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4.2: Photosynthesis

Difficulty Level: At Grade Created by: CK-12

Lesson Objectives

  • Explain the importance of photosynthesis.
  • Write and interpret the chemical equation for photosynthesis.
  • Describe what happens during the light reactions and the Calvin Cycle.

Check Your Understanding

  • How are plant cells different from animal cells?
  • In what organelle does photosynthesis take place?


Almost all life on Earth depends on photosynthesis. Recall that photosynthesis is the process by which plants use the sun's energy to make their own “food” from carbon dioxide and water. For example, animals, such as caterpillars, eat plants and therefore rely on the plants to obtain energy. If a bird eats a caterpillar, then the bird is obtaining the energy that the caterpillar gained from the plants. So the bird is indirectly getting energy that began with the “food” formed through photosynthesis. Almost all organisms obtain their energy from photosynthetic organisms, either directly, by eating photosynthetic organisms, or indirectly by eating other organisms that ultimately obtained their energy from photosynthetic organisms. Therefore, the process of photosynthesis is central to sustaining life on Earth.

Overview of Photosynthesis

Photosynthesis is the process that converts the energy of the sun, or solar energy, into carbohydrates, a type of chemical energy. During photosynthesis, carbon dioxide and water combine with solar energy, yielding glucose (the carbohydrate) and oxygen. As mentioned previously, plants can photosynthesize, but plants are not the only organisms with this ability. Algae, which are plant-like protists, and cyanobacteria (certain bacteria which are also known as blue-green bacteria, or blue-green algae) can also photosynthesize. Algae and cyanobacteria are important in aquatic environments as sources of food for larger organisms.

Photosynthesis mostly takes place in the leaves of a plant. The green pigment in leaves, chlorophyll, helps to capture solar energy. And special structures within the leaves provide water and carbon dioxide, which are the raw materials for photosynthesis. The veins within a leaf carry water which originates from the roots, and carbon dioxide enters the leaf from the air through special pores called stomata (Figure below).

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

The water and carbon dioxide are transported within the leaf to the chloroplast (Figure below), the organelle in which photosynthesis takes place. The chloroplast has two distinct membrane systems; an outer membrane surrounds the chloroplast and an inner membrane system forms flattened sacs called thylakoids. As a result, there are two separate spaces within the chloroplast. The interior space that surrounds the thylakoids is filled with a fluid called stroma. The inner compartments formed by the thylakoid membranes are called the thylakoid space.

The chloroplast is the photosynthesis factory of the plant.

The overall chemical reaction for photosynthesis is 6 molecules of carbon dioxide (CO2) and 6 molecules of water (H20), with the addition of solar energy, yields 1 molecule of glucose (C6H12O6) and 6 molecules of oxygen (O2). Using chemical symbols the equation is represented as follows:

6CO2 + 6H2O \rightarrow C6H12O6+ 6O2

Oxygen: An Essential Byproduct

Oxygen is a byproduct of the process of photosynthesis and is released to the atmosphere through the stomata. Therefore, plants and other photosynthetic organisms play an important ecological role in converting carbon dioxide into oxygen. As you know, animals need oxygen to carry out the energy-producing reactions of their cells. Without photosynthetic organisms, many other organisms would not have enough oxygen in the atmosphere to survive. Oxygen is also used as a reactant in cellular respiration, which is discussed in the next lesson, so essentially, oxygen cycles through the processes of photosynthesis and cellular respiration.

The Light Reactions and the Calvin Cycle

The overall process of photosynthesis does not happen in one step, however. The chemical equation of photosynthesis shows the results of many chemical reactions. The chemical reactions that make up the process of photosynthesis can be divided into two groups: the light reactions (also known as the light-dependent reactions, because these reactions only occur during daylight hours) and the Calvin Cycle, or the light-independent reactions. During the light reactions, the energy of sunlight is captured, while during the Calvin Cycle, carbon dioxide is converted into glucose, which is a type of sugar. This is summarized in Figure below.

This overview of photosynthesis shows that light is captured during the light reactions, resulting in the production of ATP and the electron carrier NADPH. Through the Calvin Cycle, these materials are used to fix carbon dioxide into sugar. Also during the Calvin Cycle, NADP+ and ADP are regenerated.

Stage 1: Capturing Light Energy

In the first step of the light reactions, solar energy is absorbed by the chlorophyll (and accessory pigments) within the chloroplast’s thylakoid membranes. This absorbed energy excites electrons in the thylakoid membranes. The electrons are then transferred from the thylakoid membranes by a series of electron carrier molecules. The series of electron carrier molecules that transfers electrons is called the electron transport chain. During this process water molecules in the thylakoid are split to replace the electrons that left the pigment, releasing oxygen and adding hydrogen ions (H+) to the thylakoid space. As the thylakoid becomes a reservoir for hydrogen ions, a chemiosmotic gradient forms as there are more hydrogen ions in the thylakoid than in the stroma. As H+ ions flow from the high concentration in the thylakoid to the low concentration in the stroma, they provide energy as they pass through an enzyme called ATP synthase. ATP synthase uses the energy of the movement of H+ ions to make ATP. Meanwhile, highly energized electrons from the electron transport chain combine with the electron carrier NADP+ to become NADPH (Figure below). NADPH will carry this energy in the electrons to the next phase of photosynthesis, the Calvin Cycle.

