Animals are heterotrophs (consumers): they cannot make such molecules
on their own and obtain energy by eating other organisms.
Producers convert that energy into chemical energy of organic
Consumers obtain energy by feeding on those molecules.
Energy ultimately leaves the ecosystem as heat.
The chemical elements are recycled.
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.
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.
During glycolysis, a glucose molecule is broken into 2 molecules of
The pyruvate enters the citric acid cycle.
These 2 steps produce a few molecules of ATP by substrate-level
Energy stored in electrons are moved to the ETC, where much more ATP
are produced by oxidative phosphorylation.
The net gain is two ATP molecules. In addition, two NADH are also
produced; these will be processed in the ETC.
Each acetyl CoA yields one additional ATP and more electron carriers:
three NADH and one FADH.
Citric acid cycle review:
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:
Note that fermentation itself yields no ATP molecules; it serves to
recycle NADH back to NAD^+, which is reused in glycolysis.
Only 2 molecules of ATP from glycolysis are harvested, the NADH is
recycled back to NAD^+.
can all be used as fuel for cellular respiration.
Monomers of these molecules enter glycolysis
or the citric acid cycle
at various points.