What is photosynthesis and why it is important
Photosynthesis is a process during which energy from light is harvested and used to drive synthesis of organic carbohydrates from carbon dioxide and water, generating oxygen. Photosynthesis is the only way that radiant energy from the sun can be converted into organic molecules for plants and animals to consume.
Structure of Chloroplasts
Chloroplasts are specialized organelles in plant cells for the purpose of photosynthesis. Each cell may contain 1-1000 copies of chloroplasts. Chloroplasts are double membrane structure with stacked disc-like membrane structure (called thylakoids) inside the stroma. Light reactions of photosynthesis occur in thylakoids, and dark reactions occur in stroma.
Physics of light
Quantum is the elemental unit of energy. Photon is a quantum of electromagnetic energy and it is particle of light.
Light reaction and photophosphorylation
Pigments embedded on thylakoid membranes form photosystems. There are of two types: PS I P700, PS II P680. Components of photosystem I and II transfer the electrons from water to NADP via cyclic electron transfer or non-cyclic electron transfer. During electron transfer, the light energy captured by the photosynthetic organisms is transformed into the phosphate bond energy of ATP. This is called photophosphorylation. NADPH is generated during non-cyclic electron transfer.
Dark reaction – Calvin cycle
Second step of photosynthesis is called Calvin’s cycle. Because it does not require light, so it is called dark reaction. During dark reaction, the ATP and NADPH generated by light reaction are consumed to fix carbon dioxide into organic carbohydrates. The first fixed carbohydrate is a three carbon compound 3-phosphoglycerate (3PGA). The final product is a high-energy 3 carbon compound glyceraldehyde-3-phosphate (G3P) which can be used to synthesize a broad range of organic molecules. An important intermediate molecule for carbon dioxide fixation is ribulose bisphosphate (RuBP), and the enzyme catalyzing the CO2 fixation is Rubisco.
Photorespiration
Under high oxygen and low carbon dioxide conditions, Rubisco favors binding to oxygen instead of carbon dioxide, therefore the energy produced in light reactions are consumed for no productivity of organic carbohydrates. The final result is that oxygen is consumed and CO2 is produced, which mimics respiration, and therefore named photorespiration.
C4 pathway
C4 cycle is the pathway adopted by C4 plants to conserve the carbon dioxide released via photorespiration. It adopts a new enzyme (PEPC, Phosphoenol pyruvate carboxylase) which does not react with oxygen; it also adopted a separated compartment for CO2 up taking and fixation.
CO2 is taken in mesophyll cells and further fixation occurs in bundle sheath cells. Carbon dioxide is incorporated to form 4-carbon oxaloacetate.
