Photosynthesis SGN 13 No way! More like chlorophabulous!!

Photosynthesis is a metabolic process wherein light energy is used to reduce molecules of CO2 to make a 3-carbon carbohydrate, with O2 as a waste product Light energy is converted to chemical energy, which is then used to fuel endergonic, anabolic reactions The reactions result in carbon fixation – reduction of inorganic carbon to organic carbon Glucose + =

3C molecule is used to make glucose and as starting material for metabolic pathways that synthesize other organic compounds

Generally summarized as 6CO2 + 6H2O  C6H12O6 + 6O2 The carbon and oxygen of CO2 are combined with electrons and hydrogen from H2O to make the 3 C molecule; molecular oxygen comes from H2O

Students should understand photosynthesis as a redox process

Importance of 3-C molecule Glceraldehyde 3-C molecule is origin of virtually all carbon, hydrogen and oxygen in organisms, either used by the photoautotroph itself to build its body, or becoming part of the food chain and used to build the bodies of heterotrophs 3-C molecule provides primary source of all fuel for ACR Making 3-C molecule produces O2  

Students should understand molecular relationship between photosynthesis and cellular respiration (see chemical equations)

Organisms that use photosynthesis – photoautotrophs (source of energy = light; source of carbon = CO2) Some prokaryotes (cyanobacteria) Some unicellular and multicellular protists (golden and brown algae, dinoflagellates, etc.) Plants (including green algae)

Plant mesophyll cells, found primarily in the leaf, have chloroplasts – organelles where the production of the 3-C molecule takes place Students should know the anatomy of the chloroplast Students should know the basic anatomy of the leaf

Overview: Primarily 2 parts to photosynthesis as it occurs within the chloroplast Light reactions - occur in thylakoid membrane - light energy is used to drive the production of ATP and NADPH (conversion of light to chemical energy) - H2O provides electrons added in the process and in turn is oxidized to O2 Calvin Cycle/Light Independent Reactions - occurs in the stroma - uses the ATP and NADPH produced in LR to reduce CO2 to produce G3P

Light reactions Photosystems within the thylakoid membrane of the chloroplast are major players in the light reactions Photosystems are light harvesting complexes of 100’s of pigments of several different kinds (mostly chlorophylls) embedded in the thylakoid membrane

Pigments – molecules that are “excited” by photons of light

Examples of pigments are chlorophyll a and b, carotenoids Chlorophyll and accessory pigments have broad action spectrum (range over which they absorb energy) but generally reflect green light

When light photons strike these molecules they transfer energy to the pigment’s electrons, which jump from their “ground” state to an “excited” state This excited state represents potential energy (like the energy found in the electrons of a C-C and C-H bonds)

Two photosystems typically work in concert to first produce ATP and then produce NADPH

Photosystem II Central chlorophyll passes excited electrons to PEA, which passes electrons to electron transport chain, which produces ATP, in same manner as ACR An enzyme in the reaction center of PSII is stimulated by this exchange of electrons, allowing it to strip H2O of an electron (oxidizing H2O to O2); the electron is used to replace the electrons lost in the reaction center When electron has fallen to its lowest state it is used to replace electron lost by PSI

Photosystem I Electrons are excited at the reaction center PEA passes excited electron to electron transport chain ETC includes reductase that uses electrons to reduce NADP+  NADPH

The ETC leading from PSII includes electron transporters and proton pumps; these use the energy of the falling electron to create an electrochemical gradient (thylakoid space more +) The diffusion of H+ back into the stroma through ATP synthase phosphorylates ADP  ATP The deenergized electrons fill the electron hole in PS I

The short ETC leading from PS I is used to enzymatically reduce NADP+  NADPH

The pathway that includes both photosystems is called noncyclic electron flow (or noncyclic photophosphorylation)* *Versus substrate level or oxidative phosphorylation

Because the Calvin cycle (where they will be used) consumes more ATP than NADPH (9:6 for each G3P produced), NADPH buildup causes transporter in PS I ETC to hand electron back to first PS II, leading to production of more ATP but no NADPH or O2 (cyclic electron flow /photophosphorylation)

Calvin cycle - uses ATP (energy for anabolism) and NADPH (electrons) and CO2 to make G3P Occurs in the stroma

3 phases in cycle that lead to the production of G3P Carbon fixation – RuBisCo transfers CO2 to CO2 acceptor molecule (unstable 5-carbon molecule), which comes apart to produce 2, 3-carbon molecules

Reduction – reduction by NADPH adds energy rich C-H bonds and produces G3P

Regeneration of CO2 acceptor molecule – energy is used (more ATP) to regenerate unstable 5-C acceptor

Significant problem with photosynthesis is photorespiration RuBisCo has high affinity for oxygen, which competes for active site with CO2 Plants that populate wetter, cooler regions are able to keep stomates open, allowing for diffusion to provide adequate supplies of CO2; stomates also allow for escape of excess molecular oxygen Closing of stomates in hot, dry conditions to prevent dehydration leads to increased O2 and decreased CO2 within leaf, and therefore increased photorespiration (O2 replacing CO2) and decreased photosynthesis

Plants that operate in this way (decreased photosynthesis in hot, dry conditions) are called C3 plants – first product of carbon fixation is a 3-C molecule Rice, wheat, soybeans, etc.

In hotter, drier regions plants have evolved adaptation that allow for closure of stomates without debilitating photorespiration

C4 Plants - stomates can close during day without decrease in photosynthesis because… Light reaction (produces O2) and carbon fixation occurs in outer layer of mesophyll cells, but by alternate enzyme (with low O2 affinity), which produces 4 carbon compound, which acts as a CO2 carrier 4-C compound is transported to inner layers of cells where CO2 is cleaved away and reassimilated into Calvin cycle by RuBisCo Spatial separation of steps; separation of Rubisco from site of O2 production (light reactions)

Corn and sugarcane (grasses), etc

CAM Plants Stomates are opened during the night and closed during the day, saving water vapor, but interfering with gas exchange During the night the plant takes up CO2 and incorporates it into a variety of organic acids (light reactions not making O2) During day when light is available to fuel production of ATP and NADPH, CO2 is released from acids, saturating cell so CO2 can compete with O2 Temporal separation of steps; saturation or cells with CO2 when stomates closed

Pineapple, many cacti, many succulents

Make yourself be more photosynthesis every day!!! Where would we be without photosynthesis? Make yourself be more photosynthesis every day!!!