Chapter 3, Section 3: (Photosynthesis)

  • Learning Goal:
    The student will be able to…
    1. …describe how the sun supplies living things with the energy they need.
    2. …describe how the process of photosynthesis works.

    First, write down the vocab words. Then, click below to practice learning the parts of a cell.



    the way a cell captures energy from sunlight and uses it to make food

    the main photosynthetic pigment in plants, captures light and uses it to make energy

    ingredients in photosynthesis
    The sunlight bakes the water and carbon dioxide together and changes it into sugar

    water enters the plant through the roots

    carbon dioxide
    enters through the stomata (holes) on the bottom of the leaves

    the organelle that mixes ingredients so that photosynthesis can happen

    photosynthesis produces sugar (glucose) and oxygen

    a gas that exits through the bottom of the leaf through the stomata and into the air

    a sugar that is food for the plant so it can grow 

    Sources of Energy

    The process by which a cell captures energy in sunlight and uses it to make food is called photosynthesis (foh toh sin thuh sis). The term photosynthesis comes from the Greek words photo, which means “light,” and synthesis, which means “putting together.”


    Nearly all living things obtain energy either directly or indirectly from the energy of sunlight captured during photosynthesis. Grass obtains energy directly from sunlight, because it makes its own food during photosynthesis. When the zebra eats the grass, it gets energy that has been stored in the grass. Similarly, the lion obtains energy stored in the zebra. The zebra and lion both obtain the sun’s energy indirectly, from the energy that the grass obtained through photosynthesis.

    Plants manufacture their own food through the process of photosynthesis. An organism that makes its own food is called an autotroph (awt oh trahf). An organism that cannot make its own food, including animals such as the zebra and the lion, is called a heterotroph (het ur oh trahf). Many heterotrophs obtain food by eating other organisms. Some heterotrophs, such as fungi, absorb their food from other organisms.

    The Two Stages of Photosynthesis

    Photosynthesis is a complex process. During photosynthesis, plants and some other organisms use energy from the sun to convert carbon dioxide and water into oxygen and sugars. The process of photosynthesis is shown in Figure 14. You can think of photosynthesis as taking place in two stages: capturing the sun’s energy and producing sugars. You’re probably familiar with many two-stage processes. To make a cake, for example, the first stage is to combine the ingredients to make the batter. The second stage is to bake the batter. To get the desired result—the cake—both stages must occur in the correct order.

    Stage 1: Capturing the Sun’s Energy

    The first stage of photosynthesis involves capturing the energy in sunlight. In plants, this energy-capturing process occurs mostly in the leaves. Recall that chloroplasts are green organelles inside plant cells. The green color comes from pigments, colored chemical compounds that absorb light. The main photosynthetic pigment in chloroplasts is chlorophyll.

    Chlorophyll functions in a manner similar to that of the solar “cells” in a solar-powered calculator. Solar cells capture the energy in light and use it to power the calculator. Similarly, chlorophyll captures light energy and uses it to power the second stage of photosynthesis.

    Stage 2: Using Energy to Make Food

    In the next stage of photosynthesis, the cell uses the captured energy to produce sugars. The cell needs two raw materials for this stage: water (H2O) and carbon dioxide (CO2). In plants, the roots absorb water from the soil. The water then moves up through the plant’s stem to the leaves. Carbon dioxide is one of the gases in the air. Carbon dioxide enters the plant through small openings on the undersides of the leaves called stomata(stoh muh tuh) (singular stoma). Once in the leaves, the water and carbon dioxide move into the chloroplasts.

    Inside the chloroplasts, the water and carbon dioxide undergo a complex series of chemical reactions. The reactions are powered by the energy captured in the first stage. These reactions produce chemicals as products. One product is a sugar that has six carbon atoms. Six-carbon sugars have the chemical formula C6H12O6. Recall that sugars are a type of carbohydrate. Cells can use the energy in the sugar to carry out important cell functions.

    The other product of photosynthesis is oxygen (O2), which exits the leaf through the stomata. In fact, almost all the oxygen in Earth’s atmosphere was produced by living things through the process of photosynthesis.

    Click below to watch a an animation about photosynthesis.


     Click below to play a game about photosynthesis.

    Play the Game