Chloroplast Structure and Photosynthesis

Chloroplast Structure and Function

• Chloroplasts are organelles responsible for photosynthesis in plants and algae.
• They have an outer membrane, inner membrane, thylakoids (stacked in grana), and stroma.
• Light-dependent reactions occur within the thylakoids, while light-independent reactions (Calvin cycle) take place in the stroma.

Thylakoid Function and Chlorophyll

• Thylakoids are the sites of light-dependent reactions and contain the green pigment chlorophyll.
• Chlorophyll absorbs light energy, which excites electrons, initiating the electron transport chain.
• This chain ultimately leads to the production of ATP and NADPH, crucial for the Calvin cycle.

Steps in Photosynthesis

• Photosynthesis is the process of converting light energy into chemical energy, producing glucose and oxygen.
• It involves two main stages: light-dependent reactions and light-independent reactions (Calvin cycle).
• Light-dependent reactions capture light energy, split water, and produce ATP and NADPH.
• The Calvin cycle uses ATP and NADPH to convert carbon dioxide into glucose.

Light and Pigments

• Light is electromagnetic radiation visible to plants, particularly in the wavelengths of 400-700 nm.
• Pigments are molecules that absorb light. Key pigments in plants include:
  • Chlorophyll a: the main photosynthetic pigment, absorbs blue-violet light and reflects blue-green.
  • Chlorophyll b: an accessory pigment, absorbs red-blue light and reflects yellow-green.
  • Carotenoids and Xanthophyll: accessory pigments that absorb blue-purple light and reflect orange and yellow respectively.

Photosystems

• Photosystems are complexes of pigments and proteins located in thylakoid membranes.
• They capture light energy and excite electrons.
• Two types exist:
  • Photosystem II (PSII): absorbs light at 680 nm (P680) and splits water to replace lost electrons.
  • Photosystem I (PSI): absorbs light at 700 nm (P700) and accepts electrons from the electron transport chain.

Autotrophs vs. Heterotrophs

• Autotrophs (e.g., plants) produce their own food through photosynthesis.
• Heterotrophs (e.g., humans) obtain energy by consuming other organisms.
• Photosynthesis forms the basis of food chains and oxygen production, supporting all life on Earth.

Light-Dependent Reactions

• Occur in thylakoid membranes.
• Steps include:
  • Photon absorption: light energy is absorbed by chlorophyll, exciting electrons.
  • Electron transport chain (ETC): excited electrons are transferred, generating a proton gradient (H+).
  • ATP and NADPH production: H+ ions move through ATP synthase, generating ATP. Electrons reduce NADP+ to form NADPH
• Inputs: Light, water (H2O).
• Outputs: Oxygen (O2), ATP, NADPH.

Calvin Cycle (Light-Independent Reactions)

• Occurs in the stroma of chloroplasts.
• Steps include:
  • Carbon fixation: CO2 is incorporated into ribulose bisphosphate (RuBP) using the enzyme RuBisCO.
  • Reduction phase: ATP and NADPH are used to convert 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P), a precursor to glucose.
  • Regeneration phase: Some G3P molecules are used to regenerate RuBP, allowing the cycle to continue.
• Inputs: CO2, ATP, NADPH.
• Outputs: G3P (sugar precursors).

Chemiosmosis

• Chemiosmosis is the movement of H+ ions down their concentration gradient through ATP synthase, driving the production of ATP.
• This mechanism is essential in both light-dependent reactions and cellular respiration.