Biology · Plant Kingdom · NEET
Algin comes from BROWN algae (Phaeophyceae). This is the single most tested point. NCERT says exactly: algin (brown algae) and carrageen (red algae). NTA loves to swap these two, so lock it in: brown = algin, red = carrageen. Also note the brown algae Laminaria, Sargassum and Fucus have a wall coated with algin on the outside.
Carrageen (also spelt carrageenan) comes from RED algae (Rhodophyceae) only. NEET has asked this many times (2021, 2022). It is a hydrocolloid, meaning a water-holding substance. If an option says Rhodophyceae, that is correct; if it says Phaeophyceae or blue-green algae, it is wrong.
Agar (agar-agar) is obtained from the red algae Gelidium and Gracilaria. Remember these two genus names by heart, they are directly asked. Agar is used to grow microbes in the lab and to make ice-creams and jellies. Note: agar itself is technically from red algae, but NCERT names the two genera Gelidium and Gracilaria, so learn the genus names.
All three are commercial products from marine algae, but from different sources and uses. Agar = from red algae Gelidium and Gracilaria, used to culture microbes and in jellies/ice-creams. Algin = from brown algae, a hydrocolloid. Carrageen = from red algae, a hydrocolloid. Both algin and carrageen are hydrocolloids (water-holding substances); agar is named by its source genera in NCERT.
Chlorella, a unicellular green alga rich in proteins, is used as a food supplement, even by space travellers. This is a direct one-line fact from NCERT and has appeared in NEET. Do not confuse it with Spirulina, which is a cyanobacterium (blue-green, Monera), though some old keys treat it as a food alga too.
About 70 species of marine algae are used as food. NCERT names Porphyra (a red alga), Laminaria and Sargassum (brown algae) as examples. So for food, remember Porphyra, Laminaria and Sargassum. This matters for NEET because match-the-column questions pair these genera with 'food'.
Algae carry out at least half of the total carbon dioxide fixation on Earth through photosynthesis. Because they photosynthesise, they raise the level of dissolved oxygen in water around them. They are the primary producers in aquatic food chains, making the energy-rich food that all aquatic animals depend on.
Which one of the following statements is wrong?
Which of the following algae produce Carrageen?
Which of the following pairs is of unicellular algae?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Carrageen and carrageenan are the same hydrocolloid from red algae (Rhodophyceae). NEET uses both spellings, so treat them as identical.
NCERT names the red algae Gelidium and Gracilaria as the source of agar-agar. Learn both genus names because they are asked directly.
Hydrocolloids are water-holding substances. NCERT gives two examples: algin (from brown algae) and carrageen (from red algae). Both are produced in large amounts by certain marine algae and used commercially.
Chlorella, a unicellular green alga rich in proteins, is used as a food supplement, even by space travellers, per NCERT.
At least half of the total carbon dioxide fixation on Earth is carried out by algae through photosynthesis. This is why they are so important as primary producers and oxygen sources.
