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How Does Photosynthesis Work?

written by: pcwriter • edited by: SForsyth • updated: 9/11/2012

You know plants are important because of their ability to harness the sun's energy to make their own food. Yet how do they use this process called photosynthesis? With this study guide, you'll learn the basics behind the photosynthesis formula and why plants are essential to all other life forms.

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    Understanding Photosynthesis

    What is Photosynthesis?

    Plants supply the energy for all higher life forms on Earth. Herbavores such as deer and cows eat the plants. Lions, humans and other carnivores in turn eat the herbavores. In fact any food chain traced back to its original source will always begin with a plant. Without plants and their ability to make their own food through a process called photosynthesis, life would cease to exist. So what exactly is photosynthesis? And why are other life forms dependent on it for survival? You have probably seen this photosynthesis equation in your science textbook:

    carbon dioxide + energy + water = carbohydrate + oxygen

    During the photosynthesis process, plants use energy from the sun, water from the soil and carbon dioxide gas from the air to make glucose. Plants as well as all other organisms are dependent on glucose for energy. A plant's ability to use the sun's energy is due to special leaf structures called chloroplasts which contain a green pigment called chlorophyll. Chlorophyll - a molecule that gives the plant its green color- has an atom of the metal maganese at the center surrounded by a complex ring made of carbon and hydrogen with a long tail - giving it a kite-like appearance.

    Understanding How Photosynthesis Works

    Photosynthesis starts when a photon - a tiny packet of light- is absorbed by a molecule of chlorophyll. The energy of the photon is used to move an electron in the chlorophyll molecule to a higher orbit and that electron moves to another molecule. This donation of an electron is the energy imput that starts the whole chain of reactions going.

    The photosynthesis process is divided up into two steps.Some of these reactions happen in the light and other reactions occur in the dark. The first step is a light-dependent reaction. When the electron from the chlorophyll molecule is removed it sets off this complex series of chain reactions leading to the production of molecules that capture light energy and store it in the form of ATP. Cells tap into this stored energy to provide energy needed for basic life processes. The second step is a light-independent or dark reaction. The ATP stored in these molecules is used to drive another series of complex reactions known as the Calvin Cycle. This helps the plant use carbon dioxide from the air and water drawn up from the roots in the soil to make glucose for food. Although this second step implies light isn't necessary, production of ATP and dark and light reactions would stop if there was no light.

    Why Photosynthesis is Important

    When we eat plants or plant-eating animals, we store that energy from the glucose until it is released by cellular respiration. The formula for cellular respiration looks something like this:

    glucose + oxygen = carbon dioxide + water + energy

    Our cells take in glucose as well as oxygen from the air. Carbon dioxide, water and energy are the products formed during those reactions. Plants not only provide glucose but are also responsible for the oxygen we breathe; they give off oxygen as a waste product during the photosynthesis process. Although plants can grow and maintain themselves by making their own food, they need us too, mainly for the carbon dioxide we produce in the cellular respiration process. Although plants give off carbon dioxide during photosynthesis, they need the carbon we produce for new cell formation and energy.

    Now that you know how photosynthesis works, you should be able to answer these questions:

    Test Your Knowledge

    1. Plants use energy from the __________ to make ______________.

    2. _____________________ are special leaf structures where photosynthesis takes place.

    3. _____________________ is the substance in plants that give them their green color.

    4. Photosynthesis starts when a ____________ is absorbed by a molecule of chlorophyll.

    5. The two types of reactions that happen in photosynthesis are _________ reactions and __________ reactions.

    6. __________ is stored in the chlorophyll molecules and released for the production of glucose.

    7. _______________, _______________, and _____________________ are products formed from cellular respiration.

    8. ___________________ is a waste product of photosynthesis.

    9. Plants depend on us for ______________ ______________, which they use for new cell formation and energy.

    10. What is the basic formula for photosynthesis?


    1. sun, glucose

    2. chloroplasts

    3. chlorophyll

    4. photon

    5. light, dark

    6. ATP

    7. carbon dioxide, water, energy

    8. oxygen

    9. carbon dioxide or carbon

    10. carbon dioxide + energy + water = carbohydrate + oxygen