1. Cellular Respiration
© Lisa Michalek

2. Cellular Energy Most of the foods we eat contain usable energy.
Much of the energy is stored in proteins, carbohydrates, and fats.
Cells transfer energy in organic compounds to ATP through a process called cellular respiration.
Oxygen in the air makes the production of ATP more efficient.
Metabolic processes that require oxygen are called aerobic.
Metabolic processes that do not require oxygen are called anaerobic (without air).

3. The Steps of Cellular Respiration
Cellular respiration is the process cells use to produce the energy in organic compounds.
Cellular respiration can be summarized by the following equation:
enzymes
C6H12O O → 6CO H2O + energy
glucose oxygen carbon water ATP
gas dioxide

4. The Steps of Cellular Respiration
Cellular respiration occurs in two steps.
Step 1 – Glucose is converted to pyruvate, producing a small amount of ATP and NADH.
Step 2 – When oxygen is present, pyruvate and NADH are used to make large amounts of ATP (aerobic respiration). Aerobic respiration occurs in the mitochondria of all cells. When oxygen is not present, pyruvate is converted to either lactate or ethanol and carbon dioxide.

5. Step One: Breakdown of Glucose
The primary fuel for cellular respiration is glucose, which is formed when carbohydrates such as starch and sucrose are broken down.
If too few carbohydrates are available to meet an organism’s glucose needs, other molecules, such as fats, can be broken down to make ATP.
One gram of fat contains more energy than two grams of carbohydrates.
Proteins and nucleic acids can also be used to make ATP, but they are usually used for building important cell parts.

6. Glycolysis
In the first step of cellular respiration, glucose is broken down in the cytoplasm during a process called glycolysis.
Glycolysis is an enzyme-assisted anaerobic process that breaks down one six-carbon molecule of glucose to two three-carbon pyruvate ions.
Pyruvate is the ion of a three-carbon organic acid called pyruvic acid.
The pyruvate produced during glycolysis still contains some of the energy that was stored in the glucose molecule.

7. Glycolysis
As glucose is broken down, some of its hydrogen atoms are transferred to an electron acceptor called NAD+.
This forms an electron carrier called NADH.
The electrons carried by NADH are eventually donated to other organic compounds.
This recycles NAD+, making it available to accept more electrons.

8. Glycolysis

9. Glycolysis
Glycolysis uses two ATP molecules but produces four ATP molecules, yielding a net gain of two ATP molecules.
Glycolysis is followed by another set of reactions that use the energy temporarily stored in NADH to make more ATP.

10. Step Two: Production of ATP
When oxygen is present, pyruvate produced during glycolysis enters a mitochondrion and is converted to a two-carbon compound.
This reaction produces one carbon dioxide molecule, one NADH molecule, and one two-carbon acetyl group.
The acetyl group is attached to a molecule called coenzyme A (CoA), forming a compound called acetyl-CoA.

11. Krebs Cycle
Acetyl-CoA enters a series of enzyme-assisted reactions called the Krebs cycle.
The cycle is named for the biochemist Hans Krebs, who first described the cycle in 1937.

13. Krebs Cycle
After the Krebs cycle, NADH and FADH2 now contain much of the energy that was previously stored in glucose and pyruvate.
When the Krebs cycle is completed, the four-carbon compound that began the cycle has been recycled, and acetyl-CoA can enter the cycle again.

14. Electron Transport Chain
In aerobic respiration, electrons donated by NADH and FADH2 pass through an electron transport chain.
In eukaryotic cells, the electron transport chain is located in the inner membranes of mitochondria.
The energy of these electrons is used to pump hydrogen ions out of the inner mitochondrial compartment.

15. Electron Transport Chain
Hydrogen ions accumulate in the outer compartment, producing a concentration gradient across the inner membrane.
Hydrogen ions diffuse back into the inner compartment through a carrier protein that adds a phosphate group to ADP, making ATP.
At the end of the electron transport chain, hydrogen ions and spent electrons combine with oxygen molecules, O2, forming water molecules, H2O.

16. Electron Transport Chain

17. Respiration in the Absence of Oxygen
What happens when there is not enough oxygen for aerobic respiration to occur?
The electron transport chain does not function because oxygen is not available to serve as the final electron acceptor.
Electrons are not transferred from NADH, and NAD+ therefore they cannot be recycled.
When Oxygen is not present, NAD+ is recycled in another way.

18. Respiration in the Absence of Oxygen
Under anaerobic conditions, electrons carried by NADH are transferred to pyruvate produced during glycolysis.
This process recycles NAD+ needed to continue making ATP through glycolysis.
The recycling of NAD+ using an organic hydrogen acceptor is called fermentation.
Prokaryotes carry out more than a dozen kinds of fermentation all using some form of organic hydrogen acceptor to recycle NAD+.
Two important forms of fermentation are lactic acid fermentation and alcoholic fermentation.
Lactic acid fermentation by some prokaryotes and fungi is used in the production of foods such as yogurt and some cheeses.

19. Lactic Acid Fermentation
In some organisms, a three-carbon pyruvate is converted to a three-carbon lactate through lactic acid fermentation.
Lactate is the ion of an organic acid called lactic acid.
During vigorous exercise, pyruvate in muscles is converted to lactate when muscle cells must operate without enough oxygen.
Fermentation enables glycolysis to continue producing ATP in muscles as long as the glucose supply lasts.
Blood removes excess lactate from muscles.
Lactate can build up in muscle cells if it is not removed quickly enough, sometimes causing muscle soreness.

20. Lactic Acid Fermentation

21. Alcoholic Fermentation
In other organisms, the three-carbon pyruvate is broken down to ethanol, a two-carbon compound, through alcoholic fermentation.
Carbon dioxide is released during the process.
First, pyruvate is converted to a two-carbon compound, releasing carbon dioxide.
Second, electrons are transferred from a molecule of NADH to the two-carbon compound, producing ethanol.
As in lactic acid fermentation, NAD+ is recycled, and glycolysis can continue to produce ATP.

22. Alcoholic Fermentation

23. Alcoholic Fermentation
Alcoholic fermentation by yeast, a fungus, has been used in the preparation of many foods and beverages.
Wine and beer contain ethanol made during alcoholic fermentation by yeast.
Carbon dioxide released by the yeast causes the rising of bread dough and the carbonation of some alcoholic beverages, such as beer.
Ethanol is actually toxic to yeast.
At a concentration of about 12 percent ethanol kills yeast.
Therefore, naturally fermented wine contains about 12% ethanol.

24. Production of ATP
The total amount of ATP that a cell is able to harvest from each glucose molecule that enters glycolysis depends on the presence or absence of oxygen.
When Oxygen is present, aerobic respiration occurs.
When Oxygen is absent, fermentation occurs.

25. Production of ATP