Photosynthesis Summary

Photosynthesis

Modes of Nutrition

  • Heterotrophs: Obtain nutrients by consuming other organisms.
  • Autotrophs: Produce their own nutrients.
  • Photoautotrophs: Use light as their energy source.

Photosynthesis in Plants

  • Occurs in plants, algae, and some eukaryotes/prokaryotes; primarily in plant leaves.
  • Chloroplasts within mesophyll cells are the site of photosynthesis.
  • Stomata facilitate CO2 and O2 exchange.
  • Chloroplasts have two membranes surrounding the stroma.
  • Thylakoids are sac-like structures within the stroma; grana are stacks of thylakoids.

Photosynthesis Equation

  • Reactants: Carbon dioxide, water, and light energy.
  • Products: Glucose and oxygen.
  • Photosynthesis is an endergonic and redox process.
  • Carbon dioxide is reduced to glucose; water is oxidized to oxygen.

Light Energy

  • Electromagnetic spectrum ranges in wavelength.
  • Visible light ranges from 380 nm to 750 nm.
  • Light travels in rhythmic waves, with photons as fixed quantities of energy.
  • Energy within a photon is inversely related to wavelength.

Chlorophyll and Pigments

  • Pigments absorb visible light; chlorophyll is the primary pigment in chloroplasts.
  • Chlorophyll absorbs violet, blue, and red light, reflecting green.
  • Chloroplasts contain chlorophyll a, chlorophyll b, and carotenoids.
  • When chlorophyll absorbs light, electrons are elevated to an excited state, then return to the ground state releasing energy.

Stages of Photosynthesis

  • Stage 1: Light Reactions: Light energy converted to chemical energy.
  • Stage 2: Calvin Cycle: Chemical energy used to build sugars.

Light Reactions

  • Occur in thylakoids.
  • Light energy produces ATP and NADPH.
  • Photosystems harness light energy.
  • Electron transport chain generates chemical energy.
  • Oxygen is a byproduct.
  • Photosystems contain a reaction-center complex with chlorophyll and a primary electron acceptor.
  • Light-harvesting complexes contain pigment molecules.
  • Photosystems II (P680) and I (P700) work together.
  • Electrons are passed through electron transport chains, creating a proton gradient used by ATP synthase to produce ATP.
  • NADP+ is reduced to NADPH.

Calvin Cycle

  • Occurs in stroma.
  • ATP and NADPH from light reactions are used to build sugars.
  • CO2 is the building block.
  • Three phases: carbon fixation, reduction, and regeneration of RuBP.
  • Carbon Fixation: CO2 is fixed to RuBP by Rubisco, forming 3-phosphoglycerate.
  • Reduction: ATP and NADPH transform 3-phosphoglycerate into G3P.
  • Regeneration: ATP regenerates RuBP from remaining G3P.
  • To produce one G3P, 9 ATP and 6 NADPH are used.

Photorespiration

  • Occurs when Rubisco binds oxygen instead of carbon dioxide due to stomata closure in hot/dry conditions.

C4 and CAM Plants

  • C4 plants: Spatially separate carbon fixation and Calvin cycle.
  • CAM plants: Temporally separate carbon fixation and Calvin cycle.