Nutrition Science and Everyday Application - beta v 0.1

PHOTOSYNTHESIS AND METABOLISM 137
Starch is the storage form of glucose in plants, stored in seeds, roots, and tubers for later use
as an energy source for the plant to reproduce. When a seed is buried deep in the soil, this
starch can be broken down into glucose to be used for energy for the seed to sprout. As the
seed sprouts, and shoots go above the ground and leaves start to form, the new plant can
then photosynthesize glucose for an energy source. When we eat foods that contain starch,
we must digest that starch down into single sugars (glucose) in order for the glucose to be
absorbed into the intestinal cells, where it will enter the bloodstream to be carried to all cells
of the body to use as an energy source. The basic process of digestion of these foods will be
covered in the next section.
CELLULAR RESPIRATION
All living things require energy to survive. For humans, and many other organisms, that
energy is generated by the complex interaction of photosynthesis and cellular respiration.
Cellular respiration is a key pathway in energy metabolism (the process of converting food
into energy) of all aerobic organisms. Respiration refers to breathing: taking in oxygen and
removing carbon dioxide. But ultimately, the reason we need to breathe is to provide the
oxygen needed to carry out cellular respiration in our cells and to remove the carbon dioxide
that is produced as a byproduct.
In the process of cellular respiration, energy that is stored in the food we eat is converted
to the body’s energy currency, ATP, while a small amount is lost as heat. During cellular
respiration, glucose is broken down to carbon dioxide and water; in the process, ATP is
released. Cellular respiration occurs in the mitochondria of cells and is an aerobic
process, which means that oxygen is required. It is a series of reactions that can be
summarized as follows:
glucose + 6O 2 → 6CO 2 + 6H 2 O + energy (ATP and heat)
Another way of saying this:
Cellular respiration uses:
Cellular respiration produces:
1 molecule of glucose (C 6 H 12 O 6 )
6 molecules of oxygen (6O 2 )
6 molecules of carbon dioxide (6CO 2 )
6 molecules of water (6H 2 O)
Energy
Even though glucose is the starting substance used in cellular respiration, we do not
consume only glucose as an energy source. Instead, many different kinds of food molecules
are broken down into smaller molecules, metabolized, and then enter the cellular respiration
pathway. For example, complex carbohydrates like starch are readily converted to glucose.
Fats and proteins can also be used in cellular respiration, but they must be modified before
they can feed into the process.