BiologyCBSE Class 11 · Plant Kingdom

Algae: Green, Brown and Red Algae and How They Reproduce

Updated 17 October 2026 · Fact-checked

Algae are chlorophyll-bearing, simple, thalloid, autotrophic, largely aquatic organisms. They are classed as Chlorophyceae (green), Phaeophyceae (brown) and Rhodophyceae (red) on the basis of pigments, stored food and cell wall. They reproduce vegetatively by fragmentation, asexually by spores such as zoospores, and sexually by isogamous, anisogamous or oogamous fusion.

Each algal class is named by its colour pigment: chlorophyll in green, fucoxanthin in brown and phycoerythrin in red algae, each with its own stored food.

Illustration drawn for QuestionBuddy.

Understand Algae: Classes, Pigments and Reproduction

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (fresh and marine) organisms; some live on moist stones, soil and wood, and some live in association with fungi (lichens) or animals (sloth bear). Size ranges from the microscopic unicellular *Chlamydomonas* to massive kelps, colonies such as *Volvox*, and filaments such as *Ulothrix* and *Spirogyra*.

Reproduction.

Vegetative: fragmentation (each fragment grows into a thallus).
Asexual: spores, commonly flagellated zoospores that germinate into new plants.
Sexual: fusion of two gametes, which may be similar in size (isogamous, flagellated as in *Ulothrix* or non-flagellated as in *Spirogyra*), flagellated and dissimilar in size (anisogamous, as in *Eudorina*), or one large non-motile female gamete (egg) fertilised by a smaller motile male gamete (oogamous, as in *Volvox* and *Fucus*).

Chlorophyceae: usually grass-green due to chlorophyll a and b. Many have pyrenoids (storage bodies containing starch and protein) in the chloroplasts. Body: unicellular, colonial or filamentous. Reproduce vegetatively, by flagellated zoospores, and sexually.
Phaeophyceae: brown to olive green due to the xanthophyll pigment fucoxanthin. Body from simple branched filaments (*Ectocarpus*) to profusely branched forms (kelps) that may reach 100 m. The plant body has a holdfast, a stipe and fronds (leaf-like photosynthetic organ). Asexual reproduction by biflagellate zoospores with two unequal laterally attached flagella.
Rhodophyceae: red due to r-phycoerythrin; mostly marine in warm areas, both well-lit surface water and great depths where little light penetrates. Reproduction vegetative by fragmentation, asexual by non-motile spores, sexual oogamous with complex post-fertilisation developments.

Economic importance. At least half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis, so they increase the level of dissolved oxygen in the environment; they are primary producers of aquatic food chains. Many marine algae (*Porphyra*, *Laminaria*, *Sargassum*) are used as food. Agar from *Gelidium* and *Gracilaria* is used to grow microbes and in ice-creams and jellies. Algin (brown algae) and carrageen (red algae) are hydrocolloids. *Chlorella* and *Spirulina* are unicellular algae rich in protein, used as food supplements even by space travellers.

Draw and label: a diagram of *Volvox* colony or of a brown alga body (holdfast, stipe, frond).

Algae: Classes, Pigments and Reproduction: key facts

Pigments: $G r ee n : c h l a, b ∣ B r o w n : c h l a, c, f u co x an t hin ∣ R e d : c h l a, d, p h y coer y t h r in$; Basis of classes
Food reserve: Starch green
mannitol, laminarin brown
floridean starch red; Stored food
Gametes: $I so g am o u s ∣ ani so g am o u s ∣ oo g am o u s$; Ulothrix | Eudorina | Volvox, Fucus
Phycocolloids: $A g a r (G e l i d i u m, G r a c i l a r ia) ∣ a l g in (b r o w n) ∣ c a r r a g ee n (r e d)$; Economic importance

Starch	green
mannitol, laminarin	brown
floridean starch	red

How to answer Algae: Classes, Pigments and Reproduction questions

Questions give a pigment, food or example and ask for the class, or give reproduction clues.

1Use pigment to name the class: fucoxanthin = brown; phycoerythrin = red; chlorophyll a and b = green.
2Match food reserve and wall to the class table.
3For reproduction, decide if gametes are similar (iso), unequal (aniso) or egg and sperm (oogamous).
4For uses, name the product and the alga (agar: Gelidium).
5Give two examples for each class.

Algae: Classes, Pigments and Reproduction in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Jelly and lab plates

Suppose a bakery sets a jelly dessert and a school lab grows bacteria on a solid medium, and both depend on the same plant-derived gel.

Name the gel, the algae yielding it, and the class.

Try it first, then show the solutionSolution

The gel is agar.
It comes from Gelidium and Gracilaria.
They are red algae (Rhodophyceae).

Answer: Agar from red algae.

Memory hook: Red algae set the jelly.

Tricks to remember Algae: Classes, Pigments and Reproduction

Short memory aids. Say each one aloud once, then use it on a question the same day.

Green, Brown, Red
a,b / a,c,fuco / a,d,phyco.
Use it when: Pigments
Starch, Mannitol, Floridean
Green, Brown, Red in order.
Use it when: Reserves
Iso, Aniso, Oo
Same, Different, Egg-sperm.
Use it when: Gamete types

Algae: Classes, Pigments and Reproduction: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Class and pigment · 3 marks · Short answerBoard level

Name the class and main pigments for (a) Laminaria (b) Polysiphonia (c) Spirogyra.

