4 higher-level sections hidden.
Educerie · IB Diploma · Biology
Theme C Interaction and interdependence · C1.3 Photosynthesis
What you must be able to do
| You must be able to | Level | What it looks like in the exam |
|---|---|---|
| Explain photosynthesis as the conversion of light energy to chemical energy in carbon compounds | SL, HL | "Outline the energy conversion in photosynthesis" (2 marks) |
| Write the word equation; state that hydrogen and oxygen come from splitting water | SL, HL | Paper 1A; "State the source of the oxygen" (1 mark) |
| Separate pigments by chromatography, calculate Rf values and identify pigments | SL, HL | Paper 1B chromatogram: calculate and identify (3 marks) |
| Explain why pigments absorb only some wavelengths; read absorption spectra | SL, HL | Label or interpret a spectrum (2 to 3 marks) |
| Compare absorption and action spectra; calculate rates and plot an action spectrum | SL, HL | Paper 1B data: rate calculation, plot, comparison (4 to 6 marks) |
| Design limiting-factor experiments, suggest hypotheses, identify variables | SL, HL | Paper 1B method evaluation; "Identify the independent variable" (1 mark) |
| Evaluate CO₂ enrichment experiments, including FACE, and identify controlled variables | SL, HL | Paper 1B data from an enrichment study (3 to 5 marks) |
| Describe photosystems and the advantage of a pigment array | HL only | "Outline the structure of a photosystem" (3 marks) |
| Explain photolysis, chemiosmosis in thylakoids, NADP reduction, cyclic and non-cyclic flow | HL only | Section B: "Explain the light-dependent reactions" (7 marks) |
| Explain carbon fixation by Rubisco, TP synthesis, RuBP regeneration and the products of the Calvin cycle | HL only | "Explain the Calvin cycle" (5 to 6 marks) |
| Explain the interdependence of the light-dependent and light-independent reactions | HL only | 3-mark "explain" |
Before you start
You need enzymes from C1.1 and ATP from C1.2; at HL you also need chemiosmosis from C1.2, because the thylakoid uses the same trick as the mitochondrion. You need the structure of a chloroplast from B2.2: a double membrane around a fluid stroma, with stacks of flattened sacs called thylakoids, each enclosing a space called the lumen.
1The idea in one paragraph
Photosynthesis turns light energy into chemical energy. Pigments in the chloroplast absorb particular wavelengths of light; that energy splits water, and the hydrogen from the water converts carbon dioxide into glucose and other carbon compounds, while the oxygen is released as a by-product. How fast it happens depends on whichever of light, carbon dioxide or temperature is in shortest supply. At HL you learn the two linked stages: the light-dependent reactions in the thylakoid membranes, which make ATP and reduced NADP, and the Calvin cycle in the stroma, which spends them to fix carbon.
2Light in, carbon compounds out
Photosynthesis is the production of carbon compounds in cells using light energy. Its essential act is an energy transformation: light energy becomes chemical energy in the bonds of carbon compounds such as glucose. Those compounds feed the plant and everything that eats it, so this one transformation supplies most of the chemical energy used by life in ecosystems.
carbon dioxide + water → glucose + oxygen (using light energy, absorbed by chlorophyll)
Two facts hidden in that equation are examined often.
The hydrogen comes from water. To turn carbon dioxide into glucose, hydrogen must be added to it. The plant gets that hydrogen by splitting water molecules using energy from light.
The oxygen comes from water, not from carbon dioxide. When water is split, the oxygen atoms are left over and released as oxygen gas. Oxygen is a by-product. This happens in plants, in algae and in cyanobacteria, the photosynthetic bacteria whose ancestors first filled the atmosphere with oxygen.
3Separating the pigments: chromatography
A leaf holds several photosynthetic pigments, and chromatography separates them so you can identify them. Either paper or thin-layer chromatography (TLC) works the same way.
- Grind leaf tissue with a solvent such as propanone to extract the pigments.
