Introduction
Plants and some other autotrophs can make their food from carbon dioxide and water in the process of photosynthesis. The energy for this process is provided by the light rays coming from the sun. The light energy captured by the electrons is liberated while they travel down the electron transport chain. This energy is used to make ATP required for chemical reactions.
Photosynthesis involves two types of reactions; light-dependent and light-independent reactions. The light-dependent reactions provide energy for the dark reactions. In this article, we will discuss in detail the light-dependent reactions of photosynthesis.
Structure of Chloroplast
It is important to get familiar with the structure of chloroplast to understand the light-dependent reactions of photosynthesis. Chloroplasts are the factories of photosynthesis present in all green plants and algae.
Chloroplasts are double membrane-bound organelles having three main components. These are;
- Envelope
- Stroma
- Thylakoid
Envelope
The envelope of chloroplasts is made up of two plasma membranes just like the mitochondria. However, unlike mitochondria, the inner membrane does not show any infoldings. Both the membranes are smooth with little inter membranous space among them.
Stroma
It is just like a matrix that covers most of the volume of chloroplasts. It is a fluid that contains proteins, ribosomes, enzymes, and small circular DNA. Stroma is the site for dark reactions of photosynthesis.
Thylakoids
Thylakoids are flattened vesicles suspended in the stroma of chloroplasts. These vesicles stack over one another to form piles called grana. These grana are connected with stalk-like structures called intergrana.
The thylakoids in grana are stacked on each other just like coins. Around 50 thylakoids are present in each granum.
Chlorophyll molecules are present on these thylakoids. They are the site for light-dependent reactions.
Chlorophyll
Sunlight during the light-dependent reactions is absorbed by photosynthetic pigments called chlorophyll. Different types of chlorophyll have been identified based on the wavelength of visible light absorbed by them. The most common chlorophylls in plants are chlorophyll a, b, c, and d.
These chlorophylls absorb violet-blue and orange-red wavelengths. Green wavelength is least absorbed by these pigments and gets either reflected or transmitted. This is the reason why chlorophylls are green pigments and why the plants appear green.
A chlorophyll molecule is made up of two main parts;
- Protoporphyrin ring
- Hydrocarbon tail
The protoporphyrin ring makes the flat head of the chlorophyll molecule. One magnesium atom is present at the centre of the ring. Light waves are absorbed by this head portion of the chlorophyll molecule.
The long hydrocarbon tail is phytol, made up of 20 carbon atoms. It is attached to one of the rings of protoporphyrin and serves to anchor the chlorophyll molecules to the core of thylakoid membranes.
Chlorophyll is the most abundant pigment present in plants. It directly takes part in light-dependent reactions. It is found in all photosynthetic organisms except bacteria. Chlorophyll also exits in several forms that differ in terms of their red peak i.e. wavelength for maximum light absorption.
Photosystems
Light-dependent reactions of photosynthesis involve the light energy is converted into chemical energy by the photosynthetic pigments. The photosynthetic pigments are not scattered on the thylakoid membranes. Rather, they are organized into clusters for better absorption and utilization of solar energy. These clusters are called photosystems.
Each photosystem is made up of two components; an antenna complex and a reaction centre.
Antenna Complex
The antenna complex functions to gather the sunlight and channel it to the reaction centre. It has many molecules of chlorophyll a, chlorophyll b and accessory pigments. Light photons strike on these molecules which channel the energy to the reaction centre.
Read more about Light Reactions of Photosynthesis
Reaction Center
The reaction centre consists of one or more chlorophyll molecules along with a primary electron acceptor. Some electron carriers of the nearby electron transport system are also present in the reaction centre.
The electrons that get excited by the photons of light are accepted by the primary electron acceptor and are then transferred to the nearby electron transport chain via associated carriers.
The electron transport system is essential for making ATP during light-dependent reactions.
Types of Photosystems
There are two types of photosystems in photosynthetic organisms. These systems differ with respect to the wavelength of maximum light absorption. These include;
- Photosystem I (PS I): This photosystem has chlorophyll a molecule in its reaction center that absorbs maximum light of 700 nm wavelength. Therefore, it is also called P700.
- Photosystem II (PS II): Its reaction center has chlorophyll a molecule that absorbs maximum light of 680 nm wavelength. Thus, it is called P680.
