Introduction to Photosynthesis: Study with Video Lessons, Practice Problems & Examples

Photosynthesis is a redox reaction where carbon dioxide (CO₂) is reduced to glucose (C₆H₁₂O₆) and water (H₂O) is oxidized to oxygen (O₂). This process occurs in chloroplasts, utilizing sunlight, CO₂, and H₂O to produce glucose and O₂. The chemical equations for photosynthesis and cellular respiration are nearly opposites, highlighting their interdependence. Photosynthesis produces the reactants for cellular respiration, while cellular respiration generates the reactants for photosynthesis, emphasizing the cyclical nature of energy flow in ecosystems.

Introduction to Photosynthesis

In this video, we're going to begin our introduction to photosynthesis. Photosynthesis is the process that uses energy from sunlight to synthesize or build sugars such as glucose. You can see that within the word "photosynthesis" itself because the root "photo" refers to light, such as sunlight. And "synthesis" refers to the ability to synthesize or build something. In this case, it's going to be sugars such as glucose. Now, as we'll see down below in our image, the process of photosynthesis uses carbon dioxide gas (CO₂) from the atmosphere, water (H₂O), and light energy or sunlight. It uses those three things to make glucose, whose chemical formula is C₆H₁₂O₆, and oxygen gas, whose chemical formula is O₂. Recall from our previous lesson videos when we introduced different organelles that the chloroplasts are green organelles that function as the site of photosynthesis. Photosynthesis is associated with chloroplasts, whereas cellular respiration is associated with mitochondria. Also recall from our previous lesson videos that photosynthetic organisms, or organisms that are capable of performing photosynthesis, are also called autotrophs. This is because autotrophs are able to make their own food without having to consume other living organisms. Autotrophs include photosynthetic organisms. Let's take a look at our example image down below to look at the overall chemical equation for photosynthesis. In this image, you'll notice, over here on the left, we have the organelle that is the chloroplast. When we zoom into the chloroplast, we can see that it really functions as the site of photosynthesis. As mentioned above in our lesson, photosynthesis takes carbon dioxide gas (CO₂) from the atmosphere, water (H₂O), and sunlight or solar energy from the sun. It's able to convert these three relatively simple reactants into sugars such as glucose. Here we have glucose, and glucose's chemical formula is C₆H₁₂O₆. Oxygen gas is also created as a byproduct. This oxygen gas would either be released into the environment by the plant or it would be used by the plant itself to perform cellular respiration. This concludes our brief introduction to photosynthesis, and we'll continue to talk more and more about photosynthesis as we move forward in our course. I'll see you all in our next video.

The main product of photosynthesis is:

a) Glucose.

b) Oxygen.

c) Carbon dioxide.

d) Water.

e) Sunlight.

• A. Glucose.
• B. Oxygen.
• C. Carbon dioxide.
• D. Water.
• E. Sunlight.

Photosynthesis is a Redox Reaction

In this video, we're going to talk about how photosynthesis is really just a redox reaction, which recall from our previous lesson videos just means that it involves the transfer of electrons between molecules. And so once again, the overall chemical equation for photosynthesis is a redox reaction. And recall from our previous lesson videos, redox reactions, we can remember by remembering Leo the lion goes ger, and that's because Leo reminds us of substances that lose electrons are oxidized, whereas substances that gain electrons are reduced. And so if you remember, "Leo the lion goes ger," you'll be good on the redox reactions. And so really photosynthesis is a redox reaction, and what we'll see is that by the end of the process of photosynthesis, carbon dioxide or CO₂ is going to get reduced. And so that means that it is going to gain electrons. Whereas water, on the other hand, we'll see is going to get oxidized, which means that water is going to lose electrons.

Let's take a look at our example image down below at the chemical equation for photosynthesis to get a better understanding of this. And so what you'll notice is that over here, we have carbon dioxide gas, which is one of the reactants of photosynthesis, and it turns out that there's 6 carbon dioxide gas molecules that are going to react, and there are also going to be 6 water molecules that are going to react. And so the carbon dioxide, water, and sunlight or solar energy are the reactants, and the products are going to be a sugar such as glucose whose chemical formula is C₆H₁₂O₆, and this is the chemical structure of glucose right here. And then also 6 oxygen gas molecules are going to be produced as well. And so this here represents the entire process of photosynthesis, and what you'll notice is that the carbon dioxide here is actually getting reduced, meaning that it is going to gain electrons as it's used to form glucose. And the oxygen I'm sorry, the water molecule here is going to get oxidized as it's converted into oxygen gas. And so that means the water molecule is going to be losing electrons. It's going to be, essentially providing or supplying the electrons, that are needed to reduce carbon dioxide and form glucose.

