This document discusses the classification of life and the hierarchy of life forms. It notes that biologists categorize organisms into groups and subgroups to make their study easier. Classification is based on characteristics like morphology, anatomy, biochemistry, and ecology. All living things share common themes of organization, information processing, energy and matter transformation, and interactions at different hierarchical levels. Cells are the basic unit of life, and while they can differ, they all descend from earlier cells and share common features. Organisms are classified as unicellular or multicellular depending on whether they are composed of single or multiple cells.

1. Classification, why and how?
• Living world is full of diversity and complexity
• Organisms are categorized in different super
groups and subgroups to make their study easier
• Biology is an ongoing enquiry about the nature of
life and classification of life forms is a part of it.
• Biologist use different criterions for classification
based on the characteristics of life.
• Morphology – External structure and appearance
• Anatomy – Internal structure and appearances
• Biochemical pathways – various chemical
reactions occurring inside the body
• Ecological – Various adaptation to suit their
environmental requirements

2. Hierarchy of life

3. Common thread weaves this hierarchy
• The study of life reveals unifying themes
• Organization - living organisms are highly ordered
in structure
• Information – life processes involves expression
and transmission of information
• Energy and matter processing - Life requires
transfer and transformation of energy and matter
• Interactions at every level of hierarchy -
Interaction leads to emergence of new properties
in biological systems
• Idea of evolution makes logical sense of
everything we know about living organisms

4. The Cell
• Cells are as fundamental to biology as atom is for
chemistry
• Cell is the simplest collection of matter that can
be considered a living entity
• All cells are related by their descent from earlier
cells
• Although cells can differ substantially from one
another they share common features
• Discovered by Robert hook in 1665
• Cell theory was developed by Schlieiden and
Schwann in 1839

6. Classification based on cellularity
Unicellular organisms
• Composed of single cell
• Microscopic in nature
• Division of labor is at
organelle level
• Includes both eukaryotes
and prokaryotes
• Life span is short
• Asexual reproduction is
predominant
• Bacteria, amoeba,
paramecium, yeast
Multicellular organisms
• Composed of more than
one cells
• Mostly macroscopic
• Division of labor is at
cellular level
• Only eukaryotes
• Shows longer lifespan
• Both asexual and sexual
mode of reproduction
• Humans, elephants, whales,
trees