Photosynthesis Basics - Study Guide

How Plants Make Food - Key Concepts

Photosynthesis is the set of chemical reaction by which plants and other autotrophs convert energy from sunlight into chemical food.
Photosynthesis is the set of chemical reaction by which plants and other autotrophs convert energy from sunlight into chemical food. Dorling Kindersley, Getty Images

Learn about photosynthesis step-by-step with this quick study guide. Start with the basics:

Quick Review of the Key Concepts of Photosynthesis

  • In plants, photosynthesis is used to convert light energy from sunlight into chemical energy (glucose). Carbon dioxide, water, and light are used to make glucose and oxygen.

     

  • Photosynthesis is not a single chemical reaction, but rather a set of chemical reactions. The overall reaction is:

     6CO2 + 6H2O + light → C6H12O6 + 6O2
     
  • The reactions of photosynthesis can be categorized as light-dependent reactions and dark reactions.

     

  • Chlorophyll is a key molecule for photosynthesis, though other cartenoid pigments also participate. There are four (4) types of chlorophyll: a, b, c, and d. Although we normally think of plants as having chlorophyll and performing photosynthesis, many microorganisms use this molecule, including some prokaryotic cells. In plants, chlorophyll is found in a special structure, which is called a chloroplast.

     

  • The reactions for photosynthesis take place in different areas of the chloroplast. The chloroplast has three membranes (inner, outer, thylakoid) and is divided into three compartments (stroma, thylakoid space, inter-membrane space). Dark reactions occur in the stroma. Light reactions occur the thylakoid membranes.

     

  • There is more than one form of photosynthesis. In addition, other organisms convert energy into food using non-photosynthetic reactions (e.g. lithotroph and methanogen bacteria)

    Products of Photosynthesis

    Steps of Photosynthesis

    Here is a summary of the steps used by plants and other organisms to use solar energy to make chemical energy:

    1. In plants, photosynthesis usually occurs in the leaves. This is where plants can get the raw materials for photosynthesis all in one convenient location. Carbon dioxide and oxygen enter/exit the leaves through pores called stomata. Water is delivered to the leaves from the roots through a vascular system. The chlorophyll in the chloroplasts inside leaf cells absorbs sunlight.

       

    1. The process of photosynthesis is divided into two main parts: light dependent reactions and light independent or dark reactions. The light dependent reaction happens when solar energy is captured to make a molecule called ATP (adenosine triphosphate). The dark reaction happens when the ATP is used to make glucose (the Calvin Cycle).

       

    2. Chlorophyll and other carotenoids form what are called antenna complexes. Antenna complexes transfer light energy to one of two types of photochemical reaction centers: P700, which is part of Photosystem I, or P680, which is part of Photosystem II. The photochemical reaction centers are located on the thylakoid membrane of the chloroplast. Excited electrons are transferred to electron acceptors, leaving the reaction center in an oxidized state.

       

    3. The light-independent reactions produce carbohydrates by using ATP and NADPH that was formed from the light-dependent reactions.

    Photosynthesis Light Reactions

    Not all wavelengths of light are absorbed during photosynthesis. Green, the color of most plants, is actually the color that is reflected. The light that is absorbed splits water into hydrogen and oxygen:

    H2O + light energy → ½ O2 + 2H+ + 2 electrons

    1. Excited electrons from Photosystem I can use an electron transport chain to reduce oxidized P700. This sets up a proton gradient, which can generate ATP. The end result of this looping electron flow, called cyclic phosphorylation, is the generation of ATP and P700.
    1. Excited electrons from Photosystem I could flow down a different electron transport chain to produce NADPH, which is used to synthesize carbohydratyes. This is a noncyclic pathway in which P700 is reduced by an exicted electron from Photosystem II.
    2. An excited electron from Photosystem II flows down an electron transport chain from excited P680 to the oxidized form of P700, creating a proton gradient between the stroma and thylakoids that generates ATP. The net result of this reaction is called noncyclic photophosphorylation.
    3. Water contributes the electron that is needed to regenerate the reduced P680. The reduction of each molecule of NADP+ to NADPH uses two electrons and requires four photons. Two molecules of ATP are formed.

    Photosynthesis Dark Reactions

    Dark reactions don't require light, but they aren't inhibited by it, either.

    For most plants, the dark reactions take place during daytime. The dark reaction occurs in the stroma of the chloroplast. This reaction is called carbon fixation or the Calvin cycle. In this reaction, carbon dioxide is converted to sugar using ATP and NADPH. Carbon dioxide is combined with a 5-carbon sugar to form a 6-carbon sugar. The 6-carbon sugar is broken into two sugar molecules, glucose and fructose, which can be used to make sucrose. The reaction requires 72 photons of light.

    The efficiency of photosynthesis is limited by environmental factors, including light, water, and carbon dioxide. In hot or dry weather, plants may close their stomata to conserve water. When the stomata are closed, the plants may start photorespiration. Plants called C4 plants maintain high levels of carbon dioxide inside cells that make glucose, to help avoid photorespiration. C4 plants produce carbohydrates more efficiently than normal C3 plants, provided the carbon dioxide is limiting and sufficient light is available to support the reaction. In moderate temperatures, too much of an energy burden is placed on the plants to make the C4 strategy worthwhile (named 3 and 4 because of the number of carbons in the intermediate reaction). C4 plants thrive in hot, dry climates.Study Questions

    Here are some questions you can ask yourself, to help you determine if you really understand the basics of how photosynthesis works.

    1. Define photosynthesis.
    2. What materials are required for photosynthesis? What is produced?
    1. Write the overall reaction for photosynthesis.
    2. Describe what happens during the cyclic phosphorylation of photosystem I. How does the transfer of electrons lead to the synthesis of ATP?
    3. Describe the reactions of carbon fixation or the Calvin cycle. What enzyme catalyzes the reaction? What are the products of the reaction?

    Do you feel ready to test yourself? Take the photosynthesis quiz!