Biology · Plant Kingdom · NEET
| Gamete size | Isogamy: both the same | Oogamy: very unequal (large egg + small sperm) |
| Male vs female | Isogamy: cannot tell apart | Oogamy: clearly egg and sperm |
| Motility | Isogamy: both may swim (Ulothrix) or both still (Spirogyra) | Oogamy: egg is still, sperm swims |
| NCERT example | Isogamy: Ulothrix, Spirogyra | Oogamy: Volvox, Fucus |
| Evolution level | Isogamy: most primitive | Oogamy: most advanced |
It is only about how the two fusing gametes compare. Isogamy: the two gametes are alike in size and shape (you cannot tell male from female). Anisogamy: the two gametes are of different sizes but often both can move. Oogamy: the gametes are very different — one is a large, non-motile (still) female gamete (egg) and the other is a small, motile (swimming) male gamete. This is a favourite NEET question because it looks simple but the examples get mixed up.
No. This is the most common mistake. In isogamy the gametes are the SAME size, but they may or may not have flagella. In Ulothrix the isogametes are flagellated (motile). In Spirogyra the isogametes are non-flagellated (non-motile). So 'same size' is the rule for isogamy, not 'can swim'. NCERT gives both examples on purpose.
Spirogyra is isogamous. Its two gametes are similar in size, so it is isogamy — even though the gametes are non-motile (no flagella). Do not confuse 'non-motile' with 'different'. Motility is a separate thing from size.
NCERT names Volvox and Fucus for oogamy. Remember 'Volvox and Fucus need a Female egg' — both have a big still egg and a small swimming sperm. Chara also shows oogamy. For anisogamy NCERT gives Eudorina. For isogamy remember Ulothrix (with flagella) and Spirogyra (without flagella).
'Oo' means egg (from the Greek word for egg, like in 'oocyte' and 'oogenesis'). So oogamy literally means 'egg-marriage' — one gamete has clearly become a large egg. That is why oogamy is the most advanced type: male and female gametes are now clearly different. This word-hook helps you never mix oogamy with the others.
Oogamy is the most advanced. The order of evolution goes isogamy → anisogamy → oogamy. As we move along, the two gametes become more and more different, ending with a large still egg and a small moving sperm. This same pattern (isogamy, anisogamy or oogamy) is seen in green algae and brown algae, which NEET has tested.
Read the following statements about members of Phaeophyceae and choose the correct set: A. Asexual reproduction occurs usually by biflagellate zoospores. B. Sexual reproduction is in oogamous method only. C. Stored food is either mannitol or laminarin. D. Major pigments are chlorophyll a, c and carotenoids and xanthophyll. E. Vegetative cells have a cellulosic wall covered by algin.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Isogamy = two gametes of the same size. Anisogamy = two gametes of different sizes. Oogamy = a large still female egg fusing with a small swimming male gamete.
Isogamy: Ulothrix (flagellated) and Spirogyra (non-flagellated). Anisogamy: Eudorina. Oogamy: Volvox and Fucus.
No. Oogamy is seen in many plants and algae too, such as Volvox, Fucus and Chara. All higher plants also show oogamy because they form a large egg and a small male gamete.
Isogamy is the most primitive (simplest) type, because the gametes are still identical. Oogamy, with a clear egg and sperm, is the most advanced.
Because the examples are easy to swap by mistake, and because NCERT clearly says green and brown algae can be isogamous, anisogamous OR oogamous — a common trap in match-the-column and statement questions.
