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 – 2-8; 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 d, and phycoerythrin

Stored food –­ Floridean starch

Cell wall – Cellulose, pectin, and polysulphate esters

Flagella number – Absent