Chloroplasts, pigments and limiting factorsEdexcel International A Level Biology: Revision notes
Section 1
Chloroplast structure and role
A chloroplast is a biconvex organelle, 2 to 10 μm long, found in palisade and spongy mesophyll cells and guard cells. It has:
- A double membrane (envelope) that controls what enters and leaves.
- Thylakoids: flattened, membrane-bound sacs containing chlorophyll. Stacks of thylakoids form grana, joined by lamellae (intergranal membranes). The large membrane area holds the photosystems, electron carriers and ATP synthase for the light-dependent reactions.
- The stroma: the fluid matrix with the enzymes (including RUBISCO) of the Calvin cycle.
- Starch grains and lipid droplets as stores, plus 70S ribosomes and a loop of DNA so the chloroplast can make some of its own proteins.
The role of the chloroplast is to carry out photosynthesis, transferring light energy into the chemical energy of organic molecules.
Section 2
Photosynthetic pigments
Pigments absorb light in the thylakoid membranes, arranged in photosystems.
- Chlorophyll a (the primary pigment): absorbs mainly red and blue-violet light and is found at the reaction centre.
- Chlorophyll b and carotenoids (carotene, xanthophyll) are accessory pigments. They absorb other wavelengths and pass the energy to chlorophyll a.
Green light is mostly reflected, so leaves appear green.
Section 3
Absorption and action spectra
An absorption spectrum plots the absorbance of a pigment against wavelength. Chlorophyll a absorbs strongly at about 430 nm (blue) and 660 nm (red), and weakly in green.
An action spectrum plots the rate of photosynthesis against wavelength for the whole plant or leaf.
The action spectrum follows the absorption spectra of the pigments combined, but is broader than the chlorophyll a spectrum alone because accessory pigments absorb extra wavelengths. This is evidence that those wavelengths drive photosynthesis.
Do not mix up the two: an absorption spectrum is for a pigment (absorbance), an action spectrum is for photosynthesis (rate).
Section 4
Chromatography and Rf values
Pigments can be separated by paper or thin-layer chromatography. A concentrated extract is spotted on a pencil line, and the paper stands in a solvent below that line. The solvent carries each pigment up the paper at a different rate, depending on its solubility in the solvent and how strongly it is attracted to the paper.
Rf = distance moved by the pigment ÷ distance moved by the solvent front
The Rf value is between 0 and 1 and has no units. Worked example: a spot 6.3 cm from the line when the solvent front has moved 9.0 cm gives Rf = 6.3 ÷ 9.0 = 0.70. Measure to the centre of each spot, and mark the solvent front immediately because it evaporates. Carotene runs furthest and chlorophyll b the least.
Keep the origin line above the solvent level, and give Rf values to 2 decimal places with no units.
Section 5
Limiting factors and Core Practical 10
A limiting factor is the factor in shortest supply, so it restricts the rate of photosynthesis. Increasing it raises the rate until another factor becomes limiting.
- Light intensity: drives the light-dependent reactions. At low intensity it is limiting.
- Wavelength: red and blue light give the highest rates.
- Carbon dioxide concentration: needed for carbon fixation.
- Temperature: affects enzymes in the Calvin cycle: the rate rises to an optimum, then falls as enzymes denature.
Core Practical 10: use an aquatic plant such as Cabomba or Elodea in sodium hydrogencarbonate solution. Measure oxygen produced (bubbles counted or volume collected in a given time). Vary one factor at a time: lamp distance for light intensity, coloured filters for wavelength, a water bath for temperature, hydrogencarbonate concentration for CO₂. Control the others, use a heat shield or water bath so the lamp does not heat the plant, and repeat to calculate means. Light intensity is proportional to 1 ÷ distance².
Bubble counts are only an estimate of the rate because bubbles vary in size. Collecting the oxygen in a gas syringe gives a more accurate volume.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Chloroplasts, pigments and limiting factors
- A student extracts pigments from spinach leaves using propanone and applies a concentrated spot to a pencil line near the bottom of a strip of chromatography paper. The paper is placed in a solvent, which runs 9.0 cm from the pencil line to the solvent front. Four pigment spots separate. Their centres are 8.5 cm (carotene), 7.2 cm (xanthophyll), 6.3 cm (chlorophyll a) and 5.4 cm (chlorophyll b) from the pencil line.Explain why the pencil line must be above the level of the solvent in the container, and why the position of the solvent front must be marked as soon as the paper is removed.2 marks
- A plant physiologist measures two spectra for a leaf. The absorption spectrum of extracted chlorophyll a shows strong absorption of blue light (about 430 nm) and red light (about 660 nm) and very little absorption of green light (about 550 nm). The action spectrum for photosynthesis in the leaf has peaks at similar wavelengths, but the rate of photosynthesis in green light is higher than the chlorophyll a absorption alone would predict.Use the information to explain why a plant lit only with green light photosynthesises slowly but not at zero rate.2 marks
- An electron micrograph of a palisade mesophyll cell shows several chloroplasts. Each chloroplast is surrounded by a double membrane and contains stacks of flattened thylakoids joined by membranes called lamellae, a fluid stroma, small starch grains, and ribosomes and a loop of DNA.Describe how the structure of a chloroplast is adapted to its role in photosynthesis.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).