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Lesson Video: The Light-Dependent Reactions Biology

In this video, we will learn how to describe the reactions that occur in the light-dependent stage of photosynthesis and recall the products formed.


Video Transcript

In this video, we will learn about the light-dependent reactions of photosynthesis. We will learn about the electron transport chain and the series of reactions that occur to produce ATP in plant cells.

Plants like all living organisms need food to keep them alive and healthy. But unlike animals, plants cannot move around to hunt or gather their food; instead they carry out photosynthesis. Photosynthesis is the process by which plants convert carbon dioxide and water into glucose and oxygen. We can learn a little bit about photosynthesis just from the word alone. Photo- means light and synthesis means to make. So, photosynthesis refers to a process that makes food using light.

Photosynthesis in plants has two main stages: one stage that is completely dependent on light being available and one that is not. So even though one stage of photosynthesis happens independently of light, the overall reaction will not happen unless there is a light source present. Let’s take a look at the stage of photosynthesis which directly uses light energy, which we refer to as the light-dependent reactions. The light-dependent reactions take place in the chloroplasts of plant cells. Chloroplasts are organelles primarily found within the cells of plants and some algae.

The light-dependent stage of photosynthesis relies on photosystems to absorb light, and these photosystems are found within the membranes of the thylakoids inside the chloroplast. So this is where the light-dependent reactions take place. Let’s have a look at the structures involved in the light-dependent reactions in a bit more detail.

Within the thylakoid membrane, we find two photosystems. These photosystems contain photosynthetic pigments, primarily the pigment chlorophyll. These pigments absorb the light needed for the light-dependent reactions. There are also other structures; these include electron carriers, proton pumps, the enzyme NADP+ reductase, and the enzyme ATP synthase. We’ll learn more about the role these structures play throughout the video. So, let’s go through the reactions that make up the light-dependent stage of photosynthesis.

First, photosystem two takes in light energy. This may seem confusing: why is photosystem two appearing before photosystem one? This is actually just because of the order in which they were discovered, with photosystem one being discovered and named before they found the second one.

The absorption of light energy by photosystem two initiates two events. First, electrons within the photosystem become excited and move to a higher energy level. They are then transferred to the next electron acceptor in the electron transport chain.

Secondly, a molecule of water is split into an oxygen atom and two hydrogen ions. The splitting of water using light energy is called photolysis or photolysis. The photolysis of water also releases electrons. These electrons replace those which have moved from photosystem two to the next electron acceptor and just like before can absorb light energy and become excited.

Now, we can follow the progress of these electrons. These electrons move down the structures that make up the electron transport chain, and energy is released during this process. This energy is used to actively transport hydrogen ions from the stroma of the chloroplast to the inside of the thylakoid, otherwise known as the thylakoid space. The electrons are then moved to photosystem one by an electron carrier. Photosystem one, much like photosystem two, absorbs light energy. Then, the electrons in photosystem one become excited. They are passed to electron carriers and then to the enzyme NADP+ reductase. Here, the coenzyme NADP+ is reduced, but what does this mean?

In chemistry, reduction is the gain of electrons. In this reaction, the coenzyme NADP+ gains two electrons and a hydrogen ion to form reduced NADP, also called NADPH. This is an important reaction, as NADPH is a key coenzyme in the next stage of photosynthesis, the light-independent stage. You might remember that earlier on in the process, hydrogen ions were being actively transported from the stroma and into the thylakoid space. This means that the interior of the thylakoid contains a higher concentration of hydrogen ions than the stroma. The difference in the electrical charge and the number of hydrogen ions across the thylakoid membrane is called an electrochemical gradient.

We would expect the hydrogen ions to move down their electrochemical gradient by diffusion. This means they would move from an area of high concentration to an area of low concentration. However, the hydrogen ions cannot just diffuse through the membrane; instead the hydrogen ions move through the inner channel of the enzyme ATP synthase. This movement of ions down their electrochemical gradient is called chemiosmosis. But why is this important? The movement of hydrogen ions through ATP synthase is coupled to the phosphorylation of ADP. In this process, ADP, or adenosine diphosphate, gains a phosphate group to form ATP or adenosine triphosphate.

