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Catabolism is the process of breaking down larger molecules into smaller ones and this 'always' releases energy. Anabolism is the process of building up larger molecules, effectively a polymerisation process, and 'always' takes energy.

My question is, how can my teacher assert that it always must take or release energy? Infact, I can think of a synthesis reactions - the polymerisation of ethene - that is exothermic. I can't think of any chemical reason why the breaking down of a substance must be exothermic.

we eat food and do get energy from it. Thus it must still be true in the general case. Can someone explain why this is true generally?

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    $\begingroup$ Because ALL reactions need energy, go into what activation energy is. Or are you asking why anabolic reactions are endergonic, in which case the answer is because the products have more chemical bonds and chemical bonds require energy to form. This question is a better fit for chemistry not biology. $\endgroup$ – John Apr 10 at 2:02

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