What photosynthesis is and how it was discovered
Every meal you eat, directly or indirectly, comes from a green plant that made food from air, water and sunlight. A leaf is a tiny kitchen that runs on light.
Photosynthesis is the process by which green plants, algae and some bacteria use light energy to make carbohydrates from carbon dioxide and water, releasing oxygen. It is the only major process that brings energy from the Sun into the living world.
In the figure : Photosynthesis in one line: light, carbon dioxide and water give sugar and oxygen.
Rule 1: Overall reaction — 6 CO2 + 12 H2O, in light and with chlorophyll, gives C6H12O6 + 6 H2O + 6 O2. Water is written on both sides because 12 molecules are used and 6 are newly made. The oxygen released comes from water, not from carbon dioxide.
- Easy : Photosynthesis needs carbon dioxide, water, light and chlorophyll; remove any one and it stops.
- Medium : If 12 molecules of water are used and 6 are formed again, the net water used is 6 molecules.
- Exam-type : Q: From which molecule does the released oxygen come? A: From water.
Rule 2: Early experiments — Priestley (1770s) showed that a mint plant can restore air in which a candle had stopped burning. Ingenhousz showed that light and the green parts are essential. Sachs showed that glucose made in leaves is stored as starch. Engelmann split light with a prism and saw aerobic bacteria gather at blue and red regions of an alga, proving these are the most effective colours. Van Niel showed that bacteria using H2S release sulphur, suggesting that in plants the hydrogen source is water and the oxygen comes from it.
- Easy : Priestley showed that plants can 'purify' stale air; Ingenhousz showed that this needs sunlight.
- Medium : Bacteria collecting in blue and red light along an alga showed where oxygen release was greatest.
- Exam-type : Q: Who showed that starch is a product of photosynthesis? A: Sachs.
Oxygen released in photosynthesis comes from water. Light, CO2, water and chlorophyll are all needed.
Thinking the oxygen comes from carbon dioxide. It comes from the splitting of water.
Memory clue: P-I-S-E-V — Priestley (air), Ingenhousz (light), Sachs (starch), Engelmann (colours), van Niel (water source).
Spot the difference
| Scientist | Finding |
|---|---|
| Priestley | Plants restore air spoiled by a burning candle |
| Ingenhousz | Sunlight and green parts are needed |
| Sachs | Glucose is stored as starch |
| Engelmann | Blue and red light are most effective |
| Van Niel | Oxygen comes from water |
Practice questions
- 1Name the four raw requirements of photosynthesis.
- 2What gas is released in photosynthesis?
- 3Who linked photosynthesis with starch formation?
- 4Which experiment showed that blue and red light are most effective?
- 5Write the overall equation of photosynthesis in words.
- 6Why is water shown on both sides of the equation?
- 7What did van Niel's work on sulphur bacteria suggest?
- 8A plant kept in a closed jar in darkness cannot make food even with water and CO2. Why?
Show answers (try them yourself first)
- 1Carbon dioxide, water, light and chlorophyll.
- 2Oxygen.
- 3Sachs.
- 4Engelmann's experiment with an alga, a prism and aerobic bacteria.
- 5Carbon dioxide and water, with light and chlorophyll, give glucose, water and oxygen.
- 6Twelve molecules are used and six are produced again, so the net use is six.
- 7That the hydrogen comes from water and the released oxygen comes from water.
- 8Photosynthesis needs light energy.
Site of photosynthesis and the pigments
A solar panel works only where it is exposed to sunlight and has the right material. In a leaf, the 'panels' are chloroplasts packed with pigments.
Photosynthesis takes place in the green parts of plants, mostly the leaves, inside chloroplasts. Chloroplasts contain grana (stacks of thylakoids) where the light reactions occur, and the stroma where sugar is made.
In the figure : A chloroplast has grana for the light reaction and stroma for sugar synthesis.
