Photosynthesis: Light Reactions I Flashcards by Audrey Viel

Photosynthesis

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

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Photosynthesis Equation

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

oxidation/reduction processes
H2O is oxidized
CO2 is reduced

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Oxidation/reduction separate processes

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

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Oxidation/reduction related processes

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

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Chloroplast

primary light transducer
converts light to chemical energy
site of photosynthesis

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Chloroplast structure

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

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Parts of the chloroplast

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

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Transport in the Chloroplast

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)

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Phosphate / Phosphate Ester transporter

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

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Dicarboxylate transporter

not strict exchanger

supply ATP to chloroplast in the dark

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Chloroplast Metabolic/Genomic Autonomy

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

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Wavelength + Energy

Low wavelength, high energy

high wavelength, low energy

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Energy of a photon formula

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

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Absorption of Light: 3 laws

(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.

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Photon Absorption

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)

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Reduction Potential

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

Five fates of the excited state

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

1st Fate: Heat

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

				rapid (10 -9 sec)

2nd Fate: Fluorescence

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

3rd Fate: Triplet State and Phosphorescence

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

4th Fate: Energy transfer

transfer to neighboring molecule

energy transferred, NOT electron

5th Fate: Photosynthesis

transfer of energy AND electron

Absorption Spectrum

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

Excited State

a series of different energy levels

each level is reached by different wavelengths

Photosynthetic Pigments

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

Types of chlorophyll

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

chl a (structure)

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

How do we know chl a is involved in photosynthesis?

- 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

chl a

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

Carotenoids

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

Roles of Carotenoids

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.

superoxide radicals

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

Xanthophyll cycle

``` 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