Photosynthesis and Cyanobacteria CCMs

When understanding carbon capture facilitated by biological organisms, it is first crucial to learn how photosynthesis actually works. So let's jump into it!

Stomata are tiny pores on the surface of plant leaves that allow for gas exchange of CO2 and O2. Once CO2 enters the plant, photosynthesis occurs in organelles called chloroplasts. Photosynthesis has two parts; light dependent reactions and light-independent reactions.

Light-Dependent Reactions 

The light dependent reactions take in the thylakoid membranes of the chloroplast. Photosystems are chlorophyll and protein molecules. In the first step of the light-dependent reactions, chlorophyll in Photosystem II absorbs sunlight and therefore electrons. The electrons, being in an excited state, move through the thylakoid membrane via proteins in Photosystem II and an electron transfer chain is formed. Next, negatively charged electrons in the thylakoid membrane attract positively charged H+. When this happens, water molecules split and the electrons released replenish Photosystem II. Oxygen that used to be part of water molecules is released and exits into the atmosphere. Chlorophyll in Photosystem I absorbs sunlight and this, once more, creates an electron transfer chain. Electrons help bind NADP+ and H+ to create NADPH which is responsible for carrying hydrogen. By this time, high amounts of H+ have accumulated in thylakoid membranes attracted by electrons. H+ diffuses through protein ATP synthase and then helps bind ADP to P to create ATP.

Light-Independent Reactions (Calvin Cycle)

A 5-carbon molecule called RuBp bonds with CO2 to form a 6-carbon molecule with the help of an enzyme called RuBisCO. ATP and NADPH produced during light-dependent reactions break the 6-carbon molecule into two 3-carbon molecules called phosphoglycerides (PGAs). Some of these PGAs rearrange to create glucose, but others recombine to make new 5-carbon molecules which keeps the cycle going. 

Cyanobacteria CO2 Concentrating Mechanisms

Cyanobacteria, known for their carbon sequestration abilities, do not have chloroplasts, so their process of creating glucose varies from plants. Cyanobacteria have thylakoid membranes in their cytoplasm that contain structures called carboxysomes. Carboxysomes are microcompartments with highly selective protein shells. They contain enzymes such as RuBisCO and carbonic anhydrase, critical for carbon fixation. Carbonic anhydrase converts -HCO3 into CO2. This is because -HCO3, having a negative charge, can easily enter the carboxysome, but CO2 is required for glucose to be produced. Carbonic anhydrase activity is also crucial for concentrating CO2 in the carboxysome in order to avoid wasteful photorespiration if RuBisCO reacts with O2.