Copyright
W. T. (William Thompson) Sedgwick.

An introduction to general biology online

. (page 4 of 20)
Online LibraryW. T. (William Thompson) SedgwickAn introduction to general biology → online text (page 4 of 20)
Font size
QR-code for this ebook


bulk, to change its form, the two ends being brought nearer

together (Fig. 16). The visible change
of form is here supposed to be due to an
invisible change of molecular arrange-
ment, and this in turn to be coincident
with chemical action taking place in the
living substance.

A striking and beautiful example
of movement in protoplasm occurs in
B the simple organism known as Amceha
(Fig. S-I, p. 159). The entire body of
this animal consists of a mass of naked
protoplasm enclosing a nucleus, or
sometimes two ; in other words, it is a

FiG.76.-change of form in a single naked Cell. The protoplasm of
contracting muscle. A, mus- qx\ active Amocha is iu a statc of cease-

cle in the ordinary or extend- . ,.

ed state; B, the same muscle less movement, contractmg, expanding,
when contracted. (Diagram.) fl^^^jj^g^ ^nd changing the fomi of the

animal to such an extent that it is known as the '' Proteus''
animalcule. The whole movement is a kind of Hux. A })ortion
of the protoplasm flows out from the mass, making one or more
prolongations {pseudopods) into which the remainder of the
protoplasm finally passes, so that the whole body advances in the




A



28



PROTOPLASM AND THE CELL.



direction of the flow. If particles of food be met with, the

protoplasm flows around them, and when thej have been digested

within tlie body, the protoplasm flows onward, leaving the refuse

behind. Hour after hour and day after day this flowing may

go on, and there is perhaps no

more fascinating and suggestive

spectacle known to the biologist.

A similar change of form is ex-

liibited by the colorless corjDuscles

of amphibian and other blood, in

which it may be observed, though

far less satisfactorily, if Afnoehce

cannot be obtained. Among plants,

protoplasmic movements of perhaps

equal beauty may be observed.

One of the simplest is known as the

rotation of protoplasm, wliich may





n-



'?,»•■*■ •••.,'ov .•.-.'<:"- <J ■■:■■*. 0-




Fig. 17.— a cell of a stonewort (NiteU
la) showing the rotation of proto-
plasm; the arrows show the direc-
tion of the flow, m, membrane of
the cell; n, nucleus, opposite to
which is a second ; p, protoplasm ; v,
large central vacuole filled with sap.



i






5>~ .:-;.■ .-V^^i.^: •,'^:f>



r




Fig. 17a.— Two cells and a part of a
third from the tip of a "leaf" of a
stonewort, showing rotation of the
protoplasm in the direction of the
arrows.



be studied to advantage in rather young cells of stoneworts {Cliara
or Nitella). These cells have the form of short or elongated
cylinders which are often pointed at one end (Fig. 17). The



PROTOPLASMIC MO VEMENTS.



29



protoplasm is surrounded by a delicate membrane which thus
forms a sac enclosing the protoplasm. In very young cells the
protoplasm entirely tills the sac ; but as the cell grows older a
drop of liquid appears near the centre of the mass and increases
in size until the protoplasm is reduced to a thin layer {jyinmor-
dial utricle)^ lining the inner surface of the memljrane (compare
Fig. 2). In favorable cases the entire mass of protoplasm is
seen to be flowing steadily around the inside of the sac, as in-
dicated by the arrows in Fig. IT. It moves upwards on one
side, downwards on the opposite side, and in 02)posite directions
across the ends, forming, an unbroken circuit. The flow is ren-
dered more conspicuous by various granules and other lifeless
masses floating in the protoplasm and by the large oval nucleus
or nuclei, all of which are swept onward by the current in its
ceaseless round. A similar rotation of protoplasm occurs in many
other vegetal cells, one of the best examples being the leaf-cells
of Anacharis.

A second and somewhat more intricate kind of movement in
vegetal protoplasm is known as circulation. This differs from
rotation chiefly in the fact that the protoplasm travels not only in
a peripheral stream but also in strands which run across through
the central space (vacuole) and thus form a loose network. Cir-




Fio. 18.— Flower-cluster {a) and single stamen (h) of a cultivated spidervrort {Trades'
cantia). 7i, hairs upon the stamen, a, slightly reduced ; b, slightly enlarged

dilation is well seen in cells composing the hairs of various ])lants,
such as the common nettle (Urtica), the spiderwort (Trades-



30



PROTOPLASM AND THE CELL,



cantia)^ tlie liolljliock {Altlima)^ and certain sj^ecies of gourds
{Cucxii'ljitci). It may be conveniently studied in the liairs upon
the stamens of the cultivated spiderwort {Tradescantia). The
fiower of this plant is shown in Fig. 18, «, and one of the
Btamens with its hairs at h. Each hair consists of a single row

A




1^.



