WHAT IS PHOTOSYNTHESIS? The process that occurs in green plants,whereby solar energy is converted into chemicalenergy and stored as organic molecules bymaking use of carbon dioxide, sunlight, andwater. Water and Oxygen are formed asbyproducts Photosynthesis can be summarized in thefollowing equation:6 CO2 + 12 H2O + Light energyC6H12O6 + 6 O2 + 6 H2O(glucose)
WHO OR WHAT CAN PHOTOSYNTHESIZE?Photoautotrophs can photosynthesize.Include: Green plants, algae, cyanobacteriaand green protists.PlantsGreen protistsCyanobacteriaAlgae
WHY DO PLANTS PHOTOSYNTHESIZE?To provide nutrients and oxygen forheterotrophs.Heterotrophs are dependent onautotrophs, because they cannot producethere own food.
WHAT PART OF THE PLANT ISRESPONSIBLE FORPHOTOSYNTHESIS? Photosynthesis occurs in the chloroplastsof plant cells. The chloroplasts are mainly concentratedin the mesophyll cells of leaves. Chloroplast contain chlorophyll – greenpigment that absorbs sunlight. Chlorophyll fill the space in the thylakoidmembrane.
RAW MATERIALS OFPHOTOSYNTHESIS• The raw materials ofphotosynthesis are: water,carbon dioxide andsunlight.
HOW RAW MATERIALSREACH THE CHLOROPLASTS Water is absorbed through the root hair into thexylem of the roots, into the xylem of the stem,through the xylem of the leaves into the mesophyllcells and finally into the chloroplasts. Carbon dioxide diffuses from the atmospherethrough the stomata, into the intercellularairspaces in the leaves, and finally into thechloroplasts of the mesophyll cells. The chlorophyll and other pigments in thethylakoid membrane absorb the solar energy todrive photosynthesis
PHOTOSYNTHESIS CONSIST OF 2STAGES:LIGHT REACTION PHASE(Dependent on light)DARK PHASE/ CALVIN CYCLE(Light independent)
LIGHT REACTION PHASETakes place in the thylakoids of the chloroplasts.Chlorophyll absorbs solar energy from the sun.When a chlorophyll pigment absorbs light (photonof energy), it excites the electrons, which goesfrom ground state to an excited state, which isunstable, but can be used as potential energy.When unused excited e- fall back to the groundstate, photons and heat are given off.
The electrons are excited in thephotosystems fount in the thylakoidmembrane.This potential energy is then used firstly tosplit water – into hydrogen & oxygen.2H2O 2H2 + O2 Oxygen is released as a byproduct –diffuse through stomata into atmosphere.The hydrogen reduces NADP+ to NADPHSome energy is then used tophotophosphorylate ADP to generate ATP.ADP + P ATP
CALVIN CYCLE Carbon dioxide diffuses through thestomata of the leave and finally into thestroma of the chloroplast. The carbon dioxide is accepted by a 5Cmolecule called ribulose biphosphate(RuBP) which then forms an unstable 6Ccompound. 6C compound dissociates into 2 x 3Ccompounds called phosphoglycerate(PGA)
PGA is then reduced to phosphoglyceraldehyde(PGAL/ G3P) by accepting a phosphate from ATPand a hydrogen electron from NADPH.Thus changing ATP back to ADP and NADPH toNADP. PGAL are now used for the following reactions: Some PGAL are used to make RuBP again,so that the cycle can start over again. Some PGAL are used to form hexose sugarslike glucose and fructose. Which combine toform disaccharides and polysaccharides.* The carbohydrates can then be converted toother biological compounds like proteins or fatsby adding mineral salts like nitrates andphoshates.
CO2 +RuBP(5C)6C compound2x PGA (3C)PGALATP = ADP + PNADPH = NADP + H
.Fig. 10-21LightReactions:Photosystem IIElectron transport chainPhotosystem IElectron transport chainCO2NADP+ADPPi+RuBP 3-PhosphoglycerateCalvinCycleG3PATPNADPHStarch(storage)Sucrose (export)ChloroplastLightH2OO2
THE NATURE OF SUNLIGHTLight is a form of energy =ELECTROMAGNETIC ENERGY/ELECTROMAGNETIC RADIATIONThe electromagnetic energy travel in waves.Distance between crests of electromagneticwaves = WAVELENGTHWavelength range from ≤ 1nm (gamma rays) –≥ 1 km (radio waves)The entire range of radiation wavelengths =ELECTROMAGNETIC SPECTRUM
The most important part for life is the visiblelight (380nm – 750nm)We can see this light as various colours.Light consist of particles = PHOTONSPhotons have energy- The shorter the wavelength the greater the energy of the photon.Therefore violet light has more energy than redlight.Photosynthesis are driven by visible light of thesun.
MAIN PIGMENTS USED DURINGPHOTOSYNTHESIS: Chlorophyll a – Absorb violet, blue and redlight. Reflects and transmits green light (thatis why plant leaves appear green) Chlorophyll b – Absorb violet, blue and redlight. Reflects and transmits green light (thatis why plant leaves appear green). Carotenoids – Play an accessory role inphotosynthesis. They are shades of yellow andorange and able to absorb light in the violet-blue-green range. These pigments becomenoticeable in the fall when chlorophyll breaksdown.
HOW A PHOTOSYSTEMHARVESTS LIGHT The thylakoid membrane of a chroloplastcontains several photosystems. A photosystem consist of a protein complexcalled a reaction-centre complex surrounded byseveral light harvesting complexes. Study the diagram to understand the process oflight harvesting.
THYLAKOID SPACE(INTERIOR OF THYLAKOID)STROMAe–PigmentmoleculesPhotonTransferof energySpecial pair ofchlorophyll amoleculesThylakoidmembranePhotosystemPrimaryelectronacceptorReaction-centercomplexLight-harvestingcomplexes
The Importance of Photosynthesis:A ReviewEnergy entering chloroplasts as sunlight getsstored as chemical energy in organic compoundsSugar made in the chloroplasts supplies chemicalenergy and carbon skeletons to synthesize theorganic molecules of cells.Plants store excess sugar as starch in structuressuch as roots, tubers, seeds, and fruitsIn addition to food production, photosynthesisproduces the O2 in our atmosphere