ly above a green LED light source (wavelength $530\\text{ nm}$).\n* Record the time taken for the leaf discs in each syringe to rise to the surface of the solution.\n\n![An experiment setup with two syringes containing sodium hydrogen carbonate solution and leaf discs, illuminated from below by a blue LED and a green LED respectively.](https://assets.mathsgenie.co.uk/aqa-qb/6d8cb424-8a31-4e57-9af7-a022e20f5efe.png)","difficulty":5,"difficultyBand":"medium","totalMarks":8,"sourceTitle":null,"calculatorUse":null,"parts":[{"id":"6680e4a4-9f5b-4926-b25f-266f23161796","label":"a","content":"Identify **three** variables (other than leaf species) that the student must control to ensure that the data collected for the two light wavelengths is valid.","markscheme":"- Concentration / volume of the sodium hydrogen carbonate ($NaHCO_3$) solution B1\n- Light intensity / distance of the LED light source from the syringe B1\n- Initial temperature of the solution B1\n- Size, surface area, or thickness of the leaf discs B1","marks":3,"order":0,"markingRubric":null,"explanation":{"methods":[{"id":"controlled_variables","label":"Identify controlled variables","steps":[{"type":"text","order":1,"title":"Set the variables","content":"The wavelength is the **independent variable**: it is the factor changed between the two syringes. A **controlled variable** is a factor kept the same so that any difference in photosynthesis rate is a fair comparison. The question asks for **any three** suitable controls.","highlights":[{"text":"how the wavelength of light affects the rate of photosynthesis","location":"specification"}]},{"type":"text","order":2,"title":"Control the solution","content":"Keep the concentration and volume of the sodium hydrogen carbonate solution the same in both syringes. This controls the carbon dioxide supply, so it earns **B1**.","highlights":[{"text":"$$0.2\\%$ sodium hydrogen carbonate solution","location":"specification"}]},{"type":"text","order":3,"title":"Control light intensity","content":"Keep the light intensity the same, for example by keeping each LED the same distance from its syringe. Otherwise a different brightness could affect the rate, so this earns **B1**.","highlights":[{"text":"Place the first syringe directly above a blue LED light source","location":"specification"},{"text":"Place the second syringe directly above a green LED light source","location":"specification"}]},{"type":"text","order":4,"title":"Control temperature","content":"Start both solutions at the same temperature. Temperature affects the rate of photosynthesis, so keeping it constant earns **B1**. An equally valid replacement for any one of these three controls is keeping the leaf discs' size, surface area or thickness the same.","highlights":[{"text":"Record the time taken for the leaf discs in each syringe to rise to the surface of the solution.","location":"specification"}]}]}],"generatedAt":"2026-07-22T11:10:00.169Z","modelVersion":"gpt-5.6-terra+diagrams-1","explanationVersion":"v3"},"options":[]},{"id":"3d1a0e49-2c6b-4172-a1ce-5352eeb16530","label":"b","content":"Explain the biological process that causes the leaf discs to rise to the top of the syringe.","markscheme":"- Oxygen ($O_2$) is produced during the light-dependent stage of photosynthesis M1\n- This oxygen gas accumulates in the air spaces of the leaf tissue, decreasing the overall density of the discs and making them buoyant A1","marks":2,"order":1,"markingRubric":null,"explanation":{"methods":[{"id":"photosynthesis_explanation","label":"Explain the leaf discs rising","steps":[{"type":"text","order":0,"title":"Identify the gas produced","content":"The leaf discs contain chlorophyll and are illuminated by the LEDs, so they carry out photosynthesis. In the **light-dependent stage** of photosynthesis, water is split and oxygen gas, $O_2$, is produced. This earns **M1**.","highlights":[{"text":"Explain the biological process that causes the leaf discs to rise to the top of the syringe.","loc

Photosynthesis

Question 2

Photosynthesis Questions

Photosynthesis

Question 2

Photosynthesis Questions