Rhodophyceae (Red Algae): Phycoerythrin and Floridean Starch
Biology · Plant Kingdom · NEET
Rhodophyceae are the red algae. They look red because of a red pigment called r-phycoerythrin. They store their food as floridean starch (which is like amylopectin and glycogen). Most are marine and multicellular. Memory hook: "RED = R-phycoerythrin + floRIDEAN starch." Both the pigment and the food start with the same idea as the group name.
The three NEET facts for Rhodophyceae: red pigment r-phycoerythrin, stored food floridean starch, and non-motile spores and gametes (no flagella). Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium.
Your doubts, answered
Why are members of Rhodophyceae called red algae?
They are called red algae because a red pigment named r-phycoerythrin is present in large amount (predominates) in their body. This one pigment gives the whole group its red colour, and it is also the top NEET fact. So if the question says 'red pigment', the answer is phycoerythrin, and the class is Rhodophyceae.
What is floridean starch and how is it different from normal starch?
Floridean starch is the stored food (reserve carbohydrate) of red algae. NCERT says it is very similar in structure to amylopectin and glycogen. So do not write 'starch' or 'laminarin' or 'mannitol' for red algae — the exact word NEET wants is floridean starch. Green algae store starch, brown algae store laminarin or mannitol, and red algae store floridean starch.
Do red algae have flagella or motile gametes?
No. Red algae reproduce vegetatively by fragmentation, asexually by non-motile (non-flagellated) spores, and sexually by non-motile (non-flagellated) gametes. This is a favourite NEET trap. So a match like 'Polysiphonia — uniflagellate gametes' is WRONG, because Polysiphonia is a red alga and its gametes have no flagella.
Where do red algae live?
Most red algae are marine (live in the sea), with more of them found in warmer areas. They can grow both in well-lit water near the surface and at great depths where very little light reaches. The deep-water fact matters because phycoerythrin can absorb the blue light that reaches deep water, so they survive where other algae cannot.
Are red algae single-celled or multicellular?
The red thalli of most red algae are multicellular, and some have a complex body organisation. So do not pick red algae like Gelidium or Gracilaria when a question asks for unicellular algae — those are multicellular. Chlorella (a green alga) is the classic unicellular example instead.
What products do we get from red algae?
From red algae (Rhodophyceae) we get the hydrocolloid carrageen (a water-holding substance) and agar (from Gelidium and Gracilaria), used to grow microbes and in ice-creams and jellies. Careful trap: algin comes from BROWN algae, carrageen comes from RED algae. NEET often swaps these two.
⚠️ The NEET trap ✗ Polysiphonia has uniflagellate (motile) gametes, or red algae store starch/laminarin. ✓ Polysiphonia is a red alga and its gametes are non-motile (no flagella). Red algae store floridean starch, and their red pigment is r-phycoerythrin. 🧠 Red algae are the 'no-flagella' club: non-motile spores AND non-motile gametes. If you see 'flagellate gametes — Polysiphonia/red algae', it is the wrong match NEET wants.
Real NEET questions
2023
Which classes of algae possess pigment fucoxanthin and pigment phycoerythrin, respectively?
A · Chlorophyceae and Rhodophyceae
B · Rhodophyceae and Phaeophyceae
C · Phaeophyceae and Chlorophyceae
D · Phaeophyceae and Rhodophyceae ✓
Solution: Fucoxanthin is the brown pigment of Phaeophyceae (brown algae). Phycoerythrin (r-phycoerythrin) is the red pigment of Rhodophyceae (red algae). So the order is Phaeophyceae and Rhodophyceae. Remember: red pigment = phycoerythrin = red algae.
2018
Which one is wrongly matched?
A · Gemma cups – Marchantia
B · Biflagellate zoospores – Brown algae
C · Uniflagellate gametes – Polysiphonia ✓
D · Unicellular organism – Chlorella
Solution: Polysiphonia is a red alga (Rhodophyceae). Red algae reproduce sexually by non-motile (non-flagellated) gametes, so 'uniflagellate gametes – Polysiphonia' is wrong. The other three pairs are all correct.
2016
Which of the following is incorrectly matched?
A · Ectocarpus - Fucoxanthin
B · Ulothrix - Mannitol ✓
C · Porphyra - Floridian Starch
D · Volvox - Starch
Solution: Ulothrix is a green alga and stores starch, not mannitol, so this pair is wrong. Note the correct pair: Porphyra (a red alga) stores floridean starch. Mannitol belongs to brown algae.
Solved Plant Kingdom NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the pigment responsible for red colour in Rhodophyceae?
The red pigment r-phycoerythrin. Its dominance (predominance) in the body gives red algae their red colour, and this is the most common NEET fact from this topic.
What is the reserve food of red algae?
Floridean starch, which NCERT describes as very similar to amylopectin and glycogen in structure. Green algae store starch, brown algae store laminarin or mannitol, red algae store floridean starch.
Give some examples of red algae.
Polysiphonia, Porphyra, Gracilaria and Gelidium are red algae. Gelidium and Gracilaria give us agar.
Are red algae motile?
No. They reproduce by fragmentation, by non-motile spores (asexual) and by non-motile gametes (sexual). No flagella are involved.
Do algin and carrageen both come from red algae?
No. Carrageen comes from red algae (Rhodophyceae), but algin comes from brown algae (Phaeophyceae). NEET often reverses these to trick you.
