Introduction to Biology: Branches, Levels, and Characteristics of Living Things

Introduction to Biology

Biology is the science dedicated to the study of living things and phenomena related to life in all its aspects. Aristotle was the creator of biology. The theory of spontaneous generation explains asexual reproduction, and he studied and described more than 500 species of animals. Michael Christoph Hanov was the first person who used the term biology for the first time.

Branches of Biology

  • Bacteriology: studies of bacteria
  • Zoology: studies animals
  • Botany: studies plants
  • Mycology: studies fungi
  • Protozoology: studies protozoa
  • Ornithology: study of birds
  • Herpetology: study of reptiles
  • Mammalogy: the study of mammals
  • Ichthyology: study of fish

Levels of Organization of the Studied Organism

  • Biochemistry/Molecular Biology: molecules that make up living things
  • Cytology: study of cells
  • Histology: studying the groups of cells that form tissues
  • Physiology: studies the function of cells and organs
  • Anatomy: study of living things in terms of the structure of their organs
  • Ecology: study the interaction of living things with the environment
  • Paleontology: origin of life

Other Branches

  • Chemistry: knows the structure of both living things and biomolecules (carbohydrates, lipids, protein, and nucleic acids)
  • Physics: allows us to study microscopic organisms
  • Mathematics: allows us to know how a population grows, velocity of diseases, etc.
  • Geography: helps us to analyze the world on a dynamic planet that has been in constant change since origin
  • Informatics: helps us to store the great amount of information

Cell Theory

The theory that states that all living things are made up of cells (James Watson, Francis Crick, and Rosalind Franklin).

How DNA Was Discovered

With the help of x-ray crystallography, quantum physics, and their great creativity.

Characteristics of Living Things

  • Living things have different levels of complexity in their levels of organization
  • Levels:
    • Cellular level: made up of cells, considered the smallest unit of life
    • Tissue level: made up of specialized tissues
    • Organ level: organs have similar structure and function
    • Individual or multicellular organism level: made up of all the apparatus and systems that allow us to carry out all the functions necessary for life
    • Population level: group of individuals of the same species, living in the same habitat that interchange genes through reproduction
    • Ecosystem level: communities interact between themselves and with their environment
  • Living things interchange materials and energy (one or more cells)
  • Unicellular: only one cell, bacteria or amoebas
  • Multicellular: more than one cell
  • Kinds of Reproduction:
    • Asexual: happens when identical organisms originate from the progenitor without fertilization
    • Sexual reproduction: requires the union of a sperm with an ovule that will give rise to a zygote and then to an embryo
  • Irritability: capable of reacting to changes in the surroundings
  • Homeostasis: things have the characteristic of responding to changes to motion their internal environment stable
  • Adaptation: change the process of natural selection
  • Evolution: is the result of the selections of those characteristics that are favored in the environment
  • Why Cells Die? Because of age or because of deterioration


  • Genome: the genetic material of any particular organism
  • Biological elements: the chemical elements found in each of the living things
  • Primary biological elements: form the highest percent of the living things mass: carbon, lipids, oxygen, and nitrogen
  • Biomolecules: result from the combination of several bioelements to form components (carbohydrates, lipids, proteins, nucleic acids, and vitamins)


Lipids are organic molecules that are insoluble in water, with an oily, waxy, or fatty consistency, whose functions are to store energy, to participate in muscle contraction, as insulators, and make up part of the cell membranes. There are three kinds of lipids:

  1. Fatty acids and waxes: made up of carbon, hydrogen, and oxygen
  2. Phospholipids: also contain phosphorus and nitrogen
  3. Steroids: characterized by being made up of fused carbon rings

Fats, which are made up of two or three fatty acids and glycerol, are a rich source of energy, they are found in butter and vegetable oil. Phospholipids are made up of a polar head with a phosphate group and two nonpolar fatty acid tails. Steroids are also lipids that make part of cells; cholesterol is the most abundant steroid in animal cells and is used in the production of the bile salts, vitamin D, and steroid hormones such as testosterone and estrogen.


Proteins are organic compounds made up of chains of amino acids, they are the most diverse molecules in living things, form the structural part of the body, like keratin, which makes up nails, scales, hooves, hair, and feathers. Enzymes are proteins in charge of different chemical reactions. Hemoglobins’ function is transportation since it is in charge of carrying oxygen to all body cells.

Nucleic Acids

These are organic molecules that carry the genetic message of the cells and are made up of subunits called nucleotides in which the sugar of one nucleotide is bonded to the phosphate group of another.