- Put a small, concentrated spot of the extract on a pencil line (the origin) near the bottom of the paper or TLC plate.
- Stand it in a shallow layer of running solvent, below the origin, in a closed container.
- As the solvent rises, each pigment is carried a different distance, depending on how soluble it is in the solvent and how strongly it sticks to the paper or plate.
- Remove it before the solvent reaches the top and mark the solvent front at once, before it evaporates.
Each pigment is identified by its colour and by its Rf value.
Rf = distance moved by the pigment ÷ distance moved by the solvent front, both measured from the origin
Rf has no units and is always between 0 and 1. Figure 1 shows a set of invented results.
| Pigment | Colour | Typical position |
|---|---|---|
| Carotene | orange-yellow | nearest the solvent front: highest Rf |
| Xanthophyll | yellow | between carotene and the chlorophylls in many solvents |
| Chlorophyll a | blue-green | middle |
| Chlorophyll b | yellow-green | below chlorophyll a |
Exact Rf values change with the solvent, the paper and the temperature, so identify by colour and relative position first and use the Rf to confirm. Measure to the centre of each spot.
4Why pigments absorb some wavelengths and not others
Light arrives as packets of energy, and each wavelength carries a different amount: shorter wavelengths (violet, blue) carry more energy than longer ones (red). When a pigment molecule absorbs light, the energy raises one of its electrons to a higher energy level. That is excitation, and the excited electron is the first step in turning light energy into chemical energy.
An electron can only jump between particular energy levels, so a pigment can only absorb light whose energy matches the gap. Light of other wavelengths passes through or is reflected. That is why each pigment absorbs only some wavelengths, and why pigments are coloured: you see the wavelengths they do not absorb.
An absorption spectrum is a graph of how much light a pigment absorbs at each wavelength. Figure 2 shows three. The guide asks for both wavelengths and colours on the horizontal axis.
Chlorophylls a and b absorb strongly in the blue and the red and hardly at all in the green, which is reflected; that is why leaves look green. The carotenoids (carotene and xanthophylls) absorb in the blue and blue-green, filling part of the gap. Pigments other than chlorophyll a are called accessory pigments: they widen the range of wavelengths the plant can use.
5Absorption spectra and action spectra
An action spectrum is a graph of the rate of photosynthesis at each wavelength of light. You make one by shining light of one wavelength at a time on a plant, all at the same intensity, and measuring the rate: oxygen produced or carbon dioxide used per unit time.
Here is how one set of invented measurements is turned into a rate: a pondweed in blue light (450 nm) released 38 mm³ of oxygen in 5 minutes.
Do that for every wavelength, plot rate (or % of maximum) against wavelength, join the points, and you have an action spectrum. Figure 3 places one over the absorption spectrum of all the pigments together.
| Absorption spectrum | Action spectrum | |
|---|---|---|
| What it shows | how much light the pigments absorb | how fast photosynthesis goes |
| How it is measured | light passing through a pigment extract | oxygen made or CO₂ used by a living plant |
| Similarity | peaks in blue and red, trough in green | peaks in blue and red, trough in green |
| Difference | falls close to zero in the green | stays above zero in the green |
The similarity is the evidence that the absorbed light is what drives photosynthesis: the wavelengths absorbed most are the wavelengths that give the highest rates. The differences exist because the action spectrum reflects all the pigments working together, including accessory pigments that pass on energy from wavelengths chlorophyll absorbs poorly, and because not every photon absorbed is used with the same efficiency.
6Limiting factors, and how to investigate them
Three factors control the rate of photosynthesis in a well-watered plant.
- Light intensity: supplies the energy for the light-dependent reactions.
- Carbon dioxide concentration: the substrate that is fixed.
- Temperature: controls the rate of the enzyme-catalysed reactions, especially Rubisco's (section 10).
