There are several types of lipoproteins. Chylomicrons
are made in the small intestine and transport dietary lipids to
adipose cells for storage (see Figure 14.8). Very-low density
lipoproteins (VLDLs) transport triglycerides from liver cells
to adipose cells for storage. After After depositing some tri-
glycerides, VLDLs become LDLs. Low-density lipoproteins
(LDLs) carry 75% of the cholesterol in blood and deliver it to
cells throughout the body. LDL is known as “bad” cholesterol
because it can lead to fatty plaques in blood vessels. Blood
LDL levels below 130mg/dL are desirable. High-density
lipoprotein (HDL) carries excess cholesterol from body
cells to liver cells for disposal. HDL is considered “good”
cholesterol and blood levels over 40 mg/dL are desirable.
Now that we have seen what happens to lipids, let’s
take a closer look at protein catabolism.
Proteins Can Also Be
Metabolized to Create ATP
During digestion, proteins are broken down into amino
acids, which are absorbed in the small intestine. Unlike
carbohydrates and lipids, amino acids cannot be stored.
Instead, they are used to make new proteins (see Chapter 3),
converted to glucose or triglycerides, or catabolized to
make ATP. Furthermore, when the new proteins wear
out, cells degrade them to amino acids, which are either
metabolized or recycled to make new proteins. Cells take
up amino acids when stimulated by insulin-like growth fac-
tor (IGF) and insulin. During amino acid catabolism, the
amino group is removed first (via deamination), yielding
highly toxic ammonia; the liver converts ammonia to urea,
which is excreted into the urine. The remaining organic
component is some intermediate in cellular respiration,
mostly Krebs cycle components . These Krebs cycle inter-
mediates are then metabolized completely to make ATP.
Some amino acids can be converted into glucose, fatty
acids, or ketone bodies (see Figures 14.13 and 14.14).
Hormones and Chemical Levels
Regulate Metabolic Activities
If carbohydrates, lipids, and proteins can be broken down,
stored, or interconverted, how does the cell know what to
do? Regulation of metabolism is based on the levels of ATP
in the cell and is controlled by the enzymatic reactions of
certain key metabolic products:
Glucose-6-phosphate (G6P). The cell traps glucose
inside by transferring a phosphate from ATP to glucose.
G6P can be metabolized to form ATP or converted into
other types of molecules needed by the body, depending
on cell ATP level and blood glucose levels.
P a t h w a y s f o r m a k i n g a n d b r e a k i n g d o w n lip id s •
F ig u r e 1 4 . 1 4
Nutrients Are Metabolized in a Number of Ways 421