Unit 2: Cellular Structure & Chemistry
Indiana State Standard 3 & 4
Cells & Energy: Cellular Respiration
Concept 1/Cellular Respiration
Lesson Objectives
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Write and explain the chemical formula for cellular respiration.
Explain the two states of cellular respiration.
Compare photosynthesis with cellular respiration.
Why do you need to breathe?
Of course if you didn't breathe, you couldn't survive. Why do you need air to live? You need the gas
oxygen to perform cellular respiration to get energy from your food.
What is Cellular Respiration?
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. But cells do not "eat" apples or candy bars, these foods need to be broken
down so that cells can use them. Through the process of cellular respiration, the energy in food is
changed into energy that can be used by the body's cells. In other words, glucose and oxygen are
converted into ATP, carbon dioxide, and water. ATP, or adenosine triphosphate, is chemical energy
the cell can use. It is the molecule that provides energy for your cells to perform work, such as moving
your muscles as you walk down the street.
A video of cellular respiration can be seen at http://www.youtube.com/watch?v=nkRcdfmHqqI (7:58).
The Process of Cellular Respiration
Cellular respiration is the process of extracting energy in the form of ATP from the glucose in the
food you eat. How does cellular respiration happen inside of the cell? Cellular respiration is a three
step process. Briefly:
1. In stage one, glucose is broken down in the cytoplasm of the cell in a process called glycolysis.
2. In stage two, the pyruvate molecules are transported into the mitochondria. The mitochondria
are the organelles known as the energy "powerhouses" of the cells (Figure below). In the
mitochondria, the pyruvate, which have been converted into a 2-carbon molecule, enter the
Krebs cycle. Notice that mitochondria have an inner membrane with many folds, called
cristae. These cristae greatly increase the membrane surface area where many of the cellular
respiration reactions take place.
3. In stage three, the energy in the energy carriers enters an electron transport chain. During this
step, this energy is used to produce ATP.
Oxygen is needed to help the process of turning glucose into ATP. The initial step releases just two
molecules of ATP for each glucose. The later steps release much more ATP.
Most of the reactions of cellular respiration are carried out in the mitochondria. [Figure1]
The Reactants
What goes into the cell? Oxygen and glucose are both reactants in the process of cellular respiration.
Oxygen enters the body when an organism breathes. Glucose enters the body when an organism eats.
The Products
What does the cell produce? The main product of cellular respiration is ATP. Waste products include
carbon dioxide and water. Carbon dioxide is transported from your mitochondria out of your cell, to
your red blood cells, and back to your lungs to be exhaled.
When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules
of ATP. This only occurs in the presence of oxygen.
The Chemical Reaction
The overall chemical reaction for cellular respiration is 1 molecule of glucose (C6H12O6) and 6
molecules of oxygen (O2) yields 6 molecules of carbon dioxide (CO2) and 6 molecules of water (H20).
Using chemical symbols the equation is represented as follows:
C6H12O6 + 6O2
6CO2 + 6H2O
ATP is generated during the process. Though this equation may not seem that complicated, cellular
respiration is a series of chemical reactions divided into three stages: glycolysis, the Krebs cycle, and
the electron transport chain.
Glycolysis
Stage one of cellular respiration is glycolysis. Glycolysis is the splitting, or lysis of glucose. Glycolysis
converts the 6-carbon glucose into two 3-carbon pyruvate molecules. This process occurs in the
cytoplasm of the cell, and it occurs in the presence or absence of oxygen. During glycolysis a small
amount of NADH is made as are two ATP. The NADH temporarily holds energy, which will be used
in stage three.
The Krebs Cycle
In the presence of oxygen, under aerobic conditions, pyruvate enters the mitochondria to proceed into
the Krebs cycle. The second stage of cellular respiration is the transfer of the energy in pyruvate,
which is the energy initially in glucose, into two energy carriers, NADH and FADH2. A small amount
of ATP is also made during this process. This process occurs in a continuous cycle, named after its
discover, Hans Krebs. The Krebs cycle uses a 2-carbon molecule (acetyl-CoA) derived from pyruvate
and produces carbon dioxide.
The Electron Transport Chain
Stage three of cellular respiration is the use of NADH and FADH2 to generate ATP. This occurs in two
parts. First, the NADH and FADH2 enter an electron transport chain, where their energy is used to
pump, by active transport, protons (H+) out of the thylakoid. This establishes a proton gradient across
the thylakoid membrane. These protons then flow back into the thylakoid by facilitated diffusion.
During this process, ATP is made by adding inorganic phosphate to ADP. For each glucose that starts
cellular respiration, in the presence of oxygen (aerobic conditions), 36-38 ATP are generated. Without
oxygen, under anaerobic conditions, much less (only two!) ATP are produced.
Connecting Cellular Respiration and Photosynthesis
Notice that the equation for cellular respiration is the direct opposite of photosynthesis (Figure below).
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 or cellular respiration worldwide helps
to keep atmospheric oxygen and carbon dioxide at stable levels.
Cellular respiration and photosynthesis are direct opposite reactions. Some of the ATP made in the
mitochondria is used as energy for work, and some is lost to the environment as heat. Can you explain
what is depicted in this diagram? [Figure2]
Lesson Summary
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Cellular respiration is the breakdown of glucose to release energy in the form of ATP.
A summary of cellular respiration can be viewed at http://www.youtube.com/watch?v=wqqYIgY40OE
(8:50).
Review Questions
Recall
1. What is the purpose of cellular respiration?
2. Where is glucose broken down to form ATP?
3. Where is glucose broken down to form ATP?
4. Write the chemical reaction for the overall process of cellular respiration.
Apply Concepts
3. What are the products of alcoholic fermentation?
4. Write the chemical reaction for the overall process of cellular respiration.
5. What produces more ATP, aerobic or anaerobic cellular respiration? What is the purpose of
fermentation?
7. Why is the cellular respiration equation the opposite of the photosynthesis equation?
Summary
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Most of the steps of cellular respiration take place in the mitochondria.
Oxygen and glucose are both reactants in the process of cellular respiration.
The main product of cellular respiration is ATP; waste products include carbon dioxide and
water.
Practice
Use the resources below to answer the following questions
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Glycolysis at http://www.youtube.com/watch?v=piIrBw24c8M (0:44)
1. Where does glycolysis occur?
2. When glucose is broken down what is produced?
3. Does glycolysis require oxygen?
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Krebs Cycle at http://www.youtube.com/watch?v=O6bInBQXtmM (5:30)
1. Which types cells have mitochondria?
2. What is the "cristae"? Where does it occur? Why is this structure important?
3. What high energy electron carriers are produced by the Krebs cycle? Where do they carry their
electrons?
4. What is acetyl-CoA? Where does it fit into the Krebs cycle?
5. How much ATP is made by the Krebs cycle for every molecule of Pyruvate that enter the
cycle?
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Electron Transport Chain at http://www.youtube.com/watch?v=xbJ0nbzt5Kw (3:50)
1. What is the name of the protein complex that makes ATP?
2. Where does the electron transport chain in mitochondria start? Where does it end?
3. What is a "mobile transfer molecule"? What ones occur in mitochondria? What is their
function?