A limiting factor is the factor that is furthest from its optimum, and so is holding the rate back. Increasing it raises the rate; increasing any other factor does not. Figure 5(a) shows the pattern. At low light intensity the rate rises as light increases: light is limiting. Then the curve levels off, because something else, usually carbon dioxide, has become limiting. Raise the carbon dioxide and the plateau rises. Figure 5(b) shows temperature: a rise to an optimum as enzyme reactions speed up, then a steep fall as enzymes denature.
The experiment (Figure 4). An aquatic plant such as Elodea or Cabomba is placed in water. The dependent variable is the rate of photosynthesis, measured as the volume of oxygen collected per minute, or the number of bubbles counted per minute (quicker, less accurate, since bubbles vary in size). Choose one independent variable and vary it:
| To vary | How |
|---|---|
| Light intensity | move a lamp to different distances; intensity falls as distance increases. Use a heat shield or an LED so the temperature does not change with it. Or use a light meter |
| Carbon dioxide concentration | add different concentrations of sodium hydrogencarbonate solution, which releases carbon dioxide into the water |
| Temperature | stand the tube in a thermostatically controlled water bath at different temperatures |
Everything else is a controlled variable: kept the same because it would also change the rate. For a light experiment that means temperature, carbon dioxide concentration, the same piece of plant, and the time allowed for the plant to adjust before each reading.
Hypotheses. A hypothesis is a provisional explanation that can be tested. "As light intensity increases, the rate of photosynthesis will increase, because more light energy is available for the light-dependent reactions, until another factor becomes limiting" is a good one: it predicts a direction and gives a reason. The guide makes a nature-of-science point: a hypothesis can come first, from theory, and then be tested; or it can come after an experiment, as a way of explaining results already seen. Either way it is provisional, and it needs repeated testing before it is trusted.
7Carbon dioxide enrichment: predicting the future
Atmospheric carbon dioxide is rising, now over 420 parts per million. Because carbon dioxide is often the limiting factor, biologists want to know how much faster crops and forests will photosynthesise and grow as it rises. Carbon dioxide enrichment experiments grow plants at raised concentrations to find out.
Enclosed experiments grow plants in greenhouses or closed chambers with added carbon dioxide. Temperature, water, light and CO₂ concentration are easy to control, but the conditions are artificial: no wind, no normal pests, roots often restricted in pots.
FACE (free-air carbon dioxide enrichment) experiments work in the field. A ring of pipes surrounds a plot of crop or natural vegetation and releases carbon dioxide into the open air, with sensors and the wind direction used to keep the concentration inside the ring at a target level above today's. The results are more realistic, but rainfall, temperature and pests cannot be controlled, and the experiments are expensive. FACE results have generally shown smaller increases in growth than enclosed experiments suggested, which is exactly why field experiments matter.
The nature-of-science point: control of variables is easier in a laboratory, but some questions can only be answered in the field. Be ready to name a controlled variable: in a greenhouse, temperature or water supply; in a FACE study, the plant variety, with control rings identical except for the added carbon dioxide.
8HLPhotosystems
SL students can skip to section 12.
Pigments do not work alone. In the thylakoid membranes of chloroplasts, and in the membranes of cyanobacteria, they are organised into photosystems. A photosystem is a molecular array of hundreds of chlorophyll and accessory pigment molecules, held in precise positions by proteins, around a reaction centre: a special pair of chlorophyll a molecules. Photosystems are always located in membranes. Figure 6 shows the arrangement.
When any pigment in the array absorbs light, the energy is passed from molecule to molecule until it reaches the reaction centre. There, the energy excites an electron so strongly that the reaction centre emits it, handing it to an electron acceptor. That emitted electron is the start of the light-dependent reactions.
Why an array, and not single pigment molecules? A single chlorophyll molecule, however well lit, could not perform any part of photosynthesis. Four reasons:
- It would absorb light only rarely. Hundreds of pigments gather light over a wide area and funnel it to one reaction centre, so the reaction centre is excited many times a second, even in dim light.
