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Lecture 2: LIGHTREACTION

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38 Slides

LEARNING OUTCOMEUpon mastering the materials of this lecture,students should be able to1. Explain the characteristics of light as a source

of energy in the process of photosynthesis2. Calculate the quantity of solar radiation at the

top of earth’s atmosphere3. Explain the process of NADPH formation in the

conversion of radiation energy to be chemicalenergy

4. Explain the process of ATP formation in theconversion of radiation energy to be chemicalenergy

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LECTURE OUTLINE

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What is photosynthesis? The process of converting solar energyinto chemical energy. The process of CO2 reduction intocarbohydrates (sugars) at the expense ofNADPH & ATP The process responsible for removal of ~200 billion tons of C from the atmosphereyearly.

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2 H + 1/2

Water-splittingphotosystem

Reaction-center

chlorophyll

Light

Primaryelectronacceptor

Energyto make

Primaryelectronacceptor

Primaryelectronacceptor

NADPH-producingphotosystem

Light

NADP

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How the Light Reactions Generate NADPH and ATP

PS II PS I

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Photosynthesis has two major phases:1. The absorbance of light and production of chemical forms of

energy (light reactions)2. The fixation and reduction of carbon and other oxidized

molecules (dark reactions)

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Solar energy is the ultimate source of energy forlife on earth Solar energy is created at the core of the sunwhen hydrogen atoms are fused into helium bynuclear fusion . Temperatures of the sun are about 15,000,0000K at the core,and about 5,8000K at the photosphere(radiative surface of the sun)

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The solar energy is then transmitted in the form ofelectromagnetic radiation Radiation is the transfer of energy through somematerial or through space in the form ofelectromagnetic waves Electromagnetic waves are the self-propagating, mutualoscillation of electric and magnetic fields.Electromagnetic waves move electromagnetic energythrough space (either empty or filled with transparentmatter)

Most of the electromagnetic radiation emittedfrom the sun's surface lies in the visible bandcentered at 500 nm9/1/2016 8

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The Electromagnetic Spectrum

Shortest wavelengths(Most energetic photons)Longest wavelengths(Least energetic photons)

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The Sun appears to have been active for 4.6billion years and has enough fuel to go on foranother five billion years or so. At the end of its life, the Sun will start to fusehelium into heavier elements and begin to swellup, ultimately growing so large that it willswallow the Earth. After a billion years as a red giant, it willsuddenly collapse into a white dwarf -- the finalend product of a star like ours. It may take atrillion years to cool off completely.

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2.1 Stefan–Boltzmann Law This law states that the power emitted perunit area of the surface of a black body isdirectly proportional to the fourth power ofits absolute temperature. That isR=T4R = radiation flux (W.m-2 = J.m-2.s-1)= emissivity (01)= Stefan-Boltzmann constant (5,67032 x 10-8 W.m-2.K-4)T = absolute temperture (273 + 0C).

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Apply Stefan-Boltzmann Law To Sun and EarthR=T4 Sun (5800 0K)RS = (5.67 x 10-8 W/m2 K4) * (5800 0K)4= 64,164,532 W/m2 Earth (300 0K)RE = (5.67 x 10-8 W/m2 K4) * (300 0K)4= 459 W/m2

Sun emits about 160,000 times more radiationper unit area than the Earth because Sun’stemperature is about 20 times higher thanEarth’s temperature 6000/300 = 209/1/2016 12

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Inverse Square LawThe amount of radiation passing through a specific areais inversely proportional to the square of the distance ofthat area from the energy source.I = E(4R2)/(4r2)I = Irradiance at the surface of

the outer sphereE = Irradiance at the surface of

the object (Sun)R = 6.96 x 105 km (Radius of the Sun)r = 1.5 x 108 km (Average Sun-Earth Distance)I = 64,164,532 W/m2 x(6.96 x 105 )2 /(1.5 x 108 )2

I = 1382 W/m2 (The generally accepted solar constant of1368 W/m2 is a satellite measured yearly average)

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Radiation emitted by a human body The net power radiated is the difference betweenthe power emitted and the power absorbed:Pnet = Pemit − PabsorbApplying the Stefan–Boltzmann law,R = (T4-T04)A = the total surface area of an adult is about 2 m²,

= the mid- and far-infrared emissivity of skin and mostclothing is near unity, as it is for most nonmetallicsurfaces.T = skin temperature is about 33°C, but clothing reducesthe surface temperature to about 28 °C when theambient temperature is 20 °C (T0 )9/1/2016 14

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Hence, the net radiative heat loss with = 0.97 and = 5.67×10−8 W m−2 K−4is about Pemit =0.97* 5.67×10−8 W m−2 K−4 *2 m2(273+28)4= 902.92 W.m-2 or J.s-1 Pabsorbed =0.97* 5.67×10−8 W m−2 K−4 *2 m2(273+20)4= 810.69 W.m-2 or J.s-1 Pnet = 902.92-810.69 = 92.23 J.s-1 92.23 x 24 hours x 60 minutes x 60 seconds =7,968,672 J/day = 7.97 MJ/day

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Skintemperatureand ambienttemperature

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The air temperaturein Malang, onaverage, is 270C What is thetemperature of theskin? How much is the netradiative heat loss?

Why are plants green?

Transmitted light

WHY AREPLANTS GREEN?

