Chapter 8 Photosynthesis: Energy from the Sun Biology 101 Tri-County Technical College Pendleton, SC

The Chemical Equation  6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 –Not quite correct, but it will do  Light and chlorophyll omitted from equation but absolutely essential  Conversion of light energy into chemical energy  Conversion of inorganic substances into an organic substance

The Two Pathways  Photosynthesis occurs in chloroplasts of photosynthetic eukaryotic cells  Consists of LIGHT and DARK reactions –Light reactions aptly named but not so the dark  **Key: Dark reactions require products of light reaction  Light reactions produces ATP and NADPH  Dark reactions produces sugar

More Redox…hot damn!!  Light reactions composed of 2 photosystems  PS I and PS II  Can run either cyclic or noncyclic electron flow  Light reactions are series of redox reactions using electron carriers in thylakoid membrane and chemiosmosis  Depending on “what’s” running, can make ATP and NADPH

Photon Power  Light is form of electromagnetic radiation  Wave-theory explains most of what we know about light –Waves composed of discrete packets of energy called photons  Wavelength is distance from peak of one wave to peak of the next wave  Humans see in range of nm  Below 400 is ultraviolet, above 700 is infrared

Light, cont.  **Shorter the wavelength, the greater the energy (energy is inversely proportional to wavelength)  Three things can happen when photon meets a molecule –Bounce off (be reflected) –May pass through molecule (transmitted) –May be absorbed by molecule  If absorbed, it disappears but not its energy— energy acquired by molecule absorbing photon

Light III  Molecule is raised from ground state to excited state  An electron is boosted into a higher orbital –May fall back and emit that energy as light –May be so excited it is lost to the molecule  Molecules that absorb wavelengths in visible spectrum called pigments  Let’s talk about reflection and absorption

Chlorophyll Absorption  In plants, 2 chlorophylls predominate –a and b  Absorb in red and blue wavelengths (their absorption spectrum)  If only chlorophyll pigments were active in photosynthesis, MOST of visible spectrum would be unused  Accessory pigments absorb photons between red and blue wavelengths and transfer that energy to the chlorophylls (action spectrum for photosynthesis)

Absorption Visual

Action Spectrum Visual

Photosynthesis and Pigments  Chlorophyll a and b  Carotenoids (B-carotene & others) absorb in blue/blue-green and appear deep yellow  Phycobolins (phycoerythrin & phycocyanin) absorb in yellow-green, yellow, and orange

Warms my heart…  Pigment molecule absorbs photon and gets excited  Can return to ground state by emitting energy as fluorescence or pass energy along to another pigment molecule  Pigments arranged into antenna systems –Excitation passed from one pigment to another until reaches reaction center –Center is always chlorophyll a molecule that absorbs longest wavelengths

Fluorescence Visual

Excitation, cont.  Light energy absorbed is passed along as an electron  Ground state chlorophyll not much of reducing agent, but excited chlorophyll (Chl+) is –Chl+ can react with an oxidizing agent  Electron(s) can be boosted out of PS beginning series of redox reactions

E & E Visual

Photosystem  Accumulation of pigment molecules held together by proteins in right orientation for light absorption  Pigment molecules located so they can pass excitation to next pigment molecule in system  Eventually winds up at reaction center which (in plants) is always a chlorophyll a molecule –Absorbs at highest wavelength of all pigments in system

Photosystem, cont.  Only reaction center can “boost” electron out of PS to electron carriers in ETC in thylakoid membrane  Depending on PS being used and pathway, electron(s) can “wind up” in an electron carrier or be used in ETC (chemiosmosis) to eventually manufacture ATP

Photophosphorylation  Chalk talk time on PS I and cyclic electron flow  Chalk talk time on noncyclic electron flow utilizing PS I and PS II  One can bet the farm they will need this info: Product(s) of cyclic electron flow using PSI, product(s) of noncyclic electron flow using PS I and II, and why is PS II needed in the first place?

Light Reactions Data  6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2  Water molecules split in noncyclic electron flow to replace electrons lost by PS II  Diatomic oxygen produced (2 waters split = 1 O 2 )  NOT shown by balanced formula are NADPHs and ATPs needed by dark reactions to “fix” a sugar

Comparing Chemiosmosis  Works in both oxidative and photophosphorylation  Uses an ETC and redox reactions  This active proton transport = proton motive force (DOES 3 THINGS!!!!!)  In mitochondrion, H+s pumped OUT of matrix into intramembranous space  In chloroplast, H+s pumped into interior of thylakoid (from stroma to inside of thylakoid)  ATPase and synthesis of ATP

Calvin-Benson Cycle  Often called the “dark reactions”  Incorporate CO 2 into a sugar  Occurs in stroma of chloroplast, why?  Is a cycle, so something must already be “there”  Uses high-energy compounds made during light reactions to reduce carbon dioxide to carbohydrate  Three processes make up the cycle

Fixation of CO 2  RuBP (ribulose bisphosphate) already present in stroma; starts the “cycle”  Is a 5-carbon compound with two phosphates  Rubisco “fixes” carbon from CO 2 into RuBP  Forms unstable 6-carbon biphosphate molecule  splits into two 3-carbon phosphated molecules called PGA (phosphoglyceric acid)

Fixed CO 2 into Carb  Series of reactions involves phosphorylation using ATP and a reduction using NADPH  Product of interest is G 3 P (PGAL)  Two G 3 P can be combined into one glucose  In typical leaf, about 1/3 winds up in starch –2/3s winds up converted to sucrose  Transported out to other organs where hydrolyzed to glucose/fructose for various uses  Carbons from glucose can be used to make AAs, lipids, and precursor of nucleic acids  How important is this process?

Regeneration of RuBP  Most of G 3 P (10 of them) end up as regenerated RuBP so cycle can continue  For every turn of the Calvin-Benson cycle, with one CO 2 “fixed”, the acceptor (RuBP) gets regenerated  **Takes 6 turns of the Calvin-Benson cycle to make ONE glucose molecule

Calvin-Benson Visual

What’s the Cost?  12 ATP required to change 12 3-phosphoglyceric acid to 12 1,3 diphosphoglyceric acid  12 NADPH required to finish conversion of 1,3 diphosphoglyceric acid to G 3 P  2 of the 12 G 3 P converted to one glucose  Other 10 are converted to 6 ribulose 5-phosphate  Requires another 6 ATP to convert ribulose 5- phosphate into RuBP so cycle can continue  Just have to understand the question..NOW and later

Fickle finger of fate..so to speak  Balanced formula already provided and should be KNOWN  CO 2 is “fixed” into G 3 P (takes 2 to make a glucose)  H 2 O split to provide electrons for PS II during noncyclic electron flow  Some protons (H+s) from splitting water wind up in carbohydrate  O 2 generated by splitting of water and is released to environment (stomata)

Part and Parcel…  Glucose produced used by plants to make other compounds beside sugars –AAs, lipids, precursors of nucleic acids  Most of stored energy in these products released by glycolysis and cellular respiration during plant growth, development, and reproduction  Much plant matter ends up being consumed by animals –Glycolysis and cellular respiration in animals releases free energy from plant matter for use in animal cells

See 3, See 4, See AM  Calvin-Benson cycle is the C3 Cycle  Photorespiration is a big problem for C3 plants  Some plants have a C4 cycle and others use CAM  C4 plants fix oxaloacetate and cycle it into Calvin- Benson  CAM plants fix oxaloacetate, convert it into malic acid and use it to provide carbons for Calvin- Benson