78 MHR • Unit 1 Metabolic Processes
Anaerobic Cellular Respiration
When oxygen is not present, there is no electron
acceptor to remove electrons at the end of the
electron transport chain. As a result, no further
oxidation of NADH and FADH 2 is possible. No H+
ions can be pumped into the intermembrane space.
Without the oxidative coenzymes NAD+and FAD,
the Krebs cycle stops. The transition reaction, which
requires NAD+, also stops. Pyruvate can enter the
mitochondrion, but it cannot be processed. Theconcentration of pyruvate inside the mitochondrion
increases until it matches the concentration in the
cytosol. As a result, there is no net movement of
pyruvate into the mitochondrion. This leads to a
high level of pyruvate in the cytosol, where one
of two possible fermentation pathways may be
followed. These pathways are examples of anaerobic
cellular respiration, the production of ATP in
the cell without the use of oxygen. Figure 3.14
compares aerobic and anaerobic cellular respiration.Biology Magazine TECHNOLOGY • SOCIETY • ENVIRONMENT
Nutrition and Cellular Energy
Most people know that a healthy, balanced diet is
important for good health. Dieticians recommend that
the average person should consume a balanced diet in
which 60 to 65 percent of daily energy requirements
come from carbohydrates, 30 to 35 percent from fat, and
the remainder from protein. What role does each type of
food play in the production of cellular energy?Carbohydrates
Carbohydrates are essential to cellular respiration. They
break down to form simple sugars such as glucose, which
is the primary reactant for glycolysis. Glucose undergoes
glycolysis to produce pyruvic acid, which combines with
coenzyme A (CoA) to form acetyl-CoA. Acetyl-CoA enters
the Krebs cycle to produce ATP molecules.
Excess carbohydrates are converted into glycogen, a
large molecule that is stored in muscle and liver cells.
When the body requires energy, the glycogen can be
broken down into molecules of glucose to begin glycolysis.
Good sources of dietary carbohydrates include pasta,
rice, whole grains, and bread. Dieticians usually advise
choosing sources of complex carbohydrates (such as
brown rice or whole-wheat bread) over sources of refined
carbohydrates (such as sugar, white rice, or white bread)
whenever possible. Complex carbohydrates contain
vitamins and dietary fibre, which are missing from refined
carbohydrates.Fats
Fats belong to the group of substances known as
lipids. In the body, lipids are hydrolyzed into glycerol
and fatty acids.
Glycerol is converted to PGAL and then enters
glycolysis. The pyruvic acid produced can then enter
the Krebs cycle in the mitochondria to produce ATP.
Fatty acids may be stored as tissue fat or be
converted to a two-carbon fragment that can enter the
mitochondrion to form acetyl CoA. The acetyl CoA canthen also enter the Krebs cycle in the mitochondria to
produce ATP.
Sources of dietary fats include butter, cream, margarine,
and oil, and any food item prepared with these
ingredients. Fats are also found in meats and nuts. Health
professionals usually recommend choosing unsaturated
fats such as olive or canola oil over saturated fats such
as butter or margarine.Protein
Once in the body, protein is broken down into amino
acids. The amino acids contribute to the production of
cellular energy via several routes.
Amino acids can be converted into glucose or glycogen
to enter glycolysis, eventually entering the Kreb’s cycle
as pyruvic acid.
Amino acids can also be converted into lipids and
stored in the body’s fatty tissue.
Amino acids are used for numerous other functions within
the body, including the synthesis of new tissue proteins,
nucleic acids, hormones, and antibodies.
Dietary protein is found in meat, including red meat,
poultry, and seafoods. Protein is also found in eggs and
milk products. You can get protein from plants in the form
of beans, peas, and nuts. Cereals and pasta also contain
certain kinds of protein.Dietary carbohydrates, fats, and protein come from a wide
variety of foods.