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Metabolism, Cell Respiration and Photosynthesis (AHL) - Photosynthesis

Grade 11IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Photosynthesis occurs in two main stages: the light-dependent reactions in the thylakoid membranes and the light-independent reactions (Calvin cycle) in the stroma of the chloroplast.

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Light-dependent reactions involve the absorption of light by photosystems (PSIIPSII and PSIPSI). This leads to the photolysis of H2OH_2O, which releases O2O_2 as a waste product and provides electrons for the electron transport chain.

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Photophosphorylation is the production of ATPATP using energy derived from light. In non-cyclic photophosphorylation, both ATPATP and reduced NADPNADP (NADPH+H+NADPH + H^+) are produced. In cyclic photophosphorylation, only ATPATP is produced via PSIPSI.

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Chemiosmosis in chloroplasts involves the pumping of H+H^+ ions into the thylakoid space, creating a proton gradient. Protons flow back into the stroma through ATPATP synthase to generate ATPATP.

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The light-independent reactions begin with carbon fixation: CO2CO_2 reacts with ribulose bisphosphate (RuBPRuBP, a 5C5C sugar) catalyzed by the enzyme RubiscoRubisco to form two molecules of glycerate-3-phosphate (GPGP or G3PG3P, 3C3C).

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In the Calvin cycle, GPGP is reduced to triose phosphate (TPTP) using ATPATP and NADPHNADPH. One-sixth of the TPTP is used to synthesize organic molecules like glucose (C6H12O6C_6H_{12}O_6), while five-sixths are used to regenerate RuBPRuBP using ATPATP.

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Chloroplast structure is adapted to its function: the thylakoids have a small internal volume to quickly increase the H+H^+ gradient, and the stroma contains all the enzymes necessary for the Calvin cycle.

📐Formulae

6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2

2H2O→4e−+4H++O22H_2O \rightarrow 4e^- + 4H^+ + O_2

NADP++2H++2e−→NADPH+H+NADP^+ + 2H^+ + 2e^- \rightarrow NADPH + H^+

ADP+Pi→ATPADP + P_i \rightarrow ATP

Rate of Photosynthesis=Change in O2 or CO2Time\text{Rate of Photosynthesis} = \frac{\text{Change in } O_2 \text{ or } CO_2}{\text{Time}}

💡Examples

Problem 1:

Calculate the number of ATPATP and NADPHNADPH molecules required to produce one molecule of glucose (C6H12O6C_6H_{12}O_6) via the Calvin cycle.

Solution:

1818 molecules of ATPATP and 1212 molecules of NADPHNADPH.

Explanation:

To produce one molecule of glucose, 66 molecules of CO2CO_2 must be fixed. Each CO2CO_2 fixation requires 33 ATPATP (22 for reduction of GPGP to TPTP and 11 for regeneration of RuBPRuBP) and 22 NADPHNADPH (for reduction). Therefore, 6×3=186 \times 3 = 18 ATPATP and 6×2=126 \times 2 = 12 NADPHNADPH are needed.

Problem 2:

Explain the role of the thylakoid space in the light-dependent reactions.

Solution:

The thylakoid space (lumen) acts as a reservoir for protons (H+H^+).

Explanation:

Because the volume of the thylakoid space is very small, the accumulation of protons from the photolysis of H2OH_2O and the action of the electron transport chain quickly creates a steep electrochemical gradient (pHpH gradient) compared to the stroma. This gradient drives the synthesis of ATPATP through ATPATP synthase (chemiosmosis).

Problem 3:

If a plant is exposed to light but the concentration of CO2CO_2 is suddenly reduced to zero, what happens to the levels of RuBPRuBP and GPGP?

Solution:

RuBPRuBP levels will increase, and GPGP levels will decrease.

Explanation:

RuBPRuBP is the substrate that reacts with CO2CO_2 to form GPGP. Without CO2CO_2, RuBPRuBP cannot be converted into GPGP, causing RuBPRuBP to accumulate. Simultaneously, the existing GPGP is still being reduced to TPTP and eventually used to regenerate RuBPRuBP, but no new GPGP is being produced.

Photosynthesis - Revision Notes & Key Diagrams | IB Grade 11 Biology