Rule 1: Where each stage happens — Grana (thylakoid membranes) hold the pigments and carry out the light-dependent stage, making ATP and NADPH. The stroma holds the enzymes that use ATP and NADPH to fix CO2 into sugar. Stroma lamellae connect grana and also hold some membrane reactions.
- Easy : Light is trapped in grana; sugar is made in stroma.
- Medium : ATP and NADPH made in the grana move to the stroma to be used.
- Exam-type : Q: Where are the enzymes for sugar synthesis located? A: In the stroma.
Rule 2: The four main pigments — Chlorophyll a (bright yellow-green in a solvent) is the main pigment. Chlorophyll b (yellow-green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange) are accessory pigments: they absorb other wavelengths and pass the energy to chlorophyll a, and also protect it from strong light.
- Easy : Paper chromatography of a leaf extract separates the four pigments as bands.
- Medium : Chlorophyll a is the only pigment that directly takes part in the reaction; the others pass energy to it.
- Exam-type : Q: Which pigment is the main photosynthetic pigment? A: Chlorophyll a.
Rule 3: Absorption and action spectrum — Chlorophyll a absorbs mainly in the blue and red regions. A graph of the rate of photosynthesis against wavelength is the action spectrum; it closely follows the combined absorption of all the pigments, showing that photosynthesis is highest in blue and red light.
- Easy : Green light is mostly reflected, so leaves look green.
- Medium : At a wavelength where absorption is high, the rate of photosynthesis is also high.
- Exam-type : Q: Why are leaves green? A: Chlorophyll absorbs little green light and reflects it.
Grana: light stage. Stroma: sugar stage. Chlorophyll a is the main pigment; the rest are accessory.
Saying accessory pigments do nothing. They widen the range of light used and protect chlorophyll.
Memory clue: Grana = Glow, Stroma = Sugar — Glow (light reaction) in grana, Sugar (dark reaction) in stroma.
Spot the difference
| Part | Role |
|---|---|
| Grana | Light reaction; ATP and NADPH made |
| Stroma | Carbon fixation; sugar made |
| Chlorophyll a | Main pigment |
| Accessory pigments | Absorb other light and protect chlorophyll a |
Practice questions
- 1Where in the cell does photosynthesis occur?
- 2What are grana?
- 3Name the main photosynthetic pigment.
- 4Name two accessory pigments.
- 5What is an action spectrum?
- 6Which colours of light are absorbed most by chlorophyll a?
- 7Why does a leaf appear green?
- 8Give two roles of accessory pigments.
Show answers (try them yourself first)
- 1In chloroplasts.
- 2Stacks of thylakoids in the chloroplast.
- 3Chlorophyll a.
- 4Chlorophyll b, xanthophylls or carotenoids (any two).
- 5A graph of the rate of photosynthesis against wavelength of light.
- 6Blue and red.
- 7Green light is reflected or transmitted, not absorbed.
- 8They absorb other wavelengths and pass energy to chlorophyll a, and protect chlorophyll from photo-oxidation.
Light reaction: photosystems and the electron flow
Think of a relay race in which a runner receives energy at one end, passes the baton along a track and gives energy at each step. In the light reaction, electrons are the baton.
The light reaction (photochemical phase) traps light and makes ATP and NADPH, and releases oxygen. It uses two photosystems, PS I and PS II, each with a light-harvesting complex and a reaction centre of chlorophyll a. The reaction centre of PS I absorbs best at 700 nm (P700) and that of PS II at 680 nm (P680).
In the figure : Non-cyclic flow of electrons through PS II, carriers and PS I to NADP+.
Rule 1: Non-cyclic electron flow (Z scheme) — Light excites PS II, and electrons leave P680 and pass through a chain of carriers to PS I. Light excites PS I and its electrons are passed on to NADP+, which becomes NADPH. Electrons lost by PS II are replaced by electrons from water. This zig-zag route is the Z scheme. Both ATP and NADPH are produced.
- Easy : Electrons move from water to PS II, to PS I and finally to NADP+.
- Medium : For one NADPH formed, two electrons must pass through the whole chain.
