The overall reaction of photosynthesis
This unit is A Level (IA2) content, and it assumes everything from the AS units, especially carbohydrates from the 1a notes, enzymes from the 2a notes and plant cells from the 4a notes.
It is examined on the Unit 4 paper, WBI14, together with the rest of Topics 5 and 6; its core practical, CP10 (the rate of photosynthesis in an aquatic plant), is also examined as a method on the Unit 6 practical paper, WBI16.
This first section gives you the big picture that every later section fills in.
The idea: storing hydrogen in a fuel
Every living thing needs two things from its food: energy, and carbon atoms to build new molecules.
A plant gets neither by eating.
It has sunlight, which is abundant but cannot be stored, and it has water and carbon dioxide, which hold almost no useful energy.
Photosynthesis is how the plant turns the first into a store built from the other two.
Learn this precise description of the reaction word for word: photosynthesis requires energy from light to split apart the strong bonds in water molecules, storing the hydrogen in a fuel (glucose) by combining it with carbon dioxide and releasing oxygen into the atmosphere.
Read it as three linked steps.
1. Splitting water needs energy. The bonds between hydrogen and oxygen in a water molecule are strong. Breaking them is "uphill": energy must be put in, and light supplies it. That is why the reaction stops in the dark.
2. The hydrogen is stored in a fuel. The hydrogen taken from water is added to carbon dioxide. Adding hydrogen to a molecule is reduction, and a reduced carbon compound such as glucose is a fuel: it holds chemical energy that can be released later.
3. Oxygen is the leftover. Once the hydrogen has been taken from water, the oxygen atoms are left. They join in pairs as O₂ and diffuse out of the leaf into the atmosphere.
The balanced equation sums all of this up:
The equation is only a summary.
Glucose is not made in one step, and glucose is not even the first sugar made: the first product that leaves the cycle is a three-carbon sugar called GALP, from which glucose is built.
The later sections show the steps.
Two stages, one handover
Photosynthesis runs as two linked stages in the chloroplast, shown below.
Stage
Where
Needs
Gives
Light-dependent reactions
thylakoid membranes
light energy, water, ADP + Pi, NADP
oxygen (by-product), ATP, reduced NADP
Light-independent reactions (Calvin cycle)
stroma
carbon dioxide, ATP, reduced NADP
GALP (then glucose), ADP + Pi, NADP
The two stages map straight onto that description.
The light-dependent reactions are where light energy splits water (photolysis) and releases oxygen; the energy is caught in ATP, and the hydrogen is carried away on the coenzyme NADP as reduced NADP.
The light-independent reactions are where that hydrogen is "stored in a fuel": ATP and reduced NADP are used to reduce carbon dioxide.
Neither stage can run for long without the other, because each one regenerates what the other needs.
Why it matters: the mirror of respiration
Respiration (the 7a notes) does the reverse job.
It splits glucose, releases carbon dioxide, and reunites the hydrogen with atmospheric oxygen to form water, releasing the stored energy.
So the hydrogen is taken from water by light in the chloroplast and handed back to oxygen in the mitochondrion.
The total energy stored in one mole of glucose by photosynthesis (about 2870 kJ) is the same as the total energy released when it is completely respired.
This one reaction underpins almost every food chain on Earth (the productivity sections at the end of this unit), it put the oxygen into the atmosphere, and it removes carbon dioxide, which links it to the carbon cycle and climate change in the 5b notes.
Going further (not examined): the oxygen released comes from water, not from carbon dioxide.
In 1941 Ruben and Kamen gave algae water labelled with the heavy oxygen isotope ¹⁸O and found the label in the oxygen given off; when the label was in the carbon dioxide instead, the oxygen released was not labelled.
- Each marking point is worth one mark, shown as (1) in these notes. A header in a scheme sets the cap, for example "an explanation that includes three of the following points", and the list usually holds more points than marks.
- A "/" separates alternative wordings for one point. ACCEPT, IGNORE and DO NOT ACCEPT lines widen a point, mark words that neither gain nor lose, and name wording that loses it. "ACCEPT converse" means the same idea argued the other way round also scores.
- "DO NOT PIECE TOGETHER" means a point must be complete in one statement; an OR between two sets of points gives two routes to full marks.
- WBI14 prints no "first answer only" rule: a wrong answer next to a right one can cancel it. Give exactly as many answers as asked.
- In a calculation a correct answer alone gains full marks, but working earns the first mark when the answer is wrong, and an error is carried forward within a part. A stated format (standard form, two significant figures, a whole number, a ratio "... : 1") is worth the answer mark by itself.
- The one or two asterisked 6-mark items on WBI14 are marked in three levels; within a level the mark is set by how many points you make and how many you extend in the context of the question.
WBI14 prints no formula sheet, and none of these has been printed in a WBI14 stem, so learn them all:
- magnification = image size ÷ actual size (the chloroplast section)
- Rf = distance moved by the pigment from the origin ÷ distance moved by the solvent front from the origin (the chromatography section)
- volume of gas in a capillary tube = πr²l (core practical 10)
- light intensity ∝ 1 ÷ d² (core practical 10)
- NPP = GPP − R (the productivity section)
- efficiency of transfer (%) = energy (or biomass) transferred to the next trophic level ÷ energy (or biomass) in the previous trophic level × 100 (the last section)
- percentage change = change ÷ original value × 100
A formula that WBI14 does print when it is used (a sphere's surface area, the growth rate constant) is not in this unit.
Learn the first line word for word.
- The overall reaction of photosynthesis: requiring energy from light to split apart the strong bonds in water molecules, storing the hydrogen in a fuel (glucose) by combining it with carbon dioxide and releasing oxygen into the atmosphere.
- Reduction: the gain of hydrogen (or electrons) by a molecule; carbon dioxide is reduced when hydrogen from water is added to it.
- Photolysis: the splitting of water using light energy, giving protons, electrons and oxygen.
1a notes: glucose as a hexose; starch as a store; condensation reactions.
2a notes: enzymes, active sites and the effect of temperature (RUBISCO and every Calvin cycle step are enzyme-catalysed); active transport needs ATP.
2b notes: nucleotides (ATP is a nucleotide).
4a notes: the chloroplast in the plant cell; starch and cellulose; nitrate and magnesium ions.
WBI14 papers have set 8 to 17 marks a paper on AS content, often inside photosynthesis questions.
A biology magazine says that "plants turn carbon dioxide into oxygen".
Explain why this description is wrong, using the overall reaction of photosynthesis.
(3)
Full-mark answer:
- energy from light is used to split the bonds in water molecules (1)
- the oxygen released into the atmosphere comes from the water / not from carbon dioxide (1)
- the hydrogen from water is combined with carbon dioxide to form glucose / the carbon dioxide becomes part of a fuel (1)
A weaker answer: "Plants take in carbon dioxide and give out oxygen in photosynthesis, which is the opposite of us."
This scores 0.
It restates the claim and never says where the oxygen comes from or what happens to the carbon dioxide.
Invented data: a green alga was grown in two flasks.
In flask 1 the water contained the heavy isotope ¹⁸O; in flask 2 the carbon dioxide contained it.
The oxygen gas released was collected.
Flask
¹⁸O in the water / %
¹⁸O in the carbon dioxide / %
¹⁸O in the oxygen released / %
1
0.85
0.20
0.84
2
0.20
0.68
0.21
Explain what these results show about the source of the oxygen released in photosynthesis.
(3)
- in each flask, the ¹⁸O in the oxygen released matches the ¹⁸O in the water: 0.84 % with 0.85 %, and 0.21 % with 0.20 % (1)
- it does not match the carbon dioxide (0.21 % released when the carbon dioxide held 0.68 %) (1)
- so the oxygen comes from the splitting of water (photolysis), and the oxygen atoms of carbon dioxide end up in the sugar (and in water made in the Calvin cycle) (1)
A weaker answer: "Flask 1 has more ¹⁸O in the oxygen than flask 2."
This is true but scores 0: it describes one difference and draws no conclusion.
An Explain on data needs the comparison and the biological reason.
A pondweed releases 360 cm³ of oxygen in one hour.
One mole of gas occupies 24 dm³ at room temperature, and the relative molecular mass of glucose is 180.
(a) Calculate the mass of glucose made in that hour, in mg. Give your answer to two significant figures. (2)
(b) Making one mole of glucose stores 2870 kJ. Calculate the energy stored in this glucose, in kJ. (1)
1. Moles of oxygen: mol; moles of glucose: mol (1)
2. Mass: g 450 mg (1)
3. (b) Energy: , so 7.2 kJ (1)
Writing 0.45 (no unit, in g) when mg was asked, or 450.0 when two significant figures were asked, costs the answer mark.
What the number means: this is the glucose made beyond what the plant respired in that hour, because some oxygen made in photosynthesis is used straight away in the plant's own respiration.
The productivity section calls the total "gross" and the surplus "net".
