Key Terms

TermMeaning
Cellular respirationThe process by which organisms break down glucose into a form that the cell can use as energy
ATPAdenosine triphosphate, the primary energy carrier in living things
MitochondriaThe eukaryotic cell structure where cellular respiration occurs
CytoplasmJelly-like substance that fills the space between organelles
AerobicProcess that requires oxygen
AnaerobicProcess that does not require oxygen
FermentationAn anaerobic pathway for breaking down glucose

Cellular respiration

Cellular respiration can occur both aerobically (using oxygen), or anaerobically (without oxygen).
During aerobic cellular respiration, glucose reacts with oxygen, forming ATP that can be used by the cell. Carbon dioxide and water are created as byproducts.
The overall equation for aerobic cellular respiration is:
In cellular respiration, glucose and oxygen react to form ATP. Water and carbon dioxide are released as byproducts.
The three stages of aerobic cellular respiration are glycolysis (an anaerobic process), the Krebs cycle, and oxidative phosphorylation.

Fermentation

Some organisms are able to continually convert energy without the presence of oxygen. They undergo glycolysis, followed by the anaerobic process of fermentation to make ATP.
  • Muscle cells can continue to produce ATP when oxygen runs low using lactic acid fermentation. However, this often results in muscle fatigue and pain.
  • Many yeast use alcoholic fermentation to produce ethanol. For this reason, humans have domesticated yeast to use for many commercial purposes including baking as well as beer and wine production.

Aerobic vs anaerobic respiration

AerobicAnaerobic
ReactantsGlucose and oxygenGlucose
ProductsATP, water, CO2_2ATP and lactic acid (animals); or ATP, ethanol, and CO2_2 (yeast)
LocationCytoplasm (glycolysis) and mitochondriaCytoplasm
StagesGlycolysis (anaerobic), Krebs cycle, oxidative phosphorylationGlycolysis, fermentation
ATP producedLarge amount (36 ATP)Small amount (2 ATP)

Common mistakes and misconceptions

  • Anaerobic respiration is a normal part of cellular respiration. Glycolysis, which is the first step in all types of cellular respiration is anaerobic and does not require oxygen. If oxygen is present, the pathway will continue on to the Krebs cycle and oxidative phosphorylation. However, if oxygen is not present, some organisms can undergo fermentation to continually produce ATP.
  • Plants undergo cellular respiration. Many people believe that plants undergo photosynthesis and animals undergo respiration. Really, plants do both! Plants simply undergo photosynthesis first as a way to make glucose. Animals don't need to photosynthesize since they get their glucose from the food they eat.
  • Cellular respiration is not simply 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.
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