Molecular Biology - 8.3 Photosynthesis Reactions "detailed"/HL

Question 1

Overview of steps/processes in photosynthesis:

Answer 1

1. Light dependent phase:
   a) non-cyclic photophosphorylation
   b) Cyclic photophosphorylation
   c) phosphorylation in terms of chemiosomosis
2. Light INdependent phase *Calvin cycle”
   a) Carbon fixation
   b) Reduction
   c) regeneration of RuBP

Question 2

Light dependent phase - understandings

Answer 2

https://ib.bioninja.com.au/higher-level/topic-8-metabolism-cell/untitled-2/light-dependent-reactions.html

• Light-dependent reactions take place in the thylakoid membranes and the space inside them
• Reduced NADP and APT are produced in these reactions
• Absorption of light by photosystems generates excited electrons
• Photolysis of water generates electrons for use in the LD reactions
• Transfer of excited electrons occurs between carriers in thylakoid membranes
• Excited electrons from Photosystem II are used to contribute to generating a proton gradient
• ATP synthase in thylakoids generates ATP using the proton gradient
• Excited electrons from Photosystem I are used to reduce NADP

1. The LDR occurs ON the thylakoid membrane and may occur by either cyclic or non-cyclic processes
2. In both processes, light excites chlorophyll (clustered in photosystems) which realise electrons that pass through an electron transport chain, making ATP (photophosphorylation)

Question 3

LDP = Non-cyclic photophosphorylation

Answer 3

1. Electrons are generated by the photolysis of H2O
2. Chlorophyll in photosystems I and II absorbs light which triggers the release of high-energy electrons (photoactivation)
3. The e- from PII pass along a series of carriers (electron transport chain), producing ATP via chemiosmosis
4. the e- from PI reduce NADP+ to generate NADPH + H+
5. e- lost from PI are replaced by e- from PII
6. e- lost from PII are replaced by electrons generated by the photolysis of H2O (oxygen is produced as a by-product)

(SEE DIAGRAM)
https://ib.bioninja.com.au/higher-level/topic-8-metabolism-cell/untitled-2/photophosphorylation.html

Question 4

LDP = Cyclic photophosphorylation

Answer 4

(Only PI is involved in Cyclic photophosphorylation!!!)
(CP DOES NOT produce NADPH + H+ which is needed for the light-independent reactions = can be used to make chemical energy (ATP) from light BUT not to make organic molecules)

1. The high energy e- released by photoactivation pass along an e- transport chain (producing ATP) before returning to PI

(SEE DIAGRAM(S)!)
https://ib.bioninja.com.au/higher-level/topic-8-metabolism-cell/untitled-2/photophosphorylation.html

Question 5

LDP = phosphorylation in terms of chemiosomosis

Answer 5

1, As the electrons (released from chlorophyll) cycle through the e- transport chains located on the thylakoid membrane they lose energy

2. This free energy is used to pump H+ ions from the stroma into the thylakoid
3. The build-up of proteins inside the thylakoid creates an electrochemical gradient (or proton motive force)
4. The H+ ions return to the stroma via the transmembrane enzyme ATP synthase, which uses the potential energy from the proton motive force to convert ADP and an inorganic phosphate (Pi) into ATP

(SEE DIAGRAM FOR MORE INTO AND BETTER UNDERSTANDING)

Question 6

Light INdependent phase - understandings

Answer 6

https://ib.bioninja.com.au/higher-level/topic-8-metabolism-cell/untitled-2/light-independent-reactions.html

• Light-independent reactions take place in the stroma
• In the LIR a carboxylase catalyses the carboxylation of ribulose bisphosphate
• Glycerate 3-phosphate is reduced to triose phosphate using reduced NADP and ATP
• Triose phosphate is used to regenerate RuBP and produce carbohydrates
• Ribulose bisphosphate is reformed using ATP

1. The LIR occurs in the stroma and uses the ATP and NADPH + H+ produced by the LDR (non-cyclic)
2. LIR is also known as the Calvin cycle and occurs via 3 main steps:
   a) Carbon fixation
   b) Reduction
   c) regeneration of RuBP

Question 7

LIP - Carbon Fixation

Answer 7

1. The enzyme rubisco (RuBP carboxylase) catalyses the attachment of CO2 to the 5C compound ribulose bisphosphate (RuBP)
2. The unstable 6C compound that is formed immediately breaks down into 2 3C molecules called glycerate-3-phosphate (GP)
3. A single cycle involves 3 molecules pf RuBP combining w/ 3 molecules of CO2 to make 6 molecules of GP = 3x ROUND = 1 CYCLE

(SEE CALVIN CYCLE DIAGRAM !!!!)
https://ib.bioninja.com.au/higher-level/topic-8-metabolism-cell/untitled-2/light-independent-reactions.html

Question 8

LIP - Reduction

Answer 8

1. Each GP molecule (from carbon fixation) is phosphorylated by ATP and reduced by NADPH + H+
2. This converts each GP molecule into a triose phosphate (TP_ called glyceraldehyde phosphate

(SEE CALVIN CYCLE DIAGRAM !!!!)
https://ib.bioninja.com.au/higher-level/topic-8-metabolism-cell/untitled-2/light-independent-reactions.html

Question 9

LIP - Regeneration of RuBP

Answer 9

1. For every six molecules of TP produced, only one is used to form half a sugar molecule (need two cycles to form complete glucose)
2. The remaining TP molecules are reorganised to regenerate stocks of RuBP (it keeping it going around the cycle) in a reaction that involves ATP
3. With RuBP regenerated, the cycle will repeat many times and be used to construct chains of sugars for use by the plant

(SEE CALVIN CYCLE DIAGRAM !!!!)
https://ib.bioninja.com.au/higher-level/topic-8-metabolism-cell/untitled-2/light-independent-reactions.html

Question 10

Calvin Cycle - Lollipop Experiment

Answer 10

(https://ib.bioninja.com.au/higher-level/topic-8-metabolism-cell/untitled-2/calvin-cycle.html)

he light independent reactions are also collectively known as the Calvin cycle – named after American chemist Melvin Calvin

Calvin mapped the complete conversion of carbon within a plant during the process of photosynthesis

Calvin’s elucidation of photosynthetic carbon compounds is commonly classed the ‘lollipop experiment’

This is due to the fact that the apparatus he utilised was thought to resemble an upside-down lollipop

Lollipop Experiment

Radioactive carbon-14 is added to a ‘lollipop’ apparatus containing green algae (Chlorella)
Light is shone on the apparatus to induce photosynthesis (which will incorporate the carbon-14 into organic compounds)
After different periods of time, the algae is killed by running it into a solution of heated alcohol (stops cell metabolism)
Dead algal samples are analysed using 2D chromatography, which separates out the different carbon compounds
Any radioactive carbon compounds on the chromatogram were then identified using autoradiography (X-ray film exposure)
By comparing different periods of light exposure, the order by which carbon compounds are generated was determined
Calvin used this information to propose a sequence of events known as the Calvin cycle (light independent reactions)