The molecular level of life. Chemical composition of the cell
Lessons 3–4 · 2 lessons · A. G‘afurov, A. Abdukarimov, J. Tolipova, O. Ishankulov, M. Umaraliyeva, I. Abdurahmonova. Biology, Grade 10, 1st edition. “Sharq” publishing and printing joint-stock company, Tashkent, 2017
4
Chemical composition of living organisms and its constancy
Textbook: pp. 16–19
GoalExplain the chemical composition of the cell, groups of elements, and the functions of mineral salts and water.
Living organisms contain biogenic elements. By quantity they are divided into macroelements (C, O, H, N – about 98%; S, P, Ca, Na, K, Cl, Mg, Fe) and microelements (I, Cu, Co, F, Zn and others; below 0.001%). For example, iron carries oxygen in haemoglobin, iodine is part of thyroid hormones and fluorine is found in tooth enamel. Mineral salts occur as ions at different concentrations inside and outside the cell, creating the potential difference needed for nerve impulses and muscle contraction; some anions (for example phosphate and bicarbonate) keep the cell medium slightly alkaline – this is buffering. Water makes up most of body mass and, being a polar molecule (dipole), dissolves polar substances, which are called hydrophilic (table salt, sugars, amino acids); hydrophobic substances that do not dissolve in water include lipids and cellulose. Hydrogen bonds between neighbouring water molecules give water a high heat capacity; water is also the medium that carries substances around the organism.
Worked examples
If a 250 g tissue sample contains 80% water, the mass of water is 250 · 80 : 100 = 200 g.
Table salt dissolves well in water because Na⁺ and Cl⁻ ions are surrounded by polar water molecules; fat, being non-polar, does not dissolve and forms a layer on the water.
Class activity
“Element and its job”. Students pair cards: iron – haemoglobin, magnesium – chlorophyll, iodine – thyroid hormones, calcium – bone and blood clotting.
Practice
1
Which four elements are most abundant in the cell and which compounds do they form?
C, O, H, N; they form proteins, nucleic acids, carbohydrates and lipids.
2
A student’s body mass is 60 kg and water makes 65% of it. How many kg of water is that? (Number only, kg.)
39
3
Explain the role of mineral salts in conducting nerve impulses.
The concentrations of Na⁺, K⁺ and Ca²⁺ differ inside and outside the cell, creating a potential difference; the passage of an impulse is based on changes of this difference.
4
Why are buffer systems needed in the cell? Which ions form them?
Reactions in the cell are sensitive even to small changes in acidity; buffers hold it at a slightly alkaline level. Inside the cell: H₂PO₄⁻ and HPO₄²⁻; in blood plasma: HCO₃⁻.