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Plant Physiology > Chapter 7: Light Reactions > Flashcards

Chapter 7: Light Reactions Flashcards

1
Q

what is photosynthesis?

A
  • the synthesis of light
  • powers cellular processes in the plant
  • energy sources for organisms and downstream ecological processes
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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)
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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)
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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
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5
Q

how does light behave?

A
  • as a wave = many wavelengths and frequencies

- particle = energy

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6
Q

wavelength formula

A

c = lambda x v

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7
Q

particle formula

A
  • E = h x v

h = Planck’s cnstant

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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
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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

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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

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11
Q

how do chlorophyll molecules move to a less excited state?

A
  • heat energy
  • fluorescence
  • energy transfer
  • photochemistry
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12
Q

heat energy

A
  • heat emitted from molecules

- very fast, sometimes faster than fluoresce

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13
Q

fluorescence

A

photon of lower energy (longer wavelength) re-emitted

  • remission of photons
  • changing the nature of light
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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
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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

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16
Q

carotenoids

A
  • accessory pigments
  • help to processes excess energy
  • appear orange
  • linear molecules associated w/ thylakoid membrane
  • have a higher energy state
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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
Q

cytochrome b6f

A
  • connect PS1 and PS2 because they are physically separated

- oxidizes plastohydroquinone and delivers electron to plastocyanin

26
Q

plastocyanin and plastoquinone

A

deliver electrons between the two photosystems and cytochrome

27
Q

how is energy delivered to the reaction center?

A

antenna system

- typically 200-300 chlorophylls per reaction center

28
Q

how is energy transferred to the reaction center?

A
  • fluorescence resonance energy transfer
  • non-radiative process and no chemical change, analogous to tuning forks
  • very efficient way to transfer energy
29
Q

photosystem 2

A

oxidizes water to O2 in the thylakoid lumen, releasing protons into the
lumen, passes electrons to plastoquinone, forming plastohydroquinone

30
Q

photosystem 1

A

oxidizes plastocyanin, and passes electron to ferredoxin, which reduces
NADP+ to NAPDH

31
Q

ATP synthase

A

produces ATP as protons diffuse through it from lumen to stoma

32
Q

Steps of light reaction

A
  • 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
Q

some herbicides can block electron transport

A
  • DCMU block reduction of the plastoquinone acceptor

- Paraquat competes with ferredoxin acceptors of PSI

34
Q

light from the sun

A
  • irradiance
  • 4.5 lbs of energy per second
  • not all used, some lost to space
35
Q

energy transfer

A
  • not the same as heat transfer
  • passing physical energy to physical energy
  • cannot directly use into other processes
36
Q

how do pigments work?

A
  • 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
Q

quantum yield

A
  • 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
Q

how fast per unit of light increased can you photosynthesize?

A
  • for a single photochemical reaction = quantum yield is close 1 (.95)
  • 10 photons of light = 1 O2
    .1 quantum yield for oxygen
39
Q

efficiency

A

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
Q

energy conservation

A

energy absorbed per chemical energy produced

Energy conservation absorbed to chemical energy = 27%

41
Q

what is the ratio of PS2 to PS1 and why is it like this?

A
  • 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

Plant Physiology (21 decks)

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