Through the tunnel – like needle of a syringe, you fall suddenly and uncontrollably enter in a blood vessel in the arm of a volunteer. You continuously float together with large, bouncy, and elastic red blood cells until you notice that the liquid turns to yellow in color. You know that you have reached already the blood plasma, so you switch on your headlamp and observe the cells of the epithelial tissues that line the wall of the blood vessel. Their cell membranes seem to be made of millions of small balloons. These are the hydrophilic heads of the (1) __________ molecules that make up most of the membrane surface. Through the transparent surface, you can see their flexible, (2)__________ tails projecting inward toward the interior of the cell membrane, and beyond them an inner layer of (3)__________ molecules with their tails pointing toward you.
Through the tunnel – like needle of a syringe, you fall suddenly and uncontrollably
enter in a blood vessel in the arm of a volunteer. You continuously float together with
large, bouncy, and elastic red blood cells until you notice that the liquid turns to
yellow in color. You know that you have reached already the blood plasma, so you
switch on your headlamp and observe the cells of the epithelial tissues that line the
wall of the blood vessel. Their cell membranes seem to be made of millions of small
balloons. These are the hydrophilic heads of the (1) __________ molecules that make
up most of the membrane surface. Through the transparent surface, you can see
their flexible, (2)__________ tails projecting inward toward the interior of the cell
membrane, and beyond them an inner layer of (3)__________ molecules with their
tails pointing toward you. Here, there are (4)__________ proteins embedded on the cell
membrane; some rest lightly on the surface, but most project all the way into the
interior of the cell. The cell membrane is indeed a (5)__________ mosaic, the proteins
are embedded like the pieces of a picture, but you can see that they are free to move
around. You push on one of the proteins, and it bobs like an iceberg. Some of the
phospholipids and proteins have (6)__________ attached to them which serve as an
effective interaction with the aqueous environment that surrounds the cell. You
notice that one of the proteins has a dimple in its surface. Just then a minute, plump
molecule floating in the blood plasma installs in a depression of a protein. The
molecule is a hormone, a chemical signal, and the dimpled protein is the
(7)__________ that enables the cell to respond to it.
In your light beam, you can see the sparkle and shimmer of many molecules, large
and small, in the blood and pass through the cell membrane. You see that the
transparent gas of oxygen is moving from the plasma and enters the cell interior.
This movement is (8)__________; which it occurs through biological membrane, it is
called (9)__________ transport. Similarly, carbon dioxide is flowing out of the cell,
down its (10)__________ gradient, from the cell interior, where it is (11)__________
concentrated, to the blood, where it is (12)__________ concentrated.
You note that water molecules are passing through the cell membrane equally in
both directions. The total concentration of solutes in the cell and in the blood must
18
be equal; the solution must be (13)__________. You signal the control team to inject a
small amount of concentrated salt solution into the blood, making the blood slightly
(14)__________ relative to the cell contents. This causes water to flow (15)__________
the cell, until the two solutions are again in equilibrium. This diffusion of water
through a (16)__________ permeable membrane is called (17)__________.
Some sugar molecules floating in the blood are simply too large to pass easily through
gaps in the cell membrane like the much smaller water molecules can. These sugar
molecules slowly glide past on the cell membrane and pass through pores in special
(18)__________ proteins. This is a type of passive transport, because the molecules
move down a concentration gradient without the expenditure of (19)__________.
Because transport proteins help out, it is called (20)__________ diffusion.
Your chemscanner detects that the cell interior is concentrated with potassium ions.
Transport proteins here and there in the membrane are able to move potassium ions
into the cell against the concentration gradient. This must be (21)__________
transport; the cell expends (22)__________ to provide energy to “pump” the potassium
ions into the cell.
Suddenly there is a tug at your foot. You look down to see your flipper engulfed by a
rippling membrane. A leukocyte the size of a building quickly holds you against the
wall of the blood vessel. The phospholipids of its cell membrane are pressed against
your face mask. The cell is engulfing you, protecting the body from a foreign invader!
Taking in a substance in this way is called (23)__________, more specifically
(24)__________, if the substance is a solid particle. Suddenly the pressure diminishes,
and you are inside the leukocyte, floating free in a membrane – enclosed bag, or
(25)__________. Another sac is approaching; it is a (26)__________, full of digestive
enzymes. You manage to get your legs outside of the vacuole and move it back toward
the inner surface of the cell membrane. As the vacuole joins with the cell membrane,
you pull your feet freely and you glide away from the impatient cell, realizing
that(27)__________ released you as fast as endocytosis engulfed you!
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