Chloroplasts and Photosynthesis

Overview of Photosynthesis §

• Photosynthesis was thought to have first developed from photoautotrophic prokaryotic cells that were assimilated by other prokaryotic cells.
  • Although originally consuming Hydrogen Sulfide, the ones assimilated were able to make use of water.
  • The chloroplast is the byproduct of this assimilation.

Chloroplasts §

• Within plants, chloroplasts are predominantly located in the mesophyll cells.
  • They are lens shaped and giant relative to other organelles.
  • They arise by fission from existing chloroplasts or proplastids.
• The chloroplast consists of membranous sacs called thylakoids arranged into grana
  • Thylakoids have a space inside of them called the lumen.
  • The thylakoid membrane contains the chlorophyll and protein complexes that power the processes of the chloroplast
    • The membrane is high protein, low phospholipid, and high galactose-containing glycolypids which makes the layer highly fluid (see more here).
• The stroma pertains to the space outside of the thylakoids. It contains enzymes for carbohydrate synthesis.
  • The stroma contains small double stranded circular DNA molecules and ribosomes.

Photosynthesis §

• Photosynthesis mainly consists of light dependent and light independent reactions.

Light Dependent Reactions §

• Energy from sunlight as photons is absorbed and stored as chemical energy in and .
• When a photon strikes a chlorophyll molecule, an electron in the molecule becomes excited, which then fuels the rest of the reaction (similar to the Electron Transport Chain)
  • This process absorbs most of the energy from the light.
  • Chlorophyll itself absorbs red and blue light, hence why it is colored green.
  • Another pigment called Carotenoids absorb blue and green regions. They act as secondary light collectors, and draw excess energy away from excited chlorophyll molecules.
• Each chlorophyll is organized into photosynthetic units with only one reaction-center chlorophyll being responsible for the transfer of electrons. The remaining chlorophyll act as absorption units. * Photosystem II boosts electrons from an energy level below that of water to a midway point * Photoinhibition is a phenomenon which involves the formation of a very strong oxidizing agent in PS-II due to high intensity light. * Photosystem I raises the energy level to that above . * PS-I was capable of cyclic photophosphorylation which produces ATP on its own by recycling used electrons. * It is thought that this gives additional ATP for carbohydrate synthesis.

Light Independent Reactions §

• Carbohydrates are synthesized from using the and synthesized from the light dependent reactions.
• These reactions are much more rapid and do not require light.

C3 Plants §

• Makes use of RUBISCO to fix with ,
  • It is one of the most important molecules that can fix inorganic carbon.
  • It is one of the most abundant, and yet one of the most inefficient enzymes.
    • It causes photorespiration which releases previously fixed molecules. This is more apparent in hot, dry conditions due to the plants closing their stomata to avoid water loss but also gas exchange.
    • Photorespiration appears to be unavoidable since favors over .
    • It becomes more apparent when the concentration of in the cell becomes too high.
• The fixing of carbon to organic carbohydrates follows the Carbon Cycle that comprises of:
  • The carboxylation of to form , a three carbon molecule.
  • The reduction of to the level of a sugar by forming using and .
  • Regeneration of using .
• The Calvin cycle is regulated. The enzymes in the pathway are more active in alkaline .

C4 Plants §

• Makes use of PEP Carboxylase to fix with PEP to form oxaloacetic acid, a four Carbon molecule.
  • PEP carboxylase continues to operate at much lower levels and is not inhibited by .
  • It can, therefore, bypass photorespiration more effectively.
  • However, this comes at the cost of requiring more and than the C3 pathway.
• It releases extra which is then consumed in a regular C3-pathway.

CAM Plants §

• Adapted for very hot and dry habitats.
• They make use of PEP Carboxylase for carbon fixation. However, they carry out the light dependent reactions and carbon fixation at different times of the day.

Links §

• Mitochondrion - a similar organelle.

• Cellular Respiration - for a related process to Photosynthesis

• Karp Ch. 6

  • 6.6 - 6.8 - specific operations of the two Photosystems and photophosphorylation.
  • 6. 9 - 6.10 - specifics of the dark operations in C3 / C4 and CAM plants.