(^28)
(^) Adenine (^)
(^) NH (^2)
3 Phosphate groups
(^) N (^) N
H^
O O O N^ N H (^)
O^2 P (^) O P O P O CH 2 O (^)
O^2 O^2 O^2 H H (^)
H H
®^
(^) OH OH
Lear ni ng^
Ribose sugar
(^)
Cen gage
(^)
Adenosine triphosphate (ATP) molecule
©^
Figure 2- 12 The structure of the adenosine
triphosphate (ATP) molecule.
MOVEMENT OF MATERIALS
INTO AND OUT OF CELLS
The plasma membrane of cells is a selectively perme-
able membrane. This means that only selected materials
are capable of getting into and out of cells. The chemi-cal
structure of the cell membrane is responsible for this
quality. The cell membrane is composed of an outer and
inner layer of protein with a double phospholipid layer in
between. This chemical arrangement allows water to pass
into and out of the cell with ease. However, water is not the
only material needed for the cell’s survival. Cells need food
like sugars, amino acids to make proteins, and nutrients like
the mineral salts. Materials pass through the cell’s
membrane in three different ways: diffusion, osmosis, and
active transport.
Diffusion
Diffusion is the movement of molecules through a me-
dium from an area of high concentration of those mole-
cules to an area of low concentration of those molecules.
As an example of diffusion, think of a closed perfume
bottle in a room. Within the stoppered bottle, perfume
molecules are in constant motion; they are in the liquid and
the gaseous state. Those in the gaseous state are in faster
motion than those in the liquid state. In the air of the room,
there are also molecules in motion such as water vapor,
oxygen, nitrogen, and carbon dioxide gas. When the
perfume bottle is opened, perfume molecules randomly
move out of the bottle and randomly bump or collide with
those other molecules in the air. The col-lisions are like
bumping billiard balls on a pool table. The random
collisions eventually bump the perfume
Chapter 2
molecules toward the walls of the room and eventually
throughout the room. If the perfume bottle is opened at one
end of the room and you are standing at the -opposite end
of the room, you would eventually smell the per-fume once
the molecules reached your end of the room. A person
standing near the perfume bottle when it was opened would
smell the perfume molecules before you did. The random
collisions of diffusing molecules are -referred to as
Brownian movement after Sir Robert Brown, an
English scientist who described this kind of movement in
1827.
Despite the randomness of these collisions, over time
there is a net displacement of perfume molecules from
areas of high concentration (on and near the perfume
bottle) to areas of low concentration (at the other end of the
room). This is diffusion. Eventually, the proportion of
perfume molecules being bumped back to the perfume
bottle will equal the proportion of perfume molecules being
bumped away from the bot-tle and the molecules will be
evenly spread through-out the room.
Temperature has an effect on diffusion. The higher the
temperature, the faster the movement. Think of a chunk of
ice. Low temperature keeps the molecules moving very
slowly, so the water is in a solid state. As temperature
increases, molecular motion increases and the water moves
to a liquid state. The ice melts. Continued heating, such as
putting a pot of water on a stove, increases molecular
motion even further so that the water becomes water vapor
and moves into the gaseous state.
An example of an important diffusion in the human
body is the uptake of oxygen by the blood in the lungs and
the release of carbon dioxide gas to the lungs from the
blood. Blood returning to the lungs is low in oxygen but
high in carbon dioxide gas as a result of cellular respi-
ration. When we breathe in air, we take in oxygen gas, so
the lungs have lots of oxygen but little carbon dioxide gas.
The oxygen moves from an area of high concentration (the
lungs) to an area of low concentration (the blood) by
diffusion. Similarly, the carbon dioxide gas moves from an
area of higher concentration (the blood) to an area of low
concentration (the lungs) by diffusion. We exhale to get rid
of the carbon dioxide gas now in the lungs.
Osmosis
Osmosis (oz-MOH-sis) is a special kind of diffusion.
Osmosis- pertains only to the movement of water mol-
ecules through a selectively permeable membrane
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