Chapter 3 - Photosynthesis

Question 1

The electromagnetic spectrum

Answer 1

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)

Question 2

Pigments

Answer 2

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

Question 3

Absorption spectra of photosynthetic pigments

Answer 3

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

Question 4

Action spectra of photosynthesis

Answer 4

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

Question 5

Chloroplast structure

Answer 5

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

Question 6

Chromatography

Answer 6

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

Question 7

Photoactivation of photosystem II

Answer 7

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

Question 8

Photolysis of water

Answer 8

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

Question 9

Electron transport

Answer 9

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

Question 10

Photoactivation of photosystem I

Answer 10

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

Question 11

Reduction of NADP+

Answer 11

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

Question 12

Cyclic photophosphorylation

Answer 12

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

Question 13

Melvin Calvin

Answer 13

Calvin determined the light dependent reactions while a professor at Berkeley
he got the 1961 Nobel prize in chemistry for this experiment

Question 14

Experiment setup

Answer 14

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

Question 15

Experiment results

Answer 15

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

Question 16

The C3 Cycle

Answer 16

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)

Question 17

Carbon fixation

Answer 17

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

Question 18

Reduction to glyceraldehyde phosphate (PGAL)

Answer 18

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

Question 19

Synthesis of carbohydrates and other products

Answer 19

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

Question 20

Synthesis of carbohydrates and other products

Answer 20

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

Question 21

Photorespiration problem

Answer 21

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

Question 22

Photorespiration results

Answer 22

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

Question 23

C3 plants

Answer 23

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

Question 24

C4 plants

Answer 24

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

Question 25

C4 cycle

Answer 25

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

Question 26

Minimizing photorespiration

Answer 26

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

Question 27

CAM

Answer 27

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

Question 28

C3, C4, and CAM plants

Answer 28

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

Question 29

Light intensity

Answer 29

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

Question 30

Wavelength

Answer 30

photosynthesis works better with red or blue light

this is because the primary photosynthetic pigment is chlorophyll, and it absorbs light at these wavelengths

Question 31

Temperature

Answer 31

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

Question 32

Carbon dioxide concentration

Answer 32

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

Question 33

Oxygen concentration

Answer 33

as the concentration of oxygen goes up, the rate of photosynthesis goes down
this is due to the effect of photorespiration