Metabolism: anabolism and catabolism
Metabolism is the full set of chemical reactions inside a living cell. It has two sides.
- Anabolism (assimilation): small molecules are joined to make big ones, such as amino acids to proteins or sugars to starch. It uses energy.
- Catabolism (dissimilation): big molecules are broken into small ones, such as glucose into CO2 and water. It gives out energy.
The energy from catabolism is saved in a small molecule called ATP (the cell's energy coin). Anabolism spends ATP. Both sides work all the time, and enzymes speed up each reaction.
Autotrophs, heterotrophs and mixotrophs
Living things are grouped by how they get food.
- Autotrophs ("self-feeders") make food from simple things like CO2 and water. Plants, algae, and some bacteria are autotrophs.
- Heterotrophs ("other-feeders") cannot make their own food and eat other living things or their remains. Animals, fungi and most bacteria are heterotrophs.
- Mixotrophs can do both. Euglena, a tiny pond organism, makes food in light and absorbs food in the dark. Some carnivorous plants, such as sundew, also photosynthesise and catch insects for minerals.
Photosynthesis and chemosynthesis
Both processes make sugar from CO2, but the energy source differs.
Photosynthesis
Plants, algae and some bacteria use light energy trapped by green chlorophyll in chloroplasts:
CO2 + water + light โ glucose + oxygen
It feeds almost every food chain and gives us the oxygen in air.
Chemosynthesis
Some bacteria use chemical energy from oxidising simple substances such as hydrogen sulphide, ammonia or iron compounds. No light is needed. They live in the deep sea near vents, in soil and in caves. Nitrifying bacteria in soil are chemosynthetic: they help plants get nitrogen.
Quick compare: Photo = light energy; Chemo = chemical energy.
Stages of energy metabolism
When a cell breaks down glucose to release energy, it happens in three stages.
- Preparatory stage: big food molecules (starch, fats, proteins) are cut into small ones (glucose, fatty acids, amino acids), mostly in the gut or in digestive compartments. Almost all the energy is lost as heat. No ATP.
- Glycolysis (anaerobic): in the cytoplasm one glucose (6 carbon atoms) splits into two smaller pieces (pyruvate, 3 carbon atoms each). This makes 2 ATP and needs no oxygen.
- Oxygen (aerobic) stage: in the mitochondria the pieces are fully broken to CO2 and water, making about 28 more ATP.
Total with oxygen: about 30 ATP per glucose. (Some books say 36 or 38; newer measurements give about 30 to 32. Check what your teacher uses.)
Equation: glucose + 6 O2 โ 6 CO2 + 6 water + energy (ATP).
Anaerobic and aerobic breakdown
Aerobic means with oxygen; anaerobic means without oxygen.
- Aerobic: uses all three stages. Glucose โ CO2 + water. About 30 ATP.
- Anaerobic (fermentation): only glycolysis plus one more step. About 2 ATP per glucose.
- Lactic acid fermentation: glucose โ lactic acid. In tired muscles, and in curd bacteria.
- Alcoholic fermentation: glucose โ alcohol + CO2. In yeast, used for bread and for making wine and beer.
Anaerobic is far less efficient: about 2 against 30 ATP. That is why complex animals depend on oxygen, while some bacteria and yeast can live without it.
Try it: the blow-up bread test
Try it (with an adult). Put half a teaspoon of dry yeast and one teaspoon of sugar in a small bottle with warm water. Stretch a balloon over the neck. Wait 30 minutes.
- What happens to the balloon? (It fills with CO2 from alcoholic fermentation.)
- Make a second bottle with no sugar. Does the balloon fill? Why not?
- Run on the spot for 1 minute. Feel your muscles and breathing. Which route did they use?
In the 3D, predict the ATP for 3 glucose with full oxygen (3 x 30 = 90), then slide and check.
Key formulas and definitions
- Photosynthesis: CO2 + water + light โ glucose + oxygen
- Aerobic: glucose + 6 O2 โ 6 CO2 + 6 H2O + energy (about 30 ATP)
- Anaerobic: glucose โ lactic acid (or alcohol + CO2) + 2 ATP
- Stages: 1 preparatory (no ATP), 2 glycolysis (+2 ATP), 3 aerobic (about +28 ATP)
- Autotroph = makes own food; heterotroph = eats ready food; mixotroph = both
Worked examples
1. How many ATP do 4 glucose molecules make with oxygen (use 30 per glucose)?
4 x 30 = 120 ATP.
2. A muscle cell breaks 10 glucose molecules without oxygen. How many ATP does it make, and how many fewer than with oxygen?
Without oxygen: 10 x 2 = 20 ATP. With oxygen: 10 x 30 = 300 ATP. Difference = 300 โ 20 = 280 ATP fewer.
3. Is a Venus flytrap an autotroph, a heterotroph or a mixotroph? Explain.
Mixotroph. It makes food by photosynthesis in its green leaves, and also catches insects, mainly to get nitrogen and minerals missing in poor soil.
Common mistakes
- Thinking plants do not respire. Plants do both photosynthesis and respiration, in all cells all the time.
- Mixing up respiration (breaking glucose to release energy in cells) with breathing (moving air in and out).
- Saying energy is 'made'. Energy is changed from one form to another, here from food to ATP.
- Believing anaerobic means no energy. It gives 2 ATP per glucose, much less than aerobic.