• Photosynthesis has two important inputs: carbon dioxide and water. Both of these are used differently to create sugars (CH2O)n

CO2 + H2O = CH2O + O2

• • The process of converting carbon dioxide (CO2) in the atmosphere into organic carbon, like sugar, is called carbon fixation. The plant will use the carbon and oxygen from gaseous carbon dioxide to incorporate them into sugars.

  • (Aerobic) photosynthesis involves splitting water molecules into hydrogen and oxygen atoms (called hydrolysis) in the presence of sunlight.

    • The process keeps the hydrogen but discards the water's oxygen atoms, which are released and become the O2 that we breathe and use in respiration

    • Some prokaryotic organisms, such as the green sulfur bacteria, conduct anaerobic photosynthesis by splitting hydrogen sulfide [H2S](instead of water) to make carbohydrates. Instead of oxygen, they release sulfur.

CO2 + H2S = CH2O + S2

• In plants, photosynthesis takes place within an organelle called the chloroplast, where (simply put) the hydrogen from water is combined with the carbon dioxide from the air, to make sugars. It's more complicated than that...

  • Organisms that "fix carbon" in the presence of light are called autotrophs (e.g., plants, cyanobacteria, many algae)

  • Organisms that survive by consuming the carbon created by autotrophs are called heterotrophs (e.g., animals, fungi)

  • Chemosynthesis is the fixing of carbon in the absence of sunlight

    • These organisms use other energy sources; this process is found in archaea and some bacteria

RuBisCO

• RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the world's most abundant enzyme and is the first major player in carbon fixation.

• This enzyme is used by autotrophs, from bacteria to plants.

• The Good, the Bad, and the Ugly: RuBisCO has tricky chemistry such that it can react with either carbon dioxide or oxygen. 

  • The Good: Binding with CO2 leads to carbon fixation and the creation of sugars (glucose). 

  • The Bad: RuBisCO can also bind with oxygen if levels are sufficiently high within plant tissues.

  • The Ugly: Binding with O2 leads to photorespiration, which greatly reduces the photosynthetic capacity of many plants

• Therefore, many autotrophs have mechanisms to increase the concentration of CO2 around the enzyme, to prevent O2 binding and photorespiration

  • Some groups of plants have evolved different metabolic pathways, called C4 and CAM carbon fixation (see below), to increase photosynthetic efficiency. This is particularly found in drought-adapted plants that close their stomata to prevent moisture loss, thus reducing the amount of carbon dioxide inside the plant that RuBisCO can fix. 

  • Algae have a structure called a pyrenoid that concentrates carbon dioxide around the chloroplast. Carbon dioxide is frequently limiting in water bodies and slow to repopulate.

Adaptive Value of C3 Carbon Fixation

• C3 photosynthesis is more efficient than C4 and CAM plants under cool and moist conditions, and under normal light because it requires less machinery (fewer enzymes and no specialized anatomy).

Disadvantages of C3 Carbon Fixation

• In hot and dry environments, C3 plants close their stomata, and RuBisCO adds O2 (not CO2) to RuBP, causing photorespiration, producing a product that cannot be used in photosynthesis.

Examples

• Many plants in temperate zones have C3 metabolism (e.g., eudicot trees, wheat, rice, barley, etc.)