Biology is a branch of science that deals with the study of living organisms

Aristotle is called the father of biology

The term “biology” was coined by Lamarck and treviranus in 1802

Characteristics of living beings

  • Cellular structure
  • The cell is the building blocks of the living body
  • It is the structural and functional unit of life
  • This complex arrangement lacks in non-living organisms
  • Metabolism
  • It’s the salient dynamic feature of all living beings
  • It’s the sum total of all chemical reactions occurring in the body of an organism
  • Metabolism can be categorized into two halves or phases namely Anabolism and catabolism
  • Anabolism
  • It is the constructive phase as in this phase, protoplasm and other complex substance is formed which help in the growth and development of the body
  • The examples are a synthesis of protein from amino acids, glucose formation from photosynthesis
  • Catabolism
  • It is the destructive phase in which complex molecules are broken down into simpler ones
  • Resulting in the release of energy
  • The examples are a breakdown of starch to form glucose which later on broken into water, carbon-di-oxide and release energy
  • Nutrition
    1. All living organism require nutrition for growth and development
    1. Intake of food particles of provide energy and material for its life sustaining activities it’s called nutrition
  • Growth and development
  • The life of every organism begins from a cell
  • It further divides to form many cells, tissues which form organs, organ system and finally a complete body.
  • The phenomenon is called growth which is the distinctive feature of an organisms
  • reproduction
  • It is the salient feature of living organisms
  • Every organism reproduces for maintaining the continuity of life
  • The examples are plant produces seeds, which germinate to produce the same plant again, likewise, mammals give birth to the child
  • Sensitivity
  • All organisms are sensitive to environmental changes
  • They have consciousness for their protection, feeling of hot and cold, withdrawal of organs after pinching off thorns and touch with acid

The cell

  • The cell was discovered by Robert Hooke who also coined the term cell in his book called the “micrographia”
  • Max Schultze proposed that the living matter of an organism is protoplasm and the cell is simply an accumulation of protoplasm limited by an outer membrane, containing a nucleus

Cell theory

  • The cell theory was stated by Schleiden and Schwann.
  • It was later modified by Rudolf Virchow
  • The principles are as follows
  • All living organisms are composed of one or more cell
  • Cells are the basic unit of structure and function in an organisms
  • New cells arise from pre-existing cells
  • On the basis of types of nuclei, all the living cells can be classified into two groups
  • Prokaryotic

These are primitive cells lacking a true nucleus and most of the other cells lacking a true nucleus and most of the other cell organelles. The examples are the bacterial cell, blue-green algae, myoplasm

  • Eukaryotic

These have the true nucleus and membrane-bound cell organelles. These are present in unicellular and multicellular plants and animals

A prokaryotic and eukaryotic cell

                                                Structural organization in cells

Cell wall

  • Plant’s cell contains cell wall, while animal cells lack it
  • The typical cell wall contains four middle lamellae, primary cell wall, secondary cell wall, tertiary cell wall
  • It is made up of cellulose, hemicellulose, pectin, lignin
  • The middle lamella of a plant cell wall is composed of calcium and magnesium pectate
  • The main function of the cell wall is to protect the cytoplasm and cell membrane from external shock
Cell wall

cell membrane or plasma membrane

  • The cell membrane or plasma membrane is a biological membrane, that separates the interior of all cells from the outside environment in animal cells
  • The cell membrane is selectively membrane to ions and organic molecules and controls the movement of substances in and out of cells
  • The basic function of the cell membrane is to protect the cell from its surroundings
  • It is involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signalling
  • It serves as the attachment surface for serval extracellular structures, including the cell wall, glycocalyx and intracellular cytoskeleton
  • The cell membrane contains receptor, which recognizes specific hormones and conveys the information to the interior of the cell
  • The most accepted model of a membrane in fluid mosaic model proposed by singer and Nicolson
  • According to this model, a membrane consists of a continuous bilayer of phosphide molecules in which globular proteins are embedded


  • The living substance of a cell is called a protoplasm
  • It is differentiated into regions nucleoplasm and cytoplasm
  • The soluble part of the cytoplasm forms the background material or ground substance between the cell organelles
  • It is considered a living matter because it has the power of internal growth
  • It shows streaming movement called cyclosis
  • It remains alive at 0-60C but at the higher temperature, it dies and coagulates into a solid mass

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