Metabolism
Metabolism is the sum of all chemical reactions in a cell, made up of anabolism (building larger molecules, requiring energy) and catabolism (breaking down molecules, releasing energy), all controlled by enzymes.
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What metabolism means
Content standard 5.1 defines metabolism as the total of all the chemical reactions that occur inside a living cell to keep it alive, growing and functioning. These reactions do not happen randomly, they occur as organised sequences called metabolic pathways, where the product of one enzyme-controlled reaction becomes the substrate for the next.
Anabolism versus catabolism
| Feature | Anabolism | Catabolism |
|---|---|---|
| Direction of reaction | Builds larger, complex molecules from smaller ones | Breaks down larger, complex molecules into smaller ones |
| Energy change | Requires (absorbs) energy | Releases energy |
| Examples | Photosynthesis, protein synthesis from amino acids | Respiration, digestion of food |
How metabolism links to ATP
Catabolic reactions such as respiration release energy that is captured and stored in ATP molecules. Anabolic reactions such as protein synthesis then use the energy released from ATP to build new molecules.
This is why ATP is described as the cell's energy currency, it links the energy-releasing and energy-requiring halves of metabolism together, so a cell with a high rate of anabolism, such as a growing or actively dividing cell, also needs a correspondingly high rate of catabolism to keep supplying ATP.
An example metabolic pathway
A plant illustrates how anabolism and catabolism connect within one organism. During the day, photosynthesis (an anabolic pathway) builds glucose from carbon dioxide and water using light energy.
The same plant's cells then use respiration (a catabolic pathway) to break some of that glucose back down, releasing the energy as ATP to power growth, active transport and other cell activities. The glucose not immediately respired may instead be converted into starch for storage, another anabolic step, showing how one metabolic pathway can feed directly into the next.
How it is examined
Exam questions typically give a named process, such as photosynthesis, respiration, digestion or protein synthesis, and ask you to classify it as anabolism or catabolism with a reason. You may also be asked to explain the relationship between metabolism and enzymes, or to explain how ATP links anabolic and catabolic reactions.
Metabolism obeys the conservation of energy
Metabolism never creates or destroys energy, it only converts it from one form to another. Catabolic reactions such as respiration release the chemical energy stored in bonds within glucose, transferring part of it into a usable form in ATP and losing the rest as heat, which is why an actively metabolising tissue, such as contracting muscle, warms up.
Anabolic reactions then use the chemical energy stored in ATP to form new bonds when building larger molecules such as proteins or starch. Because some energy is always lost as heat at each conversion step, a cell must continually carry out catabolism to keep resupplying the ATP that anabolism consumes.
Common misconceptions
Worked exam-style question
Question. A germinating seed has no leaves yet, so it cannot photosynthesise, but it still grows a root and a shoot using food stored in its cotyledons. (a) Name the type of reaction that breaks down the stored food in the cotyledons, and state whether it releases or requires energy.
(b) Name the type of reaction that builds new plant tissue for the growing root and shoot. (c) Explain how the two reactions in (a) and (b) are linked.
Model answer. (a) The stored food is broken down by respiration, a catabolic reaction, which releases energy. (b) New tissue is built by anabolic reactions such as protein synthesis and cell division, which require energy.
(c) The energy released by respiration is captured and stored in ATP; the anabolic reactions that build new tissue then use the energy released from this ATP to form new molecules, linking the energy-releasing and energy-requiring halves of the seedling's metabolism.
Practice question
A person who has not eaten for several hours starts to break down stored glycogen in the liver into glucose to keep their blood glucose level steady. State whether this is an example of anabolism or catabolism, and justify your answer.
Exam tip
Key terms
- ATP, the molecule that stores and transfers usable chemical energy between catabolic and anabolic reactions.
- Cellular respiration, the main catabolic pathway that releases energy from food to produce ATP.
- Photosynthesis, the main anabolic pathway in green plants, building glucose using light energy.
- Enzyme, a biological catalyst that controls the rate of each reaction in a metabolic pathway.
Source:SRC-DSKP-EN
Frequently asked questions
What is the difference between anabolism and catabolism?
Why are enzymes essential to metabolism?
More for Metabolism and Enzymes
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