ATP is the energy-carrying molecule of all living cells, plants included. The breakdown of ATP and associated bond formations act as a constantly available and rapid supply of energy. Because this phosphorylation of ADP is coupled with the movement of hydrogen ions through ATP synthase, we can say that ATP is synthesized using chemiosmosis. So now, we’ve reached the end of the light-dependent reactions. Let’s quickly recap the major reactants and products.

We know that a molecule of water, H2O, is split using light energy into oxygen, hydrogen ions, and electrons. This oxygen is actually the oxygen that we as humans breathe. Later, NADP+ accepts two electrons and a hydrogen ion to form reduced NADP or NADPH. Finally, ADP gains a phosphate group to form ATP via chemiosmosis. The process that we have been looking at so far can also be referred to as noncyclic photophosphorylation. Let’s break this down to understand what that means.

Photo- refers to light, and phosphorylation refers to the addition of a phosphate group. We know that in a light-dependent reactions, light energy is required and ADP is phosphorylated to form ATP. Noncyclic means that this process is linear in that it only proceeds in one direction. However, there can also be cyclic photophosphorylation. In cyclic photophosphorylation, only photosystem one is used.

The electrons that are excited by the absorption of light are essentially recycled. These electrons do not get passed on to NADP+ reductase to be gained by NADP+. Instead, the electrons that were excited by light energy in photosystem one move through electron carriers and return to photosystem one to repeat the cycle again. Hydrogen ions continue to be actively transported into the thylakoid space and then move through ATP synthase. So, ATP is still generated.

Now that we’ve learned about the light-dependent reactions of photosynthesis, let’s try some practice questions.

What is the creation of ATP from the phosphorylation of ADP using light energy known as? (A) Dephosphorylation, (B) oxidative phosphorylation, (C) photophosphorylation, (D) chemiosmosis.

To help us answer this question, let’s break down the reactions being described. Phosphorylation refers to the addition of a phosphate group. When ADP is phosphorylated, it forms the energy-carrying molecule ATP. In the light-dependent stage of photosynthesis, this phosphorylation of ADP is catalyzed by the enzyme ATP synthase. This process will only happen when light is available. This is actually where we get the photo- part of photosynthesis from. Photo- means light and synthesis means to make. So, we can assume from this that reactions which require light energy will typically have the word part photo- present.

Looking back at our answers, we can see that one option summarizes the process of phosphorylating a molecule using light energy. Therefore, the creation of ATP from the phosphorylation of ADP using light energy is known as photophosphorylation, or answer choice (C).

The diagram provided shows a basic outline of the light-dependent reactions. As electrons move from photosystem two to photosystem one, how are they replaced? (A) By the reduction of NADP+, (B) by the photolysis of water, (C) by the absorption of light energy, (D) by the movement of H+ ions.

The light-dependent stage of photosynthesis uses light energy to initiate a series of reactions, which eventually results in the production of ATP. First, photosystem two absorbs light energy, which excites the electrons contained within the chlorophyll pigment in the photosystem. The electrons then move from one structure of the electron transport chain to another. This means the electrons that were transported from photosystem two need to be replaced.

When light energy is absorbed by photosystem two, it’s used to excite electrons, but it’s also used to split water molecules. Each water molecule is split into oxygen, two hydrogen ions, and two electrons. The splitting of water using light energy is called photolysis. The electrons produced by the photolysis of water replace the electrons that move from photosystem two to photosystem one. Therefore, our correct answer is answer choice (B). The electrons are replaced by the photolysis of water.

Let’s conclude our video with some key points. Photosynthesis is the process by which plants convert carbon dioxide and water into oxygen and glucose using light energy. The light-dependent reactions of photosynthesis occur primarily in the thylakoid membrane. These reactions are initiated by the absorption of light by photosystems. ATP is formed by the phosphorylation of ADP, which is coupled to the movement of hydrogen ions through ATP synthase. The formation of ATP using light energy is known as photophosphorylation.

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