Essentials of Anatomy and Physiology

pumping of the heart. Filtration occurs when blood
flows through capillaries, whose walls are only one
cell thick and very permeable. The blood pressure in
capillaries is higher than the pressure of the surround-
ing tissue fluid. In capillaries throughout the body,
blood pressure forces plasma (water) and dissolved
materials through the capillary membranes into the
surrounding tissue spaces (see Fig. 3–3). This cre-
ates more tissue fluid and is how cells receive glu-
cose, amino acids, and other nutrients. Blood pressure
in the capillaries of the kidneys also brings about
filtration, which is the first step in the formation of
urine.

PHAGOCYTOSIS AND PINOCYTOSIS

These two processes are similar in that both involve a
cell engulfing something, and both are forms of endo-
cytosis, endomeaning “to take into” a cell. An exam-
ple of phagocytosisis a white blood cell engulfing
bacteria. The white blood cell flows around the bac-
terium (see Fig. 3–3), taking it in and eventually
digesting it. Digestion is accomplished by the enzymes
in the cell’s lysosomes.
Other cells that are stationary may take in small
molecules that become adsorbed or attached to their
membranes. The cells of the kidney tubules reabsorb
small proteins by pinocytosis(see Fig. 3–3) so that
the protein is not lost in urine.
Table 3–2 summarizes the cellular transport mech-
anisms.

THE GENETIC CODE

AND PROTEIN SYNTHESIS

The structure of DNA, RNA, and protein was

described in Chapter 2. We will review some of the

essentials here, and go a step further with a simple

description of how all of these organic molecules are

involved in the process of protein synthesis.

DNA AND THE GENETIC CODE

DNAis a double strand of nucleotides in the form of

a double helix, very much like a spiral ladder. The

uprights of the ladder are made of alternating phos-

phate groups and deoxyribose sugar molecules. The

rungs of the ladder are made of the four nitrogenous

bases, always found in complementary pairs: adenine

with thymine (A–T) and guanine with cytosine (G–C).

Although DNA contains just these four bases, the

bases may be arranged in many different sequences

(reading up or down the ladder). It is the sequence of

bases, the sequence of A, T, C, and G, that is the

genetic code. The DNA of our 46 chromosomes may

also be called our genome, which is the term for the

total genetic information in a particular species. The

human genome is believed to contain about 3 billion

base pairs, and the number of our genes is now esti-

mated to be between 20,000 and 25,000 (or perhaps as

many as 30,000, but much lower than previously

thought).

56 Cells

Table 3–3 PROTEIN SYNTHESIS

Molecule or Organelle Function

DNA

mRNA (messenger RNA)

Ribosomes

tRNA (transfer RNA)

• A double strand (helix) of nucleotides that is the genetic code in the chromosomes
  of cells.


• A gene is the sequence of bases (segment of DNA) that is the code for one protein.


• A single strand of nucleotides formed as a complementary copy of a gene in the DNA.


• Now contains the triplet code: three bases is the code for one amino acid (a codon).


• Leaves the DNA in the nucleus, enters the cytoplasm of the cell, and becomes
  attached to ribosomes.


• The cell organelles that are the site of protein synthesis.


• Attach the mRNA molecule.


• Contain enzymes to form peptide bonds between amino acids.


• Picks up amino acids (from food) in the cytoplasm and transports them to their proper
  sites (triplets) along the mRNA molecule; has anticodons to match mRNA codons.