Biomedical Engineering Reference
In-Depth Information
oligosaccharides (e.g., lactose, sucrose, maltose), and polysaccharides (e.g., glycogen). Lipids
are greasy or oily compounds that will dissolve in each other but not in water. They form
structural materials in cells and are the main reservoirs of stored energy. Proteins are the
most diverse form of biological molecules. Specialized proteins, called enzymes, make meta-
bolic reactions proceed at a faster rate than would occur if the enzymes were not available
and enable cells to produce the organic compounds of life. Other proteins provide structural
elements in the body, act as transport channels across plasma membranes, function as signals
for changing activities, and provide chemical weapons against disease-carrying bacteria.
These diverse proteins are built from a small number (20) of essential amino acids.
Nucleotides and nucleic acids make up the last category of important biological mole-
cules. Nucleotides are small organic compounds that contain a five-carbon sugar (ribose
or deoxyribose), a phosphate group, and a nitrogen-containing base that has a single or
double carbon ring structure. Adenosine triphosphate (ATP) is the energy currency of the
cell and plays a central role in metabolism. Other nucleotides are subunits of coenzymes
that are enzyme helpers. The two nucleic acids are deoxyribonucleic acid (DNA) and ribo-
nucleic acid (RNA). DNA (Figure 3.3) is a unique, helical molecule that contains chains of
paired nucleotides that run in opposite directions. Each nucleotide contains either a pyrim-
idine base—thymine (T) or cytosine (C)—with a single ring structure or a purine base—
adenine (A) or guanine (G)—with a double ring. In the double helix of DNA, thymine
always pairs with adenine (T-A) and cytosine always pairs with guanine (C-G). RNA is
similar to DNA except that it consists of a single helical strand, contains ribose instead of
deoxyribose, and has uracil (U) instead of thymine.
All cells are surrounded by a plasma membrane that separates, but does not isolate, the cell's
interior from its environment. Animal cells, such as those found in humans, are eukaryotic cells.
A generalized animal cell is shown in Figure 3.4. In addition to the plasma membrane, eukary-
otic cells contain membrane-bound organelles and a membrane-bound nucleus. Prokaryotic
cells, such as bacteria, lack membrane-bound structures other than the plasma membrane.
In addition to a plasma membrane, all cells have a region that contains DNA (which carries
the hereditary instructions for the cell) and cytoplasm (which is a semifluid substance that
includes everything inside the plasma membrane except for the DNA).
(a) (b)
FIGURE 3.3
(a) DNA consists of two chains of paired nucleotides that run in opposite directions and form a
helical structure. (b) Thymine pairs with adenine (T-A) and cytosine pairs with guanine (C-G) due to hydrogen
bonding between the bases.
