Stages of Photosynthesis - Online Tutor, Practice Problems & Exam Prep

Photosynthesis occurs in two main stages: the light reactions and the Calvin cycle. The light reactions, taking place in the thylakoids of chloroplasts, convert light energy and water into chemical energy and oxygen. The Calvin cycle, occurring in the stroma, utilizes carbon dioxide and the energy from the light reactions to synthesize glucose. Open stomata facilitate gas exchange, allowing carbon dioxide in and oxygen out, essential for these processes. Understanding these stages is crucial for grasping how plants convert solar energy into chemical energy, supporting life on Earth.

Stages of Photosynthesis

In this video, we're going to briefly introduce the stages of photosynthesis. Photosynthesis actually occurs in two stages that we have numbered down below. The first stage of photosynthesis is the light reactions, and the second stage of photosynthesis is the Calvin cycle. Moving forward in our course, we're going to talk a lot more about both the light reactions and the Calvin cycle as well. But here in this video, we're just introducing these two stages.

The light reactions, as their name implies, convert light or photons and water into chemical energy and oxygen gas. The Calvin cycle uses carbon dioxide from the atmosphere as well as the chemical energy that was formed by the light reactions to synthesize glucose, the sugar. Below in our image, we can see the stages of photosynthesis, and we know that the chloroplast acts as the site of photosynthesis. Notice that in the background, here in green, what we're showing you is the chloroplast, the site of photosynthesis. But inside of the chloroplast, photosynthesis is broken up into two stages. The first stage is the light reactions, which occur inside of the thylakoids. We'll talk a lot more about the light reactions as we move forward in our course. The second stage of photosynthesis, the Calvin Cycle reactions, occur in the stroma of the chloroplast.

What you can see here, with the light reactions, the first stage of photosynthesis, uses light and photons of light from the sun, and it also uses water. These two reactants come in and what comes out is oxygen gas and chemical energy that is used to power the Calvin cycle. That's basically what we've described here for the light reactions. Now for the Calvin cycle, notice that it's going to use carbon dioxide from the atmosphere along with chemical energy that was produced by the light reactions, and it's going to use both of those to generate glucose.

Ultimately, what you can see is that coming into the chloroplast are solar energy, water, and carbon dioxide. These are things that all come into the chloroplast. The things that come out of the chloroplast are oxygen gas and glucose, and so those are the products of photosynthesis as we've discussed in our previous videos. We're going to talk a lot more about each of these stages of photosynthesis, the light reactions and the Calvin cycle, as we move forward in our course. But for now, this here concludes our brief introduction to the two stages of photosynthesis, and I'll see you all in our next video.

Map of the Lesson on Photosynthesis

In this video, we're going to introduce our map of the lesson on photosynthesis, which is down below right here. At this point in our course, we know that photosynthesis occurs in the chloroplast. The big green structure that we have in the background represents the chloroplast. Now moving forward in our course, we're going to be talking about photosynthesis as it occurs with open stomata, and recall that the stomata are the openings or the pores that are found in the leaves.

When the stomata are open, this allows for gas exchange; it enables carbon dioxide in the atmosphere to diffuse into the leaf, and it allows oxygen that's produced from the light reactions to diffuse out of the leaf. When the stomata are open, the light reactions and the Calvin cycle reactions will proceed as normal. As we move forward in our course, we'll talk about the situation when the stomata are open, specifically we're going to talk more about the light reaction moving forward, and then after we talk about the light reaction we are going to discuss the Calvin cycle reaction and how the Calvin cycle leads to the production of glucose.

Then once we talk about the light reactions and the Calvin cycle, we'll then switch our attention to what happens when the stomata are closed, preventing gas exchange so that carbon dioxide cannot come into the plant and oxygen cannot leave the plant. Under those conditions, there's a process that's called photorespiration, C₃ photorespiration that we will talk about later in our course. And then we'll talk about how some plants have been able to evolve pathways to avoid photorespiration, including C₄ plants and CAM plants.

Once again, we'll get to talk about this portion of our map later in our course in a different video. But for now, because this is a map of the lesson on photosynthesis, you can use this image like a map to make predictions about what we're going to talk about next. So we're going to follow the leftmost branches first. We'll talk about the light reactions then we'll talk about the Calvin cycle reactions, and then after we've completed the leftmost branches, once again, we'll start to talk about the right branches.

This here concludes our introduction to the map of the lesson on photosynthesis, and we'll be able to talk more about the light reactions in our next lesson video. So, I'll see you all there.

Based on the map above, the step of photosynthesis that generates glucose sugar is…  

a) the Calvin cycle.

b) the light reactions.

c) C3 photorespiration.

d) C4 photorespiration.

• A. the Calvin cycle.
• B. C3 photorespiration.
• C. the light reactions.
• D. C4 photorestoration.

Here’s what students ask on this topic:

What are the two main stages of photosynthesis?

Photosynthesis occurs in two main stages: the light reactions and the Calvin cycle. The light reactions take place in the thylakoids of chloroplasts and convert light energy and water into chemical energy (ATP and NADPH) and oxygen. The Calvin cycle occurs in the stroma of the chloroplast and uses carbon dioxide along with the chemical energy produced in the light reactions to synthesize glucose. These stages are crucial for converting solar energy into chemical energy, which supports life on Earth.

Where do the light reactions and the Calvin cycle occur in the chloroplast?

The light reactions occur in the thylakoids of the chloroplast. Thylakoids are membrane-bound compartments that contain chlorophyll, which captures light energy. The Calvin cycle takes place in the stroma, the fluid-filled space surrounding the thylakoids. The stroma contains enzymes that facilitate the conversion of carbon dioxide and the chemical energy from the light reactions into glucose.

What is the role of stomata in photosynthesis?

Stomata are small openings or pores found on the surface of leaves. They play a crucial role in gas exchange during photosynthesis. When stomata are open, they allow carbon dioxide from the atmosphere to diffuse into the leaf, which is essential for the Calvin cycle. Simultaneously, oxygen produced during the light reactions diffuses out of the leaf through the stomata. This gas exchange is vital for the photosynthetic process to proceed efficiently.

How do the light reactions contribute to the Calvin cycle?

The light reactions contribute to the Calvin cycle by producing the chemical energy needed for the synthesis of glucose. During the light reactions, light energy is captured by chlorophyll and used to split water molecules, producing oxygen, ATP, and NADPH. The ATP and NADPH generated are then utilized in the Calvin cycle to convert carbon dioxide into glucose. Without the energy provided by the light reactions, the Calvin cycle cannot proceed.

What are the main products of photosynthesis?

The main products of photosynthesis are oxygen and glucose. During the light reactions, water is split to produce oxygen, which is released into the atmosphere. The chemical energy generated in the form of ATP and NADPH is used in the Calvin cycle to convert carbon dioxide into glucose, a sugar that serves as an energy source for the plant and other organisms that consume it.