- Different pigments absorb different wavelengths, so the array uses more of the spectrum than one pigment could.
- An isolated excited molecule would waste the energy, losing it as heat or light as the electron fell back. In the array, energy is passed on in a controlled way.
- The reaction centre sits next to an electron acceptor and the chain of carriers, so the emitted electron is captured and used. Structure in the membrane is what makes that capture possible.
There are two types, photosystem I (PSI) and photosystem II (PSII), which work in series.
9HLThe light-dependent reactions
These happen in and on the thylakoid membranes. Figure 7 shows the arrangement; follow it as you read.
Photolysis of water at photosystem II. When PSII's reaction centre emits an excited electron, it is left short of one, and it takes a replacement from water. Water is split, on the lumen side of the membrane:
This splitting by light energy is photolysis. The electrons replace those lost by PSII, and the protons (H⁺) add to the proton concentration inside the lumen. Both are used in photosynthesis; the oxygen is a waste product and diffuses out.
Oxygen-producing photosynthesis, which arose in the ancestors of cyanobacteria, had immense consequences. Oxygen accumulated in the oceans and atmosphere, rising sharply about 2.4 billion years ago. Dissolved iron was oxidised and settled out as banded iron formations; an ozone layer formed that shields the land from ultraviolet light; many organisms to which oxygen was toxic died out; and aerobic respiration, with its far greater ATP yield, became possible.
ATP production by chemiosmosis. Electrons emitted by PSII pass along a chain of electron carriers in the membrane to PSI. As in the mitochondrion, energy released as electrons flow along the chain is used to pump protons across the membrane, here from the stroma into the lumen. The lumen is tiny, so the proton concentration inside it rises steeply. Protons flow back down this gradient into the stroma through ATP synthase, which uses the energy to make ATP from ADP and phosphate, in the stroma. Making ATP with light energy is called photophosphorylation.
Reduction of NADP by photosystem I. PSI absorbs light and its reaction centre emits excited electrons too, replaced by the electrons arriving along the chain from PSII. The emitted electrons are passed, on the stroma side, to NADP. NADP accepts two electrons from PSI and one proton from the stroma to become reduced NADP. Keep the pair consistent: "NADP and reduced NADP" (or "NADP⁺ and NADPH", but never a mixture).
Cyclic and non-cyclic photophosphorylation. Figure 8 compares the two routes.
- Non-cyclic: electrons come from water via PSII, pass along the carriers to PSI, and end on NADP. Products: ATP, reduced NADP and oxygen.
- Cyclic: electrons emitted by PSI are passed back to the carriers instead of to NADP, pump more protons, and return to PSI. Only PSI is involved. Product: ATP only; no reduced NADP and no oxygen. The chloroplast uses it when it needs more ATP than reduced NADP.
The thylakoid as a system. Each process has its place: photolysis on the lumen side of PSII; protons pumped into the lumen by the carriers; ATP made by ATP synthase with its active part in the stroma; NADP reduced on the stroma side of PSI. Everything the Calvin cycle needs is released exactly where the Calvin cycle is.
10HLThe Calvin cycle
The light-independent reactions, or Calvin cycle, take place in the stroma and spend the ATP and reduced NADP to turn carbon dioxide into carbohydrate. Figure 9 shows the cycle, counted for one glucose.
1. Carbon fixation by Rubisco. Carbon dioxide combines with RuBP (ribulose bisphosphate, 5C). The product splits at once into two molecules of glycerate 3-phosphate (GP, 3C). The enzyme is Rubisco. It is the most abundant enzyme on Earth, and it needs to be: it works relatively slowly and is not effective at low carbon dioxide concentrations, so the stroma packs in a very high concentration of it.
2. Synthesis of triose phosphate. Each GP is converted into triose phosphate (TP, 3C) using ATP and reduced NADP, both from the light-dependent reactions. This is a reduction: the hydrogen from reduced NADP, which came originally from water, is added here. NADP and ADP return to the thylakoid to be recharged.