It is important to remember that one specialized electron acceptor molecule is associated with the reaction centre of each of these photosystems. The function of the electron acceptor is to trap the high energy electrons from the reaction centre and transfer them to the nearby electron transport system.
Reactions
Light-dependent reactions are the energy conservation phase of photosynthesis. The ultimate aim of these reactions is to convert light energy from the sun into chemical energy that can be utilized to make glucose during dark reactions. The chemical energy is obtained in the form of two high energy metabolites; ATP and NADPH.
The photons of light excite the electrons that flow through some specific paths, providing chemical energy. As we have studied the concept of photosystems, let us now discuss the excitation and the flow of electrons.
The electrons excited by the phot9ons can follow one of the two paths;
- Non-cyclic electron flow
- Cyclic electron flow
Both these electron paths are discussed below.
Non-cyclic Electron Flow
This type of electron flow involves both photosystems I (P700) and photosystem II (P680). The ultimate yield of this type of flow is the synthesis of ATP as well as NADPH.
- The following steps are involved in the non-cyclic flow of electrons; Light falls on the chlorophyll molecules present in the photosystem II. This light energy is channeled to the reaction center of photosystem II. Here, the photons of light excite an electron of the chlorophyll a molecule to the higher energy state. This high energy electron is captured by the primary electron acceptors of PS II. As the electron leaves the chlorophyll molecule, a gap or hole is created in it. The chlorophyll molecule becomes a strong oxidizing agent deficient in one electron.
- The electron gap in the chlorophyll molecule is filled by the water-splitting reaction. In this reaction, an enzyme splits a water molecule into two hydrogen ions, two electrons, and one oxygen atom. The electron thus released is transferred to the chlorophyll molecule of PS II. The oxygen atom immediately combines with another atom to form molecular oxygen. This splitting of water molecules during photosynthesis is called photolysis. The oxygen released during this phase is the most important source of replenishing atmospheric oxygen.
- The photoexcited electrons are transferred by the primary electron acceptor to the nearby electron transport chain. These electrons travel down the electron transport system to reach the reaction center of photosystem I (P700). The electron transport system is a series of electron carriers that are arranged in the order of increasing reduction potential. This arrangement allows the steady flow of electrons from one carrier to the next. The carriers include plastoquinone (Pq), a complex of two cytochromes (cytochrome complex), and a copper-containing protein at the end called plastocyanin (Pc).
- As the electrons move down the electron transport system, their energy is released. This energy is used to make ATP through the process of chemiosmosis. This ATP synthesis is termed as photophosphorylation as the energy for phosphorylation is driven from sunlight. As this photophosphorylation occurs during the non-cyclic flow of electrons, it is called non-cyclic photophosphorylation.
- Light photos also fall on the photosystem I and excite its electrons that are captured by the primary electron acceptor, creating a hole in P700. As the electrons reach the bottom of the electron transport system, they fill this hole in the chlorophyll a molecule of P700.
- The high energy electrons in the primary acceptor of photosystem I are transferred to another electron transport chain.The electrons are first transferred to the ferredoxin (Fd), an iron-containing electron carrier protein. An enzyme called NADP reductase then transfers these electrons to NADP. This redox reaction results in the synthesis of NADPH, a high-energy reducing agent used in the dark reactions of photosynthesis.
The non-cyclic electron flow is the predominant type of electron transport during light-dependent reactions. It results in the synthesis of ATP, NADPH, and oxygen. The path of electrons as they pass through the two photosystems appears like the alphabet Z. Therefore, the non-cyclic electron flow is also called the Z-scheme.
Cyclic Electron Flow
The photoexcited can sometimes follow an alternative path that involves only photosystem I. It is called the cyclic electron flow as the electrons move along a circular path and return to the photosystem I at the end of the path.
The cyclic electron flow involves the following steps;
- The light photons fall on the photosystem I and excite the electrons of the chlorophyll a molecule (P700) present in the reaction center. These photoexcited electrons are captured by the primary electron acceptor of photosystem I.
- The primary electron acceptor moves these electrons to an iron-containing protein called ferredoxin (Fd). It is an electron carrier that transfers these electrons to the cytochrome complex. The electrons move from cytochrome complex to the plastocyanin (Pc) and finally return to the chlorophyll molecule of photosystem I.
