Photosynthesis

Photosynthesis

​Suppose you place a plant on a sunny windowsill and water it regularly. At the same time, you place a similar plant in a dark closet and keep it watered too. The only difference between the two plants is the amount of light they receive. What do you think will happen to each plant after a few weeks? You probably predicted that the plant on the windowsill will remain healthy, while the plant placed in the closet will wilt. Light is a basic need for many plants. Why is this?

Multiple Choice

For photosynthesis to take place in green plants, plants absorb _____from the sun in the chloroplast.

• light

• water

• carbon dioxide

• glucose

​During Photosynthesis, Energy from the Sun Interacts with Matter on Earth

Plants, algae (including phytoplankton), and many microorganisms require light to live. There are a few types of plants that can live without light, but most plants will die if they do not receive adequate light. Light is a form of energy that most plants take in and use to carry out photosynthesis.

​Photosynthesis is a series of chemical reactions that produces glucose, a compound plants use as food. In this way, plants use light from the Sun to make their own food. Radiant energy from the Sun is changed to chemical energy in glucose molecules. The following diagram summarizes this process:




















Plants differ from animals by using radiant energy this way. Animals cannot use radiant energy from the Sun to make glucose. They must obtain energy from the environment in the form of food, which contains chemical energy. Animals have to eat in order to survive.

​Not All Plants Get Their Nutrients from Photosynthesis

A plant called the underground orchid is native to Western Australia. The roots and stem of this plant grow completely underground. Only the flower emerges above the surface. Is it likely that these plants use radiant energy from the Sun? If not, what form of energy do they use? This orchid is a parasite and gets its nutrients and minerals from a fungus that grows on the roots of a nearby broom bush!

​Mistletoe is a semiparasitic plant and needs the help of a host plant to grow on to provide extra nutrients.

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We can observe photosynthesis by using a simple experiment.

Photosynthesis is a chemical process. Because this process takes place at the level of molecules and atoms, we cannot observe it directly. However, we can observe the results of photosynthesis.

Joseph Priestley was one of the first scientists to observe a product of photosynthesis. He conducted an experiment in which he placed a burning candle and a mouse under a glass dome. After a few minutes, the flame went out and the mouse died. Priestley showed that the gases in this dome could not keep the mouse alive.

However, if the experiment was repeated with a plant present under the dome, the mouse remained alive. What change had the plant made to the dome?

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When he put a mint plant in the closed container with a burning candle, the flame used up the oxygen and went out. After about a month, Priestley was able to rekindle the candle.

This showed that the mint plant produced oxygen that allowed the candle to burn. The plant replaced the oxygen that had been used up by the burning candle. The mouse needed the oxygen to live. Priestley showed that the plant could produce oxygen. Oxygen is one of the by-products of photosynthesis.

​The reactants in photosynthesis are carbon dioxide and water.

The products in photosynthesis are glucose and oxygen.

Multiple Select

What are the reactants in photosynthesis?

• water

• oxygen

• glucose

• carbon dioxide

Multiple Select

What are the products in photosynthesis?

• water

• oxygen

• glucose

• carbon dioxide

​Photosynthesis occurs when water and carbon dioxide react to form oxygen and glucose. Below is the chemical equation showing this process:

​Notice that it takes six molecules of water and six molecules of carbon dioxide to make one molecule of glucose. The diagram shows the inputs and outputs of photosynthesis. Carbon dioxide in the air enters a plant through its leaves. Water in the soil enters through a plant’s roots. Oxygen exits from the leaves. Glucose can be used immediately as food or can stay behind to make other structural molecules for growth.

The process of photosynthesis is actually a complex series of many different chemical reactions. The equation shown above represents the overall reaction. This equation shows only the beginning and ending chemical compounds. There are many more steps in between.

​Photosynthesis is necessary for life on Earth.

Humans require oxygen to survive. Many other animals also require oxygen. However, Earth’s atmosphere has not always contained oxygen gas. During its first 2.3 billion years of existence, Earth had an atmosphere that contained water vapor, nitrogen, carbon dioxide, sulfur dioxide, and hydrogen sulfide. What changed that led to oxygen gas becoming part of the atmosphere?

The answer is that microorganisms capable of photosynthesis arose on the planet. These microorganisms are called cyanobacteria. Cyanobacteria live in both freshwater and saltwater environments. Millions and millions of these tiny creatures produced enough oxygen to change the composition of Earth’s atmosphere.

Oxygen became established in the atmosphere as a result of photosynthesis. Since this shift, many organisms have evolved to depend on oxygen. Photosynthesis is necessary to continue supplying oxygen to these organisms.

In addition, all life-forms depend on photosynthesis as the mechanism for capturing and using radiant energy. Plants benefit from this directly because they use photosynthesis to make the glucose molecules that are their food sources. Animals benefit indirectly because they, too, must have sources of food.

Animals either eat plants to obtain chemical energy in the form of glucose or eat other animals that eat plants. Energy moves from the Sun to plants to animals. Photosynthesis is necessary for that energy flow.

Multiple Choice

All organisms need glucose or a source of _____to carry out life functions.

• energy

• water

• glucose

• oxygen

Multiple Choice

Many plants obtain glucose through the process of ______

• energy

• photosynthesis

• carbon dioxide

• oxygen

Multiple Choice

Animals obtain glucose by eating ______

• Chemical energy

• photosynthesis

• plants

• radiant

Multiple Choice

Photosynthesizing organisms need _____energy from the sun to produce glucose.

• Chemical energy

• photosynthesis

• food

• radiant

Multiple Choice

Plants and animals depend on each other for important gases. Plants need the ______that animals exhale, while animals need the _____that plants produce.

• Chemical energy; carbon dioxide

• carbon dioxide; oxygen

• glucose; oxygen

• hydrogen; carbon dioxide