We'll be able to talk more and more about photosynthesis and exactly where and how this process works, but, what you'll notice is that this image here looks very similar to an image that we showed you when we talked about cellular respiration. And so in our next video, what we are going to do is compare and contrast the chemical equation for photosynthesis with the chemical equation for cellular respiration. So I'll see you all in that video.

Which of the following reactants is reduced during the process of photosynthesis?

a) Oxygen gas.

b) Photons of light.

c) Carbon Dioxide.

d) Water.

• A. Oxygen gas.
• B. Photons of light.
• C. Carbon dioxide.
• D. Water.

Photosynthesis vs. Cellular Respiration

In this video, we're going to compare and contrast photosynthesis versus cellular respiration. And so what you may have noticed is that photosynthesis and cellular respiration seem kind of similar when you compare their chemical equations. And so it turns out that photosynthesis and cellular respiration are both very ancient pathways that have been around for 1,000,000,000 years. And they are actually really highly connected to one another, and this is because each process produces the reactants that are necessary for the other process.

For example, photosynthesis produces products that are used as the reactants for cellular respiration, and cellular respiration produces products that are used as the reactants for photosynthesis. We'll be able to see this down below in our image. And also it turns out that photosynthesis and cellular respiration are almost exactly the opposite of each other in terms of their overall chemical equations. We'll be able to see that down below in our image as well.

In our example, we're going to be looking at the connection between cellular respiration and photosynthesis. Taking a look at the top half of the image up here, notice that it's occurring inside of the chloroplast organelle. This is the chemical equation for photosynthesis that we recently talked about in our last lesson video. Photosynthesis is going to take carbon dioxide, water, and sunlight energy from the atmosphere and convert those things into sugars such as glucose and oxygen gas as a byproduct.

But what you'll notice is that the products here of photosynthesis, which are glucose and oxygen, are actually used by mitochondria. You'll notice that glucose and oxygen here are being used by the mitochondria, and one thing to note about this image of the mitochondria is that it's actually flipped around so that the reactants are here on the right and the arrow is going from right to left. These over here are the products of cellular respiration, which we already covered in our previous lesson videos.

What you'll notice is that the reactants here of the mitochondria of cellular respiration are the products of photosynthesis. Cellular respiration converts oxygen and glucose into the products of carbon dioxide, water, and a lot of ATP energy. Carbon dioxide and water are products of cellular respiration, but they're actually used as the reactants for photosynthesis. You can see how each process is going to produce the reactants that are needed for the other process.

Now, you can also see that these two equations, the equation for photosynthesis and the equation for cellular respiration, are almost exactly opposites of each other. The reason for that is because carbon dioxide, water, and energy are all reactants for photosynthesis, whereas carbon dioxide, water, and energy are all products for cellular respiration. Remember, they're products because the arrow's going from right to left here.

In photosynthesis, glucose and oxygen are products for photosynthesis, But glucose and oxygen are reactants for cellular respiration. So really, when you compare these two equations side by side, they do line up similarly, and the only difference where they don't line up so similarly is the type of energy that's used or produced. Cellular respiration is going to be producing ATP, whereas photosynthesis is using solar energy or sunlight energy.

Another thing that you'll notice is that the substances in terms of being oxidized or reduced are flipped as well. In photosynthesis, carbon dioxide is being reduced to glucose. But down below in cellular respiration, glucose is being oxidized to carbon dioxide. And in photosynthesis, water is being oxidized to oxygen, whereas down below in cellular respiration, oxygen is being reduced to water, and so they are reversed in that respect.

