Algae are classified into three main classes – Chlorophyceae, Phaeophyceae, and Rhodophyceae. These divisions are based on the following factors:
(a) Major photosynthetic pigments present
(b) Form of stored food
(c) Cell wall composition
(d) Number of flagella and position of insertion

Class I – Chlorophyceae
Common name – Green algae
Major pigments – Chlorophylls a and b
Stored food – Starch
Cell wall composition – Cellulose
Flagella number and position – 28; equal and apical

Class II – Phaeophyceae
Common name– Brown algae
Major pigments – Chlorophylls a and c, and fucoxanthin
Stored food – Mannitol and laminarin
Cell wall composition – Cellulose and algin
Flagella number and position – 2; unequal and lateral

Class III – Rhodophyceae
Common name – Red algae
Major pigments – Chlorophylls a and b, and phycoerythrin
Stored food – Floridean starch
Cell wall – Cellulose, pectin, and polysulphate esters
Flagella number – Absent