, one of the basic ideas in biology, is the foundation for understanding the reproduction and growth of all organisms. The cell theory gives two important conclusions. Firstly, it concluded that all living organisms consist of cells and secondly, all cells come from pre-existing cells. Description of the discovery of cell comes later.


Continuity of life: The Molecular Basis of Inheritance

Even the simplest cell that cannot be seen by naked eye is incredibly complex-more intricate than a computer. Cells have many different types of shapes, basis and functions and so what specifies a particular cell is encoded inside a nucleus of cell called gene. Gene specifies work, shapes, functions and energy of cells.

Genes are formed aside DNA. DNA is a long, cable-like molecule, two long chains of building blocks, called nucleotides, wound around each other. Winding around each other like the rails of spiral staircase, the two strands of DNA make a double helix.

The continuity of life from one generation to the next-heredity – depends upon the faithful copying of a cell’s DNA into daughter cells. So, DNA is a basic unit of molecule of heredity characteristics.

The Correlation of structure and function.

Structure and function are correlated at all levels of biological organization. This theme is a guide to the anatomy of life at its many structural levels, from molecules to organisms. Analyzing a biological structure gives us clues about what it does and how it works. Conversely, knowing the function of a structure provides insight about its construction.

We can see the correlation of structure and shape of bird’s wings and their functions clearly. The aerodynamically efficient shape of a bird’s wing, chest muscle for flight efficacy, the width of chest bones to build up the muscles, air bags in the skeleton to lighten up the weight of body helps in the flight of a bird.

Diversity of Life: Evolutionary Change

The nature of diversity is a hallmark of life. Biologists have identified and named about 1.5 million species, including over 260,000 plants, almost 50,000 vertebrates (animals with backbones), and more than 750,000 insects. Thousands of newly identified species are added to the list each year. Estimates of the total diversity of life range from about 5 million to over 100 million species.

Biologists divide life’s great diversity into three great groups, called domains: Bacteria, Archaea, and Eukarya. The domains Bacteria and Archaea are composed of prokaryotes (single-celled organisms with little internal structure), while domain Eukarya is made up of eukaryotes, organisms composed of a complexly organized cell or multiple complex cells. However, Archaea seem more closely related to Eukarya than to Bacteria.

Within the Eukarya are four main groups called kingdoms. Kingdom Animalia includes human being; all plants are included in Kingdom Plantae. Kingdoms are also categorized into phylum, phylum to class, class to order, order to family, family to genus, genus to species. These are 8 great classification of life. If we ask about how do such diverse life forms of living organism form? All biologists unanimously claim it to happen through the evolution.

Unity of Life: Evolutionary Conservation

Biologists believe that all living things have descended from some simple cellular creature that arose about 2.5 billion years ago. Some of the characteristics of that earliest organism have been preserved in all things alive today. The storage of hereditary information in DNA, for example, is common to all living things.

Also, all eukaryotes possess a nucleus that contains chromosomes. The retention of these conserved characteristics in a long line of descent usually reflects a fundamental role in the biology of the organism, one not easily changed once adopted.

These five properties are the core themes in biology that are visible and we have to analyse any biological theories with respect to these themes.

Before proceeding to the next lesson, answer these questions by doing a personal research:

  1. What are the seven important properties of life?
  2. Identify each 10 hierarchy of life.
  3. What is the basic building block of life? Why is it so?
  4. How do we understand by the emergent property? Explain with an example.
  5. Identify five core themes that form the basis for all of biology.
  6. Hierarchy of life and Core themes are very important while studying biology. Why?