One thing that helps me remember which substances are oxidized and reduced is that I know that photosynthesis is going to be synthesizing something, and that is going to be a glucose molecule. Glucose is going to be synthesized using carbon dioxide. It's literally able to take carbon dioxide gas from the atmosphere and use that carbon dioxide gas to build a glucose. Glucose has a lot more chemical bonds in it than carbon dioxide. These chemical bonds are going to be made using electrons. Carbon dioxide is going to need to gain a lot of electrons or it's going to need to be reduced in order to be used to create glucose.

This helps me remember that carbon dioxide has to be reduced. It has to gain a lot of electrons for it to make all of the chemical bonds that are found in glucose. And ultimately water is going to be oxidized. That means that water is going to be losing electrons and being used to produce oxygen. Now in cellular respiration, I know that glucose has a lot of chemical bonds in it. And glucose, because it has a lot of chemical bonds in it, it needs to be broken down. It needs to be stripped of electrons. It needs to lose electrons. And when it loses electrons, it's going to be converted into carbon dioxide.

Ultimately, the oxygen here we know is going to act as the final electron acceptor in cellular respiration. And so the oxygen, because it's acting as the final electron acceptor, it's going to be gaining the electrons or it's going to be reduced and be converted into water. So those are some ways to help you remember that photosynthesis and cellular respiration are very similar to each other, almost exact opposites. And once again, the only difference is that there's sunlight energy here, but down below, there's ATP energy here. And other than that, they are pretty much exactly the opposite of each other.

This here concludes our comparison of photosynthesis and cellular respiration and we'll be able to get some practice as we move forward in our course. And we'll also continue to talk more and more about photosynthesis. So I'll see you all in our next video.

Energy used to power photosynthesis & ultimately cellular respiration originates from which energy source?

a) Glucose.

b) ATP.

c) Carbon dioxide.

d) The sun.

• A. Glucose.
• B. ATP.
• C. Carbon dioxide.
• D. The sun.

Here’s what students ask on this topic:

What is the overall chemical equation for photosynthesis?

The overall chemical equation for photosynthesis can be represented as:

$6{\mathrm{CO}}_2+6H_{2}O+\mathrm{light}\to C_6{H}_{12}{O}_6+6O_2$

This equation shows that six molecules of carbon dioxide (CO₂) and six molecules of water (H₂O) react in the presence of light energy to produce one molecule of glucose (C₆H₁₂O₆) and six molecules of oxygen (O₂).

How are photosynthesis and cellular respiration interconnected?

Photosynthesis and cellular respiration are interconnected processes that form a cycle of energy transformation and matter recycling. Photosynthesis converts carbon dioxide (CO₂), water (H₂O), and sunlight into glucose (C₆H₁₂O₆) and oxygen (O₂). The chemical equation for photosynthesis is:

$6{\mathrm{CO}}_2+6H_{2}O+\mathrm{light}\to C_6{H}_{12}{O}_6+6O_2$

Cellular respiration, on the other hand, uses glucose and oxygen to produce carbon dioxide, water, and ATP energy. The products of photosynthesis (glucose and oxygen) are the reactants for cellular respiration, and vice versa, making these processes complementary.

What role do chloroplasts play in photosynthesis?

Chloroplasts are the organelles within plant cells where photosynthesis takes place. They contain chlorophyll, the pigment that captures light energy from the sun. This light energy is then used to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂). The overall chemical reaction occurs in the thylakoid membranes and the stroma of the chloroplasts, making them essential for the photosynthetic process.

Why is photosynthesis considered a redox reaction?

Photosynthesis is considered a redox reaction because it involves the transfer of electrons between molecules. In this process, carbon dioxide (CO₂) is reduced to form glucose (C₆H₁₂O₆), meaning it gains electrons. Conversely, water (H₂O) is oxidized to produce oxygen (O₂), meaning it loses electrons. The overall redox reaction can be summarized as:

$6{\mathrm{CO}}_2+6H_{2}O+\mathrm{light}\to C_6{H}_{12}{O}_6+6O_2$

What are autotrophs and how are they related to photosynthesis?

Autotrophs are organisms that can produce their own food from inorganic substances, using light or chemical energy. Photosynthetic autotrophs, such as plants, algae, and some bacteria, use sunlight to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂) through the process of photosynthesis. This ability to synthesize their own food allows autotrophs to serve as the primary producers in ecosystems, forming the base of the food chain.