Photosynthesis

Photosynthesis

Photosynthesis occurs in many kinds of bacteria and algae, and in the leaves and sometimes the stems of green plants. The cells of plant leaves contain organelles called chloroplasts that actually carry out the photosynthetic process. No other structure in a plant cell is able to carry out photosynthesis. 

Photosynthesis takes place in three stages: (1) capturing energy from sunlight; (2) using the energy to make ATP and reducing power in the form of a compound called NADPH; and (3) using the ATP and NADPH to power the synthesis of organic molecules from CO2 in the air (carbon fixation).The first two stages take place in the presence of light and are commonly called the light reactions. The third stage, the formation of organic molecules from atmospheric CO2, is called the Calvin cycle. As long as ATP and NADPH are available, the Calvin cycle may occur in the absence of light. The following simple equation summarizes the overall process of photosynthesis:
6 CO2 + 12 H2O + light —→ C6H12O6 + 6 H2O + 6 O2 carbon water glucose water oxygen
dioxide.

Inside the Chloroplast

 
The internal membranes of chloroplasts are organized into sacs called thylakoids, and often numerous thylakoids are stacked on one another in columns called grana. The thylakoid membranes house the photosynthetic pigments for capturing light energy and the machinery to make ATP. Surrounding the thylakoid membrane system is a semiliquid substance called stroma. The stroma houses the enzymes needed to assemble carbon molecules. In the membranes of thylakoids, photosynthetic pigments are clustered together to form a photosystem. Each pigment molecule within the photosystem is capable of capturing photons, which are packets of energy.

 A lattice of proteins holds the pigments in close contact with one another. When light of a proper wavelength strikes a pigment molecule in the photosystem, the resulting excitation passes from one chlorophyll molecule to another. The excited electron is not transferred physically—it is the energy that passes from one molecule to another. A crude analogy to this form of energy transfer is the initial “break” in a game of pool. If the cue ball squarely hits the point of the triangular array of 15 pool balls, the two balls at the far corners of the triangle fly off, but none of the central balls move. 

The energy passes through the central balls to the most distant ones. Eventually the energy arrives at a key chlorophyll molecule that is touching a membrane-bound protein. The energy is transferred as an excited electron to that protein, which passes it on to a series of other membrane proteins that put the energy to work making ATP and NADPH and building organic molecules. The photosystem thus acts as a large antenna, gathering the light harvested by many individual pigment molecules.

The Role of Light

The role of light in the so-called light and dark reactions was worked out in the 1930s by C. B. van Niel, then a graduate student at Stanford University studying photosynthesis in bacteria. One of the types of bacteria he was studying, the purple sulfur bacteria, does not release oxygen during photosynthesis; instead, they convert hydrogen sulfide (H2S) into globules of pure elemental sulfur that accumulate inside themselves. 

The process that van Niel observed was CO2 + 2 H2S + light energy →(CH2O) + H2O + 2 S The striking parallel between this equation and Ingenhousz’s equation led van Niel to propose that the generalized process of photosynthesis is in fact CO2 + 2 H2A + light energy →(CH2O) + H2O + 2 A In this equation, the substance H2A serves as an electron donor. In photosynthesis performed by green plants, H2A is water, while among purple sulfur bacteria, H2A is hydrogen sulfide.

 The product, A, comes from the splitting of H2A. Therefore, the O2 produced during green plant photosynthesis results from splitting water, not carbon dioxide. When isotopes came into common use in biology in the early 1950s, it became possible to test van Niel’s revolutionary proposal. Investigators examined photosynthesis in green plants supplied with 18O water; they found that the 18O label ended up in oxygen gas rather than in carbohydrate, just as van Niel had predicted:
CO2 + 2 H218O + light energy —→ (CH2O) + H2O + 18O2 In algae and green plants, the carbohydrate typically produced by photosynthesis is the sugar glucose, which has six carbons.

 The complete balanced equation for photosynthesis in these organisms thus becomes 6 CO2 + 12 H2O + light energy —→ C6H12O6 + 6 O2 + 6 H2O. We now know that the first stage of photosynthesis, the light reactions, uses the energy of light to reduce NADP (an electron carrier molecule) to NADPH and to manufacture ATP. The NADPH and ATP from the first stage of photosynthesis are then used in the second stage, the Calvin cycle, to reduce the carbon in carbon dioxide and form a simple sugar whose carbon skeleton can be used to synthesize other organic molecules. 

The Calvin Cycle

Photosynthesis is a way of making organic molecules from carbon dioxide (CO2). These organic molecules contain many C—H bonds and are highly reduced compared with CO2. To build organic molecules, cells use raw materials provided by the light reactions: 

1. Energy. ATP (provided by cyclic and noncyclic photophosphorylation) drives the endergonic reactions.
2. Reducing power. NADPH (provided by photosystem I) provides a source of hydrogens and the energetic electrons needed to bind them to carbon atoms. Much of the light energy captured in photosynthesis ends up invested in the energy-rich C—H bonds of sugars.

 

 Reactions of the Calvin Cycle

In a series of reactions, three molecules of CO2 are fixed by rubisco to produce six molecules of PGA (containing 6 × 3 = 18 carbon atoms in all, three from CO2 and 15 from RuBP). The 18 carbon atoms then undergo a cycle of reactions that regenerates the three molecules of RuBP used in the initial step (containing 3 × 5 = 15 carbon atoms). This leaves one molecule of glyceraldehyde 3-phosphate (three carbon atoms) as the net gain. 

The net equation of the Calvin cycle is:
3 CO2 + 9 ATP + 6 NADPH + water —→ glyceraldehyde 3-phosphate + 8 Pi + 9 ADP + 6 NADP+ With three full turns of the cycle, three molecules of carbon dioxide enter, a molecule of glyceraldehyde 3-phosphate (G3P) is produced, and three molecules of RuBP are regenerated. We now know that light is required indirectly for different segments of the CO2 reduction reactions. 

Five of the Calvin cycle enzymes—including rubisco—are light activated; that is, they become functional or operate more efficiently in the presence of light. Light also promotes transport of three-carbon intermediates across chloroplast membranes that are required for Calvin cycle reactions. And finally, light promotes the influx of Mg++ into the chloroplast stroma, which further activates the enzyme rubisco.