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Introduces how plants use sunlight to produce sugars.

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Photosynthesis & Light Reactions

What can a tiny plant do that you can't do? Are plants the only organisms that perform photosynthesis?

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. Some protists, such as algae seen below, and some bacteria can also use the sun's energy to make their own food. This alga has chloroplasts and photosynthesizes just like a plant.

Humans are heterotrophs, organisms that get their energy by consuming plants, animals or both. They don’t make their food; however, the sun is the ultimate source of all energy for all organisms.

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 a producer, which means they are able to make, or produce, their own food. They also produce the "food" for other organisms. Plants are also classified as 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?  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 (C6H12O6), and oxygen. The process can be summarized as: in the presence of sunlight, carbon dioxide + water → glucose + oxygen. The formula: 6CO2 + 6H2O Light & Chlorophyll → C6H12O6 + 6O2

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 (protest) 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 can be heard at http://www.youtube.com/watch?v=C1_uez5WX1o (1:52).


  • ATP (adenosine triphosphate): Usable form of energy inside the cell.
  • autotroph: Organism that produces complex organic compounds from simple inorganic molecules using a source of energy such as sunlight.
  • cellular respiration: Process of breaking down glucose to obtain energy in the form of ATP.
  • chloroplast: Organelle that carries out photosynthesis in plants.
  • glucose: Simple sugar with the chemical formula C6H12O6; a product of photosynthesis.
  • heterotroph: Organism which obtains carbon from outside sources.
  • photosynthesis: 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.
  • producer: Organism that produces food (glucose) for itself and other organisms.


  • 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.


  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?

The Process of Photosynthesis

In the Presence of Sunlight, Carbon Dioxide + Water → Glucose + Oxygen

Photosynthesis takes place in the organelle of the plant cell known as the chloroplasts. Chloroplasts are one of the main differences between plant and animal cells. Animal cells do not have chloroplasts, so they cannot photosynthesize. Photosynthesis occurs in two stages. During the first stage, the energy from sunlight is absorbed by the chloroplast. Water is used, and oxygen is produced during this part of the process. During the second stage, carbon dioxide is used, and glucose is produced.

Chloroplasts contain stacks of thylakoids, which are flattened sacs of membrane. Energy from sunlight is absorbed by the pigment chlorophyll in the thylakoid membrane. There are two separate parts of a chloroplast: the space inside the chloroplast itself, and the space inside the thylakoids (Figure below).

  • The inner compartments inside the thylakoids are called the thylakoid space (or lumen). This is the site of the first part of photosynthesis.
  • The interior space that surrounds the thylakoids is filled with a fluid called stroma. This is where carbon dioxide is used to produce glucose, the second part of photosynthesis.



The chloroplast is the photosynthesis factory of the plant.

The Reactants

What goes into the plant cell to start photosynthesis? The reactants of photosynthesis are carbon dioxide and water. These are the molecules necessary to begin the process. But one more item is necessary, and that is sunlight. All three components, carbon dioxide, water, and the sun's energy are necessary for photosynthesis to occur. These three components must meet in the chloroplast of the leaf cell for photosynthesis to occur. How do these three components get to the cells in the leaf?

  • Chlorophyll is the green pigment in leaves that captures energy from the sun. Chlorophyll molecules are located in the thylakoid membranes.
  • The veins in a plant carry water from the roots to the leaves.
  • Carbon dioxide enters the leaf from the air through special openings called stomata (Figure below).


Stomata are special pores that allow gasses to enter and exit the leaf.

The Products

What is produced by the plant cell during photosynthesis? The products of photosynthesis are glucose and oxygen. This means they are produced at the end of photosynthesis. Glucose, the food of plants, can be used to store energy in the form of large carbohydrate molecules. Glucose is a simple sugar molecule which can be combined with other glucose molecules to form large carbohydrates, such as starch. Oxygen is a waste product of photosynthesis. It is released into the atmosphere through the stomata. As you know, animals need oxygen to live. Without photosynthetic organisms like plants, there would not be enough oxygen in the atmosphere for animals to survive.

The Chemical Reaction

The overall chemical reaction for photosynthesis is 6 molecules of carbon dioxide (CO2) and 6 molecules of water (H2O), with the addition of solar energy. This produces 1 molecule of glucose (C6H12O6) and 6 molecules of oxygen (O2). Using chemical symbols, the equation is represented as follows: 6CO2 + 6H2O → C6H12O6+ 6O2. Though this equation may not seem that complicated, photosynthesis is a series of chemical reactions divided into two stages, the light reactions and the Calvin cycle.

The Light Reactions

Photosynthesis begins with the light reactions. It is during these reactions that the energy from sunlight is absorbed by the pigment chlorophyll in the thylakoid membranes of the chloroplast. The energy is then temporarily transferred to two molecules, ATP and NADPH, which are used in the second stage of photosynthesis. ATP and NADPH are generated by two electron transport chains. During the light reactions, water is used and oxygen is produced. These reactions can only occur during daylight.


  • chlorophyll: Pigment that absorbs sunlight and gives plants their green color.
  • chloroplast: Organelle that carries out photosynthesis in plants.
  • glucose: Simple sugar with the chemical formula C6H12O6; a product of photosynthesis.
  • light reactions: First stage of photosynthesis in which light energy from the sun is captured and changed into chemical energy that is stored in ATP and NADPH.
  • products: End results of a chemical reaction.
  • reactants: Molecules that come together to start a chemical reaction.
  • stomata: Special pores in leaves; carbon dioxide enters the leaf and oxygen exits the leaf through these pores.
  • stroma: Fluid that fills the interior space that surrounds the thylakoids in the chloroplast.
  • thylakoid: Stack of flattened sacs of membrane in the chloroplast.


  • Photosynthesis occurs in the chloroplast of the plant cell.
  • Carbon dioxide, water, and the sun's energy are necessary for the chemical reactions of photosynthesis.
  • The products of photosynthesis are glucose and oxygen.


Use the resources below to answer the following questions.

Photosynthesis at http://www.youtube.com/watch?v=RNufj-64OO0 (7:08)

  • How do autotrophs differ from heterotrophs? How are they the same?
  • What do plants do with most of the sugar they produce during photosynthesis?
  • How do decreasing levels of CO2 affect plants? How do you think increasing levels of CO2 affect plants?

Photosynthesis at http://www.youtube.com/watch?v=mpPwmvtDjWw (2:41)

  • Where do plants get the raw materials for photosynthesis?
  • What do plants take up through their roots? Which of these substances are used for photosynthesis?
  • Where does the chemical reactions of photosynthesis take place?


  1. Describe the structures of the chloroplast where photosynthesis takes place.
  2. What would happen if the stomata of a plant leaf were glued shut? Would that plant be able to perform photosynthesis? Why or why not?
  3. What are the reactants needed to perform photosynthesis? The products?

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