OVERVIEW: Biological systems cannot keep going unless energy keeps flowing
through them. Plants use sunlight to make sugars by photosynthesis. All cells
use the energy stored in sugars by breaking them down into pyruvate, a process
called glycolysis. Pyruvate
is then broken down further by a repeating series of reactions called the
Krebs cycle. The largest amount of energy is released with the help of
an electron transport chain.
- Catabolism: The breakdown of energy-rich
molecules such as glucose, a process that releases energy
- ATP: Most biological processes that require energy use
Adenosine TriPhosphate (ATP). Energy-producing reactions synthesize
ATP from Adenosine DiPhosphate (ADP) and inorganic phosphate
(Pi).
Most ATP comes from the catabolism of glucose, including glycolysis, the Krebs
cycle, and electron transport.
- Glycolysis: the breakdown of sugars to pyruvate
Glucose + 2 ATP + 2 NAD+ + 2 ADP
+ 2 Pi ——>
2 Pyruvate + 4 ATP + 2 NADH + 2 H+
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- Aerobic metabolism: If oxygen is present, pyruvate forms Acetyl
coenzyme-A, which then enters the Krebs cycle.
- Anaerobic metabolism: Pyruvate undergoes fermentation:
- In microorganisms, ethyl alcohol is formed:
Pyruvate ——> CO2 + acetaldehyde
+ NADH + H+
——> ethyl alcohol
+ NAD+
- In muscles:
Pyruvate + NADH + H+ ——>
lactic acid (or lactate) + NAD+
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