- Exam-type : Q: Where do the electrons of PS II come from? A: From the splitting of water.
Rule 2: Splitting of water — Water splits near PS II on the inner (lumen) side of the thylakoid: 2 H2O gives 4 H+, 4 electrons and O2. The electrons replace those lost by P680, the protons stay in the lumen and add to the proton gradient, and oxygen is released. The reaction needs manganese, calcium and chloride ions.
- Easy : Splitting water gives the plant electrons, protons and oxygen.
- Medium : Four electrons are released when two water molecules split, producing one O2.
- Exam-type : Q: Which ions are needed for water splitting? A: Manganese, calcium and chloride.
Water is the electron source and the origin of released oxygen. PS II (P680) works before PS I (P700) in the electron path.
Reversing the order: electrons go from PS II to PS I, though PS I was discovered first and is named first.
Memory clue: 680 before 700 — The smaller number goes first: P680 (PS II) then P700 (PS I).
Spot the difference
| Feature | PS II | PS I |
|---|---|---|
| Reaction centre | P680 | P700 |
| Electron source | Water | Electrons from PS II |
| Produces | O2 and protons | NADPH |
| Position in path | First | Second |
Practice questions
- 1What products does the light reaction give?
- 2What do P680 and P700 stand for?
- 3What is the source of electrons in non-cyclic flow?
- 4Where does water splitting occur?
- 5Which molecule finally accepts the electrons in non-cyclic flow?
- 6Why is the pathway called the Z scheme?
- 7Write the water-splitting reaction.
- 8If a chemical blocked water splitting, what would happen to PS II?
Show answers (try them yourself first)
- 1ATP, NADPH and oxygen.
- 2The reaction centres of PS II and PS I, absorbing best at 680 nm and 700 nm.
- 3Water.
- 4On the lumen side of the thylakoid membrane, associated with PS II.
- 5NADP+, forming NADPH.
- 6Because the energy diagram of the electron path looks like the letter Z.
- 72 H2O gives 4 H+, 4 electrons and O2.
- 8It could not replace its lost electrons, so the electron flow and oxygen release would stop.
Cyclic flow and ATP formation by chemiosmosis
Water held behind a dam can turn a turbine when released. In a thylakoid, protons held in a small space drive a molecular turbine that makes ATP.
Photophosphorylation is the making of ATP from ADP and phosphate using light energy. Chemiosmosis is the explanation: a proton gradient across the thylakoid membrane drives ATP synthase to make ATP.
In the figure : Chemiosmosis: protons in the lumen flow out through ATP synthase to make ATP.
Rule 1: How the proton gradient builds up — Protons accumulate in the lumen because (a) water splits there, (b) carriers move protons from the stroma into the lumen as electrons pass, and (c) NADP+ reduction takes protons from the stroma. So the lumen becomes acidic and the stroma less so.
- Easy : More protons inside the thylakoid than outside makes a gradient.
- Medium : Reducing NADP+ uses protons from the stroma, which helps keep the gradient.
- Exam-type : Q: Where are protons in higher concentration, lumen or stroma? A: Lumen.
Rule 2: ATP synthase — ATP synthase has an F0 part embedded in the membrane forming a channel, and an F1 part projecting into the stroma. When protons flow through F0 down the gradient, F1 changes shape and makes ATP from ADP and phosphate. ATP forms on the stroma side, close to where it is needed.
- Easy : Protons flowing through the channel produce ATP, like water turning a wheel.
- Medium : One ATP synthase can make many ATP as long as the gradient is kept up.
- Exam-type : Q: Which part of ATP synthase makes ATP? A: The F1 part.
Rule 3: Cyclic photophosphorylation — When only PS I works, an excited electron leaves PS I, passes through carriers and returns to PS I. No NADPH is formed and no oxygen is released, but a proton gradient forms and ATP is made. It takes place in stroma lamellae, which lack PS II and NADP reductase, and is useful when more ATP is needed.
- Easy : Cyclic flow gives only ATP.
