Photosynthesis (Higher Level)

The Two Stages of Photosynthesis: A Preview

The equation for photosynthesis is a deceptively simple summary of a very complex process. Actually, photosynthesis is not a single process, but two processes, each with multiple steps. These two stages of photosynthesis are known as the light reactions (the photo part of photosynthesis) and the Calvin cycle (the synthesis part).

The diagram above is an overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes are the sites of the light reactions, whereas the Calvin cycle occurs in the stroma. The light reactions use solar energy to make ATP and NADPH, which function as chemical energy and reducing power, respectively, in the Calvin cycle. The Calvin cycle incorporates CO2 into organic molecules, which are converted to sugar. A smaller version of this diagram will reappear in several subsequent figures as a reminder of whether the events being described occur in the light reactions or in the Calvin cycle.

The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Light absorbed by chlorophyll drives a transfer of electrons and hydrogen from water to an acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate), which temporarily stores the energized electrons. Water is split in the process, and thus it is the light reactions of photosynthesis that give off O2 as a by–product. The electron acceptor of the light reactions, NADP+, is first cousin to NAD+, which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADP+ molecule. The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons along with a hydrogen nucleus, or H+. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two compounds: NADPH, a source of energised electrons (“reducing power”), and ATP, the versatile energy currency of cells. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle.

The Calvin cycle is named for Melvin Calvin, who, along with his colleagues, began to elucidate its steps in the late 1940s. The cycle begins by incorporating CO2 from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation . The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired energized electrons in the light reactions. To convert CO2 to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions. Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the NADPH and ATP produced by the light reactions. The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions, or light–independent reactions, because none of the steps requires light directly. Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the light reactions provide the NADPH and ATP that the Calvin cycle requires. In essence, the chloroplast uses light energy to make sugar by coordinating the two stages of photosynthesis.

The thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the stroma. In the thylakoids, molecules of NADP+ and ADP pick up electrons and phosphate, respectively, and then are released to the stroma, where they transfer their high–energy cargo to the Calvin cycle. The two stages of photosynthesis are treated in this figure as metabolic modules that take in ingredients and crank out products. Our next step toward understanding photosynthesis is to look more closely at how the two stages work, beginning with the light reactions.

The light reactions convert solar energy to the chemical energy of ATP and NADPH.

Chloroplasts are chemical factories powered by the sun. Their thylakoids transform light energy into the chemical energy of ATP and NADPH. To understand this conversion better, we need to know about some important properties of light.

Chlorophyll and light absorption

Chlorophyll absorbs light from the visible part of the electromagnetic spectrum. Chlorophyll is made up of a number of different pigments: chlorophyll a, chlorophyll b, chlorophyll c along with other pigments such as carotenoids. Each of these absorb different wavelengths of light so that the total amount of light absorbed is greater than if a single pigment were involved. Not all wavelengths of light are absorbed equally. An absorption spectrum is a graph showing the percentage absorption plotted against wavelength of light (Fig 1). An action spectrum is a graph showing the rate of photosynthesis plotted against wavelength of light (Fig 1). The similarity between the absorption spectrum and the action spectrum shows that red (650- 700nm) and blue (400-450nm) wavelengths, which are absorbed most strongly, are also the wavelengths which stimulate photosynthesis the most. Green light (550mm) is mostly reflected.

1. Light energy is absorbed by chlorophyll molecules in PSI and PSII.

2. The electrons in the chlorophyll molecules are boosted to a higher energy level and are emitted.

3. The loss of electrons from PSII stimulates the loss of electrons from water i.e. it stimulates the splitting or photolysis of water. O2 is given off.

4. The electron from PSII passes through a series of electron carriers. At each transfer some energy is released.

5. This energy is used by cytochromes to pump protons (H+ ions) from the stroma across the thylakoid membranes. This sets up an electrochemical or H+ gradient. The H+ ions then diffuse back through a protein which spans the thylakoid membrane. Part of this protein acts as an enzyme - ATP synthetase - which uses the diffusion of H+ to synthesise ATP.

6. The electrons emitted from PSI may:

a) Pass down through the same carrier molecules as the electrons from PSII, again generating ATP. Before returning to PSI. Thus electrons are cycled (PSI i carriers i PSI i carriers etc. The energy to begin this cycle came from light (photo) and is used to convert ADP to ATP i.e. to phosphorylate ADP (add a phosphate). Hence this process is called cyclic photophosphorylation (CPP). Or

b) Combine with the hydrogen ions (protons) released from the photolysis of water to reduce nicotinamide adenine dinucleotide phosphate (NADP), forming NADPH. Non cyclic photophosphorylation (NCP) occurs when electrons are emitted from water and then pass to PSII i carrier (with ATP production) i PSI i carriers i NADPH.

7. Reactions 1-6 make up the Light Dependent Stage. The ATP and NADPH produced diffuse into the stroma where the Light Independent Stage occurs (7-11).

8. CO2 combines with a 5C compound called ribulose bisphosphate. This reaction is catalysed by the enzyme RuBPC.

9. The 6C compound formed immediately splits into two molecules of glycerate-3-phosphate (GP).

10. The GP molecules are converted into molecules of triose phosphate (TP) using energy from ATP and the hydrogen atom from NADPH i.e. the two useful products of the LDS are now used up in the LIS.

11. Some of the TP is used to regenerate RuBP.

12. The rest of the TP is used to produce other essential substances which the plant needs - fats, proteins etc.