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2.2 LightLight is an electromagnetic waveWhat is the electromagnetic wave?• It is electricity and magnetism moving through the space• Light was known to be a wave which is a continuouslyrepeating change or oscillation in matter or in a physicalfield• After producingelectromagnetic waves ofother frequencies, it wasknown to be anelectromagnetic wave aswell.9/1/2016 19

LIGHT AS A WAVE Wavelength () – the distance between crests (ortroughs) of a wave Frequency (v) – the number of crests (or troughs) thatpass by each second. Speed (c) – the rate at which a crest (or trough) moves(3.105 km/s). Crest

Trough

l• Maxwell calculated the speed ofpropagation of electromagneticwaves and found:

This is the speed of light in a vacuum.9/1/2016 20

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2.3 Quantum TheoryLight as particles• Light comes in quanta of energy called photons – little

bullets of energy. A photon is a quantum or irreducible quantity of

electromagnetic radiation.• By the 1900's the wave model was accepted by

scientist as how light moved.• Ideas of quantum theory were developed when

classical physics (the wave model) could not explainseveral physical phenomena observed in beginning ofthe 20th century

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1. light until further heating, then it will glowred, yellow then "white" hot.

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It also did not explain colors given off by variouselements as they burn

Black body radiation Heat capacity of solids Photoelectric effect

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Planck's Theory Energy cannot be absorbed or emittedunless it is a complete packet. Planck's theory states that atoms canonly absorb or release energy in fixedquantum units

• The amounts of energy an object emits or absorbs arecalled quantum (quanta plural)• Related the Frequency of the radiation to the amount ofenergy. E = hν = hc/lFrequency (v) = c/h = 6.6262 x 10-34 J-s (joule-seconds)c = speed of light (3x108 m/s)

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Visible radiation: visible to our eyes (wavelength:0.4x10-6 - 0.7x10-6 m)Red = 0.65 mm, Orange = 0.60 mm, Yellow = 0.55 mm,Green = 0.50 mm, Blue = 0.45 mm & Violet = 0.40 mmCahaya dan PAR• Tanaman dalam proses fotosintesis hanya dapatmemanfaatkan pancaran radiasi matahari yang terletakpada batas panjang gelombang 400 - 700 nm• Radiasi pada batas panjang gelombang 400 - 700 nmdisebut PAR (photosynthetically active radiation) ataucahaya nampak (visible radiation)

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Dengan memasukkan harga-harga konstanta,makadimana dalam satuan nano meter (nm)

Sebagai contoh, kandungan energi 1 photoncahaya merah (= 680 nm) adalah1 J (Joule) = 107 erg; 1 c (cal) = 4,2 J; 1 eV= 1,6.10-12 erg

J

E1710.878,19

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whereNA is Avogadro's number (= 6.02 x 1023 photons/mol)h is Planck's constant (= 6.62 x 10 -34J s per photon)c is the speed of light (= 3 x 108 m.s-1).For instance, the energy of "green light" (= 550 nm) is: E = 217376.7 J = 0.2173767 MJ a mol of blue light (l = 400 nm) = 298893.0 J a mol of red light (l = 700 nm) = 170796.0 J

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chN

E A ..

m

smcphotonsJhmolphotonsNE A

9

83423

10.550

10.3.10.02.610.02.6

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Summary of Photons To describe interactions of light with matter, onegenerally has to appeal to the particle (quantum)description of light A single photon has an energy given byE = hc/λwhereh = Planck’s constant = 6.6x10-34 [J s]c = speed of light = 3x108 [m/s]

λ = wavelength of the light (in [m]) Photons also carry momentum. The momentum isrelated to the energy by:p = E/c = h/λ

Photons can be treated as“packets of light” which behaveas a particle.l

Representation of a Photon

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3.1 Introduction1. Photosynthesis is the process by which

autotrophic organisms use light energy tomake sugar and oxygen gas from carbondioxide and water

2. This is an over simplification approach as H2Onever meets CO2 directly in the photosynthesis

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6 CO2 + 6 H2O C6H12O6+ 6O2LightEnergy

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3. The reduction of CO2 to be carbohydratethrough photosynthesis requires energy(NADPH & ATP)

4. Photosynthesis can be divided into tworeactions Light Reaction Generation of NADPH2 Generation of ATP

Dark Reaction Diffusion of CO2 Reduction of CO2 C3, C4 & CAM

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5. Light Reaction Eventsa. Light Absorptionb. Pigmentsc. Electron Excitation

Fluorescence Phosphorescence

d. Electron transfer &Synthesis NADPH

e. Proton exchange &Synthesis ATP

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Chloroplasts In most plants, photosynthesis occurs primarily inthe leaves, in the chloroplasts The leaves have the most chloroplasts The green color comes from chlorophyll in thechloroplasts The pigments absorb light energy

A chloroplast contains: Stroma (a fluid) Grana (stacks of thylakoids)

The thylakoids containchlorophyll Chlorophyll is the green pigmentthat captures light forphotosynthesis9/1/2016 32

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The location and structure of chloroplasts

LEAF CROSS SECTION MESOPHYLL CELLLEAF

Chloroplast

Mesophyll

CHLOROPLAST Intermembrane space

Outermembrane

Innermembrane

ThylakoidcompartmentThylakoidStroma

Granum

StromaGrana9/1/2016 33

Pigment = a light absorbing molecule Associated with the thylakoidmembranes Chlorophyll Chl a and Chl b (Chl c in some algae)

Xanthophylls Carotenoids ß-carotene

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Phytol tail

Porphyrin ring delocalized e-

• Chl a has a methyl group (CH3)• Chl b has a carbonyl group (CHO)

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Phytol

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