Biochemistry of Photosynthesis

Biochemistry of Photosynthesis

Light-Dependent Reactions

• Occur in the thylakoid membranes of chloroplasts.
• Convert light energy into chemical energy (ATP and NADPH).
• Involves two main photosystems (PSII and PSI).
  • Photosystem II (PSII): Absorbs light, splits water (photolysis) releasing O2, and energizes electrons.
  • Electron Transport Chain (ETC): Transfers energized electrons, releasing energy to pump protons into the thylakoid lumen, creating a proton gradient.
  • ATP Synthase: Uses proton gradient to synthesize ATP from ADP and inorganic phosphate.
  • Photosystem I (PSI): Absorbs light, re-energizes electrons, and reduces NADP+ to NADPH.

Calvin Cycle

• Occurs in the stroma of chloroplasts.
• Utilizes ATP and NADPH produced in light-dependent reactions to convert CO2 into glucose.
• Consists of three main phases:
  1. Carbon Fixation: CO2 is fixed into a 5-carbon sugar (ribulose bisphosphate, RuBP) by the enzyme RuBisCO, producing 3-phosphoglycerate (3-PGA).
  2. Reduction Phase: 3-PGA is converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
  3. Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.

Chloroplast Structure

• Double-membrane organelle found in plant cells.
• Contains:
  • Thylakoids: Membrane-bound structures stacked into grana; site of light-dependent reactions.
  • Stroma: Fluid-filled space surrounding thylakoids; site of the Calvin cycle.
  • Granum: Stack of thylakoids, increasing surface area for light absorption.

Role Of Chlorophyll

• Primary pigment involved in photosynthesis, located in thylakoid membranes.
• Absorbs light, primarily in the blue (430-450 nm) and red (640-680 nm) wavelengths.
• Reflects green light, giving plants their green color.
• Essential for the conversion of light energy into chemical energy during the light-dependent reactions.

Photosynthetic Pigments

• Include chlorophyll a, chlorophyll b, carotenoids, and xanthophylls.
• Chlorophyll a: Main pigment; directly involved in light absorption and energy transfer.
• Chlorophyll b: Accessory pigment; assists in capturing light energy and broadening the absorption spectrum.
• Carotenoids: Provide additional light absorption and protect chlorophyll from photodamage by dissipating excess energy as heat.

Biochemistry of Photosynthesis

Light-Dependent Reactions

• Occur in the thylakoid membranes of chloroplasts.
• Transform light energy into chemical energy in the forms of ATP and NADPH.
• Feature two main photosystems: Photosystem II (PSII) and Photosystem I (PSI).
• Photosystem II (PSII): Initiates the process by absorbing light, splitting water (photolysis), and releasing O2 while energizing electrons.
• Electron Transport Chain (ETC): Transfers energized electrons and uses the release of energy to pump protons into the thylakoid lumen, establishing a proton gradient.
• ATP Synthase: Utilizes the created proton gradient to synthesize ATP from ADP and inorganic phosphate.
• Photosystem I (PSI): Absorbs light to re-energize electrons, which ultimately reduces NADP+ to form NADPH.

Calvin Cycle

• Takes place in the stroma of chloroplasts.
• Utilizes ATP and NADPH generated from light-dependent reactions to convert CO2 into glucose.
• Comprises three main phases:
  • Carbon Fixation: Carbon dioxide is attached to ribulose bisphosphate (RuBP) by the enzyme RuBisCO, leading to the formation of 3-phosphoglycerate (3-PGA).
  • Reduction Phase: 3-PGA is converted into glyceraldehyde-3-phosphate (G3P) using energy from ATP and electrons from NADPH.
  • Regeneration of RuBP: Some G3P is repurposed to regenerate RuBP, allowing the cycle to perpetuate.

Chloroplast Structure

• A double-membrane organelle located in plant cells.
• Contains:
  • Thylakoids: Membrane structures organized into stacks called grana; the site of light-dependent reactions.
  • Stroma: A fluid-filled area surrounding thylakoids, where the Calvin cycle occurs.
  • Granum: A stack of thylakoids that enhances the surface area for optimal light absorption.

Role Of Chlorophyll

• The primary pigment for photosynthesis, found in thylakoid membranes.
• Absorbs light most effectively in the blue (430-450 nm) and red (640-680 nm) wavelengths.
• Reflects green light, which gives plants their characteristic green color.
• Integral for converting light energy into chemical energy during light-dependent reactions.

Photosynthetic Pigments

• Includes chlorophyll a, chlorophyll b, carotenoids, and xanthophylls.
• Chlorophyll a: The main pigment directly involved in light absorption and energy transfer.
• Chlorophyll b: An accessory pigment that assists in capturing light energy and extends the absorption spectrum.
• Carotenoids: Provide additional light absorption and protect chlorophyll by dissipating excess energy as heat, thus preventing photodamage.

Test your knowledge on the biochemistry of photosynthesis, covering both light-dependent reactions and the Calvin cycle. This quiz will explore the roles of chloroplasts, photosystems, and the processes that convert light energy into chemical energy.