Chapter 3 - Photosynthesis Flashcards
The electromagnetic spectrum
photosynthesis is an endergonic process powered by visible light
visible light is the portion of the electromagnetic spectrum with wavelengths between 400 and 750 nm
smaller wavelengths have a higher frequency and higher energy (blue light)
larger wavelengths have a smaller frequency and lower energy (red light)
Pigments
plant chloroplasts contain numerous types of pigments (terpenes) that each absorb specific wavelengths of light and reflect others
multiple pigments maximize absorption of visible light
chlorophyll is usually the most concentrated pigment in plants
Absorption spectra of photosynthetic pigments
chlorophylls A and B absorb violet, blue, red light and reflect green light
carotenoids absorb blue and green light and reflect orange light
anthocyanins absorb green light and reflect red and purple light
Action spectra of photosynthesis
indicates the rate of photosynthesis at each wavelength of light in isolation
photosynthesis generally occurs best at blue and red wavelengths due to the chlorophyll, and less well at other wavelengths
from a plant that is photosynthesizing, an action spectrum can be produced, while absorption spectra are produced from a test tube
Chloroplast structure
double membrane structure
the inner membrane forms hollow disk-like sacs called thylakoids, that form stacks called granan
the fluid surrounding the thylakoids is called stroma
Chromatography
pigments are placed at the end of a chromatography strip, then dipped into a nonpolar solvent
since like dissolves like, than the more non-polar pigments will dissolve better
as the solvent moves up the paper, the non-polar molecules will move up the paper at a faster rate than the polar ones
this leads to a range of distances traveled, which can then be calculated
Photoactivation of photosystem II
light rays of various wavelengths strike PSII
various antenna pigments absorb useful wavelengths are relay this to the chlorophyll A reaction center
on average, PSII absorbs 680 nm wavelengths
a pair of the reaction center’s electrons are excited, and therefore more easily removed by an electron transport system in the thylakoid membrane
Photolysis of water
the PSII chlorophyll A reaction center molecule has been oxidized and its electron pair must be replaced
a PSII enzyme oxidizes water into an oxygen atom, 2 hydrogen ions, and a pair of electrons which reduce the PSII reaction center
the oxygen atoms combine as O2, which is then diffused out as waste through stomates
Electron transport
as the electrons pass from carrier to carrier, some of the energy is used to pump protons into the thylakoids from the stroma
the protons accumulate and eventually diffuse out to the stroma again through channels in ATP synthase complexes
forms ATP from ADP and P in the process
this is called non-cyclic photophosphorylation
Photoactivation of photosystem I
light rays of various wavelengths strike PSI
the appropriate antenna pigments absorb useful wavelengths and transfer to energy to the reaction center
on average, PSI absorbs photons with an average wavelength of 700 nm
one pair of the reaction center’s electrons is excited, and therefore more easily removed by its electron transport system
this pair of electrons is replaced by the pair from the PSII electron transport chain
Reduction of NADP+
the excited electron pair from PSI passes from carrier to carrier and reduces a molecule of NADP+
this then forms NADPH, which is used in the light independent reactions
Cyclic photophosphorylation
in primitive versions of photosynthesis, PSI works on its own
electrons from the reaction center of PSI are excited by the antenna pigments and are transferred to the first electron carrier of the PSI electron transport chain
they are then passed to the electron transport chain normally associated with PSII
the electrons return to PSI and are used to generate ATP by chemiosmosis, but not NADPH
this is why PSII is before PSI
this process is cyclic since the electrons start and end at the same place
Melvin Calvin
Calvin determined the light dependent reactions while a professor at Berkeley
he got the 1961 Nobel prize in chemistry for this experiment
Experiment setup
algae were grown in lollipop flasks and provided with radioactive CO2
they were illuminated for a few seconds, then immediately dumped into boiling alcohol, stopping the light independent reactions
the molecules of algae were then separated using 2D chromatography, then rotated 90 degrees and placed back in the solvent
this allowed for better separation of the molecules
Experiment results
placing the chromatography paper in contact with a photographic plate for several days allowed for the radioactive spots to develop the film
