Photosynthesis Flashcards
Non-cyclic phosphorylation
Light energy is absorbed by photosystems in the thylakoid membrane. When PSII absorbs light energy, two electrons become excited and are lost. These go through the electron transport chain. The series of redox reactions gives enough energy for H+ ions to be pumped from the stroma to the thylakoid. This creates an electrochemical and proton gradient. The only way the H+ ions can come back to the stroma is through the carrier proteins. This is where ATPsynthase is present. This enzyme catalyses the phosphorylation of ADP by coupling the energy of the proton motive force. The two electrons and then accepted by another photosystem, PSI. Light energy was also absorbed by PSI at the same time as PSII. Therefore, this photosystem also lost two electrons due to these becoming excited. These also go through electron transport chain and maintain the electrochemical gradient. When they pass through the carrier protein, ADP is phosphorylated again. Ultimately, these electrons react with 2H+ ions to reduce NADP. To replace the electrons lost by PSII, photolysis occurs. This is a process where one water molecule splits to form two electrons, two hydrogen ions and oxygen as a byproduct.
Cyclic phosphorylation
Light is only absorbed by PSI and therefore only ATP and redNADP is produced.
Light-independent stage- calvin cycle
Carbon dioxide reacts with ribulose bisphosphate (5-carbon) to form a 6-carbon unstable compound. The process of carboxylation is catalysed by the enzyme, RuBisCO.
This 6-carbon compound is then converted to 2 3-carbon compounds, called glycerate 3 Phosphate (GP)
After this, this compound is converted to 2 Triose Phosphate compounds. To produce this, the glycerate phosphate groups accept hydrogens from redNADP as well as use an ATP.
The triose phosphate groups are then converted to ribulose biphosphate (5-carbon). This reaction also requires an ATP. Every time, an ATP is used, it is broken down to ADP to P+
What can triose phosphate is used for
Amino acids Fatty acids Glycerol Nucleic acids and GLUCOSE
Where does the glucose come from
Carbon from carbon dioxide
Hydrogen comes from water
Oxygen come from carbon dioxide
What is the first sugar in the calvin cycle
Triose phosphate
A sugar has ‘ose’ in their name.
How many turns does the calvin cycle need to occur for a glucose molecule to form
6 times since a glucose molecule consists of 6 carbons.
How do we know that chloroplasts were initially prokaryotes
Chloroplasts are only produced from the division of other chloroplasts, which is a separate process from plant division.
Chloroplasts have their own genome consisting of a circular loop of DNA, much as is present in prokaryotes today
Chloroplasts have their own ribosomes (70s) and their own protein synthesis mechanism.
Chloroplasts contain similar pigments to photosynthesising cyanobacteria
Through what process did photosynthesising bacteria evolve to chloroplasts in eukaryotic cells
Through endosymbiosis
Features of a chloroplast
Has triple membranes
The outmost membrane allows many small ions and small molecules to pass through into the chloroplast. Whereas, the inner membrane relies on transport proteins to allow transport of certain chemicals, thus controlling molecules entering or leaving the chloroplasts, moving between the stroma and the cytosol.
The innermost membrane is arranged into a structure of thylakoid. A column of thylakoid is called granum. Multiple granum are called grana. These grana are interlinked by intergranal lamellae. A liquid surrounds the thylakoid structures. There are lipid droplets, starch grains and enzymes in the stroma that are needed for the light-independent reactions.
Where are photosystems located
In the thylakoid membranes
The granal stacks increase the surface area of the thylakoids, allowing many photosystems to be present for maximum light absorbance. This also allows multiple electron carriers and ATP synthase enzymes to carry out the light-dependent reactions.
What is a photosystem
It contains accessory and primary pigments which absorb different wavelengths of light
