How Cells Acquire Energy Chapter 6

zLight is central to the life of a plant zPhotosynthesis is the most important chemical process on Earth yIt provides food for virtually all organisms zPlant cells convert light into chemical signals that affect a plant’s life cycle Life in the Sun

Photosynthesis An Overview zChloroplasts are made up of; yStroma -liquid yThylakoids- disks containing chlorophyll zTwo stages of Photosynthesis yLight-dependent reactions - need sunlight yLight-independent reactions- do not need light

Figure 7.3B Figure 7.3C Experiment 1 Experiment 2 Not labeled Labeled Reactants: Products:

zPhotosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water AN OVERVIEW OF PHOTOSYNTHESIS Carbon dioxide WaterGlucoseOxygen gas PHOTOSYNTHESIS

zWater molecules are split apart and electrons and H + ions are removed, leaving O 2 gas yThese electrons and H + ions are transferred to CO 2, producing sugar 7.4 Photosynthesis is a redox process, as is cellular respiration Figure 7.4A Figure 7.4B Reduction Oxidation Reduction

Energy and Materials for the Reactions zLight Dependent ySunlight energy is absorbed and converted to ATP energy yWater molecules are split - releasing oxygen xNADP (coenzyme and electron carrier) picks up hydrogen and electrons to form NADPH

Energy and Materials for the Reactions zLight-Independent yATP donates energy to the cycle yNADP donates hydrogen yCO 2 donates Carbon and Oxygen yGlucose (C 6 H 12 O 6 ) made from CO 2 and Hydrogens zDifferent sites in chloroplast yStroma yGranum (in stroma) - stacks of thylakoids

Absorption Spectra z2 kinds of chlorophyll absorb wavelengths of visible light

The Chemical Basis of Color zPigments yChlorophylls a & b-green yCarotenoids- orange yPhycobilins - yellow

What Happens to the Absorbed Energy? zLight energy is passed from pigment to pigment until it reaches the reactioh center

Light-Dependent Reactions zIn Thylakoid membrane (granum) yPigments absorb photon energy yTransfer of electrons and hydrogen through electron transport system yATP and NADPH formation yReplacement of electrons to pigments

Cyclic and Noncyclic Electron Flow zCyclic Pathway yP700 xType I photosystem yElectron transfer creates ATP only zNoncyclic Pathway yP680 Electrons not cycled xType II and I photosystems yElectrons from water molecules passed along yPhotolysis- breaking water yATP and NADPH made

ATP Formation in Chloroplasts

zOxygen released from photolysis of water zH + concentration and electric gradients form across the thylakoid membrane zFlow of ions from thylakoid compartment into the stroma drives ATP formation 12H CO 2 ---> C 6 H 12 O 6 + 6H 2 O

zAn overview of photosynthesis Figure 7.5 Light Chloroplast LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma) Electrons H2OH2O O2O2 CO 2 NADP + ADP + P Sugar ATP NADPH

Light-Independent Reactions z“Synthesis” of sugar zCalvin-Benson Cycle

Calvin-Benson Cycle zCarbon Fixation yRuBP (ribulose biphosphate) attaches to carbon dioxide yRuBP carboxylase ( RUBisCO)-enzyme that puts together RuBP and carbon dioxide yPGA (Phosphoglyceraldehyde) - The 3 carbon sugar produced zStroma yDiffusion of CO 2 and O 2 through pores called stomates

Fixing Carbon zC3 Plants -normal photosynthesis yKentucky blue grass, crops zC4 Plants- hide the carbon dioxide in a 4 carbon sugar deeper in the leaf so when they need carbon dioxide they have it yCrabgrass

Fixing Carbon The C3 Pathway Evergreen trees and shrubs and nonwoody plants of temperate zones

Fixing Carbon The C4 Pathway Grasses and other tropical plants - Corn, crabgrass, and sugarcane

Fixing Carbon CAM Plants Hot dry climates - cacti, pineapple, orchids, and Succulents Stomates only open at night

Autotrophs, Humans, and The Biosphere Satellite images Red-orange signifies chlorophyll concentrations

In Conclusion zPlants and other autotrophs use carbon dioxide as their source of carbon and sunlight as the energy source zPhotosynthesis is the main process by which carbon and energy enter the web of life zPhotosynthesis has two stages: the light- dependent and the light-independent reactions

In Conclusion zLight-dependent reactions proceed in the thylakoids zLight-independent reactions occur in the stroma zChlorophyll a is the main photosynthetic pigment zAccessory pigments absorb wavelengths that chlorophyll a cannot

In Conclusion zIn chloroplasts, photosynthetic pigments are part of photosystems zThylakoid membranes have photosystems I & II zThe light-independent reactions in the stroma deal with the Calvin-Benson cycle

In Conclusion zSummary of photosynthesis

In Conclusion zPhotorespiration predominates in C 3 plants zC 4 plants fix carbon twice and CAM plants fix carbon at night zdeveloped by M. Roig