- Medium : Because water is not split in cyclic flow, no oxygen is released.
- Exam-type : Q: Which products of the light reaction are missing in cyclic flow? A: NADPH and oxygen.
Non-cyclic: ATP + NADPH + O2. Cyclic: ATP only. ATP forms on the stroma side by chemiosmosis.
Saying ATP is made inside the lumen. Protons are collected in the lumen, but ATP is released in the stroma.
Memory clue: Cyclic = Circle = Cell-ATP only — A circle returns to the start: the electron returns to PS I and only ATP is made.
Spot the difference
| Feature | Non-cyclic | Cyclic |
|---|---|---|
| Photosystems | PS II and PS I | PS I only |
| Electron source | Water | PS I itself |
| Products | ATP, NADPH, O2 | ATP only |
| Location | Grana | Stroma lamellae |
Practice questions
- 1What is chemiosmosis?
- 2Where is ATP made, on the lumen side or stroma side?
- 3What are the two parts of ATP synthase?
- 4Which photosystem works in cyclic flow?
- 5Name two reasons why protons build up in the lumen.
- 6Why is no oxygen released in cyclic flow?
- 7Where in the chloroplast does cyclic flow occur mainly?
- 8A plant needs extra ATP but no extra NADPH. Which flow would help?
Show answers (try them yourself first)
- 1ATP synthesis driven by the flow of protons down a gradient across a membrane.
- 2Stroma side.
- 3F0 (the channel in the membrane) and F1 (the head in the stroma).
- 4PS I only.
- 5Water splitting in the lumen and proton pumping by carriers from the stroma (also NADP+ reduction removes stroma protons).
- 6Because PS II does not work and water is not split.
- 7In the stroma lamellae.
- 8Cyclic flow.
The Calvin cycle: making sugar
A factory needs raw material, power and a way to recycle its tools. The Calvin cycle takes CO2, uses the power (ATP and NADPH) made earlier and recycles its own starting molecule.
The Calvin cycle (dark or biosynthetic phase) is the set of reactions in the stroma that use ATP and NADPH to turn CO2 into sugar. It does not need light directly, but it depends on the products of the light reaction.
In the figure : The Calvin cycle: carboxylation, reduction and regeneration.
Rule 1: Three steps — Carboxylation: the enzyme RuBisCO joins CO2 to the 5-carbon sugar RuBP, giving two molecules of 3-carbon PGA. Reduction: ATP and NADPH convert PGA to triose phosphate (PGAL); some leaves the cycle to make glucose, starch and other compounds. Regeneration: the rest of the triose phosphate is rearranged, using ATP, to remake RuBP.
- Easy : One CO2 enters per turn; the cycle must turn six times to make one 6-carbon sugar.
- Medium : RuBP (5 C) + CO2 (1 C) gives two PGA (3 C each): the carbon count balances at 6.
- Exam-type : Q: Which enzyme fixes CO2 in the Calvin cycle? A: RuBisCO.
Rule 2: Energy cost — For each CO2 fixed, the cycle uses 3 ATP and 2 NADPH. To make one glucose, six turns are needed, using 18 ATP and 12 NADPH. RuBisCO is the most abundant enzyme on Earth.
- Easy : Fixing one CO2 needs 3 ATP.
- Medium : Six CO2 need 6 x 3 = 18 ATP and 6 x 2 = 12 NADPH.
- Exam-type : Q: How many ATP are needed to make one glucose through the Calvin cycle? A: 18.
Fix, reduce, regenerate. First stable product is a 3-carbon acid (PGA) in C3 plants. Per CO2: 3 ATP + 2 NADPH.
Calling the Calvin cycle 'dark reaction' as if it runs at night. It needs the ATP and NADPH from the light reaction, so it stops soon after light stops.
Memory clue: C3 = 3 in PGA — The first product has 3 carbons, so the plants are called C3 plants.
