Q.Agar is a commercial product obtained from algae.
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Rhodophyceae — The Red Algae
Imagine you are snorkelling in warm tropical waters. As you go deeper, the familiar green seaweed disappears. But something else catches your eye — delicate, feathery fronds in shades of deep crimson, maroon, and even purple. These are the red algae, the Rhodophyceae.
Why are they red? Sunlight penetrates water in layers. Red light is absorbed in the first few metres, but blue-green light travels deeper. Ordinary chlorophyll cannot capture that deep blue-green light efficiently. Red algae solved this problem by evolving a special accessory pigment called r-phycoerythrin. This pigment absorbs blue-green light and transfers the energy to chlorophyll, allowing red algae to photosynthesise at depths where almost no other algae can survive. The red colour you see is the r-phycoerythrin itself — it reflects red light, so the algae look red to our eyes.
The name "Rhodophyceae" comes from Greek rhodon (rose) + phyceae (algae). It literally means "rose-coloured algae."
The Precise Statement
Rhodophyceae is a class of algae characterised by:
- The presence of r-phycoerythrin (a red accessory pigment) along with chlorophyll a and d, and other phycobilins.
- No flagellated cells at any life stage — a unique feature among major algal groups.
- Floridean starch as the stored food reserve, deposited outside the chloroplast (in the cytoplasm).
- Cell walls made of cellulose, often with agar or carrageenan (commercially valuable polysaccharides).
- Predominantly marine (over 98% of species), found from intertidal zones to great depths.
Key Examples You Must Know
| Alga | What it looks like | Why it matters |
|---|---|---|
| Polysiphonia | Fine, branched, filamentous — like a tiny red bush | Classic example for studying life cycles (triphasic: haploid, diploid, triploid) |
| Porphyra | Thin, sheet-like, reddish-purple fronds | Cultivated as nori — the wrapper around sushi rolls |
| Gelidium | Tough, wiry, branched | Major source of agar — used in microbiology as a culture medium and in food as a gelling agent |
To remember the three examples: Polysiphonia (filamentous), Porphyra (sheet-like), Gelidium (wiry). Think "PPG" — like a red paint colour code.
Why Red Algae Are Biologically Distinct
Unlike green algae (Chlorophyceae) or brown algae (Phaeophyceae), red algae never produce motile cells. No sperm with flagella, no zoospores — everything moves by water currents or passive transport. This is a major evolutionary constraint, yet red algae thrive.
Their food reserve, floridean starch, is similar to starch but is stored in the cytoplasm rather than inside chloroplasts. This is a diagnostic feature — if you stain a red algal cell and see starch grains outside the plastids, you know it is a rhodophycean. …
Several commercially valuable products are extracted from algae, and agar-agar, obtained from red algae, is one of the most widely used.
…
(a) Agar is obtained from red algae such as Gelidium and Gracilaria. (b) It is widely used as a solidifying agent in microbiological culture media.
(a) Agar-agar, a gelatinous polysaccharide, is commercially extracted from the cell walls of red algae (Rhodophyceae), most notably Gelidium and Gracilaria.
…
- CBSE 2026Set ANNUAL1 markQ.Name the group of algae in which food is stored in the form of floridean starch.
›Reveal solutionSolution
Rhodophyceae (red algae) are named for their red colour, from the pigment r-phycoerythrin masking their chlorophyll, and they characteristically store food as floridean starch.
Among the classes of algae covered under the Plant Kingdom, each group is distinguished partly by its pigments and its storage product:
- Chlorophyceae (green algae) store food as starch, inside chloroplasts.
- Phaeophyceae (brown algae) store food as complex carbohydrates called laminarin and mannitol.
- Rhodophyceae (red algae) store food as floridean starch, a polysaccharide chemically similar to amylopectin/glycogen, which accumulates outside the chloroplast, in the cytoplasm. …
- CBSE 2025Set ANNUAL1 markMCQQ.Agar is obtained from which type of algae ?(a) Green(b) Brown(c) Red(d) None of these
›Reveal solutionSolution
Agar is a polysaccharide extracted from the cell walls of red algae (Rhodophyceae).
Algae are grouped by their dominant pigments into Chlorophyceae (green), Phaeophyceae (brown) and Rhodophyceae (red). Several economically important products come from algae:
…
- CBSE 2025Set ANN1 markQ.The stored food material present in Rhodophyceae is ______ .
›Reveal solutionSolution
The stored food of red algae (Rhodophyceae) is floridean starch.
Members of Rhodophyceae, the red algae, owe their red colour to the pigment r-phycoerythrin. Their reserve food material is a complex carbohydrate called floridean starch, whose structure is similar to that of amylopectin and glycogen. (For comparison, gr …
- CBSE 2023Set ANN1 markQ.Observe the relationship of first pair and fill in the blank : algin : Brown algae Carrageen : ________
›Reveal solutionSolution
Just as algin is a cell-wall/colloidal product of brown algae, carrageenan ("carrageen") is obtained from red algae.
