From light energy to chemical energy
pp. 425–438Photosynthesis transforms light energy into chemical energy, stored in carbon compounds. This supplies most of the chemical energy needed for life processes in ecosystems.
Guiding questions
- How does photosynthesis capture and use the energy in sunlight?
- How do non-living factors affect photosynthesis?
Slides
Notes
Written for this site from the class slides. The chapter is on pages 425–438 of the course textbook. Image credits are on slide 15.
Making glucose from carbon dioxide
pp. 425–438carbon dioxide + water → glucose + oxygen. Water is split, using light energy, and its hydrogen is used to reduce carbon dioxide.
Oxygen as a by-product
pp. 425–438Plants, algae and cyanobacteria release oxygen, which comes from the splitting of water. It is a by-product of this process, and it diffuses out.
Watch out: the oxygen comes from water, and not from carbon dioxide.
Separating pigments by chromatography
pp. 425–438A leaf extract is spotted on a strip, and a solvent moves up it. Pigments separate, as they differ in solubility.
Rf = distance moved by pigment / distance moved by solvent. Identify pigments by colour and by Rf. For example, a pigment moving 36 mm with a solvent moving 80 mm has Rf = 0.45.
How pigments absorb light
pp. 425–438When a pigment absorbs a photon, an electron is excited to a higher energy level, so light energy has become chemical energy. Only some wavelengths have the right energy to do this. Chlorophyll absorbs red and blue best, and reflects green.
Absorption and action spectra
pp. 425–438| Absorption spectrum | Action spectrum | |
|---|---|---|
| Vertical axis | Percentage of light absorbed | Rate of photosynthesis |
| Similarities | Both peak in red and blue, and both are lower in green | |
| Differences | Rate does not fall as low as absorption in green, as accessory pigments and scattering help | |
Investigating limiting factors
pp. 425–438The rate is limited by whichever factor is furthest from its optimum. To investigate: move a lamp to vary light intensity, add sodium hydrogencarbonate to vary CO2 concentration, and use water baths to vary temperature. Measure oxygen production, or carbon dioxide use by following pH.
CO2 enrichment experiments
pp. 425–438Enclosed greenhouses add CO2 to a closed space, which makes it easier to control other factors. FACE experiments release CO2 over open plots, so conditions are closer to natural ecosystems. Growth often rises at first, but by less than predicted.
Videos
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Note-taking sheet
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
Quick self-check
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Practice questions
A printable worksheet covering all eight statements, from quick recall to data questions and a longer answer. It has 42 marks and takes about 45 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Data-based questions
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 13 marks in total. Your teacher has the mark scheme.
Light, carbon dioxide and wheat
8 marks
Wheat seedlings were grown at three carbon dioxide concentrations, in parts per million (ppm). The rate of photosynthesis was measured as net carbon dioxide uptake at different light intensities. Temperature was kept at 25 °C.
- State the rate of photosynthesis at 800 µmol photons m−2 s−1 and 400 ppm carbon dioxide. [1]
- Outline the effect of carbon dioxide concentration on the rate of photosynthesis. [3]
- Identify the factor limiting the rate at 1600 µmol photons m−2 s−1 for the 400 ppm plants. Give a reason. [1]
- Explain why the rate is negative in darkness. [1]
- Calculate the percentage increase in the highest rate when carbon dioxide is raised from 400 to 800 ppm. [1]
- Suggest one reason why growers may add carbon dioxide to greenhouses only when light is bright. [1]
Sun and shade leaves
5 marksLeaves of English ivy (Hedera helix) that grew in full sun were compared with leaves that grew in the shade of other leaves on the same plant. Values are means of 20 leaves, ± 1 SD.
| Sun leaves | Shade leaves | |
|---|---|---|
| Leaf area / cm2 | 18 ± 3 | 31 ± 5 |
| Total chlorophyll / mg g−1 | 1.6 ± 0.2 | 2.4 ± 0.3 |
| Chlorophyll a : chlorophyll b | 3.4 | 2.6 |
- Compare the area of sun and shade leaves. [1]
- Calculate how much more chlorophyll per gram the shade leaves contain, as a percentage. [1]
- Explain how a larger area and more chlorophyll help shade leaves. [2]
- Chlorophyll b absorbs some wavelengths that chlorophyll a absorbs poorly. Suggest why shade leaves have a lower ratio of chlorophyll a to b. [1]
Practical
Light intensity and floating leaf discs
You will need: Leaf discs, syringe without needle, dilute sodium hydrogen carbonate, lamp, ruler, beakers, timer
- Draw solution into the syringe with the discs and pull the plunger to remove air, so they sink
- Place the discs in solution at measured distances from a lamp
- Record the time for half the discs to float
- Plot rate against light intensity, and explain any plateau
Safety: Keep water away from lamps and sockets. Lamps get hot.
Carry this out with your teacher's supervision and follow your school's lab rules.
Nature of science and TOK
Nature of science
Experiments
Van Helmont grew a willow in weighed soil and concluded that it gained its mass from water. His data were good, but he missed the role of air.
Theory of knowledge
How can careful experiments lead to wrong conclusions?
He could not test for what he had not thought of. Consider how existing ideas limit how we interpret results.
Syllabus statements
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
- C1.3.1How photosynthesis turns light energy into chemical energy stored in carbon compoundsp. 425
- C1.3.2Making glucose from carbon dioxide, using hydrogen from split water moleculesp. 427
- C1.3.3Why plants, algae and cyanobacteria release oxygen during photosynthesisp. 428
- C1.3.4Using chromatography to separate and identify photosynthetic pigmentsp. 429
- C1.3.5How pigments absorb particular wavelengths of lightp. 431
- C1.3.6How absorption spectra and action spectra comparep. 432
- C1.3.7Practical ways to vary carbon dioxide, light or temperature to study limiting factorsp. 434
- C1.3.8What carbon dioxide enrichment experiments suggest about future photosynthesis and plant growthp. 438
HL extension
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