Photosynthesis: Light Reactions I Flashcards
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
Photosynthesis Equation
6H2O + 6CO2< ———-> C6H12O6+ 6O2
or
CO2 + H2O C(H2O) + O2
oxidation/reduction processes
H2O is oxidized
CO2 is reduced
Oxidation/reduction separate processes
separate H2O oxidation requires light light converted to chemical energy CO2 reduction doesn't directly require light consumes chemical energy
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
Chloroplast
primary light transducer
converts light to chemical energy
site of photosynthesis
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
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
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)
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
Dicarboxylate transporter
not strict exchanger
supply ATP to chloroplast in the dark
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
Wavelength + Energy
Low wavelength, high energy
high wavelength, low energy
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
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.
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)
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 state3rd 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
- photosynthesis
- absorb light, transfer energy (not electron) to chl
- 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