Algae are classified into three main classes based on pigmentation, stored food and cell-wall composition: Chlorophyceae (green algae), Phaeophyceae (brown algae) and Rhodophyceae (red algae). Each class yields characteristic commercial cell-wall/mucilage products:
- Brown algae (Phaeophyceae) — cell walls contain algin, used as a thickening/emulsifying agent. …
- CBSE 2023Set ANNUAL1 markMCQQ.Agar-agar is obtained from :(a) ulothrix(b) uiva(c) gelidium(d) sargassum
›Reveal solutionSolution
Agar-agar comes from Gelidium (red alga).
Agar (agar-agar) is a polysaccharide obtained from the cell walls of red algae, chiefly Gelidium and Gracilaria. It is widely used as a culture medium and food gelling agent. Ulothrix and Ulva are green algae and Sargassum …
- CBSE 2022Set TERM11 markMCQQ.AGAR-AGAR is obtained from(a) Green algae(b) Red algae(c) Brown algae(d) Blue green algae
›Reveal solutionSolution
Agar-agar is a commercial product obtained from red algae.
Algae are economically important in several ways. Agar, the gelatinous substance widely used to prepare solid culture media in microbiology laboratories (and as a vegetarian gelling agent in food), is extracted from the cell walls of certain red algae (Rhodophyceae), chiefly Gelidium and Gracilaria species. Green algae are a source of food/fodder in some cases but not a …
- CBSE 2022Set ANNUAL1 markMCQQ.The major pigments in Rhodophyceae are(a) chlorophyll a & b(b) chlorophyll a, d & phycoerythrin(c) chlorophyll a, c & fucoxanthin(d) chlorophyll a & c
›Reveal solutionSolution
Red algae (Rhodophyceae) carry chlorophyll a, chlorophyll d and the red accessory pigment phycoerythrin.
Rhodophyceae, commonly called red algae, get their characteristic red colour from the pigment r-phycoerythrin, which predominates over their green chlorophyll. Their chief pigments are chlorophyll a and chlorophyll d (not b or c, which belong to other algal classes), plus the phycobilin pigment ph …
- CBSE 2021Set ANNUAL1 markMCQQ.In which class r-Phycoerythrin is the major pigment?(a) Chlorophyceae(b) Phaeophyceae(c) Rhodophyceae(d) None of these
›Reveal solutionSolution
r-Phycoerythrin, a red pigment, dominates in class Rhodophyceae.
Algae are classified into classes mainly by their dominant photosynthetic pigments and stored food. Rhodophyceae (the red algae) contain chlorophyll a and d along with the phycobilin pigment r-phycoerythrin, which gives them their characteristic red colour and lets them absorb the blue-green light that p …
- CBSE 2020Set ANNUAL1 markQ.What is the name of red pigment found in members of Rhodophyceae?
›Reveal solutionSolution
The red pigment characteristic of Rhodophyceae is phycoerythrin.
Rhodophyceae, commonly called red algae, are mostly marine, often found at greater depths in the ocean. Their characteristic red colour is due to the dominant presence of the phycobilin pigment phycoerythrin, in addition to chlorophyll a and d, and carotenoids, which are also present but masked by the red pigment. This pigment composition allows red algae to absorb the blue-green light that penetrates to greater ocean depths, enabling photosynthesis wh …
- CBSE 2020Set ANNUAL1 markQ.Name the red pigment present in the members of Rhodophyceae.
›Reveal solutionSolution
Red algae (Rhodophyceae) get their red colour from the pigment r-phycoerythrin, an accessory pigment that predominates over their chlorophyll.
Members of Rhodophyceae (commonly called red algae) are found mostly in marine habitats, often at greater depths. Their cells contain:
- Chlorophyll a and d
- Phycoerythrin (a red pigment) — present in larger amounts, which is why these algae typically appear red. …
- CBSE 2019Set ANNUAL1 markQ.Match the column - item 'Agar-agar' (Column A) with its correct match in Column B:(a) Cytokinesis(b) Kranz-anatomy(c) Pacemaker(d) Gelidium(e) Chondrocytes.
›Reveal solutionSolution
Agar-agar is a polysaccharide extracted from red algae, especially the genus Gelidium (and Gracilaria); it is used to solidify microbiological culture media.
Red algae (Rhodophyceae) contain characteristic cell-wall polysaccharides. Agar, a gelatinous substance widely used to prepare solid growth media in laboratories, is extracted commercially from red algae such as Gelidium and Gracilaria. It is unrelated to the other listed terms: cytokinesis is cell-pl …
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