CK-12 Physical Science - For Middle School

9.4. Biochemical Reactions http://www.ck12.org

9.4 Biochemical Reactions

Lesson Objectives

• Describe photosynthesis.


• Outline cellular respiration.


• Explain the role of enzymes in biochemical reactions.

Lesson Vocabulary

• cellular respiration


• enzyme


• photosynthesis

Introduction

All living things need energy. From running a marathon to just taking a breath, energy is required. Where does
the energy come from? The answer is chemical reactions. Reactions that take place within living things are called
biochemical reactions. Two of the most important are photosynthesis and cellular respiration. Together, these two
processes provide energy to almost all of Earth’s organisms.Figure9.25 and the video at the URL below show how
the two processes are related.

http://www.youtube.com/watch?v=NJxobgkPEAo

Photosynthesis

Most of the energy used by living things comes either directly or indirectly from the sun. Sunlight provides the
energy forphotosynthesis. This is the process in which plants and certain other organisms (seeFigure9.26)
synthesize glucose (C 6 H 12 O 6 ). The process uses carbon dioxide and water and also produces oxygen. The overall
chemical equation for photosynthesis is:

6CO 2 +6H 2 O+Light Energy→C 6 H 12 O 6 +6O 2

Photosynthesis changes light energy to chemical energy. The chemical energy is stored in the bonds of glucose
molecules. Glucose is used for energy by the cells of almost all living things. Plants make their own glucose. Other
organisms get glucose by consuming plants (or organisms that consume plants). How do living things get energy
from glucose? The answer is cellular respiration.