The light phase is the first phase of photosynthesis, when light is absorbed by complexes made up of chlorophylls and proteins called photosystems (located in the chloroplast). During this phase, solar energy is converted into chemical energy.
In this series, we’ll explain what amounts to the most important biochemical process in nature since life began millions of years ago and changed the make-up of the atmosphere all over the planet. This is of course photosynthesis.
This process is responsible for our planet now having an atmosphere rich in oxygen. A consequence of this is that higher plants have ended up dominating the earth’s surface, supporting a whole host of organisms that feed off of—or are sheltered by—them. The primitive atmosphere contained very little oxygen, but it did contain other gases such as ammonium, nitrogen and carbon dioxide.
Plants found a way to transform this readily available CO2 into food with the help of sunlight. The question is: How is it possible to transform water and carbon dioxide into carbohydrates? And what role does solar radiation play in such a process? We’ll try to explain this in a way that is easy to understand.
What is photosynthesis?
Photosynthesis is the conversion of inorganic matter into organic matter with the help of the energy from sunlight (or from grow lamps in certain cases). In this process, light energy is transformed into stable chemical energy, with adenosine triphosphate (ATP) being the first molecule in which this chemical energy is stored. Later on, the ATP is used to synthesise more stable organic molecules.
It should also be pointed out that life on our planet is maintained fundamentally thanks to photosynthesis by seaweed in aquatic environments and by plants in terrestrial environments. This is because of their ability to synthesise organic matter (vital for the make-up of living things) using light, essential minerals, and macro- and micro-nutrients.
We can consider plants as being factories that make sugar and other carbohydrates using water and carbon dioxide as raw materials, and light as a source of energy. We can summarise this with a simple chemical equation if you’re so inclined:
H2O + Light + CO2 = C6H12O6
It’s chloroplasts that are responsible for creating this reaction in the plant cells. These are green-coloured structures that take on different forms (the colour is due to the presence of the pigment chlorophyll) characteristic to the plant cells. Mitochondria (organelles responsible for cellular respiration both in plants and animals) also have their own DNA and possibly originated as intracellular symbiotic bacteria.
Within these organelles there’s a chamber that contains an internal environment called stroma. The stroma houses different components, notably enzymes responsible for transforming carbon dioxide into organic matter and flat saccules called thylakoids that have a membrane containing photosynthetic pigments (chlorophylls).
Photosynthesis is not only a process, but it also encapsulates diverse and complex biochemical reactions. Generally speaking, photosynthesis can be divided into three different phases: the light phase, the dark phase, and photorespiration. To avoid information overload, this guide will explain the light phase. If you’d like to learn about the other phases, please see our article on the dark phase (including fixation, reduction and regeneration) and photorespiration.
What happens in the light phase?
The light phase is the first phase of photosynthesis. It refers to the conversion of solar energy into chemical energy. Light is absorbed by complexes made up of chlorophylls and proteins called photosystems, which are located in the chloroplasts. It’s called the light phase because it uses light energy, it can only occur in conditions of high light, whether this is natural or artificial. In dark conditions, this phase does not occur.
Photosystem I and photosystem II (PSI and PSII) are in charge of capturing light and using its energy to drive the transport of electrons through a chain of receptors. Or to put it another way, the electrons need to jump from the water molecules until they form ATP, passing through various intermediate chemical forms as in a transport chain.
The PSI and PSII capture the light, increasing the energy of the electrons to levels higher than its original state. This energy is transported through different molecules of chlorophyll, until the water is separated in the centre of photosystem II into the following components: two protons (H+), one atom of oxygen (O), and two electrons.
The oxygen will bond with the remainder of another molecule of water to create atmospheric oxygen (O2). This is what allows terrestrial animals (including humans) to breathe on the earth’s surface – no small feat!
The light energy that absorbs chlorophyll basically responds to two specific wavelengths: 680 and 700 nanometres. These two wavelengths excite one or the other photosystems. Then depending on which one detaches electrons at any given time, the path taken by photosynthesis is slightly distinct, although complementary.
Light energy in the form of photons is transmitted to the external electrons of the molecule(s) of chlorophyll, which escape from it and produce a kind of electric current inside the chloroplast when it joins the transport chain of electrons (see the following illustration).
This energy can be employed in the synthesis of ATP through photophosphorylation and the synthesis of NADPH. Both elements are necessary for the next phase, the Calvin Cycle. During this cycle, the first sugars will be synthesised, where they will serve to produce sucrose and starch. We’ll cover that in our guide to photosynthesis’ dark phase.
Understanding the two forms of photophosphorylation
There are however, two variants of photophosphorylation: cyclic and noncyclic. The one used will depend on the transit that the electrons follow through both the photosystems. The consequences of following one or the other depend principally on the production of NADPH and on the release of oxygen gas.
First of all, let’s look at photophosphorylation in schematic form.
In cyclic photophosphorylation, a molecule called plastocyanin transports the electrons (e–) to photosystem I, which also has a reaction centre and a light-dependent structure.
Once the electron has excited the reaction centre (P700 in the following diagram), the electrons that reach the PSI are again driven by light energy at a higher level of energy. They’re transported through the new chain of acceptors until they reach a final acceptor molecule. This molecule, which captures the electrons, will use them to convert the ADP (the no-energy form of ATP) and an atom of phosphorus into ATP (energy for storage).
In noncyclic photophosphorylation, the process is different. The photons influence the PSII by exciting and releasing two electrons, which are transferred to the primary electron-acceptor, pheophytin. The electrons are replaced by the first electron donor in the Z scheme, with the electrons that come from the photolysis of water inside the thylakoid.
The protons from the photolysis are accumulated inside and the oxygen is released in the form of gas. The electrons pass to a transport chain, which will invest the energy released in the synthesis of ATP.
How? Theory explains it in the following way: electrons are ceded to the plastoquinones which also capture two protons from the stroma. The electrons and protons pass to the cytochrome b6f complex, which pumps the protons to the inside of the thylakoid. In this way a great concentration of protons is obtained in the thylakoid (between these and those that result from the photolysis of the water).
In addition, the electrons from the cytochromes pass to the plastocyanin, which in turn cedes them to the PSI. With light energy, the electrons are again released and captured by the acceptor. From here, they pass through a series of molecules until they reach ferredoxin (Fd in the diagram). This molecule cedes them to the NADP+-reductase, which also captures two protons from the stroma. With the two protons and two electrons, a NADP+ is reduced in the form of NADPH + H+ (see the following diagram).
This set of processes is what is known as the light phase.
Hopefully this guide helps you understand the light phase and how it works. If so, you’re ready to move on! Check out our other guide explaining the dark phase and photorespiration here.