3. Regeneration of RuBP. Most of the TP is used to rebuild RuBP, using more ATP, so the cycle can continue. Five TP (15 carbons) are converted into three RuBP (15 carbons). If glucose is the product, count six turns:
Five-sixths of all the TP made must go back into the cycle. Only one-sixth is profit.
4. Everything else is made from the cycle's products. All the carbon in every compound in a photosynthesising organism was fixed in the Calvin cycle. TP and other intermediates of the cycle are the starting points for every other pathway. Glucose is made from TP and becomes sucrose for transport, starch for storage and cellulose for walls. Amino acids are made from cycle intermediates plus nitrogen, absorbed as nitrate or ammonium ions (and, for some, sulfur). Nucleotides need phosphate; fatty acids and glycerol for lipids come from the same carbon. Mineral nutrients from the soil supply the other elements; the carbon always traces back to the Calvin cycle.
11HLThe two stages depend on each other
The light-dependent reactions make ATP and reduced NADP; the Calvin cycle uses them and returns ADP, phosphate and NADP. Neither can keep going alone.
No light. Photosystems are not excited, so no ATP and no reduced NADP are made. GP cannot be converted to TP, RuBP is not regenerated, and the Calvin cycle stops within seconds. (If you measure them, GP builds up and RuBP falls.)
No carbon dioxide. The Calvin cycle stops, so ATP and reduced NADP are no longer used. Soon all the NADP is reduced and there is none left to accept electrons from PSI. Electrons back up along the chain, PSII can no longer pass on the electrons it emits, and PSII stops functioning, so photolysis and oxygen release stop too. A shortage of carbon dioxide stops the "light" reactions even in bright light.
The two stages are one system. The light-dependent reactions supply ATP and reduced NADP; the Calvin cycle returns ADP and NADP. Stop either and both stop.
12Where marks are lost
"The oxygen comes from carbon dioxide." It comes from water, split by photolysis. This is the single most tested fact in the subtopic.
"Plants absorb green light." Chlorophyll reflects green light; that is why leaves look green. Green light is absorbed least.
Measuring Rf from the bottom of the paper, or to the edge of the spot. Measure from the origin line, to the centre of the spot, and divide by the distance from the origin to the solvent front.
Confusing the two spectra. An absorption spectrum is about pigments absorbing light; an action spectrum is about a plant's rate of photosynthesis. Name which one a graph is before describing it.
Moving the lamp without controlling temperature. A lamp close to the plant heats it, so temperature changes along with light intensity and the result is confounded. Use a heat shield or an LED, and say so.
HL · Protons pumped into the stroma, or ATP made in the lumen. Protons are pumped into the lumen and flow back out to the stroma through ATP synthase; ATP and reduced NADP are both produced in the stroma, where the Calvin cycle is.
HL · Calling the Calvin cycle "the dark reaction" and saying it happens at night. It does not need light directly, but it stops within seconds without the products of the light-dependent reactions, so in practice it runs in the light.
13Draw it right
- Chromatogram: origin drawn in pencil, solvent below the origin, solvent front marked, each spot labelled, Rf distances measured from the origin to the spot's centre.
- Absorption or action spectrum: wavelength in nm on the x-axis with the colours marked, absorption or rate on the y-axis, peaks in the blue and the red, a trough in the green.
- Action spectrum from data: points plotted at each wavelength, joined, axes labelled with units; rate as volume per unit time or % of maximum.
- Limiting factor graph: rate rises, then plateaus; a second curve at higher CO₂ follows the same line at first and then plateaus higher. Label which factor limits each part.
- HL photosystem: many pigment molecules in a membrane around one reaction centre, energy arrows pointing inwards, an electron leaving to an acceptor.
