Biology Now, 2e

66 ■ CHAPTER 04 Life Is Cellular

CELLS

more concentrated, and osmotic movement of

water out of the cell then restores the concentra-

tion of water in the cell to its correct level.

You can imagine, then, that the concentration

of solutes within a cell in relation to the concen-

tration outside the cell is of critical importance.

When cells are surrounded by fluid with the

same solute concentration as the cell interior,

the extracellular and intracellular environ-

ments are said to be isotonic to each other (iso,

“equal”). If the extracellular environment has a

higher solute concentration, it is hypertonic to

the cell interior (hyper, “more”). If it has a lower

solute concentration than the cell’s interior, it is

said to be hypotonic to it (hypo, “less”).

The careful balance of concentrations is

particularly vital in human blood. Red blood

cells are typically in an isotonic solution in

our bloodstream, where the solute concentra-

tions inside and outside the cells are the same.

Doctors and nurses are taught to administer

an IV drip of saline, a solution of salt in water,

to dehydrated patients. If a dehydrated patient

was administered water instead of a saline solu-

tion, the water would dilute the patient’s blood,

making it hypotonic. Osmosis would then occur,

causing water to rapidly diffuse into red blood

cells to the point where the cells could burst

and die.

Most hydrophobic molecules, even fairly

large ones, can pass through the plasma

membrane via simple diffusion because they

mix readily with the hydrophobic tails that

form the core of the phospholipid bilayer. But

hydrophilic substances such as sodium ions

(Na+), hydrogen ions (H+), and larger molecules,

including sugars and amino acids, cannot cross

the plasma membrane without assistance.

These substances move across the plasma

membrane by facilitated diffusion, a type

of passive transport that requires transport

proteins (Figure 4.4, right).

Devaraj’s artificial membrane does not

currently contain any transport proteins, he

says, so it is impermeable to large hydrophilic

molecules. But small molecules such as water

can pass through his membrane via simple

diffusion.

The plasma membrane also contains recep-

tor proteins, which are sites where molecules

released by other cells can bind. The binding of

a molecule to a receptor protein starts a chain of

molecules immediately move by osmosis—that is,

they diffuse—across the plasma membrane into

the cell until the concentration of water inside

the cell is the same as the concentration on the

outside. On the other hand, when salt molecules

move out of a cell, the water molecules become

Y

ou’ve heard their

names: Ebola

virus, Zika virus,

H5N1, dengue virus.

Viruses—microscopic,

noncellular infectious

particles—are

perhaps the smallest

biological agents with

the greatest impact

on human health.

Like living organisms,

viruses reproduce

and evolve, yet they

lack some of the key characteristics of

life—which is why most scientists today

regard viruses as nonliving. For one

thing, viruses are not made up of cells. A

virus is much simpler than a cell, usually

consisting of a small piece of genetic material (for example, DNA) that

is wrapped in a protein coat. Some viruses also have an envelope, a

lipid layer usually stolen from a cell’s plasma membrane, enclosing the

central core of genetic material and protein.

Another difference, compared to living organisms, is that viruses

lack the many structures within cells that are necessary for critical

cellular functions such as homeostasis, autonomous reproduction, and

metabolism. To gain these functions, they become “body snatchers”:

Viruses use their genetic material to make the cells of the organisms

they infect do their work for them. They accomplish this feat by

invading cells, releasing their genetic material into the cell interior, and

“hijacking” the host cell’s machinery. Viruses multiply to huge numbers,

and viral offspring escape from a host cell either by causing it to burst

open or by budding off from the cell, wrapped in a layer of the host cell’s

plasma membrane.

Uniquely, unlike the case with living organisms, the genetic material

that viruses pass from one generation to the next is not always DNA;

sometimes it is RNA. Viruses are generally classified by the type of

genetic material they possess (type of DNA or RNA molecule), their

shape and structure, the type of organism (host) they infect, and the

disease they produce. The variant forms of a particular type of virus

are called viral strains or serotypes. Viruses evolve new strains within a

host so quickly that sometimes an antiviral drug or vaccine developed

to fight an older strain becomes useless against a new strain.

Viruses—Living or Not?

RNA

Protein

coat

Envelope