Photosynthesis Light-Dependent Reactions Importance of Leaves Most photosynthesis occurs in the leaves. Photosynthesis Converting energy from sunlight into chemical energy. Pigments Absorb Wavelengths of Light Absorption Spectrum Other pigments are called ACCESSORY pigments. Chlorophyll is the main photosynthetic pigment Action Spectrum Shows which wavelengths are actually the most useful for photosynthesis Photosynthesis Takes Place in Two Stages: Light-Dependent Reactions Capturing light energy Light-Independent Reactions Carbon fixation making sugars Light-Dependent Reactions ELECTRON TRANSPORT CHAIN Photosystems Clusters of photosynthetic pigments Photosystems Photosystem II (PII) Contains chlorophyll a Absorbs 680nm Photosystem I (PI) Contains chlorophyll b Absorbs 700nm Light-Dependent Reactions Photons of light excite electrons in PII These electrons are photoactivated Photoactivated electrons from chlorophyll are passed to primary electron acceptor. Light-Dependent Reactions Primary electron acceptor passes photoactivated electrons to ETC. Light-Dependent Reactions Photolysis = splitting of water with light The released electrons from water are donated to PII as replacement electrons. H 2 O 2H + + O 2 + 2e - O 2 is produced! Photolysis: Chemiosmosis As electrons are passed along ETC, a H + gradient is created in the thylakoid space. Photophosphorylation ATP Synthase creates ATP when the protons move down their concentration gradients, back into the stroma. Light-Dependent Reactions Photons are also absorbed by PI Electrons are photoactivated go to ETC Electrons from first ETC replace electrons lost at PI Light-Dependent Reactions At the end of the ETC, electron pairs bind with NADP + and H + in the stroma to form NADPH. NADPH is an electron carrier Noncyclic Photophosphorylation Includes both photosystems (I and II) Both ATP and NADPH are produced Cyclic Photophosphorylation Sometimes NADP + levels are low PII cant pass electrons to PI It passes them BACK to photosystem II Continues to make ATP, but not NADPH Light-Dependent Reactions SUMMARY of Light Reactions Energy comes from ___________________________ Electrons come from _________________________ H 2 O comes from _____________________________ O 2 comes from ______________________________ O 2 goes to __________________________________ H + comes from ______________________________ ATP comes from _____________________________ ATP goes to _________________________________ NADPH comes from __________________________ NADPH goes to ______________________________