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PHOTOSYNTHESIS Photosynthesis • process by which green plants & some organisms – seaweed, algae & certain bacteria • use light energy to convert CO2 + water glucose • all life on Earth, directly or indirectly, depends on photosynthesis as source of food, energy & O2 Autotrophs • self feeders – organisms that make their own organic matter from inorganic matter – producers • use inorganic molecules such as CO2, H2O & minerals to make organic molecules Heterotrophs • consumers – other feeders • depend on glucose as energy source – cannot produce it • obtained by eating plants or animals that have eaten plants Carbon and Energy Flow CO2 + H2O Photosynthesis Carbs Proteins Lipids + O2 Cellular (Aerobic) Respiration (ATP Produced) Food Chain • byproduct of photosynthesis is O2 • humans & other animals breathe in oxygen • used in cellular respiration Other Benefits of Photosynthesis • humans also dependent on ancient products of photosynthesis • fossil fuels – natural gas, coal & petroleum • needed for modern industrial energy • complex mix of hydrocarbons • represent remains of organisms that relied on photosynthesis millions of years ago Photosynthesis • plants produce more glucose than can use • stored as starch & other carbohydrates in roots, stems & leaves • can draw on these reserves for extra energy or building materials as needed Sites of Photosynthesis • leaves & green stems • in cell organelles – chloroplasts • concentrated in green tissue in interior of leaf • mesophyll • green due to presence of green pigment chlorophyll Chloroplasts • each cell has 40-50 chloroplasts – oval-shaped structures with double membrane • inner membrane encloses compartment filled with stroma • suspended in stroma are diskshaped compartmentsthylakoids – arranged vertically like stack of plates • one stack-granum (plural, grana) • embedded in membranes of thylakoids are hundreds of chlorophyll molecules Chlorophyll • light-trapping pigment • other light-trapping pigments, enzymes & other molecules needed for photosynthesis are also found in thylakoid membranes How Photosynthesis Works • Requires –CO2 –Water –Sunlight • Makes –O2 –Glucose How Photosynthesis Works • CO2 enters plant via pores- stomata in leaves • water-absorbed by roots from soil • membranes in chloroplasts provide sites for reactions of photosynthesis • chlorophyll molecules in thylakoids capture energy from sunlight • chloroplasts rearrange atoms of inorganic molecules into sugars & other organic molecules Photosynthesis • redox reaction • 6CO2 + 12H2OC6H12O6 + 6O2 + 6H2O in presence of light • must be an oxidation & a reduction • water is oxidized – loses electrons & hydrogen ions • carbon dioxide is reduced – gains electrons & hydrogens Photosynthesis • 2 stages • light-dependent reactions – chloroplasts trap light energy – convert it to chemical energy – contained in nicotinamide adenine dinucleotide phosphate-(NADPH) & ATP – used in second stage • light-independent reactions – Calvin cycle – formerly called dark reactions – NADPH (electron carrier) provides hydrogens to form glucose • ATP provides energy Light Energy for Photosynthesis • sun energy is radiation – electromagnetic energy • travels as waves • distance between 2 waveswavelength • light contains many colors • each has defined range of wavelengths measured in nanometers • range of wavelengths is electromagnetic spectrum • part can be seen by humans – visible light Pigments • • • • • • • • • • • light absorbing molecules built into thylakoid membranes absorb some wavelengths & reflect others plants appear green because chlorophyll-does not absorb green light – reflected back. as light is absorbedenergy is absorbed chloroplasts contain several kinds of pigments different pigments absorb different wavelengths of light red & blue wavelengths are most effective in photosynthesis other pigments are accessory pigments absorb different wavelengths enhance light-absorbing capacity of a leaf by capturing a broader spectrum of blue & red wavelengths along with yellow and orange wavelengths Pigment Color & Maximum Absoption • • • • • • Violet: 400 - 420 nm Indigo: 420 - 440 nm Blue: 440 - 490 nm Green: 490 - 570 nm Yellow: 570 - 585 nm Orange: 585 - 620 nm • Red: 620 - 780 nm Chlorophylls • Chlorophyll A – absorbs blue-violet & red light – reflects green – participates in light reactions • Chlorophyll B – absorbs blue & orange light – reflects yellow-green – does not directly participate in light reactions – broadens range of light plant can use by sending its absorbed energy to chlorophyll A Carotenoids • yellow-orange pigments • absorb blue-green wavelengths • reflect yellow-orange • pass absorbed energy to chlorophyll A • have protective function – absorb & dissipate excessive light energy that would damage chlorophylls Photosynthesis • • • • Pigments Absorb light Excites electrons Energy passed to sites in cell • Energy used to make glucose Photosystems • chlorophyll & other pigments clustered next to one another in a photosystem • when photon strikes one pigment molecule • energy jumps from pigment to pigment until arrives at reaction center Reaction Center • electron acceptor traps a light excited electron from reaction center chlorophyll • passes it to electron transport chain which uses energy to make ATP & NADPH Photosystems • two photosystems participate in light reactions • photosystems II & I Light Reactions • make ATP & NADPH • electrons are removed from molecules of water • oxygen escapes to air • electrons are passed from photosystem II to photosystem I to NADP+ • light drives electrons from H2O to NADP+ which is oxidized NADPH which is reduced Photosystem II • water is split • oxygen atom combines with oxygen from another split water forming molecular oxygen-O2 • each excited electron passes from photosystem II to photosystem I via electron transport chain Photosystem I • electron acceptor captures an excited electron • excited electrons are passed through a short electron transport chain to NADP+ reducing it to NADPH • NADP+ -final electron acceptor • electrons are stored in high state of potential energy in NADPH molecule • NADPH, ATP and O2 are products of light reactions ATP Formation-Chemiosmosis • uses potential energy of hydrogen ion concentration gradient across membrane • gradient forms when electron transport chain pumps hydrogen ions across thylakoid membrane as it passes electrons down chain that connects two photosystems ATP Formation-Chemiosmosis • ATP synthase (enzyme) uses energy stored by H gradient to make ATP • ATP is produced from ADP & Pi when hydrogen ions pass out of thylakoid through ATP synthase • photophosphorylation Calvin Cycle/Dark Reactions • light independent reactions • depend on light indirectly for inputs-ATP & NADPH • occurs-stroma of chloroplast • each step controlled by different enzyme • cycle of reactions • makes sugar from CO2 & energy • ATP provides chemical energy • NADPH provides high energy electrons for reduction of CO2 to sugar Steps of Calvin Cycle • starting material-ribulose bisphosphate (RuBP) • first step-carbon fixation • rubisco (an enzyme) attaches CO2 to RuBP • Next-reduction reaction takes place • to do this cycle uses carbons from 3 CO2 molecules • to complete cycle must regenerate beginning component-RuBP • for every 3 molecules of CO2 fixed, one G3P molecule leaves cycle as product of cycle • remaining 5 G3P molecules Calvin Cycle • regenerated RuBP is used to start Calvin cycle again • process occurs repeatedly in each chloroplast as long as CO2, ATP & NADPH are available • thousands of glucose molecules are produced • used by plants to produce energy in aerobic respiration • used as structural materials • stored
