Chapter 7: Light Reactions Flashcards
(41 cards)
1
Q
what is photosynthesis?
A
- the synthesis of light
- powers cellular processes in the plant
- energy sources for organisms and downstream ecological processes
2
Q
what are the two reactions of photosynthesis?
A
- the two reactions occurs in series and are the light reactions (thylakoid) and dark reactions (carbon fixation)
3
Q
where does photosynthesis take place? light reactions? carbon reactions?
A
- 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)
4
Q
what do the light reactions ultimately accomplish?
A
- split water to make reducing agents and produce energy (NADPH and ATP) which feed into the carbon reactions
5
Q
how does light behave?
A
- as a wave = many wavelengths and frequencies
- particle = energy
6
Q
wavelength formula
A
c = lambda x v
7
Q
particle formula
A
- E = h x v
h = Planck’s cnstant
8
Q
what can plants use for photosynthesis?
A
- 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
9
Q
absorption spectrum
A
a plot of its absorption of light against wavelength of
light, to quantify its ability to take up light across the spectrum
10
Q
how is light absorbed?
A
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
11
Q
how do chlorophyll molecules move to a less excited state?
A
- heat energy
- fluorescence
- energy transfer
- photochemistry
12
Q
heat energy
A
- heat emitted from molecules
- very fast, sometimes faster than fluoresce
13
Q
fluorescence
A
photon of lower energy (longer wavelength) re-emitted
- remission of photons
- changing the nature of light
14
Q
photochemistry
A
- 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
15
Q
what are the main light harvesting pigments for green plants?
A
- chlorophylls a and b
- complex ring structure with Mg bonded to N in the middle and a long hydrocarbon tail anchored to reaction center
16
Q
carotenoids
A
- accessory pigments
- help to processes excess energy
- appear orange
- linear molecules associated w/ thylakoid membrane
- have a higher energy state
17
Q
accessory pigments
A
light absorbed by carotenoids are
transferred to chlorophylls for photosynthesis
18
Q
action spectra
A
- 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
19
Q
who discovered the photosystems and how?
A
- 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
20
Q
what types of light and wavelengths do the photosystems absorb?
A
- PSI = far-red and P700
- PS2 = red light and P680
21
Q
Z-scheme
A
- 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
22
Q
where are the light reactions?
A
- 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
23
Q
chloroplasts
A
- bound by 2 lipid bilayer membranes (inner and outer envelope)
- have their own DNA, RNA, ribosomes
24
Q
what are the four essential proteins to photosynthesis?
A
- PS1
- PS2
- cytochrome
- ATTP-synthase
- all embedded in the thylakoid membrane via hydrophobic amino acids, extending into the stroma and lumen
25
cytochrome b6f
- connect PS1 and PS2 because they are physically separated
| - oxidizes plastohydroquinone and delivers electron to plastocyanin
26
plastocyanin and plastoquinone
deliver electrons between the two photosystems and cytochrome
27
how is energy delivered to the reaction center?
antenna system
| - typically 200-300 chlorophylls per reaction center
28
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
29
photosystem 2
oxidizes water to O2 in the thylakoid lumen, releasing protons into the
lumen, passes electrons to plastoquinone, forming plastohydroquinone
30
photosystem 1
oxidizes plastocyanin, and passes electron to ferredoxin, which reduces
NADP+ to NAPDH
31
ATP synthase
produces ATP as protons diffuse through it from lumen to stoma
32
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
33
some herbicides can block electron transport
- DCMU block reduction of the plastoquinone acceptor
| - Paraquat competes with ferredoxin acceptors of PSI
34
light from the sun
- irradiance
- 4.5 lbs of energy per second
- not all used, some lost to space
35
energy transfer
- not the same as heat transfer
- passing physical energy to physical energy
- cannot directly use into other processes
36
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
37
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
38
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
39
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
40
energy conservation
energy absorbed per chemical energy produced
| Energy conservation absorbed to chemical energy = 27%
41
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
