Ch 1 · 13.1 Photosynthesis as an energy transfer process · AS & A Level Biology 9700
Topic 1 of 2 in Photosynthesis — AS & A Level Biology 9700 — 68 lessons.
Introduction
Photosynthesis converts light energy into the chemical energy of organic molecules. It occurs in two linked stages inside the chloroplast: light-dependent reactions on the thylakoid membranes, which trap light energy as ATP and reduced NADP, and light-independent reactions in the stroma, which use these products to fix carbon dioxide into carbohydrate. The structure of the chloroplast keeps these two stages physically close but biochemically distinct. (Syllabus section 13.1.)
Key ideas — Chloroplast structure and function
🧑🏫 Teacher's explanation
Think of a chloroplast like a highly efficient factory. The thylakoids are the specialized power-generation floor where solar energy is captured to create 'energy currency' (ATP and reduced NADP). The stroma is the assembly line floor where these energy currencies are spent to build complex products like sugar. To keep the factory running, the power floor must be physically organized into stacks (grana) to maximize the surface area for solar panels (pigments), and the assembly line must be right next to the power floor so the energy doesn't dissipate during transport.
📖 Definitions
🔍 Worked example — Easy
A pigment travels 3.5 cm from the origin on a chromatography paper, while the solvent front travels 14.0 cm. Calculate the R_f value of the pigment.
🔍 Worked example — Standard
Compare the products of cyclic and non-cyclic photophosphorylation.
🔍 Worked example — Exam-style
Describe the role of Rubisco in the Calvin cycle.
⚠️ Common errors
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🎯 Exam question
Key ideas — Chloroplast pigments and spectra
Pigments in the thylakoid membranes absorb light energy and pass it to the reaction centre of a photosystem.
🧑🏫 Teacher's explanation
Think of a solar panel array on a roof. A single large panel might only be efficient at catching sunlight when the sun is directly overhead. However, if you use different types of materials—some that catch morning light, some that catch midday light, and some that catch evening light—you maximize the total energy captured. This is exactly what accessory pigments do for a plant.
📖 Definitions
🔍 Worked example — Easy
A pigment is separated using chromatography. The distance moved by the pigment is 2.4 cm, and the distance moved by the solvent front is 3.0 cm. Calculate the R_f value.
🔍 Worked example — Standard
Compare the absorption spectra of chlorophyll a and carotene. Describe how they differ in terms of wavelength absorption.
🔍 Worked example — Exam-style
An action spectrum for photosynthesis shows a peak at 680 nm and a minimum at 550 nm. Explain why the rate of photosynthesis is low at 550 nm in terms of pigment absorption.
⚠️ Common errors
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🎯 Exam question
(a) Calculate the R_f values for both pigments. (b) If the pigment with R_f = 0.8 is known to be chlorophyll a, suggest why the other pigment might be a carotenoid. [3 marks]
Key ideas — Separating pigments by chromatography
Grind a leaf with a little sand and propan-2-one (acetone) to extract the pigments. Using a fine capillary tube, put a small, concentrated spot of extract on a pencil line about 1 cm above the bottom of chromatography paper (or a thin-layer chromatography strip), letting each drop dry before adding the next. Stand the paper in a little solvent, with the spot above the solvent level, and cover. As the solvent rises it carries each pigment a different distance, depending on how soluble it is in the solvent and how strongly it sticks to the paper. Mark the solvent front before it dries.
🧑🏫 Teacher's explanation
Imagine a crowd of people walking through a shopping mall. Some people are very motivated and walk quickly to the exit (highly soluble pigments), while others stop frequently to look at shop windows (strongly adsorbed to the paper). The speed at which they move depends on how much they 'like' the mall (the solvent) versus how much they 'like' the shops (the paper).
📖 Definitions
🔍 Worked example — Easy
A pigment spot moved 3.0 cm from the origin, and the solvent front moved 12.0 cm from the origin. Calculate the R_f value.
🔍 Worked example — Standard
A student performs chromatography on leaf extract. The solvent front is at 8.0 cm. The pigments found are: Pigment A (6.0 cm), Pigment B (4.0 cm), and Pigment C (1.0 cm). Calculate the R_f values for all three pigments.
🔍 Worked example — Exam-style
A student extracts pigments from a leaf. The solvent front is at 10.0 cm. The student identifies a pigment with an R_f of 0.92. Based on the typical order of migration (carotene arrow xanthophyll arrow chlorophyll a arrow chlorophyll b), identify the most likely pigment.
