PHOTOSYNTHESIS

· Photosynthesis: The light-driven synthesis of carbohydrate (CH₂O) from CO₂.

· Photosynthetic systems also produce O₂ (from oxidation of water) for the earth atmosphere.

light

CO₂ + 2 H₂¹⁸O ® CH₂O + ¹⁸O₂ + H₂O

CHLOROPLASTS (Fig. 18-1)

· The site of photosynthesis in eukaryotes (algae and plants).

· ~1-1000 chloroplasts per cell, variable in size and shape, but typically as ~5-mm long ellipsoids.

· Contain 3 distinct types of membranes, namely outer, inner, and thylakoid membranes. The light-driven reactions take place in the thylakoid membrane.

· Three separate spaces: intermembrane space, stroma, and thylakoid lumen. The dark reactions take place in the stroma.

· Thylakoid membranes arise from invaginations of the inner membrane of developing chloroplasts, and resemble mitochondrial cristae.

· The thylakoid vesicle is arranged to have disk-like sacs, referred to as grana, stacked in piles, which are connected to each other by stroma lamellae.

· A chloroplast usually contains 10-100 grana.

· Stroma, a concentrated solution containing enzymes, DNA, RNA, and ribosomes.

CHLOROPHYLLS

· Magnesium porphyrins.

· Different chlorophylls have different absorption spectra.

Two Phases of Photosynthesis

1. Light Reactions

· Use light energy to generate NADPH, ATP and O₂.

· Occur in the thylakoid membrane, involving processes resembling ET and oxidative phosphorylation in mitochondria.

2. Dark Reactions

· Use NADPH and ATP to synthesize CH₂O from H₂O and CO₂.

· In eukaryotes, occur in the stroma.

LIGHT REACTIONS

I. Photosynthetic Reaction Centers (RC) and Light-Harvesting Complex (LHC)

· The primary (photochemical) reactions of photosynthesis take place in RC. An RC from purple photosynthetic bacterium Rhodopseudomonas viridis is a transmembrane protein containing a variety of chromophores. [bacteriochlorophyll (Bchl) a and b, nonheme Fe(II), bacteriopheophytin (BPheo) b, ubiquinone, menaquinone.] (BPheo is bacteriochlorophyll in which the Mg²⁺ ion is replaced by 2 H⁺)

· There are far more chlorophyll molecules than are contained in RCs. These chlorophyll molecules do not participate directly in the primary photochemical reaction but function as antenna chlorophylls critical in capturing photons from sun light.

· A light-harvesting complex (LHC) (or antenna complex) is a complex containing multiple membrane-bound hydrophobic proteins each containing numerous chlorophylls and other pigments. The non-chlorophyll pigments (i.e. accessory pigments such as carotenoids) absorb at wavelengths where chlorophylls do not absorb strongly.

· PRC is out-numbered by LHC.

· Energy from light captured by an antenna complex ® transfer from one to another antenna pigment molecule ® eventually trapped by PRC (occurs in < 10^-10 s, efficiency > 90%) which has a lower energy excited state than that of antenna pigment molecules. (Figs. 18-4, 7)

II. Two Photosystems and Non-cyclic E Transport (Fig. 18-12)

· All O₂-evolving photosynthetic cells contain photosystems I and II, whereas non-oxygen-evolving photosynthetic bacteria contain only one system (I).

· Photosynthesis requires the interaction of two light reactions, both of them can be driven by light at <680 nm but only one (photosystem I) can be driven by light at 680-700 nm.

Taken from Stryer.

1. Photosystem I (formation of NADPH)

· Can be excited at >680 nm but efficiency drops at >700 nm.

· Absorbs 4 quanta, generates a strong reductant and a weak oxidant.

· 4 Χ [P₇₀₀ + q → P₇₀₀* (= strong reductant) → P₇₀₀⁺ (= weak oxidant) + e]

· 4 Χ [P₇₀₀⁺ + e (from P₆₈₀*) → P₇₀₀]

· The strong reductant provides 4 e to reduce 2 NADP⁺ to 2 NADPH.

· NADPH is formed by ferridoxin-NADP⁺ reductase in stroma.

· The weak oxidant receives 4 e from the weak reductant produced by photosystem II (see below) to regenerate the ground state PSI P₇₀₀.

2. Photosystem II (formation of O₂ by oxygen-evolving center OEC)

· Excitation efficiency drops sharply at >680 nm.

· Absorbs 4 quanta, generates a weak reductant and a strong oxidant.

· 4 Χ [P₆₈₀ + q → P₆₈₀* (= weak reductant) → P₆₈₀⁺ (= strong oxidant) + e]

· 4 Χ [P₆₈₀⁺ + e (from water) → P₆₈₀].

· The strong oxidant abstracts 4 e from 2H₂O to generate 4H⁺ + O₂.

· The weak reductant provides 4 e to the weak oxidant formed by photosystem I.

3. Formation of H⁺ Gradient and ATP (Figs. 18-11, 12)

· The passage of 4e from the weak reductant of PSII to the weak oxidant of PSI is coupled to the pumping of 8 H⁺ INTO thylakoid lumen. In addition, the evolution of one O₂ from 2H₂O produces 4H⁺ in the thylakoid lumen.

· This passage involves multiple redox-active factors, including chlorophylls, cytochromes, Q, iron-sulfur clusters, and plastocyanin [a peripheral membrane protein on the thylakoid luminal surface, cycles between Cu(I) and Cu(II)].

· This H⁺ gradient is driving the ATP synthesis by ATP synthase following a mechanism similar to oxidative phosphorylation.

III. Cyclic E Transport (Fig. 18-12)

· Involves only photosystem I.

· Produces ATP but not NADPH or O₂.

· Presumably allow the cells to adjust the production of ATP relative to NADPH.

· Regulation of the partition between cyclic and noncyclic pathways is not known.

EFFICIENCY OF ATP SYNTHESIS

Non-Cyclic E Transport

· For every O₂ formed (or 8 q absorbed):

· 4H⁺ are formed in thylakoid lumen from 2H₂O ® 4H⁺ + O₂.

· In addition, 8H⁺ are pumped into thylakoid lumen when 4e pass from PSII to PSI.

· So, 12/3 = 4 ATP can be formed from the H⁺ gradient resulting from the absorption of 8 q.

· Additionally, the 2 NADPH formed by PSI are equivalent to another 6 ATP.

· The overall efficiency = 10 ATP per 8 q = 1.25 ATP/q.

Cyclic E Transport

For every 4 q absorbed by PSI, 8 H⁺ are pumped into thylakoid lumen, resulting in the formation of (8/3) ATP.

Efficiency = (8/3)/4 = 0.67 ATP/q.