Stages of Photosynthesis: Videos & Practice Problems

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 produced to synthesize glucose. Eukaryotic photosynthesis primarily involves chloroplasts, while prokaryotic photosynthesis occurs in the plasma membrane and cytoplasm. Understanding these processes is crucial for grasping how energy is transformed in living organisms.

Stages of Photosynthesis

In this video, we're going to briefly introduce the stages of photosynthesis. Photosynthesis occurs in two stages as we have numbered down below. The first stage of photosynthesis is the light reactions, and the second stage is the Calvin cycle. Moving forward in our course, we will discuss more details about both the light reactions and the Calvin cycle. But in this video, we are 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 formed by the light reactions to synthesize glucose. In our image below, 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. Inside the chloroplast, photosynthesis is broken up into two stages. The first stage, the light reactions, occur inside of the thylakoids. We will talk a lot more about the light reactions as we move forward in our course. The second stage, the Calvin cycle reactions, occur in the stroma of the chloroplast.

The light reactions use photons of light from the sun and water. These two reactants come in and what comes out is oxygen gas and chemical energy 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 uses carbon dioxide from the atmosphere along with chemical energy produced by the light reactions. Coming into the chloroplast are solar energy, water, and carbon dioxide. The products that come out of the chloroplast are oxygen gas and glucose, which are the products of photosynthesis as discussed in our previous videos.

We will 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 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. And so, notice that this map is broken up into 2 parts. We've got the left part over here and the right part over here. And so notice that the left part is actually going to represent eukaryotic photosynthesis, and the right part over here is going to represent prokaryotic photosynthesis. And so moving forward in our course, we're mainly and predominantly going to be focusing on photosynthesis as it occurs in eukaryotes that contain chloroplast organelles. And so, notice on the left-hand side that we're showing you a chloroplast organelle, which we know serves as the site of photosynthesis, and that the light reactions and the Calvin cycle reactions together make up photosynthesis. And so moving forward in our course, we'll be able to talk about photosynthesis mainly in the context of how it occurs in eukaryotes inside the chloroplast. And so one thing to note is that on the inside of the chloroplast here we have the stroma, to remind you that this is the chloroplast. And on the outside of the chloroplast, we have the cytoplasm of the cell. So this is still inside of the cell, just outside of the chloroplast organelle. Now on the right-hand side over here, we have prokaryotic photosynthesis, and we will talk about prokaryotic photosynthesis, but in a smaller amount moving forward. But one thing to notice that is a main difference between eukaryotic photosynthesis and prokaryotic photosynthesis is the location of the reactions. And so notice that the light reactions in prokaryotic organisms is going to be found in the plasma membrane of the cell itself, whereas the light reactions in eukaryotic organisms occur in the thylakoids within the chloroplast. Also notice that the Calvin cycle, in prokaryotes, occurs in the cytoplasm, whereas the Calvin cycle in eukaryotes occurs in the stroma of the chloroplast, and so the location is going to be different. And this is once again because prokaryotes do not contain these chloroplast organelles. And so once again, we'll get to talk a lot more about photosynthesis and these reactions as we move forward. We will mainly be focusing on eukaryotic photosynthesis, but we will get to talk a little bit about prokaryotic photosynthesis as well. So I'll see you all in our next video.

Based on the map above, how does photosynthesis differ in eukaryotes and prokaryotes?

• A. The order of the 2 stages is different.
• B. In prokaryotes, the product of the Calvin cycle is lactose.
• C. The location of the 2 stages is different.
• D. There is no difference in photosynthesis between the two cell types.

Answer: (C)

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 utilizes carbon dioxide along with the chemical energy produced in the light reactions to synthesize glucose. These stages are crucial for transforming solar energy into chemical energy that can be used by living organisms.

Where do the light reactions and the Calvin cycle occur in eukaryotic cells?

In eukaryotic cells, the light reactions occur in the thylakoids of the chloroplasts. Thylakoids are membrane-bound compartments where light energy is converted into chemical energy. The Calvin cycle, on the other hand, takes place in the stroma of the chloroplast. The stroma is the fluid-filled space surrounding the thylakoids, where the energy produced in the light reactions is used to fix carbon dioxide into glucose.

What are the main differences between eukaryotic and prokaryotic photosynthesis?

The main differences between eukaryotic and prokaryotic photosynthesis lie in the location of the reactions. In eukaryotes, the light reactions occur in the thylakoids within the chloroplasts, and the Calvin cycle occurs in the stroma of the chloroplasts. In prokaryotes, which lack chloroplasts, the light reactions take place in the plasma membrane, and the Calvin cycle occurs in the cytoplasm. These differences are due to the absence of chloroplast organelles in prokaryotic cells.

What are the reactants and products of the light reactions in photosynthesis?

The reactants of the light reactions in photosynthesis are light (photons) and water (H₂O). These reactions take place in the thylakoids of the chloroplasts. The products of the light reactions are oxygen gas (O₂), ATP, and NADPH. The oxygen gas is released as a byproduct, while ATP and NADPH are used as chemical energy to power the Calvin cycle.

How does the Calvin cycle utilize the products of the light reactions?

The Calvin cycle utilizes the ATP and NADPH produced during the light reactions to fix carbon dioxide (CO₂) from the atmosphere into glucose. This process occurs in the stroma of the chloroplast. ATP provides the energy, while NADPH provides the reducing power needed to convert CO₂ into glucose through a series of enzyme-mediated steps. The Calvin cycle is essential for synthesizing organic molecules that can be used for energy and growth in living organisms.