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
3
Class Sarcodina (amoeba-like protozoa)
Textbook: pp. 11–13
GoalExplain the structure and life processes of the Sarcodina using the amoeba.
New words
pseudopod, a temporary outgrowth of the cytoplasm · soxta oyoqfood vacuole where food is digested · hazm qilish vakuolasicontractile vacuole that removes excess water · qisqaruvchi vakuolacyst, a protective covering formed in bad conditions · sista
Explanation
A unicellular animal is one cell, yet it lives on its own. The typical member of the class Sarcodina is the common amoeba: it lives on the bottom of dirty puddles, is 0.2–0.5 mm long, transparent and constantly changes shape. When the cytoplasm flows, the body bulges out into temporary pseudopods; with them the amoeba moves and engulfs bacteria and tiny algae. A food vacuole forms around the food, and the undigested remains are pushed out. The amoeba breathes oxygen dissolved in water, while extra water and waste leave through the contractile vacuole. In response to a stimulus such as salt it pulls in its pseudopods and becomes round (irritability). It reproduces asexually by dividing in two, and in bad conditions forms a cyst. In the sea live foraminiferans with limestone shells and radiolarians with silica skeletons; the dysentery amoeba is a parasite that causes bloody dysentery in humans.
Worked examples
Under a microscope the amoeba has no permanent shape: in one minute it is long, then triangular. The reason is that its cytoplasm changes direction and forms new pseudopods.
Limestone deposits formed from the shells of foraminiferans that lived in ancient seas. So even tiny unicellular animals can build rocks.
Class activity
Draw the amoeba on the board in 5 stages: approaching food, pseudopod, engulfing, food vacuole, releasing the waste. Safety: pond or puddle water samples are taken by the teacher with adults only; children never drink it or put it in the mouth and wash their hands with soap after the work.
Practice
1
How does an amoeba move?
The cytoplasm flows, one side of the body bulges into a pseudopod, and the amoeba moves into it.
2
If the contractile vacuole of a freshwater amoeba stopped working, what could happen to the cell?
Extra water would not be removed and would build up, so the cell could swell and burst, because the contractile vacuole removes extra water and waste.
3
What happens to an amoeba in salty water and what does it show?
It pulls in its pseudopods and becomes round; this shows irritability.
4
Why does an amoeba form a cyst when it gets cold?
The cyst is a thick covering that protects it from bad conditions; when they improve, the amoeba emerges and goes on living.