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Chemical Energy In Food
CHAPTER 6
CELLULAR RESPIRATION
Purpose of food:
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calorie – the amount of energy needed to raise
the temperature of one gram of water one
degree Celsius.
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Cellular Respiration Overview
Cellular Respiration overview
Cellular Respiration – the process that
releases energy by breaking down food
molecules in the presence of oxygen.
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Contains 3 Pathways:
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Glycolysis
Krebs cycle
Electron transport
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The equation for cellular respiration is:
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6O2 + C6H12O6  6CO2 + 6H2O + ENERGY (ATP)
Step 1: Glycolysis
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Takes place in the cytoplasm.
Glycolysis is the process in which 1 molecule of
glucose is broken in half, producing 2
molecules of pyruvic acid.
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If cellular respiration took place in one step, all
the energy would be released at once and most
would be lost as heat.
PROBLEM: The cell has to find a way to trap the
energy a little bit at a time.
SOLUTION: Each of the three stages of cellular
respiration captures some of the chemical energy
available in food molecules and uses it to
produce ATP or energy.
Step 1: Glycolysis
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Glycolysis is so fast that cells can produce
1000’s of ATPs in milliseconds.
Glycolysis does not require oxygen.
This process has to get things going so it uses 2
stored ATP molecules.
4 ATP molecules are made during glycolysis, so a
total of 2 ATP molecules are released by glycolysis
as energy.
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Cells don’t burn glucose but gradually release the
energy from it.
Glycolysis – chemical pathway that begins the
process of releasing the energy in glucose.
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Source of raw materials used to make new
molecules
Source of energy
This is only 2% of the total chemical energy in glucose.
NAD+ is converted to NADH.
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OXIDATIVE RESPIRATION
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To get the rest of the energy from the food
molecules, the cell uses oxygen.
Aerobic – requires oxygen.
The energy-releasing pathways require
oxygen, and that is the reason we need to
breathe, to respire.
Respiration - process that involves oxygen
and breaks down food molecules to release
energy.
Step 2: Krebs cycle
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The Krebs cycle:
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Takes place in the mitochondria.
Also called the Citric acid cycle.
The carbon contained in pyruvic acid (produced
by glycolysis) is broken down into carbon
dioxide.
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Two high energy electron carriers are also
generated by the Krebs cycle:
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KREBS CYCLE (continued)
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The NADH and FADH2 go on to make huge
amounts of ATP in the presence of oxygen,
and the CO2 is eventually released when you
exhale.
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ENERGY AND EXERCISE
Aerobic cellular respiration produces 36 total ATP
molecules from each glucose molecule (37% efficient).
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Remainder of energy from glucose is released as heat (body
feels warm after exercise).
Complex carbs are broken down into simple sugars that are
converted to glucose.
Lipids and proteins broken down into molecules that enter
glycolysis or Krebs cycle.
Breathing and respiration:
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Remember, the final electron acceptor for the electrons
produced in respiration is oxygen.
Without oxygen, all ATP synthesis in mitochondria stops and
body tries to make ATP by glycolysis alone (not sufficient for
most cells).
Electron transport occurs in the mitochondria.
The high energy electrons are passed from the
carriers in the Krebs cycle (NADH, FADH2) to the
electron transport chain.
High energy electrons are passed from one carrier
protein to the next.
The electron transport chain uses these high
energy electrons to convert ADP into ATP or
energy.
At the end of the chain, an enzyme combines the
electrons with H+ and oxygen to form water, a byproduct of electron transport.
Photosynthesis vs Respiration
The processes are almost opposites.
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Eating food:
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NADH
FADH2
Step 3: Electron Transport
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This CO2 is considered a waste product and is
released from the cell.
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Equations are opposites.
Photosynthesis deposits energy.
Respiration withdraws energy.
Cellular respiration takes place in all
eukaryotes and some prokaryotes.
Photosynthesis occurs only in plants, algae,
and some bacteria.
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Fermentation
Occurs when oxygen is NOT present.
The respiration that occurs in the absence of
oxygen is called anaerobic respiration or
fermentation b/c it does not require oxygen.
In fermentation, cells perform glycolysis followed
by fermentation.
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Combination of glycolysis and fermentation
produces 2 ATP molecules from one molecule of
glucose.
There are two main types of fermentation:
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Pyruvic Acid + NADH  alcohol + CO2 + NAD+
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Krebs cycle and electron transport DO NOT occur.
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Alcoholic Fermentation
A.
B.
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Used by yeast (facultative anaerobes).
Forms ethyl alcohol and carbon dioxide as
wastes.
CO2 produced causes bread dough to rise,
bubbles in beer.
Alcoholic Fermentation
Lactic Acid Fermentation
Lactic acid fermentation
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Pyruvic Acid + NADH  lactic acid + NAD+
This process regenerates NAD+ so that glycolysis
can continue to make small amounts of ATP.
Lactic acid fermentation happens in many cells
including human muscle cells during rapid exercise.
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Muscles are not getting enough oxygen to produce all the
ATP they need through Krebs cycle and electron
transport, so they use lactic acid fermentation to get ATP
made.
Lactic acid buildup causes the painful, burning sensation
in your muscles after intense activity.
Bacteria undergo lactic acid fermentation to make foods
like sour cream, yogurt, buttermilk, and sauerkraut.
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