H2 Bio - Shedding Light On Photosynthesis FAQs

In this post, through answering some selected FAQs, we'll cover most of the content you need to know for the topic of Photosynthesis while  providing tips on how to answer H2 Bio questions with exam efficiency. The FAQs covered here aren't exhaustive - we've left out higher level ones requiring you to synthesize your knowledge such as comparison qns, cross-topic qns, qns involving the DCPIP indicator, cyanide etc - as the questions here are enough to cover the content. Model answers to the FAQs we've not included will be made available only for our students ;)

Whether you are the type of student who has the tendency to under or over-elaborate, the tip we have for you is to ask yourself before answering any question: which idea or concepts should you make reference to, and which keywords or phrases are associated with that concept?

Under non-cyclic photophosphorylation for example, you could make reference to how 1. Resonance energy transfer 2.excitation of electron 3. Electron transport chain (ETC) 4. NADP reduction 5. Electron from PSII fills up electron hole of PSI. 6. Chemiosmosis 7. Photolysis

Bio Essay questions can usually be divided into several sub-questions with a cap on the maximum marks on each section. For the below question, we can divide the Light reaction into non-cyclic and cyclic photophosphorylation with a reasonable cap of 4-5m and 1-2m respectively.

Outline the main stages of the Light reaction [6m]

• In non-cylic photophosphorylation, light absorbed by pigment molecules and energy transferred to special chlorophyll a molecules in reaction centre of Photosystem II via resonance
• Special chlorophyll a molecules excited, promote electron to higher energy level, accepted by primary electron acceptor
• Primary electron acceptor transfer electrons down cytochrome electron carriers of electron transport chain, until NADP is reached
• NADP combine with electron from ETC II and H+ from photolysis and reduced to NADPH
• In a similar way, electron from PSI gets promoted and transferred down electron transport chain until it reaches and fills the electron hole in PSII
• Energy releases from exergonic fall of electrondown ETC I coupled to formation of ATP via chemiosmosis
• Electron hole of PSI filled by electron from photolysis; 2H20 --> 4H+ + 4e- + O2

For cyclic photophosphorylation, we make reference to the ideas that the 1. electron from PSI is recycled 2. Only ATP but not NADPH produced

• In cyclic photophosphorylatin, electron from primary electron acceptor of PSI transferred to ETC II and is recycled to PSI
• Chemiosmosis occurs and only ATP is produced

Explain the roles of the thylakoid membrane in photosynthesis

• The increasingly electronegative arrangement of electron carriers facilitates the transfer of electron down the ETC 
• Folded into granum to increase surface area, so that more photosystems can be embedded
• Hydrophobic core impermeable to H+, hence proton gradient can be established for chemiosmosis, whereby [H+] higher in thylakoid space relative to stroma
• Contains ATP synthase which provides hydrophilic channel for H+ to flow down concentration gradient via facilitated diffusion, becoming activated and phosphorylating ADP to ATP, which is used in Calvin Cycle (students often leave out the last part where ATP is used in the Calvin Cycle)
• Compartmentalises enzymes involved in Calvin Cycle in stroma which provides optimal pH and environment for enzymes to function

Explain how the proton motive force is generated and maintained

• {H+] relatively higher in thylakoid space relative to stroma, resulting in concentration gradient
• Photolysis in thylakoid space; 2H20 --> 4H+ + 4e- + O2
• Proton pump pumps H+ against concentration gradient from stroma to thylakoid space (FYI, the proton pump is known as the b6f complex - it is found in ETC II, and is notably involved in cyclic photophosphorylation)
• Reduction of NADP to NADPH in stroma removes H+; NADP+ + H+ + 2e- --> NADPH; hence [H+] lower in stroma
• Hydrophobic core of thylakoid membrane impermeable to H+, hence H+ cannot flow down concentration gradient via simple diffusion

Explain roles of ATP and NADP in photosynthesis

• Hydrolysis of ATP provides energy to drive Calvin Cycle; energy required to rearrange atoms in G3P to regenerate RuBP for CO2 fixation to continue
• ADP is recycled to the light reactions so that it can be phosphorylated to produce more ATP (When asked about the role of ATP, you first mention its direct role in the Calvin Cycle, then proceed to mention the role of its cousin ADP in the light reactions. Likewise, when asked about the role of NADP, you get the first mark from stating its direct role in the light reactions, and the second mark from its role in the Calvin Cycle, as seen below)
• NADP is a hydrogen carrier (and co-enzyme) and final electron acceptor of ETC I, NADP reduction to NADPH is necessary for light reactions to proceed
• NADPH provides H and reducing power to convert glycerate phosphate (GP) to glyceraldehyde-3-phosphate (G3P), which is used to regenerate RuBP for CO2 fixation to continue (notice that the production of G3P is always linked to the regeneration of RuBP in photosynthesis, because RuBP is necessary for CO2 fixation and hence photosynthesis to proceed.)

Outline the stages of the Calvin Cycle

• Carbon fixation: CO2 combines with RuBP (5C) in presence of the Rubisco enzyme;
• to give an unstable 6C compound which breaks down into 2 glycerate phosphate (GP) molecules
• NADPH and ATP is used to reduce and convert GP to glyceraldehyde-3-phosphate (G3P)
• For every 6 G3P produced, 5 used to regenerate RuBP for CO2 fixation to continue, 1 converted to storage molecule eg. sugars and lipids

Discuss named factors that influence the rate of photosynthesis, with reference to the term 'limiting factors'

• Limiting factor refers to factor in shortest supply;
• Which determines the rate of reaction
• Rate of photosynthesis can be measured as rate of CO2 fixation
• Light intensity affects the light-dependent stage of photosynthesis
• At low light intensities, light intensity is the limiting factor
• As light intensity increases, more electrons are boosted from PSI and PSII, hence more ATP and NADPH are produced and used in the Calvin Cycle, thus rate of CO2 fixation and photosynthesis increases
• At high light intensities, rate of photosynthesis plateaus as some other factor becomes limiting (the idea that some other factor becomes limiting, or light intensity no longer is the limiting factor is one that students tend to miss, perhaps because it is too 'duh'. Note however that you only need to bring in this idea once ie. you don't have to repeat this idea for CO2 and temperature later)
• CO2 is a major limiting factor as atmospheric [CO2] is low
• As [CO2] increases, rate of effective collisions with RuBP and Rubisco enzyme increases, and rate of enzyme-substrate complex formation increases (Rubisco is an enzyme and it would be safer or at least there would be no harm from invoking an enzymes explanation)
• Rate of CO2 fixation and hence photosynthesis increases

  

  FYI, Data response questions like this are common

  
  
• As temperature increases, rate of effective collision between CO2, RuBP and Rubisco increases, hence rate of enzyme-substrate complex formation and CO2 fixation increases
• Rate of photosynthesis doubles with every 10C increase in temperature until optimal temperature is reached
• Beyond optimal temperature, Rubisco denatures and rate of photosynthesis falls;
• As 3D conformation of active site is altered so RuBP and CO2 no longer able to bind
• O2 competes with CO2 for active site of Rubisco (note that O2 is not by definition a 'limiting factor' and is technically not relevant in answering this particular question. Nonetheless, if the question did not restrict you to limiting factors, O2 is a factor you can bring in)
• As [O2] increases, availability of active sites for CO2 to bind falls, hence rate of CO2 fixation and photosynthesis falls

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