theses spots were cut out of the paper and the radioactive molecules were redissolved and identified
Calvin then repeated the process with longer illumination of the algae to find all the radioactive organic carbon molecules
The C3 Cycle
occurs in the chloroplast stroma
the first molecule Calvin could identify had 3 carbons, so this cycle is called of the C3 cycle, or the Calvin cycle
begins and ends with the 5 carbon sugar RuBP (ribulose bisphosphate)
Carbon fixation
rubisco (an enzyme) combines 3 CO2 with 3 RuBP forming three unstable 6 carbon compounds
these then spontaneously split into six 3 carbon molecules of 3-phosphoglycerate
this step therefore converts inorganic compounds (CO2) to organic compounds
Reduction to glyceraldehyde phosphate (PGAL)
the free energy of ATP and the electrons from NADPH are transferred to the 3-phosphoglycerate
this then forms PGAL, which can be later converted into glucose
Synthesis of carbohydrates and other products
5 out of the 6 PGAL are used to regenerate 3 molecules of RuBP
this process also requires 3 ATP, which is converted into 3 ADP and 3 phosphates
one ATP per RuBP that is made
Synthesis of carbohydrates and other products
2 PGAL out of each 12 generate in the C3 cycle enter the metabolic pool
the PGAL are then converted to glucose by reversing the glycolysis pathway
PGAL can also be converted to glycerol, fatty acids, amino acids, and other metabolites
in order to produce amino acids, PGAL requires amino groups that are obtained from the soil
Photorespiration problem
rubisco can accept O2 in place of CO2
similar to competitive inhibition, except that a reaction still happens, just with the wrong substrate
in hot, dry conditions, plants close their stomata to conserve water
CO2 can’t get into the leaf, and O2 can’t get out, so the oxygen outcompetes the CO2 for the rubisco active site
Photorespiration results
rubisco adds oxygen to RUBP and the product splits into a 3 carbon molecule (PGAL) and a 2 carbon molecule that exits the cycle, and is broken down with no benefit to the plant
photorespiration reduces photosynthetic output by siphoning away as much as 50% of the carbon fixed by the C3 cycle
C3 plants
in normal C3 photosynthesis, the light dependent and light independent reactions both take place in the same chloroplasts in the same mesophyll cells
bundle-sheath cells just form a layer around the leaf veins and don’t photosynthesize
C4 plants
mesophyll cells and bundle sheath cells form concentric layers around the leaf veins
the mesophyll cells and bundle sheath cells have different types of chloroplasts
the light dependent reactions happen in the mesophyll cells
the light independent reactions happen in the bundle sheath cells
C4 cycle
C4 mesophyll cells have the enzyme PepCO that carboxylates phosphoenolpyruvate, making oxaloacetate
oxaloacetate is then converted to malate and exported to adjacent bundle sheath cells through plasmodesmata
the malate is decarboxylated in bundle sheath cells
Minimizing photorespiration
PepCo has no attraction for O2, so the CO2 can be fixed even when the stomata are closed
as well, by releasing CO2 right next to rubisco, photorespiration does not occur
this gives C4 plants (corn) an advantage over C3 plants in drought ridden areas
CAM
occurs in plants such as cacti
stomata are closed during the day to conserve water
the stomata open at night, and the CO2 is fixed by PepCo, forming various organic acids
the organic acids formed at night are decarboxylated the next day, releasing CO2 for the C3 cycle, preventing photorespiration
C3, C4, and CAM plants
all C3, C4, and CAM plants use the C3 cycle
in C4 plants, the initial carbon fixation is in a different place than the C3 cycle
in CAM plants, the initial carbon fixation is at a different time than the C3 cycle
Light intensity
as light intensity increases, the rate of photosynthesis also increases
however, it only increases to the point where something else becomes a limiting factor
if plants are illuminated excessively, then they might overheat
Wavelength
photosynthesis works better with red or blue light
this is because the primary photosynthetic pigment is chlorophyll, and it absorbs light at these wavelengths
Temperature
as temperature increases, the rate of enzyme catalyzed reactions also increases
faster diffusion of substrates and products into/away from the active site
increased flexibility of the enzyme
after a certain point, the enzymes become denatured and photosynthesis stops
Carbon dioxide concentration
as the concentration of carbon dioxide increases, the rate of photosynthesis increases
however, it only increases up to the point where something else becomes the limiting factor
Oxygen concentration
as the concentration of oxygen goes up, the rate of photosynthesis goes down
this is due to the effect of photorespiration
