Photosynthesis
How Are Photosynthesis and Cellular Respiration Related?
Teacher Note: Connections
In this concept, students analyze the systems of photosynthesis and of cellular respiration, by defining the boundaries of each, as well as inputs (reactants) and outputs (products). To do so, students use models to simulate the flow of energy, matter, and interactions within cellular systems at different scales. Encourage students to summarize their learning about each system and its components and processes by drawing pictures accompanied by writing a few phrases or a sentence. Access this strategy by clicking the Professional Learning tab on Science Techbook. Click Strategies & Resources, then Spotlight On Strategies (SOS). Journals is found underneath Key Ideas and Details.
As students read and comprehend complex texts, view the videos, and complete the interactives, labs, and other Hands-On Activities, have them summarize and obtain scientific and technical information. Students will use this evidence to support their initial ideas on how to answer the Explain Question or their own question they generated during Engage. Have students record their evidence using My Notebook.
Autotrophs and Producers
Organisms, such as plants, that can photosynthesize are called autotrophs. Autotrophglossary term (opens in a new window) means “self-nourisher.” Autotrophs can make complex food substances from simple compounds. Plants use the simple compounds and energy from the sun to produce food. Not only do autotrophs produce their own food, but they also store these prepared food substances. This provides an available source of chemical potential energy in the form of organic molecules that other organisms can consume. Autotrophs produce the food that provides energy to food chains. For this reason, autotrophs are also called producers. Photosynthesisglossary term (opens in a new window) is the process that initiates, and provides biomass to, most of Earth's food chains.
By utilizing solar energyglossary term (opens in a new window), photosynthesizing organisms convert carbon dioxide and water into oxygen and carbohydrates. Carbohydrates are organic molecules. Carbohydrates consist of carbon, hydrogen, and oxygen. Carbohydrates store and are a source of a lot of energy. The most common carbohydrates are sugars and starches. Oxygen is a by-product of photosynthesis. Photosynthetic organisms release oxygen into the atmosphere. Animals can use this oxygen for respiration.
Plants are the main photosynthetic organisms. They include trees, flowers, grasses, herbs, vines, ferns, mosses, and shrubs. Other photosynthetic organisms include algae, some archaea, and bacteria. These are commonly found in freshwater ecosystems, the upper layers of marine environments, and salty marshes. Scientists believe microorganisms first released oxygen into Earth’s atmosphere billions of years ago. They think that before that time, Earth’s atmosphere contained very little oxygen. Photosynthesis is responsible for today’s oxygen-rich atmosphere.
Teacher Note: Misconception
Students may think that photosynthesis occurs only in green plants. Explain that photosynthesis requires a pigment that is able to absorb light. Chlorophyll is the main pigment in plants that absorbs light, and it causes the green color in most plants. However, there are other pigments found in brown or red plants and algae that absorb other wavelengths of light. Photosynthesis also occurs in some Archaea and bacteria.
Teacher Note: Practices
In this item, students construct and present a written argument about the similarities and differences between cellular respiration and photosynthesis by sorting features of both based on data and evidence about the natural world. These pathways share many common characteristics but evolved independently. Help students think critically about the characteristics of cellular respiration and photosynthesis by using the Myth Bustin’! strategy. Present students with different statements about cellular respiration or photosynthesis and have them cite evidence to refute it. The Myth Bustin’! strategy is found on the Professional Learning tab. Click on Strategies & Resources, then Spotlight On Strategies (SOS). Myth Bustin’! is found underneath Cites Evidence.
The Photosynthesis Reaction
Teacher Note: Misconception
Students may think that plants take in all needed substances through their roots. Although water and some other minerals are absorbed by the roots, plants take in carbon dioxide through leaves.
Like many chemical reactions, photosynthesis begins with a set of reactants (water and carbon dioxide) and uses energy to convert them into products (sugar and oxygen). Photosynthesis is therefore an endothermic or energy-absorbing reaction. The energy for photosynthesis comes from sunlight. The word photosynthesis literally means, “using light to build.” But how do plants capture solar energy for their use? What are the chemical reactants and products of this process? How do the reactants differ from the products in terms of energy?
