Chemical Energy and Food
• Food is the energy source for cells. The energy in food is
measured in calories.
• A calorie is the amount of energy needed to raise the
temperature of 1 gram of water 1 degree Celsius. There are
3811 calories in one gram of sugar glucose.
• The cell doesn’t burn glucose directly but converts into an
energy source, ATP, through a process that begins with the
pathway called glycolysis.
Glycolysis is the process in which one molecule of glucose is
broken in half, producing two molecules of pyruvic acid, a three
• Glycolysis takes place in the cytoplasm of a cell.
• In the process of breaking glucose in half two molecules of ATP
are needed but four molecules of ATP are produced creating a
net gain of two molecules of ATP
• 4 high energy electrons are also removed and added to NAD+
(nicotinamide adenine dinucleotide) creating NADH, which
helps pass energy from glucose to other pathways in the cell.
2 Pyruvic acid
To the electron
Chemical Pathways After Glycolysis
• When oxygen is not present, fermentation follows glycolysis
• ● Fermentation releases energy from food molecules by producing ATP in
the absence of oxygen, this is an anaerobic process.
● There are two main types of fermentation:
Lactic acid fermentation
● Alcoholic fermentation example is yeast which causes carbon dioxide to
form and make bread rise.
● pyruvic acid + NADH alcohol + CO2 + NAD+
• ● Lactic acid fermentation:
• Pyruvic acid + NADH lactic acid + NAD+
• ● Lactic acid is produced in your muscles during rapid exercise
when the body cannot supply enough oxygen to the tissues.
• ● “FEEL THE BURN” is actually the result of a build up of lactic
acid in the muscles and can become painful and lead to
Lactic Acid Fermentation
Overview of Cellular Respiration
• If oxygen is present, The Krebs Cycle and electron transport chain
• Glycolysis, the Krebs Cycle, and the electron transport chain make
up the process called cellular respiration.
• Cellular respiration is the process that releases energy by breaking
down glucose and other food molecules in the presence of oxygen
(an aerobic process).
Cellular Respiration Chemical Equation
Cellular Respiration: An Overview
Electrons carried in NADH
carried in NADH
The Krebs Cycle (Citric Acid Cycle)
• The Krebs cycle is the 2nd stage of cellular respiration.
(glycolysis is the 1st stage)
• During the Krebs cycle, pyruvic acid is broken down into
carbon dioxide in a series of energy-extracting reactions.
• The Krebs cycle is also known as the citric acid cycle, because
citric acid is one of its 1st products.
Stages Of The Krebs Cycle
1. The Krebs cycle starts when pyruvic acid formed by glycolysis enters the
2. The pyruvic acid is broken down into carbon dioxide and a 2-carbon acetyl
3. The 2 carbons of the acetyl group join a 4-carbon compound to produce
citric acid. The Krebs cycle continues in a series of reactions. In these
reactions, two energy carriers accept high-energy electrons. NAD+ is
changed to NADH, and FAD is changed to FADH2. These molecules carry
high energy electrons to the electron transport chain. The carbon dioxide
is released as a waste product.
The Krebs Cycle
The Electron Transport Chain (ETC)
• The ETC is the 3rd and final stage of cellular respiration.
• The ETC uses the high energy electrons to change ADP into ATP.
• In the ETC, high energy electrons move from one carrier protein to the
• At the end of the chain, oxygen pulls electrons from the final carrier
molecule. These electrons join with hydrogen ions, forming water.
• Each transfer along the chain releases a small amount of energy. ATP
synthase uses the energy to produce ATP.
Electron Transport Chain
Hydrogen Ion Movement
Cellular Respiration Overview
ATP Totals For Cellular Respiration
● The totals for the cellular respiration cycle are as follows:
-Glycolysis: +4 ATP – 2 ATP = Net gain 2 ATP
-Krebs Cycle and ETC: +32 ATP
-Net gain for entire cycle is 34 ATP
Note: remember there are 2 ATP used in glycolysis, therefore
only 34 ATP are realized for the cell’s further needs.
• -The total number of ATP molecules formed during cellular
respiration is 36
The Partnership Between Photosynthesis And Cellular
• The energy flows in photosynthesis and cellular respiration occur in
• On a global level, photosynthesis removes carbon dioxide from the
atmosphere and puts back oxygen.
• Cellular respiration removes oxygen from the atmosphere and puts back