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an interesting property; if it “discharges” in one place, i.e., if some oŸ the ions
were able to move through one place, so that the electric voltage is reduced there,
that electrical inÑluence makes itself felt on the ions in the neighborhood, and it
affects the membrane in such a way that it lets the ions through at neighboring
points also. 'Phis in turn affects 1t farther along, etc., and so there is a wave of
“benetrability” of the membrane which runs down the fñber when it is “excited”
at one end by stepping on the sharp stone. PThis wave is somewhat analogous
to a long sequence of vertical dominoes; 1ƒ the end one 1s pushed over, that one
pushes the next, etc. Of course this will transmit only one message unless the
--- Trang 72 ---
dominoes are set up again; and similarly in the nerve cell, there are processes
which pump the ions slowly out again, to get the nerve ready for the next impulse.
So it is that we know what we are doïng (or at least where we are). Of course
the electrical efects associated with this nerve impulse can be picked up with
electrical instruments, and because there are electrical efects, obviously the
physics of electrical effects has had a great deal of inÑuence on understanding
the phenomenon.
The opposite efect is that, from somewhere in the brain, a message is sent
out along a nerve. What happens at the end of the nerve? “There the nerve
branches out into fñne little things, connected to a structure near a musele, called
an endplate. Eor reasons which are not exactly understood, when the impulse
reaches the end of the nerve, little packets of a chemical called acetylcholine are
shot of (five or ten molecules at a time) and they affect the muscle fiber and make
1b contract—how simplel What makes a muscle contract? A muscle is a very
large number of Ñbers close together, containing two diÑferent substances, myosin
and actomyosin, but the machinery by which the chemical reaction induced by
acetylcholine can modify the dimensions of the muscle is not yet known. Thus
the fundamental processes 1n the muscle that make mechanical motions are not
known.
Biology is such an enormously wide field that there are hosts of other problems
that we cannot mention at all—problems on how vision works (what the light
does in the eye), how hearing works, etc. (The way in which £h#nking works we
shall discuss later under psychology.) NÑow, these things concerning biology which
we have just discussed are, from a biological standpoint, really not fundamental,
at the bottom of life, in the sense that even 1ƒ we understood them we still would
not understand life itself. 'To illustrate: the men who study nerves feel their work
1s very important, because after all you cannot have animals without nerves. But
you cơn have ljƒe without nerves. Plants have neither nerves nor muscles, but
they are working, they are alive, just the same. 5o for the fundamental problerms
of biology we must look deeper; when we do, we discover that all living things
have a great many characteristics in common. “The most common feature is that
they are made of celis, within each of which is complex machinery for doïng
things chemically. In plant cells, for example, there is machinery for picking up
light and generating glucose, which is consumed in the dark to keep the plant
alive. When the plant ¡is eaten the glucose itself generates in the animal a series
of chemical reactions very closely related to photosynthesis (and its opposite
effect in the dark) in plants.
--- Trang 73 ---
In the cells of living systems there are many elaborate chemical reactions,
in which one compound is changed into another and another. To give some
impression of the enormous eforts that have gone into the study of biochemistry,
the chart in Fig. 3-1 summarizes our knowledge to date on just one small part of
the many series of reactlons which occur in cells, perhaps a percent or so OÝ ïf.
Here we see a whole series of molecules which change from one to another in a
sequence or cycle of rather small steps. It is called the Krebs cycle, the respiratory
cycle. Each of the chemicals and each of the steps is fairly simple, in terms of
what change ¡is made in the molecule, but——and this is a centrally important
discovery in biochemistry—these changes are relaiiuel djficult to accomplish
ứn a laboratoru. TIf we have one substance and another very similar substance,
the one does not just turn into the other, because the bwo forms are usually
separated by an energy barrier or “hill” Consider this analogy: If we wanted
to take an object from one place to another, at the same level but on the other
side of a hill, we could push it over the top, but to do so requires the addition of
some energy. Thus most chemical reactions do not occur, because there is what
acetyl coenzyme A
S61~mmseTT—s Hạ-COO-
!Ổ HỌC COO.
:COO-;? CoA-SH Ha-COO~
Ox2loaeetate ciate no
ooˆ^ Oa,, DPNH+H
Ha DPN TC Ha-COO—~
H-C-ÔH Đo, -COO-
SQ- 4% H-COO-
L-malate cis-aconitate
H:O<‡ FUMARASE ACONITASE Yrmo
hệ ll›-COO-
hếu : CITRIC ACID CYCLE KH cSo:
! COO-? HO-CH-COO~
`" fimarate đ-isocitrate
Fe† Tflavin A. ped0BoetuosE TPNỶ
OO= TPNH+H+
FeT Tflavin GP “« Hz-COO~
H5 sọ chócöo”
SO- nã Oz¿-COO“
Succinate _ &“ oxalosuccinate
cm V CoA-SH _— Ố +
mẻ. '..ˆn
HOPO, Ở hy h2 ¡ |ThPP,LAŠ Hệ ¡
(DP);O_°_€ CoA! ci
succinyl coenzyme A ĐPNH+H+ ¬= tả Retöglutarate
Fig. 3-1. The Krebs cycle.
--- Trang 74 ---
1s called an øcfoalion energu in the way. In order to add an extra atom tO Our
chemical requires that we get it close enough that some rearrangement can OCCUT;
then it will stick. But if we cannot give it enough energy to get it close enough,
it will no go to completion, ¡i% will jusE go part way up the “hill” and back down
again. However, ¡Ÿ we could literally take the molecules in our hands and push
and pull the atoms around ïn such a way as to open a hole to let the new atom
in, and then let it snap back, we would have found another way, around the hill,
which would not require extra energy, and the reaction would go easily. Now