- HL thylakoid: PSII, carriers, PSI and ATP synthase in the membrane; photolysis in the lumen; protons pumped into the lumen; ATP and reduced NADP on the stroma side.
- HL Calvin cycle: RuBP (5C) + CO₂ → GP (3C) by Rubisco; GP → TP using ATP and reduced NADP; TP → RuBP using ATP; an arrow showing some TP leaving.
14Try it
Marks in brackets. Answers and marker's notes are at the end.
Q1. What is the source of the oxygen released during photosynthesis? 1 mark
A. Carbon dioxide
B. Water
C. Glucose
D. RuBP
Q2. A student separated the pigments from spinach leaves by paper chromatography. The solvent front moved 75 mm from the origin. An orange-yellow spot moved 70 mm, a blue-green spot 45 mm and a yellow-green spot 18 mm. (Invented data.)
(a) Calculate the Rf value of the blue-green spot. 1 mark
(b) Identify the pigments in the orange-yellow and yellow-green spots. 2 marks
Q3. A pondweed was lit with light of six wavelengths in turn, at equal intensity. The oxygen released in 5 minutes was collected at each. (Invented data.)
| Wavelength (nm) | 425 | 475 | 525 | 575 | 625 | 675 |
|---|---|---|---|---|---|---|
| Oxygen collected in 5 min (mm³) | 60 | 55 | 20 | 15 | 35 | 65 |
| Rate (mm³ min⁻¹) | 12 | 11 | 4.0 | 3.0 | 7.0 |
(a) Calculate the rate of oxygen production at 675 nm. 1 mark
(b) Describe the relationship between wavelength and rate shown by the data. 2 marks
(c) Explain why the rate at 525 nm and 575 nm is lower than at the other wavelengths, but not zero. 2 marks
(d) State one variable, other than light intensity, that should be controlled. 1 mark
Q4. A student investigates the effect of carbon dioxide concentration on photosynthesis in pondweed, using sodium hydrogencarbonate solutions of 0.0, 0.5, 1.0, 1.5 and 2.0%.
(a) Identify the independent and the dependent variable. 2 marks
(b) Suggest a hypothesis for this experiment, with a reason. 2 marks
(c) Explain why the rate may stop increasing above 1.5%. 2 marks
Q5 (HL). Explain how the light-dependent reactions produce ATP and reduced NADP. 7 marks
Q6 (HL). (a) State the substrates and the product of the reaction catalysed by Rubisco. 2 marks
(b) Explain why a lack of carbon dioxide stops photosystem II from functioning. 3 marks
15In one breath
Photosynthesis transforms light energy into chemical energy in carbon compounds: carbon dioxide + water → glucose + oxygen, with the hydrogen and the oxygen both from water, the oxygen a by-product. Chromatography separates pigments; Rf is pigment distance over solvent distance from the origin; identify by colour and Rf. Pigments absorb only wavelengths whose energy matches an electron's jump; chlorophylls absorb blue and red and reflect green; accessory pigments widen the range. The action spectrum (rate against wavelength) matches the absorption spectrum in shape, which shows absorbed light drives photosynthesis. Light, CO₂ and temperature can each be limiting; vary one, measure oxygen per minute, control the rest; hypotheses are provisional and tested repeatedly. FACE is more realistic than enclosed enrichment but harder to control. HL: photosystems are arrays of pigments in membranes that funnel energy to a reaction-centre chlorophyll, which emits an excited electron. PSII's electrons are replaced by photolysis of water, releasing oxygen as waste; carriers pump protons into the lumen and ATP synthase makes ATP as they flow back; PSI's electrons reduce NADP with a proton from the stroma; cyclic flow makes ATP only. Rubisco fixes CO₂ to RuBP making GP; ATP and reduced NADP turn GP into TP; five-sixths of TP regenerates RuBP; all carbon compounds trace back to the cycle. Without light the cycle stops; without CO₂, NADP is not regenerated and PSII stops.
