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2.11: Photosynthesis

Difficulty Level: At Grade Created by: CK-12
Practice Photosynthesis
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What can a tiny plant do that you can't do?

This tiny plant can use the energy of the sun to make its own food. You can't make food by just sitting in the sun. Plants are not the only organisms that can get energy from the sun, however. Some protists, such as algae, and some bacteria can also use the energy of the sun to make their own food.

What is Photosynthesis?

If a plant gets hungry, it cannot walk to a local restaurant and buy a slice of pizza. So, how does a plant get the food it needs to survive? Plants are producers , which means they are able to make, or produce, their own food. They also produce the "food" for other organisms. Plants are also autotrophs. Autotrophs are the organisms that collect the energy from the sun and turn it into organic compounds. So once again, how does a plant get the food it needs to survive?

Through photosynthesis. Photosynthesis is the process plants use to make their own “food” from the sun's energy, carbon dioxide, and water. During photosynthesis, carbon dioxide and water combine with solar energy to create glucose , a carbohydrate (C 6 H 12 O 6 ), and oxygen.

The process can be summarized as: in the presence of sunlight, carbon dioxide + water → glucose + oxygen.

Glucose is a sugar that acts as the "food" source for plants. The glucose is then converted into usable chemical energy, ATP , during cellular respiration . The oxygen formed during photosynthesis, which is necessary for animal life, is essentially a waste product of the photosynthesis process.

Actually, almost all organisms obtain their energy from photosynthetic organisms. For example, if a bird eats a caterpillar, then the bird gets the energy that the caterpillar gets from the plants it eats. So the bird indirectly gets energy that began with the glucose formed through photosynthesis. Therefore, the process of photosynthesis is central to sustaining life on Earth. In eukaryotic organisms, photosynthesis occurs in chloroplasts . Only cells with chloroplasts—plant cells and algal (protist) cells—can perform photosynthesis. Animal cells and fungal cells do not have chloroplasts and, therefore, cannot photosynthesize. That is why these organisms, as well as the non-photosynthetic protists, rely on other organisms to obtain their energy. These organisms are heterotrophs.

The Photosynthesis Song explaining photosynthesis, can be heard at http://www.youtube.com/watch?v=C1_uez5WX1o (1:52).

Time for Me to Leaf: Tree Chlorophyll Chromatography

Why do leaves change color each fall? This MIT video demonstrates an experiment about the different pigments in leaves. See the video at https://www.youtube.com/watch?v=_v6_5Zxdb68 .


  • All the energy used by living things on Earth came from the process of photosynthesis.
  • During photosynthesis, carbon dioxide and water combine with solar energy to create glucose and oxygen.

Explore More

Use the resource below to answer the following questions.

  1. Where does the energy for photosynthesis come from?
  2. In photosynthesis, how does the movement of electrons along the electron transport chain affect hydrogen ions (H + )? How does this compare to what happens in the mitochondria during cellular respiration?
  3. Do all organisms which carry out photosynthesis have chloroplasts? Explain your answer as fully as you can.
  4. What is the function of mobile electron carriers? What is their relationship to the embedded protein complexes in the membrane?


  1. How is the process of photosynthesis central to sustaining life on Earth?
  2. What are the two products produced by photosynthesis?
  3. What two raw materials are needed by plants in order to perform photosynthesis?




Adenosine triphosphate; usable form of energy inside the cell.


Organism that produces complex organic compounds from simple inorganic molecules using a source of energy such as sunlight.
cellular respiration

cellular respiration

Process of breaking down glucose to obtain energy in the form of ATP.


Organelle that carries out photosynthesis in plants.


Simple sugar with the chemical formula C6H12O6; a product of photosynthesis.


Organism which obtains carbon from outside sources.


Process by which specific organisms (including all plants) use the sun's energy to make their own food from carbon dioxide and water; process that converts the energy of the sun, or solar energy, into carbohydrates, a type of chemical energy.


Organism that produces food (glucose) for itself and other organisms.

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Difficulty Level:

At Grade


7 , 8

Date Created:

Nov 29, 2012

Last Modified:

Feb 17, 2015
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