HL 1 Biology

8.2c Photosynthesis; Limiting factors

Action pectrum: graph showing rate of photosynthesis at each wavelength of light.

Absorption spectrum: graph showing percentage of light absorbed at each wavelength by a pigment or a group of pigments

Action and absorption similar: photosynthesis can only occur in wavelengths of light, that chlorophyll/other photosynthetic pigments can absorb.

Discrete units of light energy: quantum of lights/ photons

Photons energy

  • shorter wavelength = greater energy
  • electron in pigment molecule can absorb photon: if energy carried by it allows electron to be raised to a higher energy level
  • too little or too much energy carried = photon cannot be absorbed
  • accessory pigments: absorb other wavelengths, then transfers light energy to chlorophyll


Measuring light Absorption by Photosynthesis Pigments (p.107)

1) The absorption of the two samples of pigments shown in the graph show similar maximum absorption of the photons and the same wavelengths in total. Both of the pigments are absorbing the same colors, red and blue and both pigments have absorbed more of the blue light than the red. The leaf represented with a red curve reflects less green light, compared to the curve in purple.

2) The absorption of the pigments from the Fagus leaf is represented with the purple curve, because it has absorbed less green light, thus the leaf is shown as green. The red curve represents the Acer (maple) leaf as it absorbs more green light, thus reflect less green is visible on the leaf.

3) Plants are using pigments that absorb light in the range 400 – 700nm and not higher or lower wavelengths because too little or too much energy carried would result photons to not be able to be absorbed.  

4) The curve for algae growing on rocky beaches having a brown color would curve down where the yellow and orange colors are plotted in the graph (600nm~700nm). This is because the algae reflects a brown color, thus absorbs less of the color brown and so results to the curve being low at the colors which make the color brown (yellow..orange..). 

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