Lessons 3–4 · 2 lessons · O. Mavlonov. Biology (Zoology), Grade 7. Revised and expanded fourth edition. “O‘zbekiston milliy ensiklopediyasi” State Scientific Publishing House, Tashkent, 2017
4
Class Flagellates
Textbook: pp. 14–17
GoalExplain the features of flagellates using green euglena and volvox.
New words
flagellum, a whip-like organ of movement · xivchinchromatophore holding chlorophyll · xromatoforaeyespot that senses light · ko‘zchacolony of connected cells · koloniya
Explanation
Flagellates are unicellular animals that move with one or several flagella; the body is covered with a firm shell. Green euglena lives in puddles and ponds, has a spindle-shaped body and is 5–10 times smaller than an amoeba. Its flagellum turns like a screw and pushes it forward, and the shell gives a permanent shape. Thanks to chlorophyll in its chromatophores euglena photosynthesises in light, but in darkness it feeds like an animal on ready-made organic matter; this trait points to a common ancestor of plants and animals. The red eyespot senses light, so euglena swims toward the lit side. It reproduces by dividing in two and forms a cyst in bad conditions. Volvox is a spherical colony of hundreds, often about a thousand, two-flagellate cells joined by cytoplasmic bridges. Trypanosoma causes sleeping sickness and leishmania causes long-lasting skin sores; both are parasites.
Worked examples
If a jar with euglena stands by a window, the water looks greener on the lit side: the euglenas sense light with the eyespot and gather there.
Each volvox cell beats its flagella in the same way, so the whole sphere rolls like a ball. Euglena is a single cell and swims by twisting its body.
Class activity
“Who is who?” game: write euglena, volvox, trypanosoma, leishmania on cards; pupils choose the matching feature (spindle-shaped, colony, blood parasite, skin sore). Safety: a skin sore that does not heal must be shown to a doctor, not treated at home.
Practice
1
One jar of euglena stood on a windowsill for a week, another in a dark cupboard. How did each one feed?
In light it photosynthesises with chlorophyll; in darkness it absorbs ready-made organic matter through its body surface.
2
What evidence lets us call volvox a colony rather than one big cell?
Hundreds to thousands of cells form one sphere and are joined by cytoplasmic bridges.
3
What does euglena use to sense light?
The red eyespot.
4
Why can euglena be compared both to a plant and to an animal?
It photosynthesises in light (like a plant), but moves by a flagellum and eats ready food in the dark (like an animal).