Check Your Understanding
- Where does the energy captured at the beginning of photosynthesis originate from?
- What is the form of chemical energy produced by photosynthesis?
- What occurs in oxidation and reduction reactions?
Introduction
How does the food you eat provide energy? When you need a quick boost of energy, you
might reach for an apple or a candy bar. Although foods with sugars can give you a quick
boost of energy, they cannot be used for energy directly by your cells. Energy is simply
stored in these foods. Through the process ofcellular respiration,the energy in food is
converted into energy that can be used by the body’s cells. In other words, glucose (and
oxygen) is converted into ATP (and carbon dioxide and water). ATP is the molecule that
provides energy for your cells to perform work, such as contracting your muscles as you walk
down the street or performing active transport. Cellular respiration is simply a process that
converts one type of chemical energy, the energy stored in sugar, into another type, ATP.
Overview of Cellular Respiration
Most often, cellular respiration proceeds by breaking down glucose into carbon dioxide and
water. As this breakdown of glucose occurs, energy is released. The process of cellular
respiration includes the conversion of this energy into ATP. The overall reaction for cellular
respiration is as follows:
C 6 H 12 O 6 + 6O 2 !6CO 2 + 6H 2 O
Notice that the equation for cellular respiration is the direct opposite of photosynthesis.
While water was broken down to form oxygen during photosynthesis, in cellular respiration
oxygen is combined with hydrogen to form water. While photosynthesis requires carbon
dioxide and releases oxygen, cellular respiration requires oxygen and releases carbon dioxide.
This exchange of carbon dioxide and oxygen in all the organisms that use photosynthesis
and/or cellular respiration worldwide, helps to keep atmospheric oxygen and carbon dioxide
at somewhat stable levels.
Cellular respiration doesn’t happen all at once, however. Glucose is broken down slowly
so that cells convert as much sugar as possible into the usable form of energy, ATP. Still,
some energy is lost in the process in the form of heat. When one molecule of glucose is
broken down, it can be converted to a net total of 36 or 38 molecules of ATP. Although the
process is not 100% efficient, it is much more efficient than a car engine obtaining energy
from gasoline.
Cellular respiration can be divided into three phases.