Algae — Classification (Chlorophyceae, Phaeophyceae, Rhodophyceae)

NCERT grounding

NCERT Class 11 Biology, Chapter 3 (Plant Kingdom), opens its treatment of Plantae with section 3.1 on algae. The very first exercise question of the chapter — "What is the basis of classification of algae?" — tells you exactly what the examiner wants you to retain: the pigments possessed and the type of stored food. The chapter then lays out the three classes in sub-sections 3.1.1 (Chlorophyceae), 3.1.2 (Phaeophyceae) and 3.1.3 (Rhodophyceae), and consolidates them in Table 3.1.

"Depending on the type of pigment possessed and the type of stored food, algae are classified into three classes, namely Chlorophyceae, Phaeophyceae and Rhodophyceae." — NCERT Class 11 Biology, Chapter 3 Summary

General features of algae

Algae are defined by NCERT as chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms — found in both fresh water and marine habitats. They also occur on moist stones, soils and wood, in association with fungi (lichen), and even on animals such as the sloth bear. Their form and size are highly variable: from colonial forms like Volvox, through filamentous forms like Ulothrix and Spirogyra, to the massive plant bodies of marine kelps.

Note carefully that cyanobacteria — the blue-green algae — are no longer counted as algae. NCERT explicitly excludes members of Monera with cell walls from Plantae, so blue-green algae belong to Monera, not to the algal classes discussed here. This is a recurring distractor in matching questions.

The three classes in detail

Each class is distinguished by a fixed combination of dominant pigment, reserve food, cell-wall composition and flagellar arrangement. Memorise these as locked clusters — NEET tests them as multi-statement sets in which one wrong attribute invalidates the whole option.

Chlorophyceae — green algae

Members may be unicellular, colonial or filamentous, and are usually grass green due to the dominance of chlorophyll a and b. The pigments are localised in definite chloroplasts, which may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species. Most members have one or more storage bodies called pyrenoids located in the chloroplasts; pyrenoids contain protein besides starch. Some store food as oil droplets. The reserve food is starch. Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose. Common examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.

Phaeophyceae — brown algae

Found primarily in marine habitats, brown algae range from simple branched filamentous forms (Ectocarpus) to profusely branched kelps reaching 100 metres. They possess chlorophyll a, c, carotenoids and xanthophylls; colour varies from olive green to various shades of brown depending on the amount of the xanthophyll pigment fucoxanthin. Food is stored as the complex carbohydrates laminarin or mannitol. The cellulosic wall is usually covered externally by a gelatinous coating of algin. The plant body is typically attached by a holdfast, with a stalk (stipe) and a leaf-like photosynthetic frond. Common examples: Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.

Rhodophyceae — red algae

The members are called red algae because of the predominance of the red pigment r-phycoerythrin, alongside chlorophyll a and d. The majority are marine, with greater concentrations in warmer areas; they occur both in well-lighted surface regions and at great depths where little light penetrates. The red thalli are mostly multicellular and some show complex body organisation. Food is stored as floridean starch, which is very similar in structure to amylopectin and glycogen. Red algae yield commercial hydrocolloids — carrageenan, and agar (obtained from Gelidium and Gracilaria), the latter used to grow microbes and in ice-creams and jellies. Common examples: Polysiphonia, Porphyra, Gracilaria and Gelidium.

Pigment, stored-food & habitat comparison

The single most examined object in this subtopic is NCERT Table 3.1. Learn it row by row; NEET converts each cell into a true/false statement.

Reproduction in algae

Algae reproduce by three routes — vegetative, asexual and sexual — and NCERT treats these identically across the three classes with class-specific flagellar detail.

Sexual reproduction is graded by gamete size and motility. In isogamy, the fusing gametes are similar in size — flagellated as in Ulothrix, or non-flagellated as in Spirogyra. In anisogamy, the two gametes are dissimilar in size, as in species of Eudorina. In oogamy, one large non-motile female gamete fuses with a smaller motile male gamete, as in Volvox and Fucus. Red algal sexual reproduction is always oogamous and is followed by complex post-fertilisation development.

Worked examples

Common confusion & NEET traps