Plants and other photosynthesizing organisms contain pigments that can absorb energy in the form of light. The main pigment in plant cells is chlorophyllglossary term (opens in a new window). Chlorophyll absorbs most of the visible spectrum except green light, which it reflects, giving plants their green color. When chlorophyll absorbs light, it absorbs light energy. This light energy is then converted to chemical potential energy in the photosynthesis reaction.
The reaction begins with water and carbon dioxide. These are converted to simple sugars and oxygen. The overall chemical reaction for photosynthesis is .
For every molecule of glucose (C6H12O6) produced, six molecules of carbon dioxide and six molecules of water are used. The carbon dioxide and water are the reactants, while the glucose and oxygen are the products. Notice that the amount of matter did not change but rather was rearranged. There are the same number of atoms on each side of the equation, making it balanced. This glucose may then be used to produce complex carbohydrates, such as starches or cellulose.
Teacher Note
Have students record each reactant and product of photosynthesis on index cards, as well as an addition sign and a yield sign. Have them create the photosynthesis reaction on their work surface using these cards. Have them keep these cards so that after they read the next section of text on cellular respiration, they will rearrange the cards to show that reaction.
Comparing Photosynthesis and Cellular Respiration
Photosynthesis is performed by plants and some microorganisms. Photosynthesis uses the energy of the sun to facilitate the reaction of water and carbon dioxide gas producing the sugar (glucose) and oxygen. They use six molecules of water and six molecules of carbon dioxide to make one molecule of glucose. If photosynthetic organisms did not exist, we would not exist. We need oxygen and organic food sources obtained from plant carbohydrates to survive. Cells metabolize the products of photosynthesis to make energy by aerobic cellular respirationglossary term (opens in a new window). Energy and matter derived from photosynthesis are used for the growth and maintenance of organisms.
Cellular respiration is a process used by all cells to produce useful energy from carbohydrates. The form of cellular respiration used in plants and most animals is called aerobic cellular respiration. Aerobic cellular respiration uses oxygen and carbohydrates. Using a series of chemical reactions aerobic cellular respiration converts the chemical energy in carbohydrates into chemical energy of another compound, adenosine triphosphate (ATP). Molecules of ATP are the preferred source of energy for the cellular activity of all organisms. ATP is needed for growth, reproduction, and maintaining homeostasis. The waste products of cellular respiration are carbon dioxide and water. The aerobic respiration of one glucose molecule produces six water molecules and six carbon dioxide molecules.
The following table compares photosynthesis with aerobic cellular respiration:
|
Photosynthesis |
Aerobic Cellular Respiration |
|
Uses energy from the sun. |
Cellular respiration releases energy. |
|
Uses ATP (made in the early steps of photosynthesis) to make carbohydrates. |
Uses carbohydrates to make ATP. |
|
Converts six molecules of carbon dioxide and six molecules of water into one molecule of glucose. |
Converts one molecule of glucose into six molecules of carbon dioxide and six molecules of water. |
|
Removes carbon dioxide from the air. |
Releases carbon dioxide into the air. |
|
Releases oxygen into the air. |
Removes oxygen from the air. |
|
Overall chemical reaction is:
|
Overall chemical reaction is: |
Teacher Note
Have students rearrange their index cards of reactants and products to show the chemical reaction for aerobic cellular respiration. Emphasize to students that the equation for photosynthesis is the reverse of cellular respiration. Ask students to consider why this is important. What would happen if plants or animals produced products that were not consumed by other organisms? Ask them to draw an illustration showing how plants and other organisms recycle the matter used in these two processes?
Photosynthesis and cellular respiration are therefore two parts of a never-ending cycle. The products of cellular respiration—carbon dioxide and water—are the reactants of photosynthesis, and vice versa. These two processes determine the amounts of carbon dioxide and oxygen in the air.
Teacher Note: Misconception
Students may think that animals only respire and plants only photosynthesize. Although most plants do photosynthesize, all plants, like all animals, respire.
