الأسبوع 6 · بقية الموضوع ١-٥ «الأهمية البيولوجية للماء»: أجزاء الخلية والأيض والإنزيمات، والموضوع ١-٦ «التكيفات البيولوجية للكائنات الحية في البيئة المائية»: التكيفات التركيبية والسلوكية والفسيولوجية والتكيف للضغط الأسموزي · 3 دروس
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Metabolism and enzymes
Integrated Science Sec 1 Term 1, Unit 1: pp. 33–42
GoalTell catabolic from anabolic reactions, and explain how enzymes speed up reactions and why temperature and pH matter.
New words
metabolism · all the chemical reactions in a living cell, in the presence of wateractivation energy · the least energy a reaction needs before it can beginenzyme · a protein that acts as a catalyst: it speeds up a reaction in a cell and is not used upsubstrate · the substance an enzyme acts on, which fits its active site
Explanation
Metabolism is made of two kinds of reactions. In catabolic reactions big molecules are broken down and release energy: for example glucose in cellular respiration, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. In anabolic reactions big molecules are built from small ones and energy is used: photosynthesis and making protein from amino acids are examples. Every reaction needs a minimum energy to start, the activation energy. Enzymes are proteins that make reactions much faster in the cell by lowering the activation energy; they are catalysts. An enzyme works like a lock and key: the substrate fits the active site of one enzyme. The enzyme comes out of the reaction unchanged, ready to work again. Each enzyme has an optimum temperature and pH. Away from them its activity falls, and if the change is large its shape is damaged and it stops working. Enzymes are not alive, and they are not “activators” but catalysts.
Worked examples
Is it catabolic or anabolic? (a) Glucose is broken down in cells and releases energy: catabolic. (b) Amino acids are joined into a protein, using energy: anabolic. (c) Photosynthesis builds glucose from CO₂ and water using light: anabolic.
Count atoms in C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Left: C 6, H 12, O 6 + 12 = 18. Right: C 6, H 6 × 2 = 12, O 12 + 6 = 18. The equation is balanced.
Read the sketch: the activity of an enzyme rises up to 40 °C, where it is highest (the optimum), then drops fast at 50 °C and is almost zero at 60 °C. The numbers are invented to show the shape. Above the optimum the enzyme’s shape is damaged, so it cannot hold its substrate.
Class activity
Yeast and balloon, with a grown-up. Mix a spoon of dried yeast and a spoon of sugar with lukewarm tap water in three small plastic bottles and stretch a balloon over each neck. Put one in a cool place, one in a bowl of warm tap water (about 35–40 °C, never hotter), and one in a bowl of cold water with ice. After an hour describe the three balloons. Explain with enzymes and temperature. Nothing is heated or tasted.
Practice
1
Is the breakdown of glucose in respiration catabolic or anabolic? Does it release or use energy?
Catabolic; it releases energy.
2
What does an enzyme do to the activation energy of a reaction?
It lowers it, so the reaction goes faster.
3
After a reaction, is the enzyme used up? Explain.
No. It stays unchanged and can work again on more substrate.
4
In the lock-and-key picture of an enzyme, what is the lock and what is the key?
The active site of the enzyme is the lock, and the substrate that fits into it is the key.
