2—8 How do cells obtain energy?
Cells and Energy Cells perform many important activities such as growing, repairing structures,
creating new cell parts, and reproducing. All of these activities require energy. Energy is the ability to
do work or cause change. Energy can be found in many forms. The sun provides energy in the form of
light. The food you eat provides energy in the form of nutrients.
Photosynthesis Plant cells have the ability to use the energy from the sun to make food. This process
is called photosynthesis. Photosynthesis occurs in the chloroplasts. During photosynthesis, plants
take in carbon dioxide and water. The sun’s energy is used to change these molecules into food and
oxygen. The food produced by the chloroplasts is sugar. It can be stored in a plant and used later as a
source of nutrients for the plant. Only organisms that have chloroplasts can perform photosynthesis.
SUGAR
Photosynthesis
Cellular Respiration
Cellular Respiration When a plant needs to use some of its stored sugar, it breaks it down into a
simple sugar called glucose. The glucose is used in a process called cellular respiration. Cellular
respiration takes place in the mitochondria of the cell. During cellular respiration, glucose is broken
down into hydrogen and carbon molecules. These molecules then combine with oxygen to form
carbon dioxide and water. This process releases energy for use by the cell.
Cells that do not contain chloroplasts cannot make their own food. They must take in energy from
their environment. Animal cells are examples of this kind of cell. For example, an elephant gets energy
from the plant materials it eats. You get energy from the foods that you eat. After the food is
digested, nutrient molecules enter the cell. The nutrients are sent to the mitochondria, and they are
used to release energy during cellular respiration.
Fermentation Sometimes respiration must take place when there is no oxygen available. Certain
plants and other organisms called yeast have the ability to carry out respiration without using oxygen.
This process is called fermentation.
During fermentation, cells break down sugar and give off carbon dioxide and alcohol molecules in
the process. Fermentation is used in baking to help make bread rise. When yeast cells are mixed in
with the dough, bubbles of carbon dioxide are produced during fermentation. These carbon dioxide
bubbles actually force the dough to expand. As a result the dough rises.
CHECKING CONCEPTS
1. Plants make food during
2. Food is turned into energy during
3. Animals must get their energy from
4. The process of fermentation does not use