7.3 Photosynthesis – Light-Dependent Reactions

How can light energy be used to make food? When a person turns on a lamp, electrical energy becomes light energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build basic carbohydrate molecules.

Absorption of Light

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum is the range of all possible frequencies of radiation (Figure 1). The difference between wavelengths relates to the amount of energy carried by them.

Figure 7.12. The sun emits energy in the form of electromagnetic radiation. This radiation exists at different wavelengths, each of which has its own characteristic energy. All electromagnetic radiation, including visible light, is characterized by its wavelength. [Image Description]

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength, the less energy it carries. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The electromagnetic spectrum (Figure 7.12) shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, which explains why both X-rays and UV rays can be harmful to living organisms.

Light energy initiates the process of photosynthesis when pigments absorb specific wavelengths of visible light. Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb. In plants, pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm; plant physiologists refer to this range for plants as photosynthetically active radiation. Light energy can excite electrons. When a photon of light energy interacts with an electron, the electron may absorb the energy and jump from its lowest energy ground state to an excited state.

How Light-Dependent Reactions Work

The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. Protein complexes and pigment molecules work together to produce NADPH and ATP. The numbering of the photosystems is derived from the order in which they were discovered, not in the order of the transfer of electrons.

Figure 7.13. A photosystem consists of 1) a light-harvesting complex and 2) a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. Credit: Rao, A., Ryan, K., Fletcher, S. and Hawkins, A. Department of Biology, Texas A&M University. [Image Description]

The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a photosystem, two types of which are found embedded in the thylakoid membrane: photosystem II (PSII) and photosystem I (PSI) (Figure 7.13). The two complexes differ on the basis of what they oxidize (that is, the source of the low-energy electron supply) and what they reduce (the place to which they deliver their energized electrons).

Both photosystems have the same basic structure; a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex, which passes energy from sunlight to the reaction center; it consists of multiple antenna proteins that contain a mixture of 300 to 400 chlorophyll a and b molecules as well as other pigments like carotenoids. The absorption of a single photon or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons.

The reaction center contains a pair of chlorophyll a molecules with a special property. Those two chlorophylls can undergo oxidation upon excitation; they can actually give up an electron in a process called a photoact. It is at this step in the reaction center during photosynthesis that light energy is converted into an excited electron. All of the subsequent steps involve getting that electron onto the energy carrier NADPH for delivery to the Calvin cycle where the electron is deposited onto carbon for long-term storage in the form of a carbohydrate. PSII and PSI are two major components of the photosynthetic electron transport chain, which also includes the cytochrome complex. The cytochrome complex, an enzyme composed of two protein complexes, enables both the transfer of protons across the thylakoid membrane and the transfer of electrons from PSII to PSI.

The reaction center of PSII delivers its high-energy electrons, one at the time, to the primary electron acceptor, and through the electron transport chain to PSI (Figure 7.15). The reaction center’s missing electron is replaced by extracting a low-energy electron from water; thus, water is “split” during this stage of photosynthesis, and PSII is re-reduced after every photoact. Splitting one H₂O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. However, splitting two molecules is required to form one molecule of diatomic O₂ gas. About 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms to support respiration.

Figure 7.15. In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons from water. The electrons travel through the chloroplast electron transport chain to photosystem I (PSI), which reduces NADP+ to NADPH. The electron transport chain moves protons across the thylakoid membrane into the lumen. At the same time, splitting of water adds protons to the lumen, and reduction of NADPH removes protons from the stroma. The net result is a low pH in the thylakoid lumen, and a high pH in the stroma. ATP synthase uses this electrochemical gradient to make ATP. Credit: Rao, A., Ryan, K., Fletcher, S. Department of Biology, Texas A&M University. [Image Description]

Generating Energy Carriers: ATP and NADPH

As electrons move through the proteins that reside between PSII and PSI, they lose energy. This energy is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. Those hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used synthesize ATP in a later step. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center, which is oxidized and sends a high-energy electron to NADP Reductase which reduces NADP+ to form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP+ into NADPH. The two photosystems work in concert, in part, to guarantee that the production of NADPH will roughly equal the production of ATP. Other mechanisms exist to fine-tune that ratio to exactly match the chloroplast’s constantly changing energy needs.

The buildup of hydrogen ions inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other.

To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP (Figure 7.15). The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure of the thylakoid.

Video 7.1. Photosynthesis: Part 5: Light Reactions | HHMI BioInteractive Video by biointeractive

Figure Descriptions

Figure 7.12. At the top, a bright photo of the sun is magnified into a horizontal rainbow bar labeled violet, blue, green, yellow, orange, red, representing visible light. Beneath it, a longer black band depicts the electromagnetic spectrum with a wavy red line whose crests spread farther apart from left to right, indicating increasing wavelength / decreasing energy. Labeled regions run left to right as gamma rays to X-rays to ultraviolet (UV) to visible light to infrared (IR) to microwaves to radio, showing that the slice of visible light occupies only a narrow band between UV and IR while the sun emits energy across many wavelengths. [Return to Figure 7.12]

Figure 7.13. The diagram titled “How a Photosystem Harvests Light” shows a Photosystem embedded in a thylakoid lipid bilayer, with the Stroma above and Thylakoid Space (Interior of Thylakoid) below. A yellow Photon strikes the outer Pigment Molecules that make up the Light-Harvesting Complexes; curved arrows labeled Transfer of Energy indicate excitation moving from pigment to pigment toward the central Reaction-Center Complex, where a Special Pair of Chlorophyll a Molecules sits at the base. From this pair, two red arrows point upward to the Primary Electron Acceptor, indicating that an electron excited by the transferred light energy is passed out of the reaction center and must be replaced for the photosystem to continue operating. [Return to Figure 7.13]

Figure 7.14. The figure compares common hydrophobic pigments in the thylakoid membrane. Panel (a) Chlorophyll a and (b) Chlorophyll b show nearly identical ring-and-tail structures; each has a red box highlighting the single difference between them. Panel (c) β-carotene is drawn as a long chain of alternating double bonds with no central ring like the chlorophylls; this pigment gives carrots their orange color. Panel (d) presents a graph of absorbance spectrum with wavelength along the x-axis and absorbance on the y-axis; three colored curves illustrate that each pigment absorbs light at different wavelengths (chlorophyll a and b strongly in the blue and red regions, and β-carotene primarily in the blue/blue-green) explaining green leaves and orange carrots. [Return to Figure 7.14]

Figure 7.15. The cutaway shows a Thylakoid Membrane separating Stroma (Low H⁺ Concentration) from Thylakoid Space (High H⁺ Concentration). At left, Light strikes Photosystem II, where H₂O is split to release O₂, H⁺, and electrons; the electrons move to Pq and through the Cytochrome Complex to Pc before reaching Photosystem I, which is hit by Light again. From PSI, electrons pass to Fd and then to NADP⁺ Reductase, reducing NADP⁺ + H⁺ → NADPH on the stromal side. Red dashed arrows indicate H⁺ being pumped into the lumen during electron flow, adding to the protons produced by water splitting, while formation of NADPH removes H⁺ from the stroma. together establishing the labeled high/low proton gradient. At center right, ATP Synthase uses this gradient as ADP + Pᵢ → ATP, and a curved arrow shows ATP (and NADPH) feeding the Calvin Cycle. [Return to Figure 7.15]