Photosynthesis I: _{Harnessing the energy of the sun}

Early experiments towards the discovery of photosynthesis

Priestley, partially credited with the discovery of elemental oxygen, found that when he placed fresh sprigs of mint leaves inside a sealed glass container, a candle would burn longer than if the leaves were not there (Figure 1). He also found that a previously extinguished candle would reignite inside a sealed jar – sometimes days after it had ceased to burn – if mint leaves were present. This caused him to suspect that the leaves were somehow “refreshing” the air inside the container.

Several years later, a Dutch scientist named Jan Ingenhousz, having heard of Priestley’s experiments, began to conduct experiments of his own. He submerged willow plants in water and saw that bubbles formed on the surface of the leaves. The bubbles, however, formed only when the experiment was conducted in the presence of sunlight. Ingenhousz later determined the gas bubbles were oxygen, but never fully understood the significance of what he had observed regarding the sunlight.

Putting it all together: Reactants and products of photosynthesis

Collectively, these chemists established the products and reactants of photosynthesis – water, oxygen, carbon dioxide, and light. But it took the musings of a German physicist named Julius Von Mayer to put the pieces together. Von Mayer, the first to propose that “energy is neither created nor destroyed,” was also first to suggest that plants derive their energy for growth from sunlight.

Von Mayer’s understanding of photosynthesis implied that the sun was the basis for all life on Earth. The sun’s chemical energy, he said, feeds the plants that in turn feed almost every living thing on the planet. He explained photosynthesis as a process that created organic molecules – sugars – from the inorganic molecules carbon dioxide and water (Liebig, 1841). He first articulated the equation as:

CO₂ + H₂O + light energy → O₂ + organic matter + chemical energy

Work by other scientists helped to establish the chemical formula of the organic products of photosynthesis, which is usually simplified as a glucose molecule: C₆H₁₂O₆. The properly balanced general formula for photosynthesis thus becomes:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

The energy of photosynthesis comes from light

The principal product of photosynthesis (sugar) is a high-energy molecule, but the reactants (carbon dioxide and water), are low-energy molecules, so the process of photosynthesis needs an energy source to drive it. Molecules called pigments absorb energy from light. The main pigment in photosynthesis is called chlorophyll. Chlorophyll exists in several different forms in different organisms. Chlorophyll a is the main photosynthetic pigment found in land plants and algae. It absorbs light in the blue/violet range of the light spectrum (wavelengths of 400-450nm) as you can see in Figure 2. It also absorbs light in the red range of the spectrum (wavelengths of 650-700nm) to a lesser degree. Green light is almost completely reflected by chlorophyll, giving plants their greenish hue.

Plants do not make equal use of all the wavelengths present in the full range of visible light – a fact first demonstrated by German plant physiologist T. W. Engelmann in 1882. He used a simple experiment to demonstrate that the blue and red wavelengths of light, in particular, were the biggest drivers of photosynthesis.

The action spectrum of photosynthesis

Engelmann split white light into its spectral components using a prism and shone it on a dish of liquid solution containing a photosynthetic green algae called Chladophora. He then released bacteria into the solution. The bacteria, which need oxygen to survive, migrated toward those areas in the dish where blue and red wavelength light was shining. Why? Because where the red and blue range of light was shining, the photosynthetic algae produced more oxygen due to increased photosynthetic activity. With this demonstration, Engelmann had established the first action spectrum of photosynthesis.

Chlorophyll a does not perfectly overlap with the action spectrum of photosynthesis identified by Engelmann (see Table 1). This led scientists to suspect that there are additional pigments in plants that absorb light at different wavelengths. Land plants have pigments such as chlorophyll b and carotene, while other photosynthetic organisms, like protists, have chlorophyll c and chlorophyll a.

Plant pigments are classified as either chlorophylls or carotenoids. Chlorophylls reflect green light while carotenoids reflect light in the red, orange, and yellow range. Carotenoids give carrots their color. They are considered an accessory pigment because they cannot transfer sunlight energy directly to the photosynthetic pathway. Carotenoids pass their absorbed energy to chlorophyll, which in turn transfers energy to the photosynthetic pathway.

Photosynthetic pigments are large, hydrophobic molecules embedded in protein pigment complexes called photosystems that work like antennas to collect the sun’s energy. In plants, the photosystems are embedded in the thylakoid membranes inside chloroplasts (Figure 3).

Phase One: The light-dependent reactions

Photosynthesis occurs in two phases: the light-dependent reactions and the Calvin-Benson Cycle (see the Photosynthesis I video below). The light-dependent reaction is the first phase, when pigments like chlorophyll harvest light energy. The Calvin-Benson Cycle uses that energy to synthesize high-energy sugar molecules from carbon dioxide. In plants and algae, the light reactions occur within the thylakoid membranes of chloroplasts. The animation below provides an overview of photosynthesis.

When a photon of light (see Light I: Particle or Wave? module) strikes a pigment molecule, its energy is transferred to the pigment and one of the pigment’s electrons becomes “excited.” When excitation of an electron occurs, it “jumps” to a higher energy state. Thus, the energy of light is “captured” by the pigment in the form of an excited electron. The excited electron can hold on to this energy only for a brief time, though. If it cannot pass the energy quickly, the electron will fall back down to a low-energy state and the energy will be given off as heat.

Within a chloroplast of a leaf, however, there are many pigment molecules packed together very tightly in structures called light-harvesting complexes, which are combinations of proteins, cofactors, and pigment molecules. The pigment molecules are constantly moving in random, Brownian motion, colliding with one another. Excited pigments transfer energy to their neighboring pigments until it reaches the reaction center, as shown in Figure 4.

Like the light-harvesting complexes, the reaction centers are also made of proteins, cofactors, and pigments, but there are two types of reaction centers: photosystem I and photosystem II. Photosystem I, so named because it was discovered first, is also referred to as P700 because the special chlorophyll a pigment molecules that form it best absorb light of wavelength 700nm. Photosystem II is also referred to as P680, because the chlorophyll molecules that form it best absorb light in the 680nm wavelength. In both cases, after either P700 or P680 become excited, either by a photon or another excited pigment molecule, one of its electrons moves to a higher energy state. The difference between these two photosystems lies in what happens next with this harnessed energy. View a video of photosystems I and II below.

Photosystem II

Even though it was discovered and named second, photosystem II is actually where the story begins. When a photon of light strikes the reaction center of photosystem II, it excites an electron that leaves and begins its journey through a series of high-energy electron acceptors and donors collectively known as the electron transport chain (ETC) as shown in Figure 5. (This particular ETC is called the cytochrome ETC, after one of the members of the chain that was discovered first.)

At the same time, two water molecules bind to a water-splitting enzyme at the reaction center of photosystem II, as seen in Figure 6. When the water molecules split, ionized hydrogen atoms (H⁺) enter the thylakoid space. An enzyme called cytochrome b6f, the next stop in the chain after photosystem II, generates more ions for the proton pump and sends the excited electrons along toward photosystem I. As the hydrogen ions accumulate within the thylakoid space, they create the H⁺ gradient that drives ATP synthesis. ATP will be used for sugar synthesis later, in the Calvin-Benson Cycle.

Oxygen atoms from the split water molecules also accumulate within the thylakoid space. Lone oxygen atoms are very reactive and rapidly combine to form molecular oxygen (O₂) that is released as a waste product of photosynthesis. Yes, each molecule of oxygen that we breathe was formed in a chloroplast somewhere as an accidental by-product of the splitting of water. Electrons are at a much lower energy state at the end of the ETC than they were at the beginning of the process. They get a badly needed boost at the reaction centers in photosystem I.

Photosystem I

Photosystem I also consists of light-harvesting complexes with lots of pigment molecules for capturing light energy. Light energy harvested from photons and intermediate-energy electrons from photosystem II flow to a special chlorophyll a molecule structure called P700 in photosystem I. Electrons jump up to a high-energy state when a photon arrives at P700, either directly from sunlight, or through a collision with an already excited pigment.

Once re-excited to a high energy level, the electrons don’t stay for long. Excited electrons leave photosystem I and flow through another ETC, but this one, called the Ferredoxin ETC, is much shorter and does not drive ATP synthesis. The Ferredoxin ETC passes the excited electrons to the high-energy electron acceptor NADP⁺, which then combines with a proton (H⁺) from the surrounding solution and forms NADPH. NADPH then delivers high-energy electrons to the Calvin Cycle for long-term energy storage in the form of sugar (Figure 7).

Phase Two: The Calvin-Benson Cycle

After the energy of light is harvested as high-energy electrons held by NADPH, these electrons are then used to synthesize high-energy sugar molecules from the low-energy starting material of carbon dioxide. The Calvin-Benson Cycle used to be called “the dark reactions” because light is not directly involved. However, this name is misleading because the products of the light reactions are required to drive the Calvin Cycle. Thus, light is required, just not directly.

So far, we’ve seen how the flow of electrons in the light reactions goes like this (note: PSI and PSII stand for photosystem I and II):

This linear path is called noncyclic electron transport. However, not all electrons flow in this linear path. Some electrons double back and return to PSII after the PSI. This is called cyclic electron flow.

Why would some electrons take the redundant path of twice being energized by PSII and twice flowing through the cytochrome ETC? The answer is found when thinking about what the ETC produces – ATP. The simple noncyclic flow of electrons produces ATP and NADPH in roughly equal amounts. However, the Calvin Cycle needs more ATP than NADPH. Thus, the extra trip through the ETC that occurs in cyclic electron flow provides a little “boost” of ATP so that the Calvin Cycle has what it needs to synthesize sugars.

In roughly 300 years, our understanding of photosynthesis has progressed from mere identification of all the basic products and reactants of photosynthesis to a detailed picture of the molecular processes involved. We have summarized in this module how electrons are harvested, energized, and stored in the covalent bonds of NADPH, a process called light reactions. In the next module, we explore the Calvin-Benson Cycle where high-energy electrons from NADPH drive synthesis of carbohydrates – the sugars that provide sustenance to nearly every living thing on the Earth.