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Summary Unit 3.2 - Photosynthesis
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  • Unit 3.2 - Photosynthesis
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Notes on photosynthetic pigments, antenna complex, light independent stage, light dependent stage, limiting factors of photosynthesis and a comparison of photosynthesis and respiration

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Document information

  • March 19, 2024
  • 5
  • 2023/2024
  • Summary

Subjects

  • photosynthesis
  • atp
  • light stage
  • dark stage
  • calvin cycle
  • limiting factors

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  • A/AS Level
  • WJEC
  • Biology
  • Unit 3.2 - Photosynthesis
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3.2 Photosynthesis

 Occurs in chloroplasts located inside mesophyll cells and guard cells
 Pigments convert light energy into chemical energy (aka glucose)
 Pigments are split into two groups:
 The chlorophylls – these absorb light in the red and blue regions of the spectrum e.g. chlorophyll
a and chlorophyll b
 The carotenoids – absorb light in blue/violet region only. They act as accessory pigments e.g.
carotene and xanthophyll
 Pigments have 2 different spectra: Absorption spectrum (indicates amount of light absorbed at
each wavelength) and Action spectrum (indicates amount of photosynthesis happening at each
wavelength
 It is advantageous for plants to have more than one pigments because they all absorb light at
different wavelengths so more of the natural light can be utilized, more photosynthesis can take
place and the plant will be more efficient
 Pigments are inside chloroplast on inner thylakoid membrane because it has a large SA so more
light can be captured
 Pigments exist in families called photosystems which consist of an antenna complex and a
reaction centre
 Function of antenna complex – trap light energy and funnel it down to the reaction centre
(pigments inc carotene and xanthophyll are in antenna complex)
 The reaction centre is made up of chlorophyll a
 Chromatography is a method used to separate out different pigments in a leaf
 Each photosynthetic pigment has an Rf value calculated using : distance moved by pigment /
distance moved by solvent
 Photosynthesis is split into two stages – The light dependent stage (inner thylakoid membrane)
and the light independent stage/dark stage/Calvin cycle (stroma of chloroplast)
 A pair of photosystems = one light harvesting unit

Light stage:

 When light shines onto leaves they rotate to move towards light
 Photosystems inside chloroplast also move to trap as much light as possible
 Photosystems are arranges into photosynthetic units which are made of photosystem 1 and 2.
Photosystem 1 is a higher energy level than 2
 Process:
o Light energy is trapped in P2 boosting electrons from the reaction centre to a higher
energy level
o Electrons are accepted by electron acceptors on thylakoid membrane
o Electrons are passed by a series of carriers to photosystem 1. The energy lost in this
process converts ADP to ATP so light energy has been converted into chemical energy
o Light energy is absorbed by photosystem 1 boosting electrons to an even higher energy
level
o Electrons accepted by another electron acceptor in the membrane
o Electrons which have previously been lost are replaced by electrons from photolysis

, o Protons from photolysis combine with NADP to make NADPH which goes on to be used
in the light independent stage
o Soe electrons from the second electron acceptor can pass back to the chlorophyl leaving
only ATP. This is called cyclic phosphorylation
 Photolysis happens as light shines onto the leaf at the same time as an electron is excited to a
higher energy level
 H2O -> 2H+ (NADPH) + 2e- (reaction centre) + ½ O2 (escapes through stomata)




 In light stage one of the products is ATP which is made in exactly the same way as it is in
respiration (chemiosmosis) but the only difference is where it happens
 It is made by the process photosynthetic photophosphorylation

Cyclic phosphorylation Non cyclic phosphorylation
Electrons can go round in a cycle and finish Electrons don’t go round in a cycle and finish
where they started in different place to where they started
Only involves one photosystem Involves photosystem 1 and 2
Products are ATP Products are ATP and NADPH which are used
in dark stage
Light independent stage:

 Occurs in the stroma of the chloroplast because there are lots of dissolved enzymes to catalyse
the reactions
 Purpose of dark stage – fix carbon dioxide gas into glucose using ATP and NADPH from the light
stage
 Calvin cycle worked out by scientist Calvin. He used radioactive carbon dioxide and fed it to
chlorella and followed the pattern of carbon atoms

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