Structure and Composition of Cell Membrane
Despite differences in structure and function, all living cells in multicellular organisms are surrounded by a cell membrane. Just like the outer layer of the skin separates the body from its environment similarly, the cell membrane, also known as 'plasma membrane,' separates the inner content from its exterior environment.
Cell Membrane
The cell membrane is known by different names like plasma membrane or cytoplasmic membrane, or biological membrane. The term "cell membrane" was first introduced by C. Nageli and C. Cramer in the year 1855. Later on, in 1931, the term "plasmalemma" for cell membrane was given by J. Plowe. The cell membrane separates the cell's internal environment from the extracellular space. This separation allows the protection of cells from their environment.
Prokaryotes vs Eukaryotes
The cell is defined as the basic structural and functional unit of life. The cell membrane bounds it. It is capable of independent existence.
The movements of single motor-protein molecules
can be analyzed directly. Using polarized laser light, it is
possible to create interference patterns that exert a cen-
trally directed force, ranging from zero at the center to a
few piconewtons at the periphery (about 200 nm from the
center). Individual molecules that enter the interference
pattern are rapidly pushed to the center, allowing them to
be captured and moved at the experimenter’s discretion.
Using such “optical tweezers,” single kinesin mol-
ecules can be positioned on a microtubule that is fixed to
a coverslip. Although a single kinesin molecule cannot
be seen optically, it can be tagged with a silica bead and
tracked indirectly by following the bead (Figure Q16–3A).
In the absence of ATP, the kinesin molecule remains at the
center of the interference pattern, but with ATP it moves
toward the plus end of the microtubule. As kinesin moves
along the microtubule, it encounters the force of the inter-
ference pattern, which simulates the load kinesin carries
during its actual function in the cell. Moreover, the pres-
sure against the silica bead counters the effects of Brown-
ian (thermal) motion, so that the position of the bead more
accurately reflects the position of the kinesin molecule on
the microtubule.
A trace of the movements of a kinesin molecule
along a microtubule is shown in Figure Q16–3B.
A. As shown in Figure Q16–3B, all movement of kine-
sin is in one direction (toward the plus end of the micro-
tubule). What supplies the free energy needed to ensure a
unidirectional movement along the microtubule?
B. What is the average rate of movement of kinesin
along the microtubule?
C. What is the length of each step that a kinesin takes
as it moves along a microtubule?
D. From other studies it is known that kinesin has two
globular domains that can each bind to β-tubulin, and that
kinesin moves along a single protofilament in a microtu-
bule. In each protofilament, the β-tubulin subunit repeats
at 8-nm intervals. Given the step length and the interval
between β-tubulin subunits, how do you suppose a kine-
sin molecule moves along a microtubule?
E. Is there anything in the data in Figure Q16–3B that
tells you how many ATP molecules are hydrolyzed per
step?
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