chapter 08

Metabolism involves the transformation of energy into energy in living things, and comprises and cellular .
- • Energy flow.
  Photosynthesis converts inorganic carbon dioxide into sugars such as glucose, using energy from light and electrons from water.
  Oxygen is released in this process.
  
  Cellular respiration uses the energy stored in glucose to produce ATP; oxygen is used as the electron acceptor.
  Carbon dioxide and water, as well as heat, are released in this process.
Cellular respiration is composed of two stages: occurs in the and the cycle occurs in .
- • Cellular (aerobic) respiration is the harvesting of energy from carbohydrates, and comprises two stages.
  1. Glycolysis occurs in the cytoplasm to break down glucose.
  2. Krebs cycle occurs in mitochondria, where oxygen acts as the final acceptor of high-energy electrons.
  The reactants are glucose and oxygen, and products are carbon dioxide, water, and ATP.
  Heat is also release in this process.
In glycolysis, is broken down into two three-carbon molecules, yielding a net of molecules of ATP per glucose. Review:
- • Glycolysis
  1. 2 molecules of ATP are used to split a 6-carbon glucose into into 2 molecules of the 3-carbon pyruvate.
  2. For each pyruvate, 2 ATP molecules are formed and 1 electron is harvested as NADH.
  3. The net output is 2 molecules of ATP and NADH per glucose.
- The pyruvates are passed into the where they are converted into acetyl-coA, which enters the cycle (citric acid cycle).
- • Producing acetyl-coA.
  In the presence of oxygen, pyruvate is converted to the 2-carbon acetyl-coA, releasing CO₂ and donating an electron to NADH.
  The acetyl-coA can enter the Krebs cycle for production of more ATP.

In the Krebs cycle, each acetyl-coA yields molecule of ATP, and several molecules of and FADH₂. Review:

•

Krebs cycle.

The 2-carbon acetyl-CoA is transferred to a 4-carbon starting material.	The resulting 6-carbon molecule yields 2 molecules each of NADH and CO₂, plus 1 ATP.	The remaining 4-carbon molecule yields another NADH and 1 FADH₂, regenerating the starting material.

The energy stored in and FADH₂ is harvested by the transport chain (ETC) to make . Review:

• Several proteins in the inner mitochondrial membrane comprise the electron transport chain (ETC).
NADH and FADH₂ donate electrons that move along the ETC.
Oxygen is the final electron acceptor, releasing water at the end of the chain.
continue

• ATP synthesis in the ETC.
Energy from the electrons finally reach an enzyme called ATP synthase to synthesize ATP.
Electrons from NADH and FADH₂ yield 32 molecules of ATP per glucose in the ETC.
The net yield during aerobic respiration is 36 ATP molecules per glucose.

In conditions, glycolysis is followed by , which regenerates the electron receptor NAD⁺, and allows to continue generating ATP.
- • Fermentation.
  In the presence of oxygen, pyruvate enters the Krebs cycle.
  In the absence of oxygen, pyruvate is reduced by fermentation, recycling NADH back to NAD⁺ and yielding either of two possible products:
  - lactate
  - ethanol
  Review:
- • Lactate fermentation.
  Some bacteria and animals produce lactate (lactic acid) as an end product.
  Review:
- • Ethanol fermentation.
  Yeasts produce ethanol and CO₂.
  Review:
Other macromolecules such as and can also enter cellular respiration via different pathways.
- • Energy from foods.
  Many different kinds of macromolecules can be broken down into their monomer building blocks which then can enter the Krebs cycle at various points.
Review exercise: