Photosynthesis Reactions Photosynthesis occurs in two stages linked by ATP and NADPH NADPH is similar to NADH seen in mitochondria; it is an electron/hydrogen carrier The complete process of photosynthesis consists of two linked sets of reactions The light reactions and the Calvin cycle. Photosynthesis Reactions The Light Reactions Occur in the grana Light energy is absorbed by the chlorophyll molecules to split water and drive the transfer of electrons and H + from water to the electron acceptor NADP + reducing it to NADPH. In this process, oxygen becomes released and ATP is produced ( as well as NADPH) 1
Photosynthesis Reactions The Calvin cycle Occurs in the stroma and uses the energy generated during the light reactions Forms sugar from carbon dioxide. This is called carbon fixation! Since this is an endergonic (energy requiring, the making of a larger molecule from a smaller one) reaction, it uses ATP for energy and NADPH for reducing power Sometimes called the dark reaction since these steps do not require light energy Figure 7.5_s3 H 2 O CO 2 Light NADP + ADP P Light Reactions (in thylakoids) ATP Calvin Cycle (in stroma) NADPH Chloroplast O 2 Sugar 2
Aspects of Light Energy Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths. Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves. The smaller the wavelength, the more energy is packed in that wave ( and the larger the wavelength, the less energy in that wave). Light behaves as discrete packets of energy called photons. A photon is a fixed quantity of light energy. Increasing energy 10-5 nm 10-3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 3 m Gamma rays X-rays UV Infrared Microwaves Radio waves Visible light 380 400 500 600 700 750 Wavelength (nm) Which of these elements of the electromagnetic spectrum are dangerous for us? 650 nm 3
Aspects of Light Energy Many forms of Life on planet earth has evolved to use only a certain part of the electromagnetic spectrum. This is called the visible light spectrum and includes the region of the electromagnetic spectrum between 380 and 750 nanometers. This spectrum includes the colors of light we can see It also includes the wavelengths that drive photosynthesis Aspects of Light Energy The way life reacts to the visible light spectrum is due to molecules that absorb the energy in the parts of the visible spectrum. Pigments : Are substances that absorb visible light The color of the pigment indicates what color is being absorbed and what color is being reflected. For example, a red apple look red because molecules (pigments) in the apple skin absorb all colors except red, being reflected back into our eyes. 4
Aspects of Light Energy Light Reflected light Chloroplast Thylakoid Why do leafs look green? Absorbed light Transmitted light Photosynthesis Pigments Thus leaves look green because the color green is reflected. This part of the electromagnetic spectrum is NOT absorbed and NOT used for photosynthesis. Those molecules in the leaves (chloroplasts) that absorb and use light energy are called the Plant pigments They absorb some wavelengths of light and Reflect or transmit other wavelengths ( the colors reflected are the colors we thus see). 5
Photosynthesis Pigments The spectrophotometer can measure what is being absorbed and what is being transmitted (not absorbed) at each wavelength. White light Refracting prism Chlorophyll solution 2 3 Photoelectric tube Galvanometer 1 4 0 100 Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 100 Blue light The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Absorption Spectrum With the use of simple spectrophotometers one can generate an absorption spectrum. An absorption spectrum of a solution of pigments Is a graph plotting light absorption (on Y-axis) versus wavelength (on X-axis). This will tell us what part of visible light is absorbed by the pigments in solution and thus what part of visible light may/is important for action of those molecules involved. 6
Photosynthesis Pigments Chloroplasts contain several different pigments in the thylakoid membrane, which absorb light of different wavelengths. Chlorophyll a Chlorophyll b Carotenoids The 3 different pigments can be isolated from greeny leaves and analyzed for what spectrum of visible light they absorb. Absorption Spectrum The combined action spectrum experiments helped reveal which wavelengths of light are photo-synthetically important. The results are shown below. RESULTS Chlorophyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Which pigment absorbs what colors? What colors are NOT absorbed? 7
Photosynthesis 3 Pigments Chlorophyll a absorbs blue-violet and red light and reflects green. Chlorophyll b absorbs blue and orange and reflects yellow-green. Carotenoids broaden the spectrum of colors that can drive photosynthesis (such as blue to violet) but reflect in the orange/red region They also appear to provide photo-protection by absorbing and dissipating excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules. Action Spectrum How effective each wavelength of light is in stimulating photosynthesis can be deduced by measuring how much oxygen is made over a certain period of time 6 CO 2 + 6 H 2 O Light energy C 6 H 12 O 6 + 6 O 2 Carbon dioxide Water PHOTOSYNTHESIS Glucose Oxygen gas Graphing the rate of oxygen production against the wavelength used creates an action spectrum ; it profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis in a leaf. 8
Action Spectrum Rate of photosynthesis (measured by O 2 release) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly. This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. Action vs Absorption Spectra Comparison between Action spectrum and Absorption spectrum Absorption Rate Photosynthesis Rate Wavelength of light (nm) 9
Action Spectrum The action spectrum for photosynthesis was first demonstrated by Theodor W. Engelmann Aerobic bacteria Filament of alga 400 500 600 700 In 1883, Theodor W. Engelmann exposed alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O 2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part in previous slide. Photosynthesis Pigments So what happens when photosynthetic pigments absorb light energy? When Pigments in chloroplasts absorb photons (capturing solar power), it increases the potential energy of the pigments electrons and sends the electrons into an excited (higher level) but unstable state. 10
Photosynthesis Pigments Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground state and release their excess energy (emission energy) as heat and some form of photon energy with less energy as initially absorbed (the basis of most fluorescence principles). Excited state Photon of light Heat Photon (fluorescence) Ground state Chlorophyll molecule Photosynthesis Pigments Solution of Chlorophyll illuminated with UV light. In normal white light, it looks green. The UV light excited electrons which now fall back to ground state, emitting photons with less energy More energy Less energy 11
The Light Reaction In the thylakoid membranes, chlorophyll molecules are organized along with other pigments and proteins into photosystems. When light is absorbed, the excited electrons are passed on to other electron acceptor molecules. The solar-powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step in the transformation of light energy to chemical energy in the light reactions. Photosynthesis Pigments There are two photosystems, called PS II and PS I, that work in concert. When PS II becomes excited by light energy, the result is splitting of water into Oxygen, H + and electrons the H + are released in the inner space of the thylakoids the electrons are funneled into an electron transport chain The electron chain passes the electrons from photosystem II to photosystem I and the energy released funnels more protons into the lumen of the thylakoids 12
Chloroplast stroma Thylakoid membrane Thylakoid inner space Photosynthesis Pigments When PS I becomes excited by light energy, the result is Reduction of NADP into NADPH by accepting electrons and H + The buildup of hydrogen high inside the thylakoid (and thus low outside) will now drive the synthesis of ATP via an ATPsynthase by the movement of hydrogen from inside to outside. This NADPH and ATP is required for the Calvin cycle that happens in the stroma. The process that results in the making of sugars. 13
A construction analogy of PS II and PSI ATP NADPH Electron transport chain ramp Photon Photon Photosystem II Photosystem I Mitochondria/ Chloroplasts So, when we compare mitochondria and chloroplasts, similar mechanisms are at work. The buildup of hydrogen drives the production of ATP. The ATP made by mitochondria is used to fuel all kinds of cellular activities The ATP made by chloroplasts is only used to fuel the Calvin Cycle in order to make sugars. 14
