Algae and how they reproduce · Biology Class 11–12 | StemPy Lab
Course 3.1·Lesson 2 of 3
Algae and how they reproduce
Algae are simple, thalloid, autotrophic plants with chlorophyll, mostly living in water. They may be single cells, colonies like Volvox or filaments like Ulothrix and Spirogyra. They reproduce by fragmentation, by zoospores, and sexually by isogamy, anisogamy or oogamy. They fix much of the world's carbon dioxide and give food, agar and algin.
about 25 min1 simulation1 practice question1 Python program5 questions to check yourself
This lesson is free to read. Enroll free to keep your place and take the tests.
Lesson 2 of 3Show all lessonsHide
Try the code and the simulations beside the lesson.
Green film on a pond
Leave a bucket of water in the sun for a week, and its sides turn green and slimy. Look at the slime under a microscope, and you see algae. They are the simplest plants.
Algae are chlorophyll-bearing, simple, thalloid, autotrophic organisms, mostly living in water. A thallus is a plant body that is not divided into a true root, stem and leaf.
Where they live
Most live in water, both fresh and marine.
Some live on moist stones, soil and wood.
Some live with fungi, in lichens, and some live on animals, such as on the fur of the sloth bear.
Forms and sizes
Algae vary hugely in form and size.
Single cells, such as Chlamydomonas.
Colonies, such as Volvox: a hollow ball of many cells.
Filaments, such as Ulothrix and Spirogyra: cells joined end to end in a thread.
Some marine forms, the kelps, form massive plant bodies.
Three ways to reproduce
Vegetative reproduction is by fragmentation. A filament breaks into pieces, and each piece grows into a new thallus.
Asexual reproduction is by spores. The most common are zoospores: they have flagella and can swim. Each zoospore settles and grows into a new plant.
Sexual reproduction is the fusion of two gametes. There are three kinds.
Three ways in which gametes fuse. Isogamy: two gametes of the same size, each with flagella. Anisogamy: two gametes of different sizes, both with flagella. Oogamy: a large egg with no flagella that stays still, and a much smaller male gamete with flagella that swims to it.
Isogamous: the two gametes are alike in size. They may have flagella, as in Chlamydomonas, or none, as in Spirogyra.
Anisogamous: the gametes are unlike in size, as in some species of Chlamydomonas.
Oogamous: a large, non-motile female gamete, the egg, fuses with a smaller, motile male gamete, as in Volvox and Fucus.
You should find that equal sizes always give isogamy, whatever the swimming switch says. Only unequal gametes need the third question.
Why algae matter
At least half of the total carbon dioxide fixation on Earth is done by algae, by photosynthesis. They also add oxygen to their surroundings.
They are primary producers: the food chains of the water start with them.
Many people eat marine algae, such as Porphyra, Laminaria and Sargassum.
Algin comes from brown algae, and carrageen from red algae. Both are used in industry to thicken liquids.
Agar, from Gelidium and Gracilaria, is used to grow microbes in a laboratory, and in ice creams and jellies.
Chlorella is rich in protein. It is used as a food supplement, even by space travellers.
Try it in Python
The program decides the kind of fusion for four algae from the sizes of their gametes and whether the larger one swims.
Practise with new numbers
Common mistakes
Calling every alga a single cell. Many are colonies or filaments, and kelps are huge.
Thinking isogamous gametes always swim. In Spirogyra they have no flagella.
Mixing up anisogamy and oogamy. In oogamy the larger gamete, the egg, does not move.
Thinking zoospores are gametes. A zoospore grows into a plant by itself, without fusing.
Remember these
Algae: simple, thalloid, autotrophic, with chlorophyll, mostly in water.
Forms: single cells, colonies (Volvox) and filaments (Ulothrix, Spirogyra).