Chloroplast, chlorophyll molecules

An intermediate-level description of the proteins that orient chlorophyll molecules in chloroplasts. 

Since chlorophyll can be removed readily from chloroplasts by mild solvent extraction, it might appear that it is simply dissolved in the lipid portion of the membranes. However, from measurements of dichroism (Gregory,226 p. Ill) it was concluded that the chlorophyll molecules within the membranes have a definite orientation with respect to the planes of the thylakoids and are probably bound to fixed structures. The absorption spectrum of chlorophyll in leaves has bands that are shifted to the red by up to 900 cm-1 from the position of chlorophyll a in acetone. Most green plants contain at least four major chlorophyll bands at 662, 670, 677, and 683 nm as well as other minor bands264 (Fig. 23-21). This fact suggested that... 

Thylakoids Stack of membranes located in the chloroplasts of plant cells. They contain the chlorophyll molecules that capture light during photosynthesis. 

The values of A,max for Chi a in vivo result from interactions between a chlorophyll molecule and the surrounding molecules, such as the proteins and the lipids in the chloroplast lamellar membranes (Fig. 1-10) as well as adjacent water molecules. Indeed, all Chi a molecules are associated with proteins in chlorophyll-protein complexes. Hydrophobic interactions among... 

The probability for resonance transfer of electronic excitation decreases as the distance between the two molecules increases. If chlorophyll molecules were uniformly distributed in three dimensions in the lamellar membranes of chloroplasts (Fig. 1-10), they would have acenter-to-center spacing of approximately 2 nm, an intermolecular distance over which resonance transfer of excitation can readily occur (resonance transfer is effective up to about 10 nm for chlorophyll). Thus both the spectral properties of chlorophyll and its spacing in the lamellar membranes of chloroplasts are conducive to an efficient migration of excitation from molecule to molecule by resonance transfer. 

The highest photon flux density normally encountered by plants occurs when the sun is directly overhead on a cloudless day, in which case the photosynthetic photon flux (PPF) for wavelengths from 400 to 700 nm is about 2000 pmol m-2 s-1 on a horizontal plane (Chapter 4, Section 4.1D). The average chlorophyll concentration in chloroplasts is approximately 30 mol m-3 (Chapter 4, Section 4.4D), and in passing through a chloroplast 2 pm thick about 30% of the incident PPF is absorbed. We can therefore estimate how often an individual chlorophyll molecule absorbs a photon ... 

Thus on average 10 photons are absorbed per second by each chlorophyll molecule in a chloroplast exposed to full sunlight. 

As we have just calculated, each chlorophyll molecule in an unshaded chloroplast can absorb a photon about once every 0.1 s. When there are 250 chlorophylls per reaction center, 12.5 of these molecules are excited every 5 ms (250 chlorophylls x 10 excitations per chlorophyll/1 s x 0.005 s). However, because the average processing time per reaction center is about 5 ms, only one of these 12.5 excitations can be used photochemically — the others are dissipated by nonphotochemical deexcitation reactions. Consequently, although the biochemical reactions leading to CO2 fixation operate at their maximum rates under such conditions of high PPF, over 90% of the electronic excitations caused by light absorption are not used for photosynthesis (Fig. 5-12). 

Figure 5-12. Efficiency of photosynthesis at various PPF levels directly incident on a chloroplast. The lines were calculated assuming 250 chlorophyll molecules per reaction center, 30% absorption of the incident PPF, and a 5 ms processing time for the biochemical reactions (see text). Under these specific assumptions, below a PPF of 160 pmol m-2 s l, all excitations are used photo-chemically (solid line) and the fraction of the time that the biochemical reactions are used increases with the PPF (dashed line) above this PPF, the biochemical reactions are used all of the time (dashed line) but the fraction of excitations processed photochemically decreases (solid line).

B. How many pmol photons m-2 s-1 of 675-nm light will be absorbed in passing through a single chloroplast How many chlorophyll molecules participate in this absorption ... 

In Chapter 5 (Section 5.4B) we noted that the processing time per CO2 fixed is about 5 ms. Eight photons are required, which are absorbed by approximately 2000 chlorophyll molecules in chloroplasts having a chlorophyll concentration of about 30 mol m-3 (see Chapter 4, Section 4.4D). Hence, the photosynthetic rate per unit volume of chloroplasts is... 

The number of chlorophyll molecules participating in this absorption for the single chloroplast is... 

Chloroplasts present in the [S2 Qb ] state, i.e., one produced by one flash without DCMU present, have a prominent B-band between 20 and 30 °C, as shown in Fig. 6 (A), trace (a). At an ANT-2p concentration of 1 (iM, corresponding to less than one ANT-2p molecule per 2000 chlorophyll molecules, i.e., less than one molecule per reaction center, the B-band is virtually abolished [Fig. 6 (A), trace (b)]. The authors also reported that the use of NM (n-methyl) ANT-2p, a structurally related but totally ADRY-inactive compound, has practically no effect on the B-band peak height (not shown). 

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