Lessons 2–3 · 2 lessons · O. Mavlonov, T. Tilavov, B. Aminov. Biology (Human body and health), Grade 8. Revised and expanded 6th edition. “O‘qituvchi” Publishing and Printing Creative House, Tashkent, 2019
3
Vital properties of the cell and the organism
Textbook: pp. 12–13
GoalExplain metabolism (assimilation, dissimilation), the properties of living organisms, the internal environment and homeostasis.
New words
assimilation, building up substances · assimilatsiyadissimilation, breaking down substances and releasing energy · dissimilatsiyaexcitability · qo‘zg‘aluvchanlikinternal environment (blood, lymph, tissue fluid) · ichki muhit
Explanation
A living organism shows metabolism, excitability, reproduction, growth, development, respiration, nutrition and excretion. In metabolism two processes run at the same time: assimilation – building the complex substances the cell needs from simple ones, and dissimilation – breaking down organic substances and releasing energy. Excitability is the cell’s change from rest to an active state in response to a stimulus: a muscle cell contracts, a nerve cell produces an impulse. Life needs nutrients, water, oxygen and a suitable environment. Blood, lymph and tissue fluid form the internal environment of the body; its composition is kept relatively constant, which is called homeostasis. For example, when water is low we feel thirsty, and excess water is removed by the excretory organs.
Worked examples
When running, muscle cells use much oxygen and glucose: dissimilation speeds up; later during rest the used reserves are rebuilt (assimilation).
Sweating on a hot day helps keep body temperature constant – an example of homeostasis.
Class activity
Draw a table with two columns: “Assimilation” and “Dissimilation”. Write 3 examples in each (for example, protein synthesis, breakdown of glucose).
Practice
1
What is the difference between assimilation and dissimilation?
In assimilation substances are built up (energy is used); in dissimilation they are broken down and energy is released.
2
What makes up the internal environment of the body?
Blood, lymph and tissue fluid.
3
What is homeostasis? Give one example.
Relative constancy of the internal environment; e.g. blood sugar and body temperature kept steady.
4
Why do you think the child’s body has relatively more water?
Their metabolism is fast and tissues are young and growing; young tissues contain more water.