The light reactions include the movement of electrons down the electric transport chain, splitting water and releasing hydrogen ions into the thylakoid space.

Stage 2: Producing Food

During the Calvin Cycle, which occurs in the stroma of the chloroplast, glucose is formed from carbon dioxide and the products of the light reactions. During the first step CO2 is attached to a 5-carbon molecule (called Ribulose-5-Phosphate, RuBP), forming a 6-carbon molecule. This reaction is catalyzed by an enzyme named RuBisCo, which is the most abundant protein in plants and maybe on Earth! The 6-carbon molecule formed by this reaction immediately splits into two 3-carbon molecules, and the 3-carbon molecule is rearranged to a 3-carbon carbohydrate. The energy and electrons needed for this process are provided by the ATP and NADPH produced earlier in photosynthesis. The "food" made by photosynthesis is formed from the 3-carbon carbohydrate. Two 3-carbon carbohydrates combine to form glucose, a 6-carbon carbohydrate. Next, the 6-carbon RuBP must be reproduced so the Calvin Cycle can start again (Figure below).

The Calvin Cycle begins with carbon fixation, or carbon dioxide attaching to the 5-carbon molecule RuBP, forming a 6-carbon molecule and splitting immediately in to two 3-carbon molecules. This is shown at the top of the figure. This carbon molecule is then reduced to a 3-carbon carbohydrate, shown at the bottom of the figure. The energy and reducing power needed for this process are provided by the ATP and NADPH produced from the light reactions. Next, RuBP must be reproduced so the Calvin Cycle can continue. The carbons are the small black circles. You can keep track of the number of carbons at each stage by counting these circles.

The 3-carbon product of the Calvin Cycle can be converted into many types of organic molecules. Glucose, the energy source of plants and animals, is only one possible product of photosynthesis. Glucose is formed by two turns of the Calvin Cycle. Glucose can be formed into long chains as cellulose, a structural carbohydrate, or starch, a long-term storage carbohydrate. The product of the Calvin Cycle can also be used as the backbone of fatty acids, or amino acids, which make up proteins.

Photosynthesis is crucial to most ecosystems since animals obtain energy by eating other animals, or plants and seeds that contain these organic molecules. In fact, it is the process of photosynthesis that supplies almost all the energy to an ecosystem.

Lesson Summary

  • The net reaction for photosynthesis is that carbon dioxide and water, together with energy from the sun, produce glucose and oxygen.
  • During the light reactions of photosynthesis, solar energy is converted into the chemical energy of ATP and NADPH.
  • During the Calvin Cycle, the chemical energy of ATP and NADPH is used to convert carbon dioxide into glucose.

Review Questions

  1. What is the energy-capturing stage of photosynthesis?
  2. What are the products of the light reactions?
  3. What are the ATP and NADPH from the light reactions used for?
  4. Where does the oxygen released by photosynthesis come from?
  5. What happens to the glucose produced from photosynthesis?
  6. Describe the structures of the chloroplast where photosynthesis takes place.
  7. What is the significance of the electron transport chain?
  8. What are the reactants required for photosynthesis?
  9. What are the products of photosynthesis?

Further Reading / Supplemental Links


ATP synthase
An enzyme that uses the energy of the movement of H+ ions to make ATP.
Calvin Cycle
The reactions of photosynthesis in which carbon dioxide is converted into glucose, which is a type of sugar; also known as the light independent reactions.
Green pigment in leaves; helps to capture solar energy.
The organelle in which photosynthesis takes place.
Photosynthetic bacteria; also known as blue-green bacteria, or blue-green algae.
electron transport chain
A series of electron carrier molecules that transfers electrons.
light reactions
The reactions of photosynthesis that only occur during daylight hours in which the energy of sunlight is captured; also known as the light-dependent reactions.
A high energy electron carrier produced during the light reactions; carries the energy in the electrons to the Calvin Cycle.
The process by which 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.
Special pores in leaves; carbon dioxide enters the leaf and oxygen exits the leaf through these pores.
Fluid in the chloroplast interior space; surrounds the thylakoids.
Flattened sacs within the chloroplast; formed by the inner membranes.

Points to Consider

  • How is glucose turned into an usable form of energy called ATP?
  • How do you gain energy from the food you eat?
  • What would provide more energy- a bowl of pasta or a small piece of candy?
  • What “waste” gas do you exhale?

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