chapter 09

Bio1151 Chapter 9 9

Living organisms require to perform cellular work.
- Plants are autotrophs (producers) and can make organic molecules by converting the energy in light to chemical bonds.
  Animals are heterotrophs (consumers): they cannot make such molecules on their own and obtain energy by eating other organisms.
Energy flows into most ecosystems as and leaves as .
- Energy flows into an ecosystem as sunlight.
  Producers convert that energy into chemical energy of organic molecules.
  Consumers obtain energy by feeding on those molecules.
  Energy ultimately leaves the ecosystem as heat.
  The chemical elements are recycled.
- At the cell level, energy from sunlight is stored in organic molecules by photosynthesis in chloroplasts.
  A byproduct of this process is oxygen.
  The organic molecules and oxygen are used by mitochondria to produce ATP in a process called cellular respiration.
  Byproducts of respiration are water and carbon dioxide, which are used as inputs for photosynthesis.
  Heat is released into the environment in this cycle.
  Exercise:
Electron transfer drives the transformation of energy and occurs through reactions.
- Oxidation-reduction reaction.redox reaction, one substance loses electrons (oxidized) and energy while another gains electrons (reduced) and energy.
Molecules such as NAD⁺ and FAD act as electron in redox reactions.
- Nicotinamide Adenine Dinucleotide, the electron carrier.
  Two electrons and one proton (H^+) is transferred to NAD^+ which is reduced to NADH.
  The NADH stores energy from the net gain of an electron that can be used to make ATP.
In cellular respiration, redox reactions occur in a series of steps called the transport chain to form .
- In cellular respiration, glucose is oxidized (loses electrons) and oxygen is reduced (gains electrons).
- The uncontrolled exergonic reaction of hydrogen with oxygen to form water releases a large amount of energy: an explosion.
- In cellular respiration, an electron transport chain (ETC) transfers the energy in electrons in a series of small steps to make ATP.
Cellular respiration comprises three metabolic stages: , acid cycle, and phosphorylation.
- Cellular respiration overview.
  During glycolysis, a glucose molecule is broken into 2 molecules of pyruvate.
  The pyruvate enters the citric acid cycle.
  These 2 steps produce a few molecules of ATP by substrate-level phosphorylation.
  Energy stored in electrons are moved to the ETC, where much more ATP are produced by oxidative phosphorylation.
- Substrate-level phosphorylation occurs when a phosphate group is transferred from a substrate molecule to ADP via an enzyme to make ATP.
- occurs in the to produce two molecules of from the metabolism of one molecule.
  - Glycolysis ("splitting of sugar") occurs in the cytosol and breaks down one glucose molecule into two molecules of pyruvate, yielding a net gain of 2 ATP and 2 NADH molecules.
  - energy investment phase
  - energy payoff phase Glycolysis summary:
  - In the energy investment phase of glycolysis, two ATP molecules are consumed to phosphorylate each glucose molecule.
  - In the energy payoff phase of glycolysis, four ATP molecules are recovered.
    The net gain is two ATP molecules. In addition, two NADH are also produced; these will be processed in the ETC.
- The is converted to CoA, which enters the acid cycle.
  - In the mitochondrion, several enzymes catalyze the conversion of 3-carbon pyruvate to 2-carbon acetyl CoA, yielding a molecule of NADH and one CO[2]. The acetyl CoA enters the citric acid cycle.
  - In the citric acid cycle, carbon dioxide (CO[2]) is released as acetyl CoA is degraded.
    Each acetyl CoA yields one additional ATP and more electron carriers: three NADH and one FADH[2].
    Citric acid cycle review:
- During phosphorylation, NADH and FADH₂ donate to the ETC to produce even more ATP.
- Chemiosmosis. As electrons move along a series of proteins in the ETC, they pump H^+ (protons) across the membrane, creating a proton gradient. The protons diffuse back (chemiosmosis) through the enzyme ATP synthase to form ATP. This process requires oxygen (the final electron acceptor) and releases water.
- Most of the ATP made by cellular respiration occurs via oxidative phosphorylation: about 32-34 ATP per glucose. Since oxygen is required to complete the citric acid cycle and oxidative phosphorylation, these two processes are known as aerobic respiration. Review:
Review: Cellular Respiration
In conditions, glycolysis may be followed by .
- Fermentation.
  In anaerobic conditions (absence of oxygen), some cells undergo fermentation instead of aerobic respiration.
  After glycolysis, pyruvate is broken down by fermentation in the cytosol via two catabolic pathways:
- ethanol
- lactic acid
  Note that fermentation itself yields no ATP molecules; it serves to recycle NADH back to NAD^+, which is reused in glycolysis.
  Fermentation review:
- Ethanol fermentation releases CO[2] as a byproduct in addition to producing ethanol.
  Only 2 molecules of ATP from glycolysis are harvested, the NADH is recycled back to NAD^+.
- Lactic acid fermentation produces lactic acid, which can be used in dairy products.
  Only 2 molecules of ATP from glycolysis are harvested, the NADH is recycled back to NAD^+.
Other organic molecules such as and can also enter cellular respiration via different pathways.
_inv_9&A How Is the Rate of Cellular Respiration Measured?
- Fuels for cellular respiration.
  Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration.
  Monomers of these molecules enter glycolysis or the citric acid cycle at various points.