Problem set #6

Question 1

What general structural/electronic characteristic is shared by organic compounds that absorb visible light?

Answer 1

compounds that absorb visible light have extensive systems of conjugated double bonds

Question 2

Chlorophyll absorbs what two colors of visible light? what color light does it not absorb?

Answer 2

Chlorophyll absorbs red and blue light….and not green

Question 3

Order the electromagnetic radiation from shortest to longest:
red light
green light
ultraviolet
microwave
infrared
x-ray
gamma rays
blue light

Answer 3

Gamma rays
x-ray
ultraviolet
blue light
green light 
red light
infrared
microwave

Question 4

of the different types of radiation… which has the highest energy…lowest energy?

Answer 4

gamma rays….microwaves

Question 5

what happens when a chlorophyll molecule absorbs a photon of red light?

Answer 5

an electron is excited from the ground state to the first excited singlet state

Question 6

list 5 different mechanisms by which an excited electron can return to a ground state configuration

Answer 6

by generation of:
heat
fluorescence
chemical work
energy transfer or phosphorescence

Question 7

photosynthesis is fundamentally an electron transfer process. Where (what molecule) do the electrons come from and where do they end up (what molecule)?

Answer 7

electrons are removed from water and transferred to NADPH in the light reactions. Electrons are transferred from NADPH to reduced forms or carbon (carbohydrates) in the dark rxns

Question 8

photosynthesis occurs into 2 separate sets of rxns…. what are they?

Answer 8

light and dark rxns

Question 9

What are 2 compounds that are products of the first set of rxns that are needed for the 2nd set?

Answer 9

NADPH and ATP

Question 10

what do each of these compounds (NADPH and ATP) do ?

Answer 10

ATP provides energy and NADPH provides reducing equivalents

Question 11

Most photons that are productively absorbed in the chloroplast are absorbed by
chlorophyll molecules in what type of complex?

Answer 11

Antenna complex (or light-harvesting complex)

Question 12

What are the three photosynthetic integral membrane protein complexes that support
electron transport from water to NADPH?

Answer 12

PSII, cytochrome b6/f and PSI

Question 13

Which photosynthetic integral membrane protein complex uses energy from light?

Answer 13

PSII and PSI

Question 14

What is the longest wavelength of light that can be used by PSII and PSI

Answer 14

680 nm for PSII and 700 nm for PSI.

Question 15

How many photons/excitons are required by PSII and PSI to complete the oxidation of two
water molecules to O2?

Answer 15

Each photosystem uses 4 excitons per O2 generated (8 total)

Question 16

How do PSII and PSI provide energy for ATP synthesis?

Answer 16

They form a proton gradient across the thylakoid membrane

Question 17

Why are quinones so important in biological oxidation/reduction reactions?

Answer 17

Quinones can accept or provide a pair of electrons at once, or one at a time.

Question 18

What does ATP synthase use to drive ATP synthesis?

Answer 18

A proton gradient across a membrane

Question 19

What is an exciton?

Answer 19

An exciton refers to the unit of energy transferred from chlorophyll molecule
to chlorophyll molecule within the antenna complex to the photosynthetic
reaction center where it is used to do chemical work (pump protons across
membrane and reduce NADP to NADPH).

Question 20

What is the step/process in the light reactions with which atrazine interferes? How does it
interfere?

Answer 20

Atrazine binds to the plastoquinone-binding site in PSII blocking further
electron transport to cytochrome b6/f and PSI effectively inhibiting the light
reactions of photosynthesis

Question 21

What is the step/process in the light reactions with which paraquat interferes? How does it
interfere?

Answer 21

Paraquat takes high-energy electrons ferredoxin reduced by PSI before they
can be used to reduce NADP to NADPH, and then reacts with oxygen to
generate superoxide. While it does effectively inhibit NADPH synthesis most
of the damage caused by paraquot is due to the superoxide production, which
causes catastrophic tissue damage when a paraquat-treated plant is exposed
to sunlight