;.\^







c ' • i; ■;'':■*! •■- ••■• .••"-••"<■ • • *1

fl-lI;f;|■:;■!•^?■•^



iifl^^vK^^



'.^^<t^?=«,,o^



^















^> '^ ■•:■■■-:*/•-■■ ^ • ••> ;->'6-. 6»>i-r
■ » ". *i \ .V ;''.,; ^ ^ ■ / *. N ft: '*■ **> ■ / /T

.^ . « - w i* '•••.&."& -■■',/ '. ■• •*, * P ^ ' »



v^ v-:-; >->-,;^\',i:^,..^?



Fig. 19.— Enlarged cells of the hairs from the stamens of the spiderwort. A, five
cells, somewhat enlarged, protoplasm not shown ; B and G, cells much more en-
larged, showing the circulation of protoplasm as indicated by the arrows ; ?i,
nucleus.

of elongated cells covered by delicate membranes and connected
by their ends. As in Nitella^ the protoplasm does not fill the
cavity of the sac, but forms a thin lining {jprimordial utricle)



CILIARY ACTION.



81



on its inner face (Fig. 19). From this laj'er delicate tlirea(l< of
protoplasm reach into and pass tlirough tlie central cavity, where
they often branch and are connected together so as to tonii a
very loose network. The nuclens {n) is embedded eitln'i- in the
peripheral layer or at some point in the network, and the threads
of the latter always converge more or less regularly to it. In
active cells currents continually flow to and fro throughcjut tiie
whole mass of protoplasm. In the threads of the netwoi-k gran-
ules are borne rapidly along, gliding now in one direction, now
in another; and although the flow^ is usually in one direction in
any particular thread, no system can be discovered in the com-
plicated movements of the whole. In the larger threads the
curious spectacle often appears of two rapid currents flowing m
opposite directions on opposite sides of the same thread. The
currents in the thread may be seen to join currents of the pe-
ripheral layer which flow here and there, but without sthe regu-
larity observed in the protoplasm of Nitella. The protoplasmic
network also, as a wdiole, undergoes a slow but steady change of
form, its delicate strands slowly ,^^i^m ^ gifc
swaying hither and thither, while
the nucleus travels slowly from
point to point.

Finally, we may consider an
example of a form of protoplas-
mic movement known as ciliary
action, which plays an important
role in our own lives and those
of low^er animals and of some
plants. The interior of the tra-
chea, or windpipe, is lined by
cells having the form shown in
Fiff. 20. At the free surface of
the cell (turned towards the cavi- ^^^ ^ ^^^^J^ Kiein.)-Three isolated

ty of the trachea) the l)rotoplasm ciliated cells from the interior <.f the
/ - 1 • 1 T X -1 windpipe of the cat. r, the cilia at the

is produced into delicate Vlbra- free end; », the nucleus: />, the proto-

tory filaments having a sickle- pi^^m. (Hitrhiy magnified.)
shape when bent; these are known as cilia {cilium, an eyelash).
They are so small and lash so vigorously as to be nearly, or quite
invisible until the movements are in some way made sluggish.




32 PROTOPLASM AND THE CELL.

The movement is then seen to be more rapid and vigorous in one
■direction than in the other, all the cilia working together like
the oars of a row-boat acting in concerted motion. By this
action a definite current is produced in the surrounding medium
(in this case the mucus of the trachea) flowing in the direction
of the more vigorous movement. In the trachea this movement
is upwards towards the mouth, and mucus, dust, etc. , are thus
removed from the lungs and windpipe. In many lower animals
and plants, especially in the embryonic state, cilia are used as
organs of locomotion, serving as oars to drive the organism
through the water. The male reproductive germs of plants and
animals are also propelled in a similar fashion.

In all these forms of vital action the protoplasm is visibly at
work. In most cases, however, no movements of tlie protoplasm
in cells can be detected. But it is certain from indirect evidence
that protoplasm is no less active in those modes of physiological
action that give no visible outward sign, as for example in an
active nerve-cell or a secreting cell. This activity being molec-
ular and chemical is beyond the reach of the microscope, but it
is none the less real ; and the play of these invisible molecular
actions is doubtless far more tumultuous and complicated tlian the
visible movements of the jDrotoplasmic mass displayed in Nitella
or in a nettle-hair. It is of the utmost importance that the stu-
dent should attain to a full and vivid sense of the reality and
energy of this invisible activity even in protoplasm which (as is
ordinarily the case) under the closest scrutiny appears to be abso-
lutely quiescent.

The Sources of Protoplasmic Energy. Whence comes the
power required for protoplasmic action, and how is it expended?
The answer to this question can be given at this point only in
very general terms. It is certain that protoplasm works by
means of chemical actions taking place in its own substance;
and it is further certain that these actions are, broadly speaking,
processes of oxidation or combustion; for in the long run all
forms of protoplasmic action involve the taking up of oxygen
and the liberation of carbon dioxide. Energy is therefore set
iree in living, active protoplasm somewhat as it is in the com-
bustion of fuel under the boiler of a steam-engine, and in this
process the protoplasm, like the coal, is gradually used up, disin-



CHEMICAL RELATIONS OF PROTOPLASM. 83

tegrates, and