Photosynthesis: Light Reactions I

Question 1

Photosynthesis

Answer 1

Most important biological phenomenon
Only energy input mechanism in the biosphere
conversion of sunlight energy into usable chemical energy
60% land, 40% oceans
uses carbon dioxide and water to produce oxygen and glucose
opposite of cellular respiration

Question 2

Photosynthesis Equation

Answer 2

6H2O + 6CO2< ———-> C6H12O6+ 6O2
or
CO2 + H2O C(H2O) + O2

oxidation/reduction processes
H2O is oxidized
CO2 is reduced

Question 3

Oxidation/reduction separate processes

Answer 3

separate
H2O oxidation
		requires light
		light converted to chemical energy
CO2 reduction
		doesn't directly require light
		consumes chemical energy

Question 4

Oxidation/reduction related processes

Answer 4

Carbon Reactions depend on Light Reactions for energy
Light Reactions depend on Carbon Reactions for electron acceptors
Light regulates some Carbon Reaction enzymes

Question 5

Chloroplast

Answer 5

primary light transducer
converts light to chemical energy
site of photosynthesis

Question 6

Chloroplast structure

Answer 6

vary in size and shape (round to oval, 4 - 10 μm x 1 μm x 2 μm)
double outer membrane (envelope)
6 - 8 nm
10-20 nm gap
outer membrane - freely permeable
inner membrane - selectively permeability
has an internal membrane system

Question 7

Parts of the chloroplast

Answer 7

Thylakoid: flattened sac, site of PET, pigments, photophosphorylation
Grana: stacks of thylakoids
Stroma: outside thylakoids (Space), CO2 fixation (reduction), has protein, DNA, ribosomes
Stroma lamellae: connects grana
outer and inner membranes

Question 8

Transport in the Chloroplast

Answer 8

permeability into/out of chloroplast is limited
gases are freely permeable
for other molecules and ions, transporters are needed (phosphate/phosphate ester transporter ,dicarboxylate transporter)

Question 9

Phosphate / Phosphate Ester transporter

Answer 9

PGA = 3-phosphoglyceric acid
DHAP = dihydroxyacetone phosphate
principal means to export carbon from chloroplast
most significant transporter
tightly regulated based on [phosphate] and [P-esters]
strict counter exchanger

Question 10

Dicarboxylate transporter

Answer 10

not strict exchanger

supply ATP to chloroplast in the dark

Question 11

Chloroplast Metabolic/Genomic Autonomy

Answer 11

Synthesize lipids, amino acids, nucleic acids
Multiple copies of DNA (circular / linear), about 120 genes
transcribes / translates own genes
under control of nuclear DNA
Genome / protein synthesis similar to bacteria
- promoters and terminators are the same
- ribosome structure and antibiotic sensitivity
- N-formylmethionine
- transcribed / translated by E. coli system
- endosymbiont theory

Question 12

Wavelength + Energy

Answer 12

Low wavelength, high energy

high wavelength, low energy

Question 13

Energy of a photon formula

Answer 13

E=hc/λ
h = 6.624 x 10-34 joule sec
c = 3 x 1010 cm / sec
each photon has a discrete energy

Question 14

Absorption of Light: 3 laws

Answer 14

(1) Grotthaus-Draper Law: for a photon to be used in a photochemical reaction, the photon must be absorbed.
(2) Einstein’s Law of Photochemical Equivalence: one photon will only excite one molecule.
(3) Einstein-Stark Law: a molecule can only absorb one photon at a time and this photon can only cause the excitation of 1 electron.

Question 15

Photon Absorption

Answer 15

when photon absorbed, e- is moved away from + nucleus
distance proportional to energy
sometimes e- is lost (return to ground state)
absorbed photon = excited (singlet) state (nanosec)

Question 16

Reduction Potential

Answer 16

measure of the ability of a compound to donate an electron
+ = readily accepts electron
- = readily donates electron

Question 17

Five fates of the excited state

Answer 17

1) heat - most common fate
2) fluorescence (light)
3) triple state and phosphorescence
4) energy transfer
5) photosynthesis

Question 18

1st Fate: Heat

Answer 18

blue light —–> second singlet state
| (heat)
v
red light —–> first singlet state
| (heat)
v
ground state

				rapid (10 -9 sec)

Question 19

2nd Fate: Fluorescence

Answer 19

(light)
first singlet
	|   (fluorescence)
        v
ground state

Question 20

3rd Fate: Triplet State and Phosphorescence

Answer 20

(electrons with parallel spin)
spin reverses and emits light
half life = 10 -3 sec

Question 21

4th Fate: Energy transfer

Answer 21

transfer to neighboring molecule

energy transferred, NOT electron

Question 22

5th Fate: Photosynthesis

Answer 22

transfer of energy AND electron

Question 23

Absorption Spectrum

Answer 23

• waves, not lines
• not all molecules absorb all wavelengths (energies) equally
• spectrum = absorption = f (wavelength)
• only a specific amount of energy will excite a molecule to excited state
• only these wavelengths will be absorbed
• slight changes in the chemical structure can change the absorption spectrum

Question 24

Excited State

Answer 24

a series of different energy levels

each level is reached by different wavelengths

Question 25

Photosynthetic Pigments

Answer 25

chlorophyll: higher plants/algae
carotenoids: higher plants/algae
phycobilins: only algae

Question 26

Types of chlorophyll

Answer 26

chl a: all plants
chl b: higher plants/some algae
chl c, d, e: only algae

Question 27

chl a (structure)

Answer 27

cyclic tetrapyrrole (porphyrin)
contains Mg +2
associated with proteins
phytol chain  -  imbedded in thylakoid
changes in interaction with membrane can change absorption spectrum

Question 28

How do we know chl a is involved in photosynthesis?

Answer 28

• compare the absorption spectrum and the action spectrum

- action spectrum is activity as a function of wavelength – to correlate absorption of light with biological functions

Question 29

chl a

Answer 29

the only one that can donate an electron
(other chls can’t)

chl b deficient  -> plants live
chl a deficient  -> plants die

only some chl a molecules can donate e –
-> these are called reaction centers

Question 30

Carotenoids

Answer 30

pigments found in almost all photosynthetic organisms
Two classes of carotenoids:
1) carotenes: C, H only (hydrocarbons)
β - carotene (most common/ food
coloring, orange)
-> Cleaving the beta-carotene in half
produces two molecules of Vitamin A

2)xanthophyll: C, H, O
many types

present in thylakoids & cytoplasm

Question 31

Roles of Carotenoids

Answer 31

1. photosynthesis
   
   • absorb light, transfer energy (not electron) to chl
2. protection
   high light + O2 —> superoxide radicals (O2-)
   How can you test that carotenoids serve a protective
   function? Disrupt it and observe the effect.

Question 32

superoxide radicals

Answer 32

they are destructive to chlorophyll
> carotenoids remove superoxide as it
is formed
or
prevent its formation
> How is it removed? with superoxide dismutase
> How is it prevented? xanthophyll cycle
(zeaxanthin violaxanthin)
violaxanthin can transfer energy to chl
zeaxanthin cannot, it can receive energy from chl
difference btwn molecules: oxygen removed to
form double bonds

Question 33

Xanthophyll cycle

Answer 33

High light:
   violaxanthin converted to zeaxanthin
   zeaxanthin receives excess energy from chl and 
   dissipates it as heat
   reduces photooxidative damage to chl

Low light:
zeaxanthin converted to violaxanthin
violaxanthin transfers energy to chl