⚠️ Common errors
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🎯 Exam question
(a) Calculate the R_f value for this pigment. [1] (b) The student identifies this pigment as chlorophyll a. State the relationship between the R_f value and the solubility of a pigment in the solvent. [1] [2 marks]
Key ideas — The light-dependent stage
The light-dependent reactions occur in the thylakoid membrane and convert light energy into chemical energy.
🧑🏫 Teacher's explanation
Think of the chloroplast as a solar-powered factory. The thylakoids are the solar panels on the roof, capturing sunlight to create 'rechargeable batteries' (ATP and reduced NADP). The stroma is the factory floor where these batteries are used to assemble complex products like sugar.
📖 Definitions
🔍 Worked example — Easy
A pigment travels 3.5 cm from the origin in a chromatography paper, while the solvent front travels 7.0 cm. Calculate the R_f value of this pigment.
🔍 Worked example — Standard
Compare the products of cyclic and non-cyclic photophosphorylation.
🔍 Worked example — Exam-style
Explain how the proton gradient is used to synthesise ATP during the light-dependent stage.
⚠️ Common errors
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🎯 Exam question
Key ideas — The light-independent stage: the Calvin cycle
The Calvin cycle takes place in the stroma and does not directly require light, but depends on the ATP and reduced NADP produced by the light-dependent stage.
🧑🏫 Teacher's explanation
Think of the Calvin cycle as a factory assembly line located in the 'office space' (stroma) of the chloroplast. The light-dependent stage (the 'power plant' in the thylakoids) sends out two essential supplies: ATP (chemical energy) and reduced NADP (high-energy electrons). Without these deliveries, the assembly line stops.
📖 Definitions
🔍 Worked example — Easy
A student observes that a plant is kept in total darkness for 24 hours. Predict the effect on the concentration of RuBP and GP in the stroma.
🔍 Worked example — Standard
Calculate the percentage increase in the concentration of a substrate if its initial concentration is 0.05 mol dm^-3 and its concentration after 10 minutes of photosynthesis is 0.08 mol dm^-3.
🔍 Worked example — Exam-style
Explain why the Calvin cycle is described as 'light-independent' even though it requires products from the light-dependent stage.
⚠️ Common errors
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🎯 Exam question
Key ideas — Linking the two stages
The Calvin cycle cannot proceed in darkness because it depends entirely on the products of the light-dependent stage:
🧑🏫 Teacher's explanation
Think of photosynthesis like a high-end restaurant kitchen. The light-dependent stage is the 'prep station' where raw ingredients (light energy) are converted into usable tools and energy packets (ATP and reduced NADP). The Calvin cycle is the 'cooking station' in the stroma, where these tools are used to assemble complex meals (carbohydrates) from simple ingredients (CO_2). If the prep station stops working because the lights go out, the chefs in the stroma quickly run out of tools and cannot finish the dishes.
📖 Definitions
🔍 Worked example — Easy
A student observes that after 10 minutes of intense light, the concentration of reduced NADP in the stroma decreases significantly. Explain why this occurs in the context of the Calvin cycle.
🔍 Worked example — Standard
Calculate the R_f value of a pigment if the distance moved by the pigment from the origin is 4.5 cm and the distance moved by the solvent front is 15.0 cm.
🔍 Worked example — Exam-style
Compare the products of cyclic photophosphorylation and non-cyclic photophosphorylation.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. In a chromatography investigation of a leaf pigment extract, the solvent front moved 9.6 cm from the origin. A green spot moved 7.2 cm and a yellow-orange spot moved 8.8 cm. (a) Calculate the R_f value of each pigment. [2] (b) Identify which spot is more likely to be a carotenoid, giving a reason. [2]
🔍 Worked example — Easy
A student performs paper chromatography on a leaf extract. The solvent front travels 10.0 cm from the origin. A pigment spot travels 2.5 cm. Calculate the R_f value.
🔍 Worked example — Standard
In a chromatography experiment, the solvent front moved 15.0 cm. Pigment A moved 12.0 cm and Pigment B moved 6.0 cm. Calculate the R_f values for both pigments.
🔍 Worked example — Exam-style
A chromatography strip shows a solvent front at 12.0 cm. A yellow pigment is found at 10.8 cm and a green pigment is found at 4.8 cm. (a) Calculate the R_f values. (b) Based on the R_f values, which pigment is more soluble in the solvent?
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Practice
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