Summary: Photosynthesis

I. Light Reactions -- require light directly

A. Photosystems I and II - many of each found in the Thylakoid Membrane

  1. Antenna Proteins bind chlorophyll and carotenoid pigment molecules that absorb light; each photosystem contains dozens of antenna proteins
  2. Reaction Center (RC) an integral membrane protein with special chlorophyll molecules where energy from light creates high energy electrons

B. NonCyclic Electron Flow

  1. PS II: Light absorbed by antenna proteins is transferred to RC called P680
    • high energy electron passes through pQ, Cytochrome b6f, and pC to the PS I RC called P700.
    • Cyt. b6f transports H+ from Stroma to Thylakoid Space
    • P680 is reduced with electrons taken from H2O as it is oxidized to O2
    • H+ gradient is used by ATP Synthetase to produce ATP
  2. PSI absorbs a second photon of light, energy is transferred to the RC, P700 where an electron is raised in energy to pass through Fd and reduce NADP+ to produce NADPH
  3. Products are: O2, ATP, and NADPH

C. Cyclic Electron Flow

  1. Only PSI adsorbs light using energy to raise an electron energy in P700 to pass through Fd to pQ through Cyt. b6f, and through pC back to P700
  2. Only produces ATP from H+ gradient created by Cyt. b6f

II. Dark Reactions a.k.a. Calvin Cycle--requires ATP and NADPH, the products of the Dark Reactions

A. Ribulose Bisphosphate Carboxylase:

  1. combines CO2 with Ribulose-1,5,-Bisphosphate (a 5 C sugar) to produce
  2. 2 molecules of 3-PhosphoGlycerate, an intermediate in Glycolysis

B. NADPH and ATP are used to drive glycolytic reaction backwards

  1. NADPH and ATP are used to reduce 3-P-Glycerate to Glyceraldehyde-3-Phosphate
  2. Some Glyc.-3-P is used to make glucose and then starch using energy from ATP
  3. Some Glyc.-3-P is rearranged to regenerate Ribulose-1,5-Bisphosphate so the cycle can continue--requires more ATP.

XXIII. PHOTOSYNTHESIS

A. Chloroplasts -- Structure (larger than mitochondria) -- Figure 4.15

1. Membranes -- 3

a) Outer Membrane -- permeable
b) Inner Membrane -- relatively impermeable. Contains transport proteins, esp. for transport of glyceraldehyde-3-Phos, the principal product of photosynthesis in chloroplasts.
c) Thylakoid Membrane -- form disklike thylakoids which stack on one another forming grana and which are connected; this increases the surface area of the thylakoid membrane system (like mito. inner membrane). Contain light-gathering and electron-transport proteins (energy producing membrane); ion impermeable.

2. Compartments

a) Intermembrane Space -- small
b) Stroma -- surrounded by inner membrane, analogous to mitochondrial matrix. Contains soluble enzymes for the dark reactions of photosynthesis. Also contain the chloroplast genetic system, DNA, ribosomes, etc. Biosynthetic reactions also take place in the stroma; fatty acid synthesis from ATP and NADPH and reduction of NO2- to NH3 for use in synthesis of amino acids and nucleotides.
c) 3. Thylakoid Space
 

B. Light Reactions -- require light.

1. Photons are captured by pigments, chlorophyll and carotenoids, in light-harvesting antennae proteins.

2. Light energy is funneled to special pigment-containing proteins called Reaction Centers

C. Electron Carriers: very similar to those in mitochondria

1. Quinones -- plastoquinone (pQ) is similar to ubiquinone in both structure and function.

2. Iron-Sulfur proteins -- Ferredoxin

3. Cytochromes -- cytochrome b 6f very similar in structure and function to cyt. bc1 (Complex III) in mitos.

4. Reaction Centers contain chlorophyll which is similar in structure to heme; the major difference is that Fe is replaced with Mg. Figure 8.9.

5. Photosystems: Reaction Centers are associated with antenna protein complexes that contain both chlorophyll molecules (chlorophyll a and chlorophyll b) as well as carotenoids (see below); the antenna protein complexes adsorb light at various wavelengths (Figure 8.7) and transfer the energy to reaction centers (Figure 8.11).

6. Plastocyanin (pC) is analogous to cyt. c, but is a copper-containing protein

D. Z-Scheme

1. Non-Cyclic Electron Flow -- 2 photons absorbed by 2 different Photosystems, Photosystem II and Photosystem I, containing two different Reaction Centers, P680 and P700. 8.12

a) P680 receives energy from red photons (collected by antennae complex) and goes to an excited state from which a high energy electron can escape and pass through a series of electron carriers. P680* can then oxidize H2O and return to the ground state, P680.

b) P700 receives an electron becoming reduced, it can then absorb a far red photon and jump to a higher energy excited state, P700*. It then gives up a high energy electron to Ferridoxin which then reduces NADP+ forming NADPH.
c) the products are both ATP (from the energy captured by cytochrome b 6f ) and NADPH

2. Cyclic Electron Flow -- only one photon absorbed by P700. Figure 8.13

a) P700 absorbs photon energy ---> P700*; the high energy electron ultimately used to reduce plastoquinone and then passes through the series of electron carriers back to P700.
b) the only product of this cycle is ATP produced from energy captured by cytochrome b 6f

F. Coupling to ATP Synthesis-- same mechanism as in mitochondria. Figure 8.14

1. Cyt. b 6f -- complex pumps H+'s from stroma into thylakoid space. pH 5 in thylakoid space; the membrane potential is very small owing to leakage of other counter ions.

2. ATP Synthetase -- very similar to mitochondrial enzyme; F1 portion out into the stroma. H+ pass from thylakoid space through ATP Synthetase into the stroma and synthetase using potential energy to produce ATP.

G. Dark Reactions -- don't require light

1. ATP and NADPH produced by light reactions are used to fix and reduce CO2 in production of carbohydrate. Sequence of reactions worked out by M. Calvin in first major use of 14C isotopic labeling -- called Calvin Cycle or Calvin-Benson Cycle. Figure 8.16, Figure 8.17

2. 1st step catalyzed by most abundant protein in world -- Ribulose Bisphosphate Carboxylase makes up 50% of chloroplast protein. It's required in such high concentrations because it works relatively slowly.

CO2 + Ribulose-1,5,-Bisphosphate -----> 2 3-Phospho-Glycerate

3. Through a sequence of reactions, 6 molecules of 3-P-Glycerate (produced from 3 molecules of Ribulose Diphosphate and 3 CO2) are reduced and rearranged at the expense of 9 ATP and 6 NADPH to produce 1 molecule of glyceraldehyde-3-Phos. and regenerating the 3 molecules of Ribulose Diphosphate.

4. Glyceraldehyde-3-Phosphate is used in the synthesis of carbohydrate or it can be reoxidized in the cytoplasm producing cytoplasmic ATP and NADPH for biosynthetic reactions. Figure 8.22


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