Lecture 15 - Metabolic Diversity

How do photosynthetic structures differ between eukaryotes and prokaryotes?

• Eukaryotes (e.g., plants, algae) have chloroplasts where photosynthesis occurs in the thylakoid membranes.

• Prokaryotes (e.g., cyanobacteria) lack chloroplasts but have infolded membranes (thylakoid-like structures) where photosynthesis takes place.

How is light energy harvested in a photosystem?

• Light-harvesting pigments absorb light and transfer energy to the reaction center (RC) pigment.

• The RC pigment excites an electron and passes it through an electron transport system (ETS).

• The ETS generates ATP and NADPH.

• A reduced molecule (H₂A) donates electrons to replace those lost from the RC pigment.

Why is O₂ made in oxygenic photosynthesis instead of something else?

Water (H₂O) is split to replace lost electrons in the reaction center, releasing O₂ as a byproduct.

• This occurs in plants, algae, and cyanobacteria.

• In anoxygenic photosynthesis (e.g., purple sulfur bacteria), molecules like H₂S are used instead, producing sulfur or sulfate.

Which photosystems are used in cyclic and noncyclic photophosphorylation?

• Cyclic photophosphorylation: Only Photosystem I (PSI) → Produces ATP only.

• Noncyclic photophosphorylation: Photosystem I (PSI) and Photosystem II (PSII) → Produces ATP and NADPH.

What are the products and steps of the Calvin cycle? How many cycles are needed to make one glucose?

Steps:

1. Carbon fixation – CO₂ is attached to RuBP by RuBisCO.

2. Reduction – ATP and NADPH are used to convert 3-PGA into G3P.

3. Regeneration – Some G3P is used to regenerate RuBP.

• 3 cycles → 1 G3P

• 6 cycles → 1 glucose

How can organisms be classified based on carbon and energy sources?

• Photolithoautotrophs: Use light for energy, inorganic molecules for electrons, and CO₂ as their carbon source. Examples include plants, cyanobacteria, and green sulfur bacteria.

• Photoorganoheterotrophs: Use light for energy, organic molecules for electrons, and organic compounds for carbon. Examples include green and purple nonsulfur bacteria.

• Chemolithoautotrophs: Use chemical energy from inorganic compounds, inorganic molecules for electrons, and CO₂ as a carbon source. An example is sulfur-oxidizing bacteria.

• Chemoorganoheterotrophs: Use chemical energy from organic compounds, organic molecules for electrons, and organic compounds as a carbon source. This group includes animals, fungi, pathogens, and most bacteria.

What are the steps of the nitrogen cycle, and what bacteria contribute to it?

1. Nitrogen Fixation – N₂ → NH₄⁺ (by cyanobacteria, Azotobacter, Rhizobium)

2. Ammonification – Organic nitrogen (dead matter) → NH₄⁺ (by decomposers)

3. Nitrification – NH₄⁺ → NO₂⁻ (Nitrosomonas), then NO₂⁻ → NO₃⁻ (Nitrobacter)

4. Denitrification – NO₃⁻ → N₂ gas (by Pseudomonas, Escherichia coli)

Effects of adding/removing inorganic molecules:

• Adding NH₄⁺ → Increases plant growth.

• Removing NO₃⁻ → Reduces nitrogen availability, slowing plant growth.