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Lesson Video: Photosynthesis Biology

In this video, we will learn to describe the process of photosynthesis, and explain the importance of the products made.

15:49

Video Transcript

In this video, we will learn about the process of photosynthesis, explore the chemical equations that describe it, and explain the importance of the products made. Then, we’ll answer some practice questions. And finally, we’ll review what we’ve learned. So, let’s make like chlorophyll in sunlight and get started.

We humans have to eat. In order to obtain the nutrients that our cells need to produce energy, we humans must constantly consume other organisms. Organisms which consume other organisms for nutrition are known as consumers. Another word for consumer is heterotroph. Hetero- is a prefix that means other or different and troph is a word part that means nutrition. Plants, on the other hand, are able to produce the molecules that they need for nutrition within their own cells. Organisms which can produce their own nutrients are called producers. Another term for producer is autotroph. Auto- is a prefix that means self.

Plants are multicellular organisms. That means that, like us and most other multicellular organisms, each plant contains many organ systems. These organ systems contain several organs. The organs are made of multiple tissues. And each tissue is made of several cells, which are the basic unit of life. Plants are able to produce the nutrients that they need to generate energy within their cells by a process called photosynthesis. Photo- is a prefix that means light, and synthesis is a word that means “to make.”

Here, we have a diagram of a simplified plant cell. The cell contains various organelles or specialized subcellular structures. Do you recall which organelle within the plant cell is responsible for carrying out photosynthesis? Absolutely! The chloroplast is responsible for carrying out photosynthesis. So now, we know that the chloroplast is the organelle responsible for carrying out photosynthesis.

Photosynthesis is a process that uses energy from light in order to generate chemical energy in the form of nutrients, specifically glucose. You may recall that the mitochondria are the organelles responsible for cellular respiration, which is a process that uses glucose to generate a molecule called ATP. Chloroplasts are filled with a complex structure of membrane-bound sacks that contain chlorophyll. Chlorophyll is a molecule that helps plants to capture light and also gives them their green color. These specialized structures within the chloroplasts, with the assistance of some important enzymes, facilitate the complex set of metabolic reactions that allow photosynthesis to convert light energy into chemical energy. Let’s take a closer look at how that works.

Since photosynthesis is a chemical reaction, we’re going to represent it with a chemical equation. Recall that in a chemical equation, you find the reactants to the left of the arrow and you find the products to the right. So first, what are the reactants in photosynthesis? Well, photosynthesis uses carbon dioxide from the air, and it uses water, often absorbed from the ground. It also uses energy from light, represented here in parentheses, because while it’s necessary for photosynthesis, the light energy is not technically a reactant. Sometimes, light is positioned over the arrow in the chemical equation or left out altogether.

As far as reactants go, we mentioned a little bit earlier that photosynthesis produces glucose. And you may already be aware that photosynthesis also generates oxygen, which is given off as a byproduct of this reaction. Worth noting here is that photosynthesis is a process that absorbs energy from light and uses that energy to convert the reactants into the products. A chemical reaction that absorbs energy is referred to as endothermic. That light energy that’s absorbed is stored in the bonds of the glucose molecule. And, as we discussed a little bit earlier, that glucose molecule is broken down during cellular respiration to produce a molecule called ATP.

Now that we’ve explored the chemical equation for photosynthesis using words, let’s try to apply the appropriate chemical symbols. The chemical symbol for carbon dioxide is CO2. One molecule of carbon dioxide contains one atom of carbon and two atoms of oxygen. The chemical symbol for water is H2O. One molecule of water possesses two atoms of hydrogen and one atom of oxygen. Light energy does not have a chemical symbol, so it will not be included in our symbol equation. The chemical symbol for glucose is C6H12O6. And you guessed it. That means that there are six atoms of carbon, 12 atoms of hydrogen, and six atoms of oxygen in one molecule of glucose. Finally, the chemical symbol for oxygen is O2 because one molecule of oxygen contains two atoms of oxygen.

So, here we’ve successfully represented our photosynthesis equation using chemical symbols, but the number of atoms of carbon, hydrogen, and oxygen in our products is higher than the number in our reactants. In order for this to be a balanced chemical equation, we need to have the same numbers of the same types of atoms in both our products and our reactants. In order to achieve that, we’re going to need to add some coefficients. Our balanced chemical equation looks like this. It takes six molecules of carbon dioxide and six molecules of water to generate one molecule of glucose and six molecules of oxygen. Try counting up the number of carbon, hydrogen, and oxygen atoms on each side of this equation to make sure that we balanced it properly.

Well, now we’ve talked a little bit about what photosynthesis is, and we’ve discussed the chemical equations that describe it. Before we move on to our practice questions, let’s learn more about where these reactants come from and where the products go.

