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.
