Chapter 7: Light Reactions Flashcards
what is photosynthesis?
- the synthesis of light
- powers cellular processes in the plant
- energy sources for organisms and downstream ecological processes
what are the two reactions of photosynthesis?
- the two reactions occurs in series and are the light reactions (thylakoid) and dark reactions (carbon fixation)
where does photosynthesis take place? light reactions? carbon reactions?
- in the leaf mesophyll
- mesophyll are cells with many chloroplasts and high concentrations of chlorophylls (light absorbing green pigments)
- light reactions = thylakoid membrane
- carbon reactions = stroma (aqueous region outside of thylakoids)
what do the light reactions ultimately accomplish?
- split water to make reducing agents and produce energy (NADPH and ATP) which feed into the carbon reactions
how does light behave?
- as a wave = many wavelengths and frequencies
- particle = energy
wavelength formula
c = lambda x v
particle formula
- E = h x v
h = Planck’s cnstant
what can plants use for photosynthesis?
- plants can only use light in the visible spectrum (400-700 nm)
- sunlight is a rain of photons of different frequencies
- different frequencies = different wavelngths
absorption spectrum
a plot of its absorption of light against wavelength of
light, to quantify its ability to take up light across the spectrum
how is light absorbed?
Absorption of light energy by pigment molecules takes place when the photons cause an
electron to move to a higher energy state— pigment molecules move to an excited state
how do chlorophyll molecules move to a less excited state?
- heat energy
- fluorescence
- energy transfer
- photochemistry
heat energy
- heat emitted from molecules
- very fast, sometimes faster than fluoresce
fluorescence
photon of lower energy (longer wavelength) re-emitted
- remission of photons
- changing the nature of light
photochemistry
- causes chemical reaction to occur
- translocation of physical energy from light into chemical energy via chemical reactions
- preferred by plants and faster than other energies
- one of the fastest reactions on earth
what are the main light harvesting pigments for green plants?
- chlorophylls a and b
- complex ring structure with Mg bonded to N in the middle and a long hydrocarbon tail anchored to reaction center
carotenoids
- accessory pigments
- help to processes excess energy
- appear orange
- linear molecules associated w/ thylakoid membrane
- have a higher energy state
accessory pigments
light absorbed by carotenoids are
transferred to chlorophylls for photosynthesis
action spectra
- used to express dynamics of light driven reactions
- the magnitude of a biological response to different wavelengths of light and the physical reaction
- do not always mean what is absorbed
who discovered the photosystems and how?
- Emmerson in the 1950s
- red drop effect: far-red light alone is inefficient in driving photosynthesis
despite high absorption of this wavelength - enhancement effect: when both red and far-red light are used together the
efficiency is greater than the sum of each applied alone
what types of light and wavelengths do the photosystems absorb?
- PSI = far-red and P700
- PS2 = red light and P680
Z-scheme
- framework centered around the two photosystems and their respective
antennae used for understanding light-reactions - Up arrows are reducing reactions, down arrows are oxidizing
- Photosystems connected by electron transport chain
where are the light reactions?
- in the thylakoid membrane where light harvesting antennae are attached
- PS2 = grana lamellae, stacked thylakoids
- PS1 = stroma lamellae, unstacked
- carbon reactions occur in the stroma surrounding the chloroplast
chloroplasts
- bound by 2 lipid bilayer membranes (inner and outer envelope)
- have their own DNA, RNA, ribosomes
what are the four essential proteins to photosynthesis?
- PS1
- PS2
- cytochrome
- ATTP-synthase
- all embedded in the thylakoid membrane via hydrophobic amino acids, extending into the stroma and lumen
cytochrome b6f
- connect PS1 and PS2 because they are physically separated
- oxidizes plastohydroquinone and delivers electron to plastocyanin
plastocyanin and plastoquinone
deliver electrons between the two photosystems and cytochrome
how is energy delivered to the reaction center?
antenna system
- typically 200-300 chlorophylls per reaction center
how is energy transferred to the reaction center?
- fluorescence resonance energy transfer
- non-radiative process and no chemical change, analogous to tuning forks
- very efficient way to transfer energy
photosystem 2
oxidizes water to O2 in the thylakoid lumen, releasing protons into the
lumen, passes electrons to plastoquinone, forming plastohydroquinone
photosystem 1
oxidizes plastocyanin, and passes electron to ferredoxin, which reduces
NADP+ to NAPDH
ATP synthase
produces ATP as protons diffuse through it from lumen to stoma
Steps of light reaction
- Light excites a reaction center chlorophyll, by direct absorption or more frequently, via energy
transfer from antennae pigments - light causes electron rearrangement and is passed to plastoquinone and is replaced by a donor
- PS2 receives electrons from oxygen-evolving complex that splits and oxidizes water which in turn releases protons into thylakoid lumen
- plastoquinone and plastocyanin carry electrons between PS1 and PS2. Q is lipid soluble and moves in the membrane while PC is water soluble and moves in lumen
- PS2 moves 2 electrons to plastoquinone which forms plastohydroquinone
- plastohydroquinone then leaves the reaction center to transfer electrons to the cytochromeb6f
- once in the cytochrome, 1 electron will go to Ps1 and the other will be cycled back to increase protons pumped across membrane
- cytochrome b6f transfers the electron to plastocyanin, which moves it to PS1
- PS1 passes an electron to ferredoxin and reduces NADP+ to NADPH; electrons are supplied by PC
- throughout this process, protons are generated to run ATP-synthase and create ATP
- both ATP and NADPH are used in the carbon reactions
some herbicides can block electron transport
- DCMU block reduction of the plastoquinone acceptor
- Paraquat competes with ferredoxin acceptors of PSI
light from the sun
- irradiance
- 4.5 lbs of energy per second
- not all used, some lost to space
energy transfer
- not the same as heat transfer
- passing physical energy to physical energy
- cannot directly use into other processes
how do pigments work?
- pigments form antennae and attach to a reaction center at one end
- 200-300 chlorophyl per a reaction
- physical energy hits the pigment and excites them and transfer energy from one pigment molecule to another
- physical energy hits the reaction center and starts the process of turning into chemical energy
quantum yield
- how fast per unit of light increased can you photosynthesize
- saturation can occur bc there are limits to metabolic functions
- as you increase light, O2 turnover increases linearly until saturated
how fast per unit of light increased can you photosynthesize?
- for a single photochemical reaction = quantum yield is close 1 (.95)
- 10 photons of light = 1 O2
.1 quantum yield for oxygen
efficiency
amount absorbed / the amount being engaged in photosynthesis
Not the same as quantum yield
- Efficiency has to do with absorption not flashes of light
energy conservation
energy absorbed per chemical energy produced
Energy conservation absorbed to chemical energy = 27%
what is the ratio of PS2 to PS1 and why is it like this?
- 1.5 to 1
- because the products of photosystem 2 powers the products of photosystem 1
- makes a product that goes into photosystem 1 - product for the second step, so you want more of the photosystem 2 products
