What is the relationship between the reactants and products of photosynthesis and cellular?

Video Transcript

A diagram linking the processes of photosynthesis and respiration in plants is provided, with two key molecules removed. Which of the following correctly replaces 1 and 2? (A) 1 oxygen, 2 carbon dioxide; (B) 1 carbon dioxide, 2 oxygen; (C) 1 hydrogen, 2 glucose; or (D) 1 water, 2 hydrogen.

Our question deals with the processes of photosynthesis and cellular respiration. The prefix photo- means light, and synthesis means to create. This reminds us that photosynthesis is how plants, algae, and some protists create their own food in the presence of light energy.

The process takes place in the plant’s chloroplasts. These special organelles are only found in photosynthetic organisms. Here, the pigment chlorophyll absorbs specific wavelengths of sunlight, allowing carbon dioxide and water to be converted into glucose and oxygen.

Cellular respiration is a process that breaks down sugars to produce energy. It occurs in organelles called mitochondria, which are found in nearly all eukaryotic organisms. Plants use a type of respiration called aerobic respiration, in which glucose must be broken down in the presence of oxygen. The result is the production of energy in the form of ATP and the release of carbon dioxide and water. Take a look at the basic equations for these two critical processes. What do you notice?

Hopefully, you spotted an interesting feature, namely, that two of the products of aerobic respiration are actually the reactants of photosynthesis, and vice versa. Now, let’s take a closer look at the provided diagram to determine the correct answer to our question.

We first need to figure out the correct molecule to replace the number 1. It needs to be a product of photosynthesis and a reactant of respiration. We’ve just learned that oxygen and glucose are produced during photosynthesis and are also required for aerobic cellular respiration. Glucose is already filled in on the diagram, so number 1 can be correctly replaced by oxygen.

To replace the number 2, we need to find a molecule that is a product of respiration and a reactant of photosynthesis. We know that respiration produces carbon dioxide and water, while photosynthesis requires these two molecules. Water is already filled in on the diagram, so the number 2 can be correctly replaced by carbon dioxide.

Therefore, the correct answer is (A). The numbers 1 and 2 in the diagram can be correctly replaced by the key molecules oxygen and carbon dioxide.

1. Photosynthesis is the process plants use to make their own food.

Like all living things, plants need energy to carry out the processes that keep them alive. They get this energy from food. Humans and most other animals are heterotrophs, meaning we have to consume other organisms—plants, other animals, or some combination of the two—for food. However, plants are autotrophs, meaning they create their own food.

Plants use sunlight to convert water and carbon dioxide into glucose and oxygen in a process called photosynthesis. In biology, this information is often expressed using a chemical equation.

Chemical equations typically show the molecules that enter the reaction (the reactants) to the left and the molecules that result from the reaction (the products) to the right, separated by an arrow that indicates a reaction taking place.

[Reactants] → [Products]

You can think of the reactants as the ingredients for preparing a meal and the products as the different dishes in that meal.

With that in mind, let’s take a look at the chemical equation for photosynthesis:

Sunlight + 6 CO2 + 6 H2O → C6H12O6 + 6 O2

CO2 = carbon dioxide
H2O = water
C6H12O6 = glucose
O2 = oxygen

*Sometimes, you’ll see sunlight, or a symbol indicating the sun, over the arrow in the equation.

Therefore, to produce one molecule of glucose (and 6 molecules of oxygen gas), a plant needs 6 molecules of carbon dioxide and 6 molecules of water.

2. The reactants of photosynthesis are carbon dioxide and water.

We’ve established that plants need carbon dioxide (CO2) and water (H2O) to produce their food, but where do these reactants come from and how do they get where they need to go inside the plant?

Plants take in carbon dioxide from the air through small openings in their leaves called stomata. Some plants (most monocots) have stomata on both sides of their leaves, and others (dicots and a few monocots) only have stomata on the underside, or lower epidermis.

Plants get water from the soil surrounding their roots, and water gets to the leaves by traveling through the xylem, part of the plant’s vascular system. In leaves, the xylem and phloem are contained in the vascular bundle.

Once inside the leaf, the carbon dioxide and water molecules move into the cells of the mesophyll, the layer of ground tissue between the upper and lower epidermis. Within these cells, organelles called chloroplasts use the carbon dioxide and water to carry out photosynthesis.

3. Light energy from the sun initiates photosynthesis in the chloroplasts of plant cells.

Plant cells have special organelles called chloroplasts, which serve as the sites for the reactions that make up photosynthesis. Their thylakoid membranes contain a pigment called chlorophyll, which absorbs photons (light energy) from the sun, initiating the light-dependent reactions that take place within the thylakoids.

During these reactions, water molecules (H2O) are broken down. NADPH and ATP—high energy molecules that power the production of glucose—are produced during the light-dependent reactions, as well. Electrons and hydrogen ions from the water are used to build NADPH. Hydrogen ions also power the conversion of ADP to ATP.

4. The products of photosynthesis are glucose and oxygen.

Did you know that oxygen is actually a waste product of photosynthesis? Although the hydrogen atoms from the water molecules are used in the photosynthesis reactions, the oxygen molecules are released as oxygen gas (O2). (This is good news for organisms like humans and plants that use oxygen to carry out cellular respiration!) Oxygen passes out of the leaves through the stomata.

The light-independent reactions of photosynthesis—also known as the Calvin cycle—use enzymes in the stroma, along with the energy-carrying molecules (ATP and NADPH) from the light-dependent reactions, to break down carbon dioxide molecules (CO2) into a form that is used to build glucose.The mitochondria in the plant’s cells use cellular respiration to break glucose down into a usable form of energy (ATP), which fuels all the plant’s activities.

After the light-independent reactions, glucose is often made into larger sugars like sucrose or carbohydrates like starch or cellulose. Sugars leave the leaf through the phloem and can travel to the roots for storage or to other parts of the plant, where they’re used as energy to fuel the plant’s activities.

What is the relationship between the products and reactants of photosynthesis and cellular?

What are the relationships between the reactants and the products of photosynthesis?

Is there a relationship between photosynthesis and cellular respiration?

Which statement describes the relationship between the reactants of photosynthesis and the products of cellular respiration?