In cellular respiration, electrons are transferred from glucose to coenzymes such
as NAD and finally to oxygen.
The energy released by this relocation of electrons is used to make ATP.
Carbon dioxide and water are given off as byproducts.
Glycolysis is in which glucose is broken down into two molecules of
As the chemical bonds in glucose are broken, electrons (and hydrogen ions) are
picked up by NAD , forming NADH.
Glucose is oxidized and NAD is reduced.
A net output of two ATP molecules are also produced for every glucose molecule
But most of the energy released by the breakdown of glucose is carried by the
electrons attached to NADH.
The pyruvate molecules are modified as they enter the mitochondrion, releasing
The altered molecules enter the citric acid cycle.
More carbon dioxide is released as the citric acid cycle completes the oxidation
Two ATPs are formed per glucose, but most of the energy released by the
oxidation of glucose is carried by NADH and FADH . 2
The NADH and FADH molec2les produced in glycolysis and the citric acid cycle
donate their electrons to the electron transport chain.
At the end of the chain, oxygen exerts a strong pull on the electrons, and
combines with them and hydrogen ions (protons) to form water.
The electron transport chain converts chemical energy of moving electrons to a
form that can be used to drive oxidative phosphorylation, which produces about
34 ATP molecules for each glucose molecule consumed. Photosynthesis
6CO +26H O 2C H O +6612 6 2
Photosynthesis transfers electrons from water to energy-poor CO molecu2es,
forming energy-rich sugar molecules (C H O ).
6 12 6
Photosynthesis uses light energy to drive the electrons from water to their more
energetic states in the sugar products, thus converting solar energy to chemical
The light reactions occur along the thylakoid membranes within the chloroplasts,
where pigments capture light energy.
The sugar-making reactions of the Calvin cycle occur in the stroma, the fluid
between the inner membrane of the chloroplast and the thylakoids.
In the light reactions, light is absorbed by chlorophyll molecules, exciting their
The energy of excited electrons is then used to join ADP and phosphate to form
NADP joins with excited electrons to form NADPH, which temporarily stores the
energized electrons. In the process, water is split and oxygen is released.
In the Calvin cycle, energy from ATP, electrons from NADPH, and carbon
from carbon diox