Lessons 7–8 · 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
8
Metabolism – the basis of cell life activity
Textbook: pp. 38–41
GoalExplain the link between plastic and energy metabolism, the stages of energy metabolism and the formation of ATP.
Metabolism is the sum of the plastic and energy reactions in the cell. Plastic metabolism (assimilation, anabolism) is the synthesis of the cell’s own complex substances from simple ones and uses energy; energy metabolism (dissimilation, catabolism) breaks substances down, releases energy and stores it in ATP. ADP + phosphate + energy → ATP; when ATP is broken down about 40 kJ/mol is released. Energy metabolism has three stages: 1) preparatory – in the digestive system and lysosomes polymers are split into monomers, and the energy is lost as heat; 2) the oxygen-free stage – glycolysis in the cytoplasm: 1 glucose → 2 pyruvate (a 3-carbon substance), net 2 ATP; without enough oxygen pyruvate becomes lactic acid in muscle, and ethanol and CO₂ in yeast (fermentation); 3) the oxygen stage – in mitochondria pyruvate is oxidised to CO₂ and H₂O. Overall: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. The textbook takes 38 ATP in total per glucose (2 + 36); present measurements give about 30–32, so the textbook value is used here as a convention in calculations.
Worked examples
When oxygen is short in a sprinter’s muscles, glycolysis produces lactic acid; only 2 ATP are obtained per glucose, so the muscle tires quickly.
When dough rises, yeast breaks down glucose without oxygen: ethanol and CO₂ form; the gas bubbles make the dough rise.
Class activity
“Energy pathway”. The class splits into three groups; each acts out one stage: what goes in, what comes out, where, and how many ATP.
Practice
1
Explain how plastic and energy metabolism depend on each other.
Plastic metabolism uses the energy of ATP, and ATP forms in energy metabolism; the enzymes for energy metabolism are made in plastic metabolism.
2
Where does glycolysis occur and what forms from 1 glucose?
In the cytoplasm; 2 molecules of pyruvate and a net 2 ATP (without oxygen pyruvate becomes lactic acid or ethanol).
3
Using the textbook value, how many ATP form if 5 glucose molecules are broken down aerobically? (Number only.)
190
4
Why does aerobic respiration yield much more energy than anaerobic?
In the anaerobic process glucose is not fully oxidised (lactic acid or ethanol keeps much of the energy); in the oxygen stage in mitochondria it is oxidised to CO₂ and H₂O and much more ATP forms.