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 need 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 Light energy Heat energy 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 depend on ancient products of photosynthesis fossil fuels natural gas, coal & petroleum for modern industrial energy represent remains of organisms that relied on photosynthesis millions of years ago

Photosynthesis plants produce more glucose than they use Stored starch & other carbohydrates in roots, stems & leaves

Sites of Photosynthesis leaves & green stems cell organelles chloroplasts concentrated in green tissue 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 disk-shaped compartments-thylakoids 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

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 + 12H2OC6H12O6 + 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 relies on a flow of energy & electrons initiated by light energy light energy causes electrons in chlorophyll pigments to boost electrons up & out of orbit hydrogens along with electrons are transferred to CO2sugar requires that H2O is split into H & O2 O2 escapes to air light drives electrons from H2O to NADP+ which is oxidized NADPH which is reduced

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 Dependent Reactions convert light energy to chemical energy & produce oxygen takes place- thylakoid membranes solar energy absorbed by chlorophyllATP + NADPH

Light Energy for Photosynthesis sun energy is radiation electromagnetic energy travels as waves distance between 2 waves- wavelength light contains many colors each has range of wavelengths measured in nanometers range of wavelengths is electromagnetic spectrum part 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 absorbedenergy 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 protective function absorb & dissipate excessive light energy that would damage chlorophylls

Light Energy light behaves as discrete packages of energy called photons fixed quantity of energy

Light Energy when pigment absorbs a photon pigment’s electrons gains energy electrons are excited unstable electrons do not stay in unstable state fall back to original orbits as electrons fall back to ground heat is released absorbed energy is passed to neighboring molecules

Photosynthesis Pigments Absorb light Excites electrons Energy passed to sites in cell Energy used to make glucose

Photosystems chlorophyll & other pigments are found clustered next to one another in a photosystem two participate in light reactions

Photosystems photosystem I & II each has specific chlorophyll at reaction center photosystem II chlorophyll P680 photosystem I chlorophyll P700 named for type of light they absorb best P700 absorbs light in far red region of electromagnetic spectrum

Reaction Center when photon strikes one pigment molecule energy jumps from pigment to pigment until arrives at 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

Reaction Center

Light Reactions during process of making ATP & NADPH electrons are removed from molecules of water passed from photosystem II to photosystem I to NADP+

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 primary electron acceptor captures an excited electron excited electrons are passed through short electron transport chain to NADP+ reducing it to NADPH NADP+ is 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

Chemiosmosis H+ H+ pH 7 pH 8 Chemiosmosis

Substrate-level Phosphorylation

Calvin Cycle light independent reactions depend on light indirectly to obtain inputs for cycle-ATP & NADPH takes place in stroma of chloroplast 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 NADPH reduces 3-phosphoglyceric acid (3-PGA) to glyceraldehye 3-phosphate (G3P) with assistance of ATP 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 are rearranged using ATP to make 3 RuBP molecules

Calvin Cycle regenerated RuBP is used to start cycle again process occurs repeatedly 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

Photosynthesis Variations plants vary in the way they produce glucose and when

C3 Plants use CO2 directly from air first organic compound produced is a 3 carbon compound 3-PGA reduce rate of photosynthesis in dry weather CO2 enters plants through pores in leaves on hot days stomata in leaves close partially to prevent escape of water with pores slightly open, adequate amounts of CO2 cannot enter leaf Calvin cycle comes to a halt no sugar is made in this situation rubisco adds O2 to RuBP 2-carbon product of this reaction is broken down by plant cells to CO2 + H20 Photorespiration provides neither sugar nor ATP

C4 Plants have special adaptations allowing them to save water without shutting down photosynthesis corn, sugar cane & crabgrass evolved in hot, dry environments when hot & dry stomata are closed saves water sugar is made via another route developed way to keep CO2 flowing without capturing it directly from air

C4 Plants have enzymes that incorporate carbon from CO2 into 4-C compound enzyme has an intense desire for CO2 can obtain it from air spaces even when levels are very low 4-C compound acts as a shuttle transfers CO2 to nearby cells -bundle-sheath cells found in vast quantities around veins of leaves CO2 levels in these cells remain high enough for Calvin cycle to produce sugar

CAM Plants pineapple, some cacti & succulent plants conserve water by opening stomata & letting CO2 in at night CO2 is fixed into a 4-C compound saves CO2 at night & releases it in the day photosynthesis can take place without CO2 needing to be admitted during the day when conditions are hot and dry

Environmental Consequences of Photosynthesis CO2 makes up 0.03% of air provides plants with CO2 to make sugars important in climates retains heat from sun that would otherwise radiate from Earth warms the Earth greenhouse effect

Global Warming CO2 traps heatwarms air maintains average temperature on Earth about 10 degrees C warmer than without it. Earth may be in danger of overheating because of this greenhouse effect CO2 in air is increasing because of industrialization when oil, gas and coal are burned CO2 is released levels in atmosphere have increased 30% since 1850 increasing concentrations have been linked to global warming slow & steady rise in surface temperature of Earth could have dire consequences for all life forms on Earth