Q.Describe the classification of algae into Chlorophyceae, Phaeophyceae and Rhodophyceae, giving their salient distinguishing features (pigments, stored food, cell wall, habitat) and two examples of each class.

Algae are sorted into three classes chiefly on the basis of their dominant photosynthetic pigments, together with supporting features such as stored food, cell-wall composition and habitat.

Chlorophyceae (green algae). Dominated by chlorophyll a and b, which give the class its green colour. They are mostly freshwater organisms. The plant body may be unicellular, colonial or filamentous, with a cellulose cell wall, and reserve food is stored as true starch, often within pyrenoids embedded in the chloroplast. Examples: Chlamydomonas (unicellular, motile, with a cup-shaped chloroplast) and Spirogyra (an unbranched filament with a spirally coiled, ribbon-shaped chloroplast).

Phaeophyceae (brown algae). Dominated by chlorophyll a and c, with the accessory pigment fucoxanthin masking the chlorophylls to give a brown colour. They are almost entirely marine. In larger members, the body differentiates into a holdfast, stipe and frond; the cell wall contains cellulose, fucans and algin, and food is stored as mannitol, laminarin and starch. Examples: Fucus and Sargassum, both large marine seaweeds with body regions differentiated for anchorage, support and photosynthesis.

Rhodophyceae (red algae). Dominated by chlorophyll a and d, with the abundant pigment phycoerythrin giving the class its red colour. Found in both marine and fresh water, including at considerable depth. The cell wall is made of cellulose and pectin, and food is stored as Floridean starch, a form unique to this class. Examples: Polysiphonia and Porphyra, both thalloid marine algae; the class is also the commercial source of agar-agar.

Overall pattern. Across all three classes, the classification is built on the same set of comparable characters -- pigments, stored food, cell-wall composition, flagellar number/position (where motile cells occur), and habitat -- applied consistently so that the enormous range of algal body forms, from a single cell to a giant seaweed, can still be sorted into three coherent, comparable classes.

Work class by class (Chlorophyceae, Phaeophyceae, Rhodophyceae), stating pigment, stored food, cell wall, habitat and two examples for each, in the same order every time.

• Mixing up the stored-food terms between classes (true starch vs mannitol/laminarin vs Floridean starch).
• Forgetting to name two examples for each class as the question specifically asks.