Spot the difference
| Step | Event | Energy |
|---|---|---|
| Carboxylation | CO2 joins RuBP to give PGA | None |
| Reduction | PGA becomes triose phosphate | ATP and NADPH used |
| Regeneration | RuBP is rebuilt | ATP used |
Practice questions
- 1Where does the Calvin cycle occur?
- 2What is the CO2 acceptor in the cycle?
- 3What is the first stable product?
- 4Name the enzyme that carries out carboxylation.
- 5How many ATP and NADPH are needed per CO2 fixed?
- 6How many turns of the cycle make one glucose?
- 7Why is the Calvin cycle called a cycle?
- 8A leaf is kept in darkness. Why will the Calvin cycle stop after some time?
Show answers (try them yourself first)
- 1In the stroma.
- 2RuBP (ribulose bisphosphate), a 5-carbon sugar.
- 3PGA, a 3-carbon acid.
- 4RuBisCO.
- 53 ATP and 2 NADPH.
- 6Six.
- 7RuBP, the starting molecule, is regenerated at the end.
- 8ATP and NADPH are no longer made by the light reaction, so they run out.
C4 pathway and photorespiration
Some grasses grow quickly in hot, dry fields while many other plants slow down. They have an extra CO2 pump that lets them work well even when their pores are partly closed.
C4 plants (such as maize and sugarcane) first fix CO2 into a 4-carbon acid, oxaloacetic acid (OAA), in mesophyll cells and then release CO2 for the Calvin cycle in bundle-sheath cells. Photorespiration is a wasteful process in which RuBisCO combines RuBP with O2 instead of CO2.
In the figure : C4 pathway: CO2 is first fixed in mesophyll and released in bundle sheath cells.
Rule 1: C4 pathway — Leaves of C4 plants show Kranz anatomy: large bundle-sheath cells arranged like a wreath around the vascular bundle. In mesophyll cells, the enzyme PEP carboxylase joins CO2 to PEP (3 C) to make OAA (4 C). OAA changes to malic or aspartic acid and moves to the bundle sheath, where it is broken down to release CO2 for the Calvin cycle. This raises CO2 concentration near RuBisCO.
- Easy : C4 plants first make a 4-carbon acid, so they are called C4 plants.
- Medium : PEP carboxylase has no affinity for O2, so it picks up CO2 even at low concentration.
- Exam-type : Q: Where does the Calvin cycle occur in C4 plants? A: In bundle-sheath cells.
Rule 2: Photorespiration — In C3 plants, when O2 is high and CO2 is low, RuBisCO binds O2 and RuBP is split into a 2-carbon compound and PGA. The process involves chloroplast, peroxisome and mitochondrion, makes no sugar and no ATP, and uses energy. C4 plants avoid it because the bundle sheath has a high CO2 concentration.
- Easy : Photorespiration lowers the yield of a C3 plant, especially in hot weather.
- Medium : C4 plants lose almost no carbon by photorespiration.
- Exam-type : Q: Why do C4 plants lack photorespiration? A: CO2 is concentrated near RuBisCO in the bundle sheath.
C4: Kranz anatomy, PEP carboxylase, OAA first product, no photorespiration, high yield, hot climate.
Thinking C4 plants skip the Calvin cycle. They still use it, but in bundle-sheath cells.
Memory clue: C4 = Kranz + 4C OAA — K for Kranz, 4 for the 4-carbon OAA.
Spot the difference
| Feature | C3 plants | C4 plants |
|---|---|---|
| First product | PGA (3 C) | OAA (4 C) |
| CO2 acceptor | RuBP | PEP |
| Leaf anatomy | No Kranz | Kranz anatomy |
| Photorespiration | Present | Almost absent |
| Optimum temperature | Lower (about 20 to 25 C) | Higher (about 30 to 45 C) |
Practice questions
- 1Name two C4 plants.
- 2What is Kranz anatomy?
- 3What is the first product of CO2 fixation in C4 plants?
- 4Which enzyme fixes CO2 in mesophyll cells of C4 plants?
- 5What is photorespiration?
- 6Why is photorespiration called wasteful?
