Cell Respiration
Objective: Students will be able to identify the location of respiration in a cell and differentiate between
respiration processes
Question: What kinds of cells carry out respiration? What is the chemical equation for respiration?
Answer – Respiration happens in all cells. Respiration is
O2 + C6H12O6  CO2 + H2O + ATP
In other words, Oxygen and Glucose are used to make ATP (cellular energy), and Carbon Dioxide
and Water are released.
Step 1: Glycolysis (“Glyco” – refers to glucose; “Lysis” – means to break down)
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Glucose enters the cell
Glycolysis takes place in the cytoplasm
2 ATP are used to break the 6-carbon glucose into 2 3-carbon molecules
NAD+ takes a Hydrogen ion from each 3-carbon molecule, becoming NADH
2 ADP take a Phosphate from each 3-carbon molecule, become ATP
End result is 2 pyruvic acid and a total gain of 2 ATP
Step 2: The Krebs Cycle (Also referred to as the Citric Acid Cycle)
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Takes place inside mitochondria.
On the way to the mitochondria, Pyruvic Acid from Glycolysis is broken down to a 2-carbon
molecule (Acetyl CoA).
The 2-carbon molecule joins with a 4-carbon molecule (oxaloacetic acid) from the previous
“turn” of the Krebs Cycle. This forms a 6-carbon molecule (Citric Acid).
NAD+ takes a Hydrogen (becomes NADH), and a CO2 is split off – results in a 5-carbon
molecule (ketoglutaric acid).
NAD+ takes a Hydrogen (becomes NADH), a CO2 is split, and ADP takes a Phosphate
(becomes ATP) – results in a 4-carbon molecule (succinic acid).
FAD and NAD+ take more hydrogens (become FADH2 and NADH), and convert the molecule
back to oxaloacetic acid.
Oxaloacetic acid joins an incoming Acetyl CoA and the cycle turns again.
Each turn of the cycle gives off 1 ATP and 2 CO2  2 ATP and 4 CO2 per glucose.
Step 3: Electron Transport Chain
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Takes place inside mitochondria.
Uses Hydrogen from NADH and FADH2 (from glycolysis and Krebs Cycle) to carry electrons
back and forth across the mitochondria’s membrane
Each time an electron is carried across the membrane, it charges an ATP.
Oxygen is used to aid this process – reacts with the Hydrogens to create water
32 ATP are created for each molecule of glucose
The process above is Aerobic Respiration – Aerobic means that oxygen must be present for it to happen
Anaerobic Respiration – occurs when there is no oxygen; referred to as “Fermentation”; 2 kinds – Lactic
Acid Fermentation and Alcoholic Fermentation.
Lactic Acid Fermentation – Occurs in animal muscle cells, as well as many kinds of bacteria.
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First step is glycolysis, which turns glucose into 2 pyruvic acid (pyruvate), and releases 2 ATP
Second step is accepting electrons/Hydrogen from NADH, turning pyruvic acid into lactic
acid (lactate)
No oxygen is used
Lactic acid build-up in animal muscles causes muscle fatigue
Lactic acid build-up from bacteria causes milk products to ferment  turns milk into cheese,
yogurt, sour cream, etc.
Alcoholic Fermentation – occurs in fungi, most notably yeast. Can occur in humans with severe diabetes
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First step is glycolysis, which turns glucose into 2 pyruvic acid (pyruvate), and releases 2 ATP
Second step is CO2 being released, which turns 3-carbon pyruvic acid into 2-carbon
acetylaldehyde
Acetylaldehyde accepts Hydrogen from NADH and becomes ethanol (ethyl alcohol)
No oxygen is used
Commonly used to make dough “rise” – CO2 gas bubbles cause the dough to puff up