Page 33 - 86 human physiology part-2

Chapter 13

modification to hydroxyproline. Uncommon elsewhere.

Similar to glutamic acid. Gln contains an amide group where Glu has a
Glutamine Q Gln
carboxyl. Used in proteins and as a storage for ammonia.
Arginine R Arg Functionally similar to lysine.
Serine and threonine have a short group ended with a hydroxyl group. Its
hydrogen is easy to remove, so serine and threonine often act as hydrogen
Serine S Ser
donors in enzymes. Both are very hydrophilic, therefore the outer regions of
soluble proteins tend to be rich with them.
Threonine T Thr Essential for humans. Behaves similarly to serine.
Essential for humans. Behaves similarly to isoleucine and leucine. See
Valine V Val
isoleucine.
Essential for humans. Behaves similarly to phenylalanine and tyrosine (see
Tryptophan W Trp
phenylalanine). Precursor of serotonin.
Behaves similarly to phenylalanine and tryptophan (see phenylalanine).
Tyrosine Y Tyr
Precursor of melanin, epinephrine, and thyroid hormones.

Dietary proteins fall into two categories: complete proteins and incomplete proteins. Complete proteins
include ample amounts of all essential amino acids. What I can eat that will include these great
complete proteins include meat, fish, poultry, cheese, eggs, and milk. Incomplete proteins contain some
but not all of the essential amino acids required by the human body. Examples of incomplete proteins
include legumes, rice, and leafy green vegetables. Someone who chooses a vegan lifestyle must be
careful to combine various plant proteins to obtain all the essential amino acids on a daily basis, but it
can be accomplished.

Ingested proteins are broken down into amino acids during digestion. They are then absorbed by
the villi of the small intestine and enter the blood stream. Our cells use these amino acids to assemble
new proteins that are used as enzymes, cell receptors, hormones, and structural features. Each protein
has its own unique amino acid sequence that is specified by the nucleotide sequence of the gene
encoding that protein (see Genetics and Inheritance). If we are deficient in even a single amino acid
then our cells cannot make the proteins they require.

Lipids

Macronutrient
Provide 9 Kcalories per gram; it is an energy-yielding nutrient

Functions are stored energy (adipose tissue), organ protection, temperature regulator, insulation
such as myelin that covers nerve cells, lipid membrane around cells, and emulsifiers to keep fats
dispersed in body fluids.

Lipids are made of organic molecules carbon, hydrogen, and oxygen. Fats consist of glycerol fatty
acids joind by an ester bond.

• A. Triglycerides composed of three fatty acids and one glycerol molecule.
• B. Saturated fatty acid fatty acid with carbon chains fully saturated with hydrogen.
• C. Monounsaturated fatty acid fatty acid that has a carbon chain with one unsaturated

248 | Human Physiology