The shape of the lesson
By the end of the lesson, learners can
- A chloroplast, part by part: Rotate the chloroplast and click each structure: the envelope, the grana, the lamellae, the stroma, the starch grains, the lipid droplets, the ribosomes and the loop of DNA.
- Pigments and the two spectra: Drag the wavelength line from 400 nm to 700 nm.
- Separating pigments: chromatography: Press Run the solvent and watch it rise up the paper, carrying the pigments different distances.
- Thylakoids (grana): light-dependent. Stroma: Calvin cycle.
- Absorption spectrum = absorbance of a pigment; action spectrum = rate.
- Accessory pigments widen the range of wavelengths used.
- Rf = pigment distance ÷ solvent distance, from the origin.
One picture every idea returns to
Photosynthesis transfers light energy into chemical energy. In the light-dependent stage, on the thylakoid membranes (stacked into grana), light energy is used to make ATP and reduced NADP. In the light-independent stage (the Calvin cycle) in the stroma, that ATP and reduced NADP are used to turn carbon dioxide into complex organic molecules. Light is absorbed by chloroplast pigments in the thylakoids: chlorophyll a (the primary pigment) and the accessory pigments chlorophyll b, carotene and xanthophyll, which widen the range of wavelengths used. An absorption spectrum shows how much light a pigment absorbs at each wavelength; an action spectrum shows the rate of photosynthesis at each wavelength. The pigments can be separated by chromatography and identified by their Rf values.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Where light becomes chemistry.. Photosynthesis is an energy transfer process: light energy ends up as chemical energy in organic molecules. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: A chloroplast, part by part | Rotate the chloroplast and click each structure: the envelope, the grana, the lamellae, the stroma, the starch grains, the lipid droplets, the ribosomes and the loop of DNA. Project tab 1 of the lab (Photosynthesis lab, the chloroplast in 3D) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Pigments and the two spectra | Drag the wavelength line from 400 nm to 700 nm. Project tab 2 of the lab (Photosynthesis lab, pigments and spectra) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Separating pigments: chromatography | Press Run the solvent and watch it rise up the paper, carrying the pigments different distances. Project tab 3 of the lab (Photosynthesis lab, chromatography of pigments) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 18 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Separating pigments: chromatography, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Learn the sites exactly: the thylakoids (membranes and spaces, in grana) are the site of the light-dependent stage; the stroma is the site of the light-independent stage. | The light-independent stage is not "the dark reaction": it needs a constant supply of products from the light-dependent stage, so it stops soon after the light goes off. |
| An R f value only identifies a pigment if it is compared with known values for the same solvent and the same paper or plate. | Change the solvent and every R f changes. Measure both distances from the origin line, not from the bottom of the paper. |
Chloroplasts and pigments: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
State the site within the chloroplast of the light-dependent stage and of the light-independent stage of photosynthesis.
Describe how the structure of a chloroplast, as seen in an electron micrograph, is related to its function.
Explain why photosynthesis is described as an energy transfer process.
An absorption spectrum for chlorophyll a has peaks at 430 nm and 662 nm. The action spectrum for the same leaf shows a high rate from 430 nm to 490 nm and a low rate from 520 nm to 600 nm. Interpret these data.
On a chromatogram the solvent front is 9.5 cm from the origin. Spot X is 6.2 cm and spot Y is 4.3 cm from the origin. Known Rf values in this solvent are: carotene 0.95, xanthophyll 0.71, chlorophyll a 0.65, chlorophyll b 0.45. Identify X and Y.
A student's chromatogram shows the pigments as one long smear with no separate spots. Suggest three improvements to the method.
Answer key and marking notes
- Light-dependent stage: the thylakoids (thylakoid membranes and thylakoid spaces), which are stacked to form grana.
- Light-independent stage (Calvin cycle): the stroma.
- Many thylakoid membranes stacked into grana give a large surface area for photosystems / pigments, electron carriers and ATP synthase.
- The thylakoid space is small, so a proton gradient builds up quickly for ATP synthesis.
- The stroma contains the enzymes of the Calvin cycle (such as rubisco), close to the thylakoids that supply ATP and reduced NADP.
- Circular DNA and 70S ribosomes make some chloroplast proteins; starch grains and lipid droplets store products.
- Light energy is absorbed by chloroplast pigments in the thylakoids.
- In the light-dependent stage it is transferred to chemical energy in ATP and reduced NADP.
- In the Calvin cycle the energy in ATP and reduced NADP is used to make complex organic molecules (such as triose phosphate, then sugars) from carbon dioxide.
- The action spectrum is high where chlorophyll a absorbs strongly (blue), so absorbed light is used for photosynthesis.
- The high rate up to 490 nm, beyond the chlorophyll a peak, is due to accessory pigments (carotene, xanthophyll, chlorophyll b) absorbing these wavelengths and passing the energy to chlorophyll a.
- The rate is low from 520 to 600 nm because little green / yellow light is absorbed by any pigment; it is reflected or transmitted.
- Rf of X = 6.2 ÷ 9.5 = 0.65, so X is chlorophyll a.
- Rf of Y = 4.3 ÷ 9.5 = 0.45, so Y is chlorophyll b.
- Both distances are measured from the origin line to the centre of the spot / to the solvent front.
- Make the origin spot small and concentrated: apply many small drops, drying between each.
- Make sure the solvent level is below the origin, so the pigments do not dissolve into the solvent in the jar.
- Cover the jar so the atmosphere is saturated with solvent vapour; or use a thin-layer plate / a more suitable non-polar solvent for better separation; or remove the paper before the solvent reaches the top.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which part of a chloroplast contains the enzyme rubisco?
- Athe stroma
- Bthe thylakoid membrane
- Cthe thylakoid space
- Dthe intermembrane space of the envelope
Which pigment is found at the reaction centre of each photosystem and is described as the primary pigment?
- Achlorophyll a
- Bchlorophyll b
- Ccarotene
- Dxanthophyll
In an electron micrograph of a leaf cell, which feature would identify an organelle as a chloroplast rather than a mitochondrion?
- Astacks of flattened membrane sacs (grana) inside a double membrane
- Ba double membrane surrounding the whole organelle
- Csmall 70S ribosomes and a loop of DNA in the matrix
- Dan inner membrane folded to form a large surface area
Why do most leaves appear green?
- Achloroplast pigments absorb little green light, so most of it is reflected or transmitted
- Bchlorophyll absorbs green light most strongly and re-emits it
- Cgreen light is used to split water and its colour is released
- Dcarotene and xanthophyll absorb all wavelengths except green and red
The figure shows a chromatogram of a leaf extract. Calculate the Rf value of spot R.
- A0.55
- B1.83
- C0.45
- D6.8
Known Rf values in the solvent used for the chromatogram in the figure are: carotene 0.95, xanthophyll 0.71, chlorophyll a 0.55, chlorophyll b 0.35. Which pigment is spot S?
- Achlorophyll b
- Bchlorophyll a
- Cxanthophyll
- Dcarotene
A student plots the absorption spectrum of an extract of chloroplast pigments and the action spectrum of the same plant. At 480 nm the action spectrum is high but chlorophyll a absorbs very little. What is the best conclusion?
- Alight absorbed by accessory pigments at 480 nm is used in photosynthesis
- Bchlorophyll a absorbs strongly at 480 nm inside the chloroplast but not in solution
- Cphotosynthesis at 480 nm does not need light to be absorbed by any pigment
- Dthe action spectrum must be wrong, because only chlorophyll a can drive photosynthesis
Plants grown in dim light under a forest canopy often have more thylakoids per granum than plants of the same species grown in full sun. Suggest the advantage.
- Amore membrane for pigments and photosystems, so more of the scarce light is absorbed
- Bmore stroma volume for rubisco, so carbon dioxide is fixed faster
- Ca thicker chloroplast that reflects more light back into the leaf
- Dless proton leakage, so fewer protons are needed to make each ATP
Quiz key
Rubisco catalyses carbon fixation in the Calvin cycle, which takes place in the stroma. The thylakoid membranes hold the photosystems, electron carriers and ATP synthase of the light-dependent stage. The thylakoid space is where protons accumulate, not where carbon is fixed. The gap between the two envelope membranes has no role in carbon fixation.
Chlorophyll a at the reaction centre emits the excited electrons that drive the light-dependent stage. Chlorophyll b, carotene and xanthophyll are accessory pigments: they absorb other wavelengths and transfer the energy towards chlorophyll a, but are not themselves the reaction centre.
Grana, stacks of thylakoids, are found only in chloroplasts. Both chloroplasts and mitochondria have a double membrane (envelope), 70S ribosomes and circular DNA, so these do not distinguish them. A folded inner membrane forming cristae is a mitochondrial feature; in chloroplasts the extra membranes are separate thylakoids, not folds of the inner envelope membrane.
All four pigments absorb mainly blue and red light; absorbance is low between about 500 and 600 nm, so green light reaches our eyes. A pigment appears the colour it does NOT absorb, so absorbing green strongly would make the leaf look red-purple. Photolysis does not depend on green light. Carotene and xanthophyll absorb mainly blue and blue-green light and appear orange and yellow; they do not absorb red.
Rf = 8.2 ÷ 15.0 = 0.55 (2 d.p.). 1.83 divides the solvent distance by the pigment distance; an Rf can never exceed 1. 0.45 is 1 minus the correct value, as if measured down from the solvent front. 6.8 is the difference between the two distances, not a ratio.
S is the lowest spot, so it has the smallest Rf (it moved about 5.2 cm of the 15.0 cm run, Rf about 0.35): chlorophyll b. Chlorophyll a is spot R (0.55). Xanthophyll is Q (0.71) and carotene, the most soluble in the non-polar solvent, is P near the front (0.95). Identifying a pigment by colour alone is less reliable than calculating Rf.
Carotene, xanthophyll and chlorophyll b absorb strongly near 450-490 nm and pass the energy to chlorophyll a at the reaction centre, so these wavelengths still drive photosynthesis. Claiming chlorophyll a behaves differently in the cell is not supported by the data. Light must be absorbed to have any effect on photosynthesis. Accessory pigments widen the range of usable wavelengths, so the action spectrum is not wrong.
Thylakoid membranes carry the pigments and photosystems, so more thylakoids give more light-harvesting capacity when light is limiting. Extra thylakoids take up space rather than adding stroma, and rubisco is not the limit in dim light. Thylakoids absorb, not reflect, the light that matters. The number of protons per ATP is set by ATP synthase, not by how many thylakoids there are.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.