8.3- Photosynthesis
What is it?
Light independent reactions
Light dependent reactions
The process of using the energy in sunlight to make food (glucose). It takes place in the chloroplast (found in plant cells ONLY)
Equation: 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2
Autotrophs store chemical energy in carbohydrate food molecules they build themselves. Most autotrophs make their food through photosynthesis using the energy from the sun
It uses the energy in sunlight to fix carbon dioxide, transferring the energy into carbohydrates and releasing oxygen in the process
These reactions occur in the thylakoid membranes of chloroplasts
Oxygen is released as a WASTE product
Protons in these reactions absorb light energy to produce ATP to split water (photolysis) and obtain H+ ions, and this energy is later changed to chemical energy that is stored in NADPH and ATP
The protons used in the light reactions come from photolysis, in which H2O molecules are broken into hydrogen ions, electrons, and oxygen atoms.
In addition, the energy from sunlight is used to pump protons into the thylakoid lumen during the first electron transport chain, forming a chemiosmotic gradient.
These reactions occur in the stroma of chloroplasts
They use ATP and H+ ions to 'fix' carbon dioxide, producing glucose
Enzymes in the stroma synthesize glucose and other organic molecules from CO2 using the chemical energy stored in ATP and NADPH
Photosystem II generates ATP
Photosystem I generates NADPH
Step 1: Carbon fixation
The Calvin cycle begins with a 5C compound called ribulose bisphosphate (or RuBP)
An enzyme, RuBP carboxylase (or Rubisco), catalyses the attachment of a CO2 molecule to RuBP
The resulting 6C compound is unstable, and breaks down into two 3C compounds – called glycerate-3-phosphate (GP)
A single cycle involves three molecules of RuBP combining with three molecules of CO2 to make six molecules of GP
Step 2: Reduction of Glycerate-3-Phosphate
Glycerate-3-phosphate (GP) is converted into triose phosphate (TP) using NADPH and ATP
Reduction by NADPH transfers hydrogen atoms to the compound, while the hydrolysis of ATP provides energy
Each GP requires one NADPH and one ATP to form a triose phosphate – so a single cycle requires six of each molecule
Step 3: Regeneration of RuBP
Of the six molecules of TP produced per cycle, one TP molecule may be used to form half a sugar molecule
Hence two cycles are required to produce a single glucose monomer, and more to produce polysaccharides like starch
The remaining five TP molecules are recombined to regenerate stocks of RuBP (5 × 3C = 3 × 5C)
The regeneration of RuBP requires energy derived from the hydrolysis of ATP
The 'Lollipop' Experiment
Calvin used Chlorella algae for this experiment, which was placed in a thin glass vessel called the ‘lollipop vessel’
The algae was given plenty of light, carbon dioxide (CO2) and hydrogen carbonate (HCO3 -) containing normal carbon
Samples of algae were taken at different time intervals
The carbon compounds were separated by chromatography and the compounds containing ^14 C identified by autoradiography
Calvin’s experiment analysed the results using autoradiograms; after only 5 seconds there was more labelled glycerate-3-phosphate than any other compound which indicates that glycerate-3-phosphate is the first product of carbon fixation
After 30 seconds a range of different labelled compounds occur showing the intermediate and final products of the light-independent reactions.