Try it first, then show the solutionSolution

Laminaria: Phaeophyceae, chlorophyll a, c and fucoxanthin (1 mark).
Polysiphonia: Rhodophyceae, chlorophyll a, d and phycoerythrin (1 mark).
Spirogyra: Chlorophyceae, chlorophyll a and b (1 mark).

Answer: Brown, red, green respectively.

Example 2: Reserve food · 2 marks · Very short answerBoard level

Name the stored food in each class of algae.

Try it first, then show the solutionSolution

Green: starch; brown: mannitol and laminarin (1 mark).
Red: floridean starch (1 mark).

Answer: Starch; mannitol and laminarin; floridean starch.

Example 3: Types of gametic fusion · 3 marks · Short answerHarder board level

Ulothrix, Eudorina and Volvox all reproduce sexually. Classify the gametic fusion in each and say how the gametes differ in size and motility.

Try it first, then show the solutionSolution

Isogamy: similar flagellated gametes, as in Ulothrix (1 mark).
Anisogamy: dissimilar flagellated gametes, as in Eudorina (1 mark).
Oogamy: a large non-motile egg fertilised by a small motile male gamete, as in Volvox or Fucus (1 mark).

Answer: Similar, dissimilar, egg and sperm.

Example 4: Brown alga body · 3 marks · Short answerHarder board level

Describe the plant body of a kelp and why it is brown.

Try it first, then show the solutionSolution

It has a holdfast, a stipe and leaf-like fronds (1 mark).
Fucoxanthin, a xanthophyll, masks the green and gives the brown colour (1 mark).
It stores mannitol and laminarin and the wall has algin (1 mark).

Answer: Holdfast, stipe, frond; fucoxanthin.

Example 5: Red algae at depth · 4 marks · Short answerHOTS / competency

Suppose a red alga is found at great depths where only blue-green light penetrates. Explain how its pigments could help.

Try it first, then show the solutionSolution

Red algae contain phycoerythrin (1 mark).
Phycoerythrin absorbs blue-green light that reaches deep water (1 mark).
Chlorophyll a alone absorbs mostly red and blue and would be less effective at depth (1 mark).
So red algae can photosynthesise in dim deep water; this explains their depth range (1 mark).

Answer: Accessory pigment extends the usable light.

Where students lose marks in Algae: Classes, Pigments and Reproduction

The slips examiners see most often, and what to write instead.

Mistake: Colour of Rhodophyceae
Called brown by mistake.
Wrong working
Red algae have fucoxanthin.
Right working
Red algae have phycoerythrin.
Do this: Fuco = brown.
Usually costs: 1 mark.
Mistake: Oogamy example
Volvox and Ulothrix mixed.
Wrong working
Ulothrix is oogamous.
Right working
Ulothrix is isogamous; Volvox oogamous.
Do this: Learn the example list.
Usually costs: 1 mark.
Mistake: Algae in Monera
Cyanobacteria called algae.
Wrong working
Spirogyra is blue-green.
Right working
Spirogyra is a green alga; blue-green forms are Monera.
Do this: Eukaryote algae only.
Usually costs: 1 mark.

Exam tips

Learn the class table of pigments, reserve food and wall.
Write two examples per class.
Name the products with their sources.
For reproduction, name the three modes and an example for each.

Practice questions on Algae: Classes, Pigments and Reproduction

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Pyrenoids are found in
(a) Chlorophyceae (b) Phaeophyceae (c) Rhodophyceae (d) bryophytes
Hint
Starch storage body.
Answer
(a).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Agar is obtained from red algae.
Reason (R): Red algae contain polysulphate esters in the wall.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Wall content.
Answer
(a).
Q3 · 2 marks · Very short answerHarder board level
Which two unicellular algae are rich in protein, and what is the advantage of using them as a food supplement?
Hint
Single-celled protein.
Answer
Chlorella and Spirulina; they are unicellular and rich in protein.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a pond sample shows a green colonial form with a large egg and small motile sperm, and a seashore sample shows a 20-metre brown plant with a holdfast.
(a) Name a likely genus for each. (1)
(b) Name the type of sexual reproduction in the first. (1)
(c) Name the parts of the second. (2)
Hint
Volvox; Laminaria.
Answer
(a) Volvox; Laminaria or another kelp. (b) Oogamous. (c) Holdfast, stipe and fronds.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a river is polluted with fertiliser and a thick green scum forms. Explain what happens to dissolved oxygen at night and in the morning, and why algae are still called oxygen producers.
Hint
Photosynthesis versus respiration.
Answer
By day algae photosynthesise and raise oxygen. At night they respire and use oxygen, and decomposition of the dead scum drains more oxygen. Overall algae fix at least half the earth's carbon dioxide and are major oxygen producers in unpolluted waters.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which pigment colours brown algae?Show answer
Fucoxanthin.
2.Where is floridean starch stored?Show answer
Red algae.
3.What is a pyrenoid?Show answer
A storage body with starch and protein.
4.Which algae yield agar?Show answer
Gelidium and Gracilaria.
5.Which alga is oogamous and brown?Show answer
Fucus.
6.What fraction of carbon fixation do algae do?Show answer
At least half.

Algae: Classes, Pigments and Reproduction: frequently asked questions

What are algae?

Simple, chlorophyll-bearing, thalloid, mostly aquatic autotrophs.

How are algae classified?

Into green, brown and red algae by pigments, stored food and cell wall.

What is zoospore?

A flagellated motile asexual spore.

What is algin?

A phycocolloid from the walls of brown algae.

Why are red algae red?

Phycoerythrin is the dominant pigment.