Understanding photosynthesis is one of the keys to unlocking the concept of energy flow and chemical cycling between different organisms and within our biosphere. We humans, as well as many other animals, breathe in oxygen and breathe out carbon dioxide. We also require glucose from the food that we consume in order to carry out cellular respiration and generate the ATP that powers almost all of our bodily functions. On the other hand, plants do their gas exchange through structures on the underside of their leaves known as stomata. Plants take in carbon dioxide from the air through each stoma. And they absorb water from the ground through their roots. Plants also absorb light energy, which they use to produce the glucose that their own cells use to produce ATP that’s used to power all of their life functions.

This process also produces excess oxygen that’s given off as waste. In this way, we see that plants produce the oxygen that humans and animals rely on. Humans and animals produce the carbon dioxide that plants utilize. And plants produce the glucose that both producers and consumers rely on to carry out cellular respiration. So, we’ve learned that, besides being used to feed consumers, glucose is used in cellular respiration for the production of ATP that plant cells need to carry out their cellular processes. Several glucose molecules can also be linked together to form starch, which is one of the ways that glucose is able to be stored. Glucose is also used for the production of cellulose, which helps to provide rigidity and structure to the plant cell wall. Glucose is also used by plants for other purposes, such as the generation of amino acids and certain lipids.

Now that we’ve learned about the process of photosynthesis, the chemical equations that describe it, and the uses of its products, we’re ready to try some practice questions.

Which of the following is the correct balanced symbol equation for photosynthesis?

This question presents us with four options for chemical equations for photosynthesis that use chemical symbols. We need to choose the one that’s correct in that the products and the reactants are in the correct places and also ensure that the chemical equation is balanced. Meaning that there are the same numbers of each type of atom on the left side of the arrow and on the right.

In order to answer this question, we’re first going to recall the word equation for photosynthesis, and then we’re gonna convert it into the correct and appropriate chemical symbols. Then, we’ll make sure that it’s properly balanced so that we can easily choose the correct answer.

Let’s recall that photosynthesis is a process carried out by plants, in which water absorbed through the roots from the soil and carbon dioxide absorbed from the air through the leaves is converted using the energy found in light into oxygen, which, luckily for us, is released into the air through the leaves, and glucose, which is consumed by some other organisms. And the plant uses for cellular respiration as well as the production of various other molecules.

Now, we have all the information we need to generate our word equation, which reads carbon dioxide and water are converted into glucose and oxygen. Now, we can replace each of these terms with the correct chemical symbol to get the symbol equation. The chemical symbol for carbon dioxide is CO2. The chemical symbol for water is H2O. The chemical symbol for glucose is C6H12O6. And the chemical symbol for oxygen is O2.

Here, I’ve drawn the molecules in this symbol equation to show that right now it’s not balanced. There are many more atoms of carbon, hydrogen, and oxygen in the products than there are in the reactants. In order to make the two sides equal, we need to add some coefficients. Luckily for us, it’s pretty easy to remember all of the coefficients for the smaller molecules are six. Glucose is a relatively large molecule, and there’s only one in this balanced chemical equation.

Now, if you were to count up all of the carbon, hydrogen, and oxygen atoms on each side of the chemical equation, you’d see that the numbers are equal. And so, the chemical equation is balanced. So now, we’re ready to choose our answer. The correct balanced symbol equation for photosynthesis is 6CO2 plus 6H2O is converted into C6H12O6 plus 6O2.

Let’s try one more practice question.

For photosynthesis to occur, energy has to be taken in from the surroundings. What type of chemical reaction is this?

When we’re discussing energy and chemical reactions, there are really two types that we’re concerned with. And those two types are called endothermic and exothermic. And I think it’s helpful to start by illustrating the difference between the two graphically. Our 𝑥-axis will represent reaction progress. And we’ll let our 𝑦-axis be stored chemical energy. So, this is what these graphs would look like.

In an endothermic reaction, the reactants have less stored chemical energy than the products, which means that energy must be absorbed or added to the reaction in order for it to occur. Endo- is a prefix that can mean to take in. And thermic make is a term that means heat or energy. In contrast, in an exothermic reaction, the products possess less stored chemical energy than the reactants. That means that as this reaction occurs, energy is released. Exo- is a prefix that can mean to be released.

One example of an endothermic reaction is photosynthesis. In photosynthesis, energy from light is stored in a high-energy product, glucose. An example of an exothermic reaction is cellular respiration. High-energy glucose releases energy as it’s broken down. And that energy is used to generate ATP molecules. Since we know that photosynthesis absorbs light energy and stores it in its product glucose, we know that it’s an endothermic chemical reaction.

Let’s go ahead and take a moment to review what we’ve learned in this video. In this video, we familiarized ourselves with both the word and the balanced chemical equations for photosynthesis. We also learned that photosynthesis is the process that plants and other autotrophs use to convert light energy into chemical energy in the form of nutrients, specifically glucose. We also learned that photosynthesis is an endothermic chemical reaction because light energy is absorbed and stored in the high-energy products.

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