CHAPTER 10 Photosynthesis

Sunlight as an Ultimate Energy Source All living things need energy Photosynthesis provides this energy  Converts light energy into chemical energy Acquired by either autotrophic or heterotrophic means

Autotrophs Heterotrophs Live without consuming anything from other living things  Require water, soil minerals, and CO2 Producers of the biosphere  Photoautotrophs  Use light as energy sources  E.g. plants, algae, protists, and bacteria Live on compounds produced by other organisms Consumers of the biosphere  Eat living organisms for energy  E.g. animals Decomposers of the biosphere  Breaks down dead organic matter  E.g. fungi

Anatomy of a Leaf Stomata allow gas exchange Veins move water from roots to leaves and sugars from leaves to roots Chloroplasts, the site of photosynthesis, Located in the mesophyll or interior leaf tissue  All green areas of plants, concentrated in leaves

Chloroplasts Double membrane bound organelle Fluid filled space called the stroma Contains multiple thylakoids, or interconnected membranous sacs  Stacked into grana  Chlorophyll pigment within  Gives plant characteristic colors  Captures energy for photosynthesis

Equation of Photosynthesis 6CO 2 + 6H 2 O + sunlight C 6 H O 2 What color line is showing reduction? oxidation?

Redox Revisited Cellular Respiration  Energy from sugar as electrons from H to O2 = H2O  Lose PE as fall to more electronegative oxygen  Mitochondria use energy released to make ATP Photosynthesis  H20 split and electrons to CO2 = sugar (reduction)  Gain PE as bond complexity increases  Requires energy = endergonic  Light provides boost

Photosynthesis: An Overview Light reactions [photo part]  Solar energy to chemical energy  Light drives transfer of e - ’s and H +  NADP + NADPH (reduction or oxidation?)  Create ATP using chemiosmosis to power photophosphorylation  NO sugar produced Calvin cycle (dark reaction) [synthesis part]  CO2 incorporated into organic molecules, carbon fixation  Add e - ’s from NADPH and ATP to reduce into carbohydrates  Makes sugar  Doesn’t need light directly

Photosynthesis

Understanding Sunlight Electromagnetic energy  Exists as discrete packets of particles called photons All wavelengths make up an electromagnetic spectrum  Wavelengths are distance between crests of waves and inversely related to amount of energy  Visible light most important to life  Detectable by human eye  Violet end is shortest waves  Red end is longest waves All combined = white light

Photosynthetic Pigments Light can be reflected, transmitted, or absorbed Chloroplasts vary in pigments  Chlorophyll a, b, and carotenoids  Violet-blue and red light most efficient for photosynthesis  Carotenoids have role in photoprotection In human eye too Action spectrum

Excitation of Chlorophyll Absorption of light elevates electrons of pigments to higher orbital ( PE)  Pigments absorb in specific range Unstable in upper orbital so ‘fall’ back quickly  Releases energy as heat White vs black cars or clothing in the South

Photosystems Protein complex with a reaction center surrounded by light-harvesting complexes Chlorophyll a always bound with reaction center molecules Other pigments with light- harvesting complexes Gather light from larger surfaces Pigments absorb photons and transfer to reaction center complex Electrons transferred to primary electron acceptor, reducing it Two types, II and I

Light Reaction Occurs in the thylakoids Two Photosystems  PS I absorbs at 700nm  PS II at 680nm Two electron flow patterns  Linear electron flow  Cyclic

Linear Electron Flow To Calvin cycle

Comparing Chemiosmosis Similarities  ETC in membranes pump protons across as e - ’s moved to more EN carriers  ATP synthase utilizes [H + gradient] Differences  M: e - ’s from organics, protons move out  P: e - ’s from H2O, protons move in

Calvin Cycle Anabolic reaction in the stroma Products from light reaction are reactants for dark (3) CO2 molecules combine to create (1) 3 carbon sugars (glyceraldehyde 3- phosphate, G3P)  Cycle must occur 3 times for 1 molecule to be made Broken into 3 steps  Carbon fixation  Reduction  Regeneration of CO2 acceptor (RuBP) CO2 RuBP G3P 3PG

Carbon Fixation 1 CO2 into stroma Attaches to ribulose bisphosphate (RuBP), a 5 carbon sugar Catalized by rubisco  Most abundant protein on Earth Forms unstable 6 carbon molecule  Immediately to (2) 3-phosphoglycerate (3PG)  2 for every 1 CO2 molecule

Reduction 3PG gains a phosphate from ATP to create 1,3- bisphosphoglycerate NADPH reduces 1,3-bisphosphoglycerate to G3P 3 cycles (3 CO2’s) create 6 G3P  Only 1 leaves (3 carbons out)  Other 5 recycled (15 carbons remain)

Regeneration of CO2 Acceptor 5 G3P are rearranged into 3 RuBP (5 carbons each)  Cost 3 ATP Capable of accepting CO2 again Overall cost of cycle  9 ATP  6 NADPH  3 CO2 2 G3P to make sugars and other fuels

Review of Photosynthesis