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Course: Class 11 Biology (India) > Unit 12
Lesson 5: Fermentation and the amphibolic pathwaySummary of Cellular Respiration
Summary of cellular respiration
Key Terms
| Term | Meaning |
|---|---|
| Cellular respiration | The process by which organisms break down glucose in order to release energy |
| ATP | Adenosine triphosphate, the primary energy carrier in living things |
| Mitochondrion | The eukaryotic cell structure where a major part of cellular respiration occurs (plural - mitochondria) |
| Cytoplasm | The contents of a cell between the plasma membrane and the nuclear envelope; includes cytosol which is the jelly-like substance that fills the space between organelles |
| Aerobic | Process that requires oxygen |
| Anaerobic | Process that does not require oxygen |
| Fermentation | An anaerobic pathway for breaking down glucose |
Cellular respiration
Cellular respiration is a biochemical process of breaking down food, usually glucose, into simpler substances. The energy released in this process is tapped by the cell to drive various energy-requiring processes.
Cellular respiration can occur both aerobically (using oxygen), or anaerobically (without oxygen).
During aerobic cellular respiration, glucose reacts with oxygen, forming carbon dioxide and water. In this process, a considerable amount of energy is released, which is trapped by the cell in ATP molecules.
The overall equation for aerobic cellular respiration is:
The three stages of aerobic cellular respiration are glycolysis, the Krebs cycle, and oxidative phosphorylation.
Fermentation
Some organisms are able to break down glucose and produce energy without the presence of oxygen. This type of cellular respiration is called anaerobic respiration. A very common type of anaerobic respiration is fermentation. This process involves glycolysis followed by one or two more reactions. The following are two types of fermentation:
- When oxygen runs low, our muscle cells can produce ATP using lactic acid fermentation. However, this can happen only for short periods of strenuous exercises, like running a sprint. In order to keep working for longer time intervals, the muscle cells have to use aerobic respiration.
- Many yeasts use alcoholic fermentation to produce ethanol. In this process, carbon dioxide is released as well. For this reason, humans have domesticated yeast to use for many commercial purposes including baking, which uses the carbon dioxide for generating a fluffy texture, as well as the production of alcoholic beverages like beer and wine.
Aerobic respiration vs fermentation
| Aerobic respiration | Fermentation | |
|---|---|---|
| Reactants | Glucose and oxygen | Glucose |
| Products | ATP, water, CO | ATP and lactic acid (animals); or ATP, ethanol, and CO |
| Location | Cytosol (glycolysis) and mitochondria | Cytosol |
| Stages | Glycolysis (anaerobic), Krebs cycle, oxidative phosphorylation | Glycolysis followed by one or two more reactions |
| ATP produced | Large amount (38 ATP) | Small amount (2 ATP) |
Common misconceptions
- Plants do not undergo cellular respiration. Many people believe that plants undergo photosynthesis and animals undergo respiration. Actually, plants do both! Plants simply undergo photosynthesis first as a way to make glucose. Animals don't have the cellular machinery to photosynthesize. Instead, they get their glucose from the food they eat.
- Cellular respiration is the same as "breathing." This can be confusing! People often use the word "respiration" to refer to the process of inhaling and exhaling. However, this is physiological respiration, not cellular respiration. The two are related processes, but they are not the same thing.
- Carbohydrates are the only source of glucose. Carbohydrates can be easily digested in our digestive system to form glucose. Then what happens if we are on a meat-only diet? The proteins from meat can be digested to form amino acids. Not only can the amino acids enter the cellular respiration process at different stages, several of them can actually be converted into glucose, in a process called gluconeogenesis.