Info

Plants, Algae

Cyanobacteria

Purple Photosynthetic Bacteria

Green Photosynthetic Bacteria

Location of the photosystem

In membranes of thylakoids, which are within the stroma of chloroplasts.

In membranes of thylakoids, located within the cell.

Within the cytoplasmic membrane; extensive invaginations in that membrane effectively increase the surface area.

Primarily within the cytoplasmic membrane; chlorosomes attached to the inner surface of the membrane contain the accessory pigments.

Type of photosystem

Photosystem I and photosystem II

Similar to photosystem II

Similar to photosystem I

Primary light harvesting pigment

Chlorophyll a

Bacteriochlorophylls

Mechanism for generating reducing power

Non-cyclic photophosphorylation using both photosystems

Reversed electron transport

Non-cyclic use of the photosystem

Source of electrons for reducing power

H2O

Varies among the organisms in the group; may include H2S, H2, or organic compounds.

CO2 fixation

Calvin-Benson cyde

Calvin-Benson cycle

Reversed TCA cycle

Accessory pigments

Carotenoids

Carotenoids, phycobilins

Carotenoids

Carotenoids

The photosystems of plants, algae, and cyanobacteria are located in special photosynthetic membranes. The photosystems of cyanobacteria are embedded in the membranes of stacked structures called thylakoids located within the cell; the phyco-bilins are contained in granules that are attached to the thylakoid membrane. Plants and algae also have thylakoids, in the stroma of the chloroplast (see figure 3.56). The similarity between the structure ofchloroplasts and cyanobacteria is not surprising considering that the organelle appears to have descended from an ancestor of cyanobacterium (see Perspective 3.1).

Light

Electron transport chain

Reaction-center chlorophyll

Chlorophyll molecules

Electron transport chain

Light

Chlorophyll molecules

Reaction-center chlorophyll

Photosystem

Figure 6.26 Photosystem The antenna complex captures the energy of light and then transfers it to reaction-center chlorophyll, which emits an electron that is then passed to an electron transport chain.

Photosystem

Figure 6.26 Photosystem The antenna complex captures the energy of light and then transfers it to reaction-center chlorophyll, which emits an electron that is then passed to an electron transport chain.

The photosystems of the purple and green bacteria are embedded in the cytoplasmic membrane. Purple bacteria have extensive invaginations in the membrane that maximize the surface area. Green bacteria have specialized structures called chlorosomes attached to the inner surface of membrane that contain the accessory pigments.

Converting Radiant Energy into Chemical Energy

The task of converting radiant energy into chemical energy is twofold. Photosynthetic organisms must produce energy in the form ofATP, but they also need to generate reducing power so they can fix CO2. Depending on the method used to fix CO2, the type of reducing power required may be either NADPH or NADH.

The process of photophosphorylation in all photosyn-thetic organisms is analogous to that of oxidative phosphoryla-tion. The high-energy electrons emitted by reaction center chlorophylls that have been excited by light are passed to an electron transport chain, which subsequently uses them to generate a proton motive force. The energy of the proton motive force is then harvested by ATP synthase to fuel the synthesis of ATP.

The mechanisms used to produce reducing power vary significantly among photosynthetic organisms. Eukaryotic cells and cyanobacteria both require NADPH for carbon fixation and share a common mechanism for generating it. Bacteria other than cyanobacteria, however, use different mechanisms.

Light-Dependent Reactions in Cyanobacteria and Photosynthetic Eukaryotic Cells

Cyanobacteria and chloroplasts have two distinct photosystems that work in tandem (figure 6.27). The sequential absorption of

Nester-Anderson-Roberts: I I. Life and Death of Microbiology, A Human Microorganisms Perspective, Fourth Edition

6. Metabolism: Fueling Cell Growth

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158 Chapter 6 Metabolism: Fueling Cell Growth f

Radiant energy

Excited chlorophyll

Proton gradient formed for ATP synthesis

Electron A ^r rrier /"""N

Radiant energy

Reaction-center chlorophyll Water-splitting enzyme

2H2O

Photosystem II

Figure 6.27 The Tandem Photosystems of Cyanobacteria and Chloroplasts Radiant energy captured by photosynthetic pigments excites the reaction-center chlorophyll, causing it to emit a high-energy electron, which is then passed to an electron transport chain. In cyclic photophosphorylation, electrons emitted by photosystem I are returned to that photosystem; the path of the electrons is shown in green arrows. In non-cyclic photophosphorylation, the electrons used to replenish photosystem I are donated by radiant energy-excited photosystem II; the path of these electrons is shown in orange arrows. In turn, photosystem II replenishes its own electrons by stripping them from water, producing O2.

Reaction-center chlorophyll Water-splitting enzyme

2H2O

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Excited chlorophyll

Proton pump

Electron carrier

Reaction center chlorophyll

Electron carrier

Reaction center chlorophyll

Electron carrier

NADP reductase

NAPH+ + H+ Radiant energy

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