Question

Diagram and describe how eukaryotic photosynthesis works. Include a labeled diagram of the appropriate cell structures, and show where each step occurs. (The level of detail for this question should be equivalent to the respiration question. This question is about the fundamental C3 pathway; you don’t need to include photorespiration or alternate photosynthetic pathways.)

(no bullet points, essay form)

Answer 1

Green Plant, cyano bacteria and most algae, are the main photosynthesizing groups in eukaryotes. By this process they convert light energy from the sun into chemical energy in the form of glucose which can be used as a fuel to drive other biological activities. The reaction is

(in the presence of light energy), This process is responsible for the production of atmospheric oxygen needed for the sustanance of life on earth.These eukaryotes have double membrane bound organelles called chloroplasts in their cells.

They have stacks of membranous sacs (or thylakoids) called grana,which are interconnected. Inside matrix of chloroplast is called stroma. There are two parts of photosynthesis- the light reaction and the dark reaction. Light reaction takes place in thylakkoids and dark reaction takes place in stroma.

Light reaction

On the thylakkoid membrane there are Photosynthetic pigments, the major one of the being Chlorophylls. Light energy is trapped by Reactions centers on the surface of thylakoid membrane. Reaction centres contain photosynthetic and accessory pigments, enzymes , proteins and cofactors. There are two types of reaction centres= Photosystem I and Photosystem II (PSI&II). The chlorophyll molecules in the PSII complex will absorb photons and are excited, which transfer the electrons to plastoquinone QB -the first mobile electron carrier, which transfers the electron to cytochrome b6f complex, from there to plastocyanin, to PSI from there to ferridoxin. Ferridoxin to ferrodoxin NADP reductase (FNR). NADPH is made by adding the electrons and hydrogen ions to NADP. In this process the electrons lost from the Chlorophyll molecules are replaced by splitting a 2 molecules of water which also releases a molecular oxygen. This will create lot of protons (hydrogen ions) which are pumped into the lumen. The hydrogen ion gradient created by electron transport is used by ATP synthase to make ATP. Thus ATP NADPH and molecular oxygen are produced in light reaction.

Dark reaction or the calvin cycle.

This part takes place in Stroma of chloroplasts. In this CO2 from atmosphere is converted to carbohydrate(C6H12O6). ATP and NADPH produced in the light reaction is used to make carbohydrate here. CO2 is captured from atmosphere by RuBP(rubulose bi phosphate, a 5C molecule) which is catalysed by the enzyme Rubisco. The result is the formation of an unstable 6C molecule, which is quickly split into a 3C molecule called 3 Phosphoglycerate or 3PG(hence called C3 Cycle). electrons are used from NADPH to reduce 3PG to G3P (glyceraldehyde 3 phosphate) which is used as a building block for glucose. G3P is also used to regenerate RuBP using ATP. NADP and ADP retUrn to light reaction centres to continue the cycle.

The cycle has to turn 6 times to produce a glucose molecule since only one CO2 molecule is fixed per cycle.