- 7Why are C4 plants more productive in hot, dry places?
- 8Which cells carry out the Calvin cycle in C4 plants?
Show answers (try them yourself first)
- 1Maize and sugarcane.
- 2Bundle-sheath cells arranged in a wreath around the vascular bundle.
- 3Oxaloacetic acid (OAA).
- 4PEP carboxylase.
- 5A process in which RuBisCO binds O2 instead of CO2, releasing CO2 and wasting energy.
- 6It makes no sugar and no ATP and uses energy.
- 7They concentrate CO2 near RuBisCO, avoid photorespiration and work at higher temperatures.
- 8Bundle-sheath cells.
Factors affecting photosynthesis
A bakery works only as fast as its slowest step: oven, mixing or staff. In the same way, photosynthesis is limited by the factor in shortest supply.
The rate of photosynthesis depends on light, CO2 concentration, temperature and water, as well as the plant's own factors such as the number and age of leaves and chlorophyll content. Blackman's law of limiting factors says that when several factors affect a process, its rate is limited by the factor nearest its minimum value.
In the figure : Four external factors that affect the rate of photosynthesis.
Rule 1: Light and carbon dioxide — At low light intensity, rate rises with light; at high intensity it levels off because other factors become limiting. Very bright light may even damage chlorophyll. CO2 is normally the most limiting factor: air has only 0.03 to 0.04 per cent. C3 plants respond to CO2 increase up to about 450 microlitres per litre, whereas C4 plants reach saturation at about 360 microlitres per litre. Some greenhouse crops (tomato, bell pepper) are grown in CO2-enriched air for better yield.
- Easy : More light increases the rate until it reaches a plateau.
- Medium : Adding CO2 to greenhouse air of a tomato crop raises the yield.
- Exam-type : Q: Why does the graph of rate against light become flat? A: Another factor, such as CO2, becomes limiting.
Rule 2: Temperature and water — Dark reactions are enzyme controlled and so respond to temperature; light reactions are less affected. C4 plants do best at higher temperatures than C3 plants. Water is needed as raw material, but a more important effect is that water stress closes stomata, which reduces CO2 entry and lowers the rate, and also makes leaves wilt.
- Easy : In very hot weather, enzymes slow down and the rate falls.
- Medium : A wilted leaf makes less food because its stomata are closed.
- Exam-type : Q: How does water shortage reduce photosynthesis indirectly? A: Stomata close, reducing CO2 supply.
Limiting factor = the one in shortest supply. CO2 is usually limiting in nature. C4 plants like heat.
Thinking more of every factor always helps. Once the limiting factor is supplied, another factor takes over.
Memory clue: L-C-T-W — Light, CO2, Temperature, Water.
Spot the difference
| Factor | Effect |
|---|---|
| Light | Rate rises, then plateaus; too much can damage |
| CO2 | C3 saturates around 450; C4 around 360 microlitres per litre |
| Temperature | Affects enzyme-controlled dark reaction |
| Water | Raw material; stress closes stomata |
Practice questions
- 1List four external factors that affect photosynthesis.
- 2What does Blackman's law say?
- 3What percentage of air is CO2 (approximately)?
- 4Which reactions are more affected by temperature?
- 5Which type of plant, C3 or C4, does better at high temperature?
- 6Why do greenhouse growers sometimes add CO2?
- 7A graph shows rate staying flat although light is increased. What may be the reason?
- 8Explain why drought reduces photosynthesis even if the cells have enough water for the reaction.
Show answers (try them yourself first)
- 1Light, CO2 concentration, temperature and water.
- 2The rate is limited by the factor that is nearest its minimum.
- 3About 0.03 to 0.04 per cent.
- 4Dark (Calvin cycle) reactions, because they are enzyme controlled.
- 5C4.
- 6CO2 is often the limiting factor, so extra CO2 raises the rate and yield.
- 7Another factor such as CO2 or temperature has become limiting.
- 8Stomata close to save water, so less CO2 enters the leaf.