Summary of Eduqas ALevel Biology Component 1, Unit 2: Photosynthesis (also applicable to WJEC). Easy to digest but comprehensive information, including diagrams and highlighted key words. Written by a Year 13 student, predicted A-A*.
Based on official Eduqas teacher guidance:
(a) the distributi...
Chloroplasts act as transducers – convert the energy of light photons into the chemical energy of ATP.
There are 10-50 chloroplasts per plant cell. Chloroplast distribution in leaf:
▪ Palisade cells have the highest density of chloroplasts as they have the highest light incidence (most
light hits them). They are column shaped, which makes them tightly packed together so light can’t pass
between them.
▪ Most light has been absorbed by the time it reaches the spongy mesophyll, so they have less
chloroplasts.
▪ The only chloroplasts in the lower epidermis are in guard cells, to provide energy for opening and
closing of stomata.
Photosynthetic pigments absorb certain wavelengths of light and reflect other wavelengths (these are
the colours we see). Chlorophyll is made up of the following pigments:
• Chlorophyll a (most abundant)– Absorbs red (2 types: 680nm & 700nm) and blue (at 450nm), appears
blue-green.
Accessory pigments:
Chlorophyll b – Absorbs 450nm & 640nm, appears yellow-green.
• Carotenoids – Absorb blue, reflect yellow (xanthophyll) and orange (carotene).
• Phaeophytin
Absorption spectra:
•
The pigments are arranged in photosystems which are embedded in thylakoid membranes:
(Electron micrograph image of the inner surface of a thylakoid membrane. Each particle represents one
photosystem 2 complex.)
, Chlorophyll a is situated in the primary pigment reaction centre.
Each photosystem contains an antenna complex of all pigment types. They absorb light at different
wavelengths : PS1 absorbs light at 700nm and PS2 at 680nm due to slightly different types of chlorophyll
a (P700 & P680).
Proportions of pigments accounts for varying shades of green found between species of plants.
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