Life Science: Chapter 2 - The Cell

THE CELL

• Cell is the basic unit of life. It is made up of life giving substance called protoplasm.
• Cell was discovered by Robert Hooke.
• Cell Theory states that "All living organisms are composed of one or more cells and all cells came from pre-existing cells.
• The study of structure and composition of cells is known as Cytology.
• Organisms consisting of only one cell are called unicellular organisms, e.g. amoeba, bacteria, etc.
• Organisms consisting of more than one cell are called multicellular organisms. They may be made of few cells (e.g. algae and fungi) to millions of cells (e.g. humans, animals, trees, etc.)
• The smallest cell is mycoplasma or PPLO (pleuro pneumonia like organism). In this cell, cell wall is absent. It is also called as joker of the microbial world.
• The largest cell is the yolk of an ostrich egg.

There are two types of cell

1. Prokaryotic Cell (Cells which do not contain neucleus)

2. Eukaryotic Cell (Cells which contain neucleus)

STRUCTURE OF CELL

In majority of the cells

Life Science: Chapter 1 - Plant and Animal Classification

CLASSIFICATION OF PLANTS

• Entire plant kingdom is divided into Cryptogamae (Non-flowering) and Phanerogamae (Flowering) depending upon the presence or absence of flowers and seeds.

Cryptogamae (Kryptos: concealed; Gamus: Marriage)

• It includes non-flowering plants such as algae, fungi, lichens, mosses and ferns.
• Classified in three parts: 

1. Thallophyta (Comes from thallus: plant body not differentiated into root, stem, body and leaves)
2. Bryophyta (Amphibian of plant kingdom) - They lack roots, flowers and seeds but root like structure rhizoids are present.
3. Pteridophyta - They have primitive vascular system.

Phanerogamae (Phaneros: Visible; Gamus: Marriage)

• They are divided into two parts:

1. Gymnospermae (Naked Seed)
2. Angiospermae (Covered Seed)

CLASSIFICATION OF ANIMALS

• When any plane passing through the central axis of the body divides the organism in two halves that are approximately mirror images it is called radial symmetry and the animals showing radial symmetry are called Radiata.
• When the body can be divided into identical left and right halves in only one plane, this kind of symmetry is called bilateral symmetry and such animals are called Bilateria.
• Animals are mainly classified into two parts: Invertebrates (animals with backbone) - Almost 99% animals fall in this group and Vertebrates (animals with backbone).
• Animals are also classified as Chordates (animals in which notochord is present) and Non-Chordates (animals in which notochord is absent).