Answers
Q1. B. Oxygen is released when water is split by photolysis. B only.
Q2. (a) 45 ÷ 75 = 0.60. no units; 0.6 accepted. (b) Orange-yellow (Rf 0.93): carotene. Yellow-green (Rf 0.24): chlorophyll b. 1 each. Identifying by colour or by position is accepted.
Q3. (a) 65 ÷ 5 = 13 mm³ min⁻¹. value and unit. (b) The rate is high in blue light (425 to 475 nm), falls to a minimum in green and yellow light (525 to 575 nm), and rises again to its highest value in red light (675 nm). 1 for high at both ends, 1 for the minimum in the middle with wavelengths quoted. (c) Chlorophyll absorbs little green and yellow light, reflecting most of it, so less light energy is absorbed and the rate is lower. The rate is not zero because accessory pigments such as carotenoids absorb some light at these wavelengths, and chlorophyll absorbs a small amount, and the energy is passed on to be used. 1 for little absorption by chlorophyll, 1 for accessory pigments or some absorption explaining the non-zero rate. (d) Any one: temperature; carbon dioxide concentration; the same plant or the same mass of plant; time allowed to adjust at each wavelength. [1.]
Q4. (a) Independent: concentration of sodium hydrogencarbonate (carbon dioxide concentration). Dependent: rate of photosynthesis, measured as volume of oxygen or number of bubbles per minute. 1 each; "oxygen" alone without "per unit time" scores 0 for the second mark. (b) As carbon dioxide concentration increases, the rate of photosynthesis will increase, because more carbon dioxide is available for fixation in the Calvin cycle, until another factor becomes limiting. 1 for a testable prediction with direction, 1 for a reason. (c) Another factor, such as light intensity or temperature, has become limiting, so extra carbon dioxide cannot increase the rate. Alternatively, Rubisco or the rate of the light-dependent reactions has reached its maximum. 1 for another factor now limiting, 1 for naming a factor.
Q5 (HL). Model answer, one idea per mark point: Light is absorbed by pigments in photosystem II and the energy is passed to the reaction centre, which emits an excited electron. The electrons lost are replaced by photolysis of water, which releases protons into the lumen and oxygen as a waste product. The excited electrons pass along a chain of carriers in the thylakoid membrane, releasing energy. This energy is used to pump protons from the stroma into the thylakoid lumen, building a proton gradient. Protons flow back into the stroma through ATP synthase, which uses the energy to produce ATP from ADP and phosphate (chemiosmosis). Photosystem I absorbs light and emits excited electrons, replaced by those from the chain. These electrons reduce NADP, which also takes a proton from the stroma, forming reduced NADP. 1 each for PSII excitation and electron emission; photolysis replacing electrons, releasing protons and oxygen; electrons along carriers; protons pumped into the lumen; a proton gradient; ATP synthase making ATP as protons return; PSI emitting electrons that reduce NADP with a proton from the stroma. Maximum 7. Protons pumped into the stroma loses the pumping mark.
Q6 (HL). (a) Substrates: RuBP and carbon dioxide. Product: glycerate 3-phosphate (GP). 1 for both substrates, 1 for the product. (b) Without carbon dioxide the Calvin cycle stops, so reduced NADP (and ATP) are no longer used. NADP is not regenerated, so photosystem I has no acceptor for its electrons. Electrons back up along the carrier chain, so photosystem II cannot pass on its excited electrons and stops, and photolysis stops. 1 for the Calvin cycle stopping so reduced NADP is not used, 1 for no NADP to accept electrons from PSI, 1 for the backlog reaching PSII.
Educerie · written from the published IB Diploma Programme Biology guide, first assessment 2025, section C1.3 Photosynthesis. Original text, examples and questions. Diagrams drawn by Educerie. Last reviewed 25 September 2026.
Check your understanding
The main ideas of this note. Tick each one you could do now, in an exam, without looking back up. Anything you cannot tick yet is the part to read again.