In the preceding chapters, we came across a large variety of organisms, both unicellular and multicellular, of the animal kingdom. In unicellular organisms, all functions like digestion, respiration and reproduction are performed by a single cell. In the complex body of multicellular animals, the same basic functions are carried out by different groups of cells in a well-organised manner. All complex animals consist of only four basic types of tissues. These tissues are organised in specific locations to form organs.
When you look around, you see both living and non-living things. You must have asked yourself – “What is it that makes an organism living?” or, “What is it that an inanimate object does not have which a living thing has?” The answer to this is the presence of the basic unit of life – the cell in all living organisms. All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms. In contrast, others, like us, are composed of many cells and are called multicellular organisms.
Cell theory and cell as the basic unit of life, structure of prokaryotic and eukaryotic cells; Plant cell and animal cell; cell envelope; cell membrane, cell wall; cell organelles – structure and function; endomembrane system, endoplasmic reticulum, golgi bodies, lysosomes, vacuoles, mitochondria, ribosomes, plastids, microbodies; cytoskeleton, cilia, flagella, centrioles (ultrastructure and function); nucleus.
There is a wide diversity in living organisms in our biosphere. A few other topics included in this chapter are How to Analyse Chemical Composition? Primary and Secondary Metabolites, Biomacromolecules, Proteins, Polysaccharides, Nucleic Acids, Structure of Proteins, Nature of Bond Linking Monomers in a Polymer, Dynamic State of Body Constituents – Concept of Metabolism, Metabolic Basis for Living, The Living State, Enzymes.
