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THE LIGHT INDEPENDENT REACTION

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1 THE LIGHT INDEPENDENT REACTION
CARBON FIXATION © 2017 Paul Billiet ODWS

2 THE LIGHT INDEPENDENT REACTION
Enzyme controlled (Q10  2) Located in the stroma of the chloroplast Occurs simultaneously with the light dependent reaction It can continue in the dark provided the necessary raw materials are available (CO2, NADPH + H+ and ATP). © 2017 Paul Billiet ODWS

3 Enzyme controlled reaction pathways
A B C D E F Ea Eb Ec Ed Ee X = Precursor (raw material) added here © 2017 Paul Billiet ODWS

4 Enzyme controlled reaction pathways
To find out the sequence of the reactions and the point at which X is added in, two approaches can be used: Label and trace the products formed through time Cut the supply of X and observe what happens to the intermediates in the pathway e.g. in studying photosynthesis, cut the CO2 supply or switch off the light so cutting the supply of ATP and NADPH+H+ © 2017 Paul Billiet ODWS

5 Calvin and Benson 1946 to 1953 Used 14C radioisotope for labelling
Unicellular algae: Chlorella and Scenedesmus Simple plants which respond quickly to changes in the environment So little time lag. Scenedesmus © 2017 Paul Billiet ODWS

6 The “Lollipop” vessel A flat-sided, round flask containing the culture of algae - provides even illumination of all the cells - permits careful control of environmental conditions (e.g. pH, temperature) - permits rapid mixing of contents - precise sampling time. © 2017 Paul Billiet ODWS

7 Labelling and tracing carbon using 14C
Add NaH14CO3 solution At timed intervals the algae are sampled and killed by dropping in hot methanol Two-way (2-dimensional) chromatography used to separate the compounds Identify radioactively labelled compounds by autoradiography. © 2017 Paul Billiet ODWS

8 Two-way (2-dimensional) chromatography
A. Mixture placed at the origin B.1st run C. Rotate the paper 90° E. Autoradiograph reveals the compound/s which are labelled with 14C D. 2nd run © 2017 Paul Billiet ODWS

9 Results 2s © 2017 Paul Billiet ODWS
Salisbury & Ross (1969) Plant Physiology Wadsworth © 2017 Paul Billiet ODWS

10 Result The compound labelled first was phosphoglyceric acid (PGA) or glycerate-3-phosphate (GP) GP is a 3 carbon sugar Therefore: Compound B + CO2  GP. © 2017 Paul Billiet ODWS

11 Light independent pathway
A B GP D E F Ea Ea Ec Ed Ee CO2 What is compound B?!!! © 2017 Paul Billiet ODWS

12 Searching for the compound that combines with carbon dioxide
Feed the algae NaH14CO3 for over 15 min All the compounds involved in photosynthesis are labelled Then reduce the level of NaH14CO3 rapidly Prediction GP levels should fall rapidly The compound that fixes CO2 should rise sharply. © 2017 Paul Billiet ODWS

13 Salisbury & Ross (1969) Plant Physiology Wadsworth
© 2017 Paul Billiet ODWS

14 Result This compound = Ribulose Bisphosphate (RBP)
RBP is a 5 carbon sugar Therefore RBP + CO2  2GP 5C C = 2x3C The enzyme controlling this important step is Ribulose Bisphosphate Carboxylase (Rubisco). © 2017 Paul Billiet ODWS

15 Light independent pathway
A RBP GP D E F RUBISCO Ea Ec Ed Ee CO2 © 2017 Paul Billiet ODWS

16 RUBISCO A large enzyme 500 000 Daltons Inhibited by oxygen
A relatively slow enzyme 50% of chloroplast protein Probably the most abundant protein on Earth Consists of two subunits (1 large + 1 small) The large subunit is coded for by a gene on chlDNA. Rubisco stick model © 2017 Paul Billiet ODWS

17 Carbon fixation and light
The light dependent pathways produce ATP and NADPH+H+ to be used in the light independent processes Illuminate the algae for 15 min then cut the light Result Glycerate-3-phosphate levels rise Other compounds, after glycerate-3-phosphate in the pathway, fall. Glycerate-3-phosphate Ribulose bisphosphate © 2017 Paul Billiet ODWS

18 Carbon fixation and light
The compound requiring the NADPH+H+ and ATP was found to be Glyceraldehyde 3-phosphate (G3P) G3P is converted to various other three carbon sugars or Triose Phosphates (TP) Some are used to produce hexoses such as fructose phosphate and glucose phosphate. © 2017 Paul Billiet ODWS

19 Light independent pathway
A RBP GP G3P E Hexoses RUBISCO Ea Ec Ed Ee CO2 12 ATP 12 NADPH + H+ © 2017 Paul Billiet ODWS

20 A curious turn When the light is cut interestingly RBP levels also fall This is for two reasons RBP requires ATP in its synthesis RBP is formed from some of the GP In other words the reaction pathway is a CYCLE. Salisbury & Ross (1969) Plant Physiology Wadsworth Glycerate-3-phosphate Ribulose bisphosphate © 2017 Paul Billiet ODWS

21 Squaring the circle! The Calvin-Benson Cycle
6 RBP (5C) 6CO2 (1C) 6 ATP 12 GP (3C) Intermediates 1 Hexose (6C) 12 TP (3C) 12 G3P (3C) 12 ATP 12 NADPH + H+ © 2017 Paul Billiet ODWS

22 PHOTOSYNTHESIS OVERVIEW
CHLOROPLAST CYTOSOL H2O O2 Light dependent reaction Light independent reaction CO2 Starch Sugars Fatty acids Amino acids ATP NADPH+H+ Triose phospates © 2017 Paul Billiet ODWS

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