Photosynthesis & Respiration
High SchoolThese two processes are the twin engines of the biosphere. Photosynthesis captures sunlight and stores it in sugar; respiration releases that stored energy to power life. Together they form a great cycle of carbon, oxygen and energy that connects every organism on Earth.
Photosynthesis: building sugar from light
In the chloroplasts of plants and algae:
It runs in two linked stages:
- Light-dependent reactions (in the thylakoid membranes): chlorophyll absorbs light, exciting electrons. Water is split (photolysis) — 2 H₂O → 4 H⁺ + 4 e⁻ + O₂ — releasing oxygen as a by-product. Electrons flow down an electron transport chain, pumping protons to drive ATP synthesis, and reduce NADP⁺ to NADPH.
- Light-independent reactions / Calvin cycle (in the stroma): the ATP and NADPH are used to fix CO₂ onto a 5-carbon sugar (RuBP) via the enzyme Rubisco, ultimately building glucose. No light is directly required, but it depends on the products of the light reactions.
Respiration: releasing the stored energy
Aerobic respiration has four stages:
| Stage | Location | Key output |
|---|---|---|
| Glycolysis | Cytoplasm | Glucose → 2 pyruvate; net 2 ATP + 2 NADH (no O₂ needed) |
| Link reaction | Mitochondrial matrix | Pyruvate → acetyl-CoA + CO₂ + NADH |
| Krebs cycle | Matrix | CO₂, NADH, FADH₂, 2 ATP |
| Oxidative phosphorylation | Inner membrane (cristae) | Electron transport + chemiosmosis → most ATP; O₂ is final electron acceptor → H₂O |
Without oxygen (anaerobic), cells rely on glycolysis alone plus fermentation: lactic acid in animals (reversible), or ethanol + CO₂ in yeast and plants. Anaerobic respiration yields only ~2 ATP per glucose — far less than aerobic.
Two halves of one cycle
Look at the two equations together: the products of photosynthesis (glucose + O₂) are exactly the reactants of respiration, and vice versa. Photosynthesis stores solar energy in chemical bonds; respiration releases it as ATP. The Sun is the ultimate power source for almost all life, and green plants are the gateway through which its energy enters the living world.
Both processes share a deep mechanism: chemiosmosis. In both the thylakoid membrane and the inner mitochondrial membrane, an electron transport chain pumps protons across a membrane, building an electrochemical gradient. Protons then flow back through the enzyme ATP synthase, which spins like a molecular turbine and forges ATP. Peter Mitchell's insight — that a proton gradient across a membrane is the universal energy intermediary — unified bioenergetics and won the 1978 Nobel Prize.
Oxygen's role in respiration is to be the final electron acceptor at the end of the chain. Without it, electrons back up, the chain stops, NADH cannot be re-oxidised, and ATP production crashes to the meagre glycolytic yield. This is why oxygen deprivation is fatal so quickly: it does not "provide" energy directly — it keeps the electron traffic flowing.
Experiment: measuring the rate of photosynthesis in pondweed
Use aquatic pondweed (Elodea/Cabomba) to test how light intensity affects photosynthesis by counting oxygen bubbles.
- Set up. Place a cut stem of pondweed, cut-end up, in a beaker of water containing dissolved CO₂ (add sodium hydrogencarbonate to ensure CO₂ is not limiting).
- Vary light intensity. Move a lamp to set distances (10, 20, 30, 40, 50 cm). Light intensity ∝ 1/distance² (the inverse-square law) — a crucial point for analysis.
- Control other factors. Keep temperature constant (use a heat-shield/water bath so the lamp doesn't warm the beaker) and CO₂ constant, so light is the only independent variable.
- Measure. Let the plant acclimatise ~5 minutes at each distance, then count oxygen bubbles released per minute from the cut stem. Repeat and average.
- Analyse. Plot bubble rate (y) against light intensity. Rate rises as light increases, then plateaus when another factor (CO₂ or temperature) becomes limiting.
- Conclude with limiting factors. The plateau demonstrates the law of limiting factors: the rate is capped by whichever required factor is in shortest supply. Below the plateau, light limits; at the plateau, something else does.
Test yourself
Q1 Where does the oxygen released in photosynthesis come from — CO₂ or H₂O? How do we know?
The oxygen comes from water, not CO₂. It is produced during the photolysis (splitting) of water in the light-dependent reactions. This was proven by isotope-labelling experiments (Ruben & Kamen, 1941): when water was labelled with heavy oxygen-18, the released O₂ was heavy; when CO₂ was labelled instead, the released O₂ was normal. So the O₂ we breathe is recycled from the water plants split.
Q2 Why does aerobic respiration yield roughly 15 times more ATP than anaerobic respiration?
Anaerobic respiration uses only glycolysis, netting ~2 ATP per glucose, and glucose is only partially broken down (to lactate or ethanol), leaving most of its energy locked up. Aerobic respiration fully oxidises glucose to CO₂ and water through the link reaction, Krebs cycle and — crucially — oxidative phosphorylation, where the NADH and FADH₂ feed electrons into the electron transport chain to make the bulk of the ATP (~30–32 total). Oxygen as final electron acceptor is what unlocks this large yield.
Q3 A greenhouse grower increases light but sees no further rise in growth. Give two possible explanations and how to test them.
By the law of limiting factors, another factor now caps the rate. Likely candidates: CO₂ concentration or temperature. Test by raising one at a time while holding the rest constant: enrich the air with CO₂ — if growth rises, CO₂ was limiting; or warm the greenhouse — if growth rises, temperature was limiting (up to the enzyme optimum). Commercial growers routinely add CO₂ and heat precisely because light alone is not enough once it exceeds the plant's other supplies.
Q4 Explain chemiosmosis and why it is described as "energetically clever."
Chemiosmosis is ATP synthesis powered by a proton gradient across a membrane. An electron transport chain pumps H⁺ to one side, creating a store of potential energy (like water behind a dam). The protons then flow back through the enzyme ATP synthase, whose rotation drives the bonding of ADP + Pi into ATP. It is clever because it converts the messy, stepwise energy of electron transfers into a single, reusable "battery" — a proton gradient — that a molecular turbine can tap efficiently. The same trick powers both mitochondria and chloroplasts.
Q5 During intense exercise your muscles switch partly to anaerobic respiration. What are the costs and how does the body repay them?
Anaerobic respiration in muscle produces lactic acid and only a little ATP. The costs: lactic acid lowers pH, contributing to fatigue, and the process is inefficient. Afterwards the body incurs an oxygen debt (excess post-exercise oxygen consumption): you breathe hard to supply extra oxygen that oxidises the lactic acid back to pyruvate (largely in the liver, converting some to glucose) and to restore ATP and oxygen stores. Unlike ethanol fermentation in yeast, animal lactic-acid fermentation is reversible, which is why it's a workable emergency system.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The process, step by step
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1Pondweed gives 30 bubbles in 3 minutes. What is the rate per minute?
- Rate = bubbles / time
- 30 / 3 = 10
Example 2Write the overall word equation for aerobic respiration.
- Glucose reacts with oxygen
- Products are CO2, water and energy
Now you try
Work each one out first, then tap to reveal the worked answer.