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Powerpoint Presentation: Light Dependant Reaction

 

Photosynthesis Index

Classifying organisms according to their carbon source and energy supplies
Photosynthesis : The Development of Ideas
The light dependent reaction
The light dependent reaction : Summary
Chlorophyll in the chloroplast
Chlorophyll and photosynthesis
Photosynthesis overview

Topic Chapters Index

 

RUBISCO Fact Sheet

  • Discovered in 1948
  • A large enzyme 500 000 Daltons
  • Inhibited by oxygen
  • A relatively slow enzyme (converts 3 molecules of substrate to products per second. An average for all enzymes is 1000 conversions s-1)
  • Found in large quantities (50% of chloroplast protein). Making it 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

 

Carbon fixation and light - What is the link between the light dependent and the light independent pathways?

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

PGA levels rise

Other compounds, after PGA in the pathway, fall

The compound requiring the NADPH+H+ and ATP was found to be Glycerate 3-phosphate (GP)

GP 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.

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.

Squaring the circle!

Glycolosis

 

PHOTOSYNTHESIS

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Carbon fixation : The light independent reaction

  • Enzyme controlled (Q10 » 2)
  • Located in the stroma of the chloroplast.
  • Occurs simultaneously with the light dependent reaction but it can continue in the dark provided the necessary raw materials are available (CO2, NADPH + H+ and ATP)

 

Enzyme controlled reaction pathways

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:

  1. Label and trace the products formed through time.

  2. Cut the supply of X and observe what happens to the intermediates in the pathway (e.g. Photosynthesis cut the CO2 supply or switch off the light, thus, cutting the supply of ATP and NADPH+H+)

 

Calvin and Benson 1946 to 1953

  • Used 14C radioisotope for labelling (available after WW II)
  • Unicellular algae: Chlorella and Scenedesmus.
    Simple plants which respond quickly to changes in the environment. So little time lag.

"Lollipop" vessel. A flat-sided, round flask containing the culture of algae
This shape:

- provided even illumination of all the cells

- permitted careful control of environmental conditions (e.g. pH, temperature)

- permitted rapid mixing of contents

- precise sampling time

 

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.

Autoradiography

A. Mixture placed at the origin

B.1st run

C. Rotate the paper 90°

Autoradiography

D. 2nd run

E. Autoradiograph reveals the compound/s which are labelled with 14C

Result

The compound that became labelled first was Phosphoglyceric Acid (or Phosphoglycerate) PGA.

PGA is a 3 carbon sugar.

Therefore: Compound B + CO2 ArrowPGA

What is compound B?!!!

 

Searching for the compound that combines with carbon dioxide

Feed the algae NaH14CO3 for over 15 min so all the compounds involved in photosynthesis are labelled.

Then reduce the level of NaH14CO3 rapidly.

Prediction

PGA levels should fall rapidly

The compound that fixes CO2 should rise sharply.

Result

This compound was found to be Ribulose Bisphosphate (RBP)

RBP is a 5 carbon sugar

 

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