6H2O + 6CO2 ----------> C6H12O6+ 6O2 Photosynthesis The process by which plants, some bacteria, and some protists use the energy from sunlight to produce sugar. 6H2O + 6CO2 ----------> C6H12O6+ 6O2 http://www.biocourse.com/mhhe/bcc/resources/concept.xsp?id=000012121&type=MOVIE

Photo = Light Synthesis = to make

Photosynthesis in nature Autotrophs: Biotic (living) producers Obtains organic food without eating other organisms Photoautotrophs Chemoautotrophs Heterotrophs: Biotic consumers Obtains organic food by eating other organisms or their by-products (includes decomposers)

Leaves and Leaf Structure A solar collector Plant cell: mesophyll Palisades: mostly photosynthetic Spongy: mostly gas exchange Gas exchange: stomata Controlled by guard cells http://www.tvdsb.on.ca/westmin/science/Biology12/Metabolic%20Processes/stoma.htm

The chloroplast Sites of photosynthesis Pigment: chlorophyll Double membrane Thylakoids = discs Grana = stacks of discs Stroma = fluid surrounding discs http://www.fw.vt.edu/dendro/forestbiology/photosynthesis.swf

Nature of Sunlight Light behaves both as a wave and a particle Particle properties (photons) seen by the photoelectric effect. Zinc exposed to ultraviolet light becomes + charged because light energy forces electrons away from the zinc. Light separates into different colors (wavelengths) by passing through a prism. E is inversely proportional to wavelength. Longer wavelengths E < shorter ones http://www.edumedia-sciences.com/a311_l2-prism.html

Chlorophyll and Antenna Pigments Absorb light energy to provide energy for photosynthesis Pigment absorbs all colors it doesn’t reflect. Wavelength reflected is color seen Chlorophyll a and b: energy from Violet-Blue and Orange-Red. Antenna pigments absorb energy that chlorophyll a and b don’t. xanthophylls (yellow) Carotenoids (orange,red) http://www.maine.gov/doc/foliage/kids/movie.html

Photosynthesis: an overview Redox process H2O is split e- (along w/ H+) are transferred to CO2 reducing it to sugar 2 major steps: Light reactions (“photo”) NADP+ (electron acceptor) to NADPH Photophosphorylation ADP ---> ATP Calvin cycle (“synthesis”) Carbon fixation: carbon into organics

Photosystems Light harvesting units found in thylakoid membranes Protein and antenna pigment complexes Two Photosystems: PSII (p680) PSI (p700) PSII comes before PSI Photophosphorylation: converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule http://www.johnkyrk.com/photosynthesis.html

Light Reaction Step 1 Photosystem II (P680) photons excite chlorophyll e- to an acceptor e- replaced by splitting of H2O (photolysis) release of O2 e-’s travel (redox) to Photosystem I down an electron transport chain (ETC) (Pq~cytochromes~Pc) as e- fall H+ pass to inside thylakoid creating a gradient which drives ADP ---> ATP

Light Reaction Step 2 Photosystem I (P700) ‘fallen’ e- replace excited e- to primary e- acceptor 2nd ETC ( Fd~NADP+ reductase) transfers e- to NADP+ ---> NADPH ...to Calvin cycle… These photosystems produce equal amounts of ATP and NADPH http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html

The Calvin cycle This process occurs in the stroma. ATP and NADPH produced by the light reactions are used in the Calvin cycle to reduce CO2 to sugar. ATP = energy NADPH = reducing agent Carbon enters as CO2 and leaves as 3 carbon sugar glyceraldehyde 3-phosphate (PGAL). 3 phases: Carbon Fixation Reduction Regeneration

Carbon Fixation Each CO2 is attached to RuBP rubisco enzyme Produces an unstable 6C compound that immediately splits into 2 molecules of 3-phosphoglycerate (PGA)

Reduction PGA gets a phosphate from the ATP producing a 1-3-bisphosphoglycerate. Electrons from NADPH reduces it to PGAL.

Regeneration PGAL rearranged to RuBP ATP used cycle continues http://www.sinauer.com/cooper/4e/animations0305.html

Calvin Cycle, net synthesis For every 3 CO2 there is one PGAL removed from the cycle PGAL can then be used by the plant to make glucose and other organic compounds http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Cyclic electron flow Alternative cycle when ATP is deficient Photosystem I used but not II produces ATP but no NADPH Why? The Calvin cycle consumes more ATP than NADPH……. Cyclic photophosphorylation

Photorespiration: Hot/dry days stomata close CO2 decrease, O2 increase in leaves O2 added to rubisco results in the breakdown of the Calvin cycle (no bueno) no ATP or food generated

Alternative carbon fixation methods C4 plants: 2 photosynthetic cells bundle-sheath & mesophyll PEP carboxylase (higher affinity than rubisco) fixes CO2 in mesophyll new 4C molecule releases CO2 grasses, corn, sugarcane, etc… CAM plants: open stomata during night, close during day (crassulacean acid metabolism) cacti, pineapples, etc.

A review of photosynthesis http://www.wiley.com/legacy/college/boyer/0470003790/animations/photosynthesis/photosynthesis.swf