If you are in Year 5 at the liceo classico (the Italian high school focused on Latin, Greek and the humanities), between October and December science usually reaches biochemistry and energy metabolism: how the cell obtains and stores energy. The oral or written test on this block generally falls six to nine weeks from now, so at the end of November or in December, and the maturità (Italy’s school-leaving exam) is in June: it is a topic you may meet again in the oral exam as a link between biology and chemistry. There is time to build a clean framework, and it is the only thing you really need.
Every year in my lessons I see the same script: the student knows the three words (photosynthesis, fermentation, respiration), but in an oral test mixes them up. They say that the Calvin cycle produces oxygen, or that fermentation takes place in the mitochondrion. The trick is to stop learning three separate chapters and learn a single grid: where it takes place, what goes in, what comes out, how much ATP.
For the detail of respiration (glycolysis, Krebs, transport chain) there is already the post on glycolysis, the Krebs cycle and oxidative phosphorylation, at university level but useful if you want to go deep. Here I stay at high school level and on the comparison.
The overall picture
Photosynthesis builds glucose using light energy; respiration and fermentation break it down to obtain ATP. The two paths are almost the inverse of one another:
This is the overall equation of photosynthesis. Read backwards (without the light, with ATP production) it is cellular respiration:
Careful, though: the overall equation is a summary, not a mechanism. In neither path does the reaction happen “in one go”: they are chains of steps, and oral tests are lost precisely there.
Photosynthesis: two phases, two places
In the chloroplast you distinguish the internal membranes, the thylakoids (stacked in grana), and the surrounding fluid, the stroma. Each zone has its own phase.
Light phase (in the thylakoids). Chlorophyll absorbs light in the photosystems. The energy is used to split water: electrons, protons and oxygen are released. The electrons travel along a transport chain, and the proton gradient that forms drives ATP synthase, which produces ATP. At the end the electrons reduce NADP⁺ to NADPH. Products to remember: ATP, NADPH, O₂.
Calvin cycle (in the stroma). It does not use light directly, but uses the ATP and NADPH produced by the light phase. In three moments:
- fixation: the enzyme RuBisCO attaches CO₂ to a 5-carbon compound (RuBP) and two 3-carbon molecules (3-PGA) form;
- reduction: with ATP and NADPH the 3-PGA becomes glyceraldehyde-3-phosphate (G3P);
- regeneration: most of the G3P is used to rebuild RuBP, so the cycle can start again.
For every 6 molecules of CO₂ fixed you get 12 molecules of G3P: 2 leave the cycle and serve to build one molecule of glucose, the other 10 regenerate RuBP. Useful final product: sugar. A very frequent mistake: the oxygen does not come from CO₂, but from the water split in the light phase.
Fermentation: glycolysis plus a “regenerator”
Without oxygen the cell cannot complete the oxidation of glucose. Glycolysis, which takes place in the cytosol, splits glucose into two molecules of pyruvate and produces 2 net ATP and 2 NADH. The problem is that glycolysis consumes NAD⁺: if the NAD⁺ runs out, glycolysis stops. Fermentation serves for this, to regenerate NAD⁺ by making the electrons of NADH go to pyruvate (or a derivative of it).
- Lactic fermentation: pyruvate becomes lactate (lactic acid). It takes place in muscles under intense effort and in yoghurt bacteria.
- Alcoholic fermentation: pyruvate loses a CO₂ and becomes acetaldehyde, then ethanol. It is that of yeasts, and explains the bubbles in bread and wine.
In both cases the yield is only 2 net ATP per glucose, because the glucose remains partially oxidised: most of the energy stays in the lactate or the ethanol. All the ATP yield of fermentation is that of glycolysis; the following step adds none.
Cellular respiration: complete oxidation
With oxygen the cell goes all the way. After glycolysis (cytosol), pyruvate enters the mitochondrion: in the matrix it is oxidised and goes through the Krebs cycle, producing CO₂, NADH and FADH₂. These carriers hand over the electrons to the transport chain in the inner mitochondrial membrane; oxygen is the final electron acceptor and turns into water. The proton gradient drives ATP synthase. Yield: about 30-32 ATP per glucose, that is about fifteen times that of fermentation.
Comparison chart
| Photosynthesis | Fermentation | Cellular respiration | |
|---|---|---|---|
| Where | Chloroplast: thylakoids and stroma | Cytosol | Cytosol (glycolysis), then mitochondrion |
| Needs oxygen? | No, it produces it | No | Yes, as final acceptor |
| Reactants | CO₂, H₂O, light | Glucose | Glucose, O₂ |
| Products | Glucose, O₂ | Lactate or ethanol + CO₂ | CO₂, H₂O |
| ATP | It consumes it in the Calvin cycle | 2 net | about 30-32 |
| Who does it | Plants, algae, cyanobacteria | Yeasts, bacteria, muscles | Almost all eukaryotes |
Note: lactic fermentation does not produce CO₂, alcoholic fermentation does. It is a test question.
Typical mistakes in oral tests
- Wrong place. Light phase in the thylakoids, Calvin in the stroma. Glycolysis and fermentation in the cytosol, Krebs in the mitochondrial matrix, transport chain in the inner membrane. If you remember the place, you remember the task too.
- “Fermentation produces ATP.” Almost: it is glycolysis that produces the ATP. Fermentation regenerates NAD⁺.
- “The Calvin cycle happens in the dark.” It is independent of light as a reaction, but depends on ATP and NADPH, which come from the light phase: without light, it soon stops.
- “Plants don’t respire.” Plants also do cellular respiration in their mitochondria, day and night: photosynthesis builds the glucose, respiration uses it.
- Saying 36 or 38 ATP. In older books you find different figures; with current estimates you write “about 30-32”. If your text gives another figure, use your book’s, but explain that it is an estimate.
An exercise to fix the ideas
A muscle under effort uses 3 molecules of glucose without enough oxygen (lactic fermentation). How many net ATP are obtained? Each glucose gives 2 ATP: net ATP. If the same glucose were completely oxidised with oxygen, the estimate would be about ATP, about fifteen times as much: that is why fermentation is a stopgap for short periods.
A second check, more conceptual: 6 molecules of CO₂ are fixed in the Calvin cycle. How many G3P leave to make one molecule of glucose? 2 G3P leave (3 carbons each), which together form the 6 carbons of glucose. The numbers add up: 12 produced, 10 regenerate RuBP, 2 leave.
How to revise in a week
Draw the grid of the table by hand without looking and fill it in; then draw a chloroplast and a mitochondrion yourself, and mark where each step takes place. The neighbouring topics worth connecting are the functional groups of organic chemistry (sugars, lactate and ethanol are organic molecules with specific groups), basal metabolic rate and energy requirement for the physiological side, and DNA: replication, transcription and translation as another molecular biology block that often comes up again in the syllabus.
Test yourself
If you want to practise on science tests and the other high school subjects, there is the free simulation on high school tests: try it after filling in the table from memory. If you find that the three paths keep getting mixed up, in an online lesson with me we can build the framework together on your textbook and on the questions your teacher asks.