When a question asks for the overall reaction "in terms of energy", use this chain in order: light energy, splitting the strong bonds in water, hydrogen stored in a fuel with carbon dioxide, oxygen released.
Each link is a separate marking point, and "plants make glucose from carbon dioxide and water" alone earns at most one.
- "The oxygen comes from carbon dioxide." It is tempting because the equation shows CO₂ going in and O₂ coming out, but the oxygen gas comes from water. The carbon dioxide's oxygen atoms end up in the sugar.
- "Photosynthesis produces energy." Energy is never produced. Light energy is transferred to chemical energy stored in glucose. Write "transfers" or "stores".
- "Plants photosynthesise instead of respiring." Plant cells respire all the time, in the light and the dark. In bright light the rate of photosynthesis is simply higher, so there is a net release of oxygen.
- "Glucose is made directly from carbon dioxide." The first sugar to leave the Calvin cycle is GALP, a three-carbon sugar; glucose is built from two GALP.
Check your understanding
1. Which describes the source of the oxygen gas released during photosynthesis? (1)
- A carbon dioxide taken in through the stomata
- B glucose made in the stroma
- C water molecules split using light energy
- D oxygen stored in starch grains
Total [1]
2. Which row shows what is used and what is made by the light-dependent reactions? (1)
Used
Made
A
water, ADP, NADP
oxygen, ATP, reduced NADP
B
carbon dioxide, ATP, reduced NADP
GALP, ADP, NADP
C
water, ATP, reduced NADP
oxygen, ADP, NADP
D
carbon dioxide, ADP, NADP
glucose, ATP, reduced NADP
Total [1]
3. A student writes: "Photosynthesis is just respiration in reverse, so a plant that photosynthesises does not need to respire." Explain two reasons why the student is wrong. (2)
Total [2]
4. The sea slug Elysia chlorotica eats a green alga. It keeps the alga's chloroplasts working inside the cells of its gut for several months. Slugs kept in a lit tank with no food lost mass more slowly than slugs kept in a dark tank with no food. Suggest an explanation for this result. (3)
Total [3]
5. A seaweed in a sealed tank released 1.2 dm³ of oxygen in a day. One mole of gas occupies 24 dm³ and the relative molecular mass of glucose is 180.
(a) Calculate the mass of glucose this represents. Give your answer in g to two significant figures. (2)
(b) The seaweed's dry mass increased by less than this mass of glucose. Give two reasons why. (2)
(c) State what happens to the carbon dioxide concentration in the tank water during the day. (1)
Total [5]
6. Describe how the overall reaction of photosynthesis stores energy from light in glucose. (3)
Total [3]
Answers
1. C (1). A is incorrect because the oxygen of carbon dioxide ends up in the sugar. B is incorrect because glucose is a product, not a source of oxygen gas. D is incorrect because starch is a store of glucose and releases no oxygen.
2. A (1). B is incorrect because it describes the light-independent reactions. C is incorrect because the light-dependent reactions make ATP and reduced NADP; they do not use them. D is incorrect because carbon dioxide is not used, and glucose is not made, in the light-dependent reactions.
3. Any two from (1 each):
- plant cells respire all the time, including in the dark, when there is no photosynthesis
- not all cells photosynthesise (roots, for example, have no chloroplasts) but all cells respire
- respiration releases energy from glucose to make ATP for processes such as active transport and protein synthesis; ATP from the light-dependent reactions is used in the Calvin cycle
- the two processes use different enzymes and pathways in different organelles
4. Any three from (1 each):
- the chloroplasts carry out photosynthesis in the light / not in the dark
- making sugars (glucose) inside the slug's cells
- the slug respires these sugars, so less of its own stored fat or protein is used
- in the dark, the slug must respire its own tissues, so it loses mass faster (Twist: an animal benefiting from photosynthesis. The stock answer "animals cannot photosynthesise" is wrong here.)
5. (a) moles of oxygen ; moles of glucose (1); mass g (1).
(b) Any two from (1 each): some glucose is used in the seaweed's respiration; carbon dioxide and water are given off, so mass is lost; part of the glucose is turned into other molecules and lost, for example in leaves that are shed; some sugar is used to make other molecules that leak out (for example mucilage).
(c) It falls / decreases (1).
6. Any three from (1 each):
- light energy is absorbed by chlorophyll
- it is used to split the strong bonds in water molecules (photolysis)
- the hydrogen (protons and electrons) is used to reduce carbon dioxide
- the hydrogen is stored in a fuel, glucose, as chemical energy
- oxygen is released into the atmosphere as a by-product