Mitochondria/ Chloroplasts In mitochondria, the production of ATP is referred to as oxidative phosphorylation via the chemiosmosis mechanisms ( the making of ATP via the use of an ETC, hydrogen gradient and the use of oxygen). An electron transport chain also produces a gradient of H + across the thylakoid membrane, which drives H + through ATP synthase, producing ATP. Because the initial energy input is light ( photo ), this chemiosmotic production of ATP is called photophosphorylation. The Importance of Photosynthesis: A Review A review of photosynthesis Light reaction Calvin cycle H 2 O CO 2 Light Photosystem II Electron transport chain Photosystem I Chloroplast O 2 NADP + ADP + P 1 ATP NADPH Light reactions: Are carried out by molecules in the thylakoid membranes Convert light energy to the chemical energy of ATP and NADPH Split H 2 O and release O 2 to the atmosphere RuBP 3-Phosphoglycerate G3P Starch (storage) Sucrose (export) Amino acids Fatty acids Calvin cycle reactions: Take place in the stroma Use ATP and NADPH to convert CO 2 to the sugar G3P Return ADP, inorganic phosphate, and NADP+ to the light reactions 15
The Importance of Photosynthesis Light H 2 O CO 2 Chloroplast NADP + Thylakoids Light Reactions Photosystem II Electron transport chain Photosystem I ADP P ATP RuBP Calvin Cycle 3-PGA (in stroma) Stroma O 2 NADPH G3P Sugars Cellular respiration Cellulose Starch Other organic compounds! The greenhouse effect operates on a global scale. Solar radiation includes visible light that penetrates the Earth s atmosphere and warms the planet s surface. Heat radiating from the warmed planet is absorbed by gases in the atmosphere, which then reflects some of the heat back to Earth. Without the warming of the greenhouse effect, the Earth would be much colder and most life as we know it could not exist. 2012 Pearson Education, Inc. 16
Sunlight Some heat energy escapes into space ATMOSPHERE Radiant heat trapped by CO 2 and other gases! The gases in the atmosphere that absorb heat radiation are called greenhouse gases. These include water vapor, carbon dioxide, and methane. Methane source 2012 Pearson Education, Inc. 17
! Remember that photosynthesis captures CO 2 into carbohydrates! The carbohydrates of plants become cellulose, wood,.! When plants, trees, animals died million of years ago, they became submerged into sediments of swamps, rivers, oceans and eventually became covered with sand, mud,! Over time the carbohydrates, woods became converted to what we know as fossil fuels. 2012 Pearson Education, Inc.! The history of the earth includes the movement of CO 2 from above the air into the deeper regions of the earth! The industrial revolution has been the beginning of increased exploration and use of fossil fuels such as coal, oils, petroleums, 2012 Pearson Education, Inc. 18
! Since 1850, there has been an increased and progressive acceleration in the usage and combustion of fossil fuels! And this implies, a faster release of the hidden CO 2 captured by those fossilized organisms resulting in a 40% increase in atmospheric CO 2 concentrations since 1850 2012 Pearson Education, Inc. These Increasing concentrations of greenhouse gases have been linked to global climate change (global warming). There has been a slow but steady rise in Earth s surface temperature and an overall change in climatic conditions in different regions of the world. Sunlight ATMOSPHERE Some heat energy escapes into space Radiant heat trapped by CO 2 and other gases 2012 Pearson Education, Inc. 19
! The presence of large amount of oceans and algae that perform photosynthesis has slowed down the effect.! CO 2 also dissolves easier in water providing a buffer for CO 2 changes! But too much CO 2 can make oceans acidic 2012 Pearson Education, Inc. GreenHouse Effect Data 20
! The predicted consequences of continued CO2 increase in the atmosphere and global climate change include melting of polar ice, rising sea levels, acidification of oceans extreme weather patterns, droughts, increased extinction rates, and the spread of tropical diseases. 2012 Pearson Education, Inc. Effects of increasing carbon dioxide and temperature on coral reefs. 2012 Pearson Education, Inc. 21
! Widespread deforestation has aggravated the global warming problem by reducing an effective CO 2 sink.! Global warming caused by increasing CO 2 levels may be reduced by limiting deforestation, reducing fossil fuel consumption, and growing biofuel crops that remove CO 2 from the atmosphere. 2012 Pearson Education, Inc. 2012 Pearson Education, Inc. 22
Global Warming Bad Quotes "Trees cause more pollution than automobiles. Quote by Ronald Reagan, 1981 The dangers of carbon dioxide? Tell that to a plant, how dangerous carbon dioxide is Quote by Santorum, presidential candidate (2012) Why are these examples of politician who obviously do not understand the importance of plants and ecology? 23