- The electrons liberate energy as the move down the electron transport chain. This energy is used to phosphorylate ADP to form ATP. This type of phosphorylation is called cyclic photophosphorylation.
This cyclic flow of electrons does no0t generate NADPH or oxygen but only ATP. It is believed that electrons follow this alternative path only when the chloroplast runs low on ATP for dark reactions and have an excess of NADPH. This excess of NADPH causes the electrons to temporarily shift flow from a non-cyclic to a cyclic path.
The cyclic flow of electrons is also regarded as a short-circuit that involves some parts of the non-cyclic path. It continues until the ATP demands of the chloroplast are fulfilled.
Summary
Light-dependent reactions are the energy conservation phase of photosynthesis in which the light energy is converted to chemical energy in the form of ATP and NADPH. These high energy molecules are used for glucose synthesis in dark reactions.
Chloroplasts are the machinery for photosynthesis. They are made up of three components;
- A double membrane envelope
- A matrix called the stroma
- Stacks of thylakoid membranes
Thylakoid membranes are the principal site for light-dependent reactions.
Chlorophyll is the photosynthetic pigment that absorbs light during these reactions. It is made up of two parts;
- A head made up of protoporphyrin ring
- A tail of hydrocarbons called phytol
The chlorophyll molecules are organized on thylakoid membranes in the form of photosystems. Each photosystem is made up of two parts;
- An antenna complex that gathers light
- A reaction center where electrons are excited and transferred to the electron transport system
Two types of photosystem are present;
- Photosystem I with chlorophyll a that absorbs maximum light at 700 nm (P700)
- Photosystem II with chlorophyll a that absorbs maximum light at 680 nm (P700)
During light-dependent reactions, the photoexcited electrons can follow one of the two paths;
- Non-cyclic flow
- Cyclic flow
The non-cyclic flow of electrons involves both photosystems. The electrons move from photosystem II to electron transport chain to photosystem I and then finally reach the NADPH. It results in the synthesis of ATP, NADPH, and oxygen.
On the other hand, cyclic flow involves only photosystem I. The photoexcited electrons move from photosystem I to the electron transport chain and back to photosystem I. It only produces ATP. The cyclic flow of electrons serves to fulfil the ATP demands of chloroplast for dark reactions.
The overall result of light reactions is the synthesis of ATP, NADPH, and oxygen molecules. The high-energy compounds ATP and NADPH are then used in the dark reactions.
Frequently Asked Questions
What is the best definition of light-dependent reactions?
Light-dependent reactions are a series of reactions taking place in the first half of photosynthesis during which light energy is captured. These reactions can take place in the presence of light only. In these reactions, light energy is captured by chlorophyll and is converted into chemical energy in the form of ATP or NADH.
What happens in light-dependent reactions?
In these reactions, sunlight falling on the thylakoid membrane of chloroplasts is absorbed by pigments such as chlorophyll. This light energy is transferred to photosystems which convert it into chemical energy.
Why is photosynthesis a light-dependent reaction?
Photosynthesis is a process by which light is used by plants to prepare their food. The energy obtained from light is used to make carbohydrates from raw materials such as water and carbon dioxide. Because light is needed as an energy source, photosynthesis is a light-dependent reaction.
Where do light-dependent reactions take place?
Light-dependent reactions take place on the thylakoid membrane of the chloroplast. These membranes are rich in chlorophyll, the light-absorbing pigment needed for light-dependent reactions.
References
- Rajni Govindjee. “The Z-Scheme Diagram of Photosynthesis”. Retrieved March 2, 2006.
- “Photosynthesis”. McGraw Hill Encyclopedia of Science and Technology. 2007. p. 472.
- Ingenhousz, J (1779). Experiments Upon Vegetables. London: Elmsly and Payne.
- Carrell CJ, Zhang H, Cramer WA, Smith JL (December 1997). “Biological identity and diversity in photosynthesis and respiration: structure of the lumen-side domain of the chloroplast Rieske protein”. Structure. 5 (12): 1613–25. doi:10.1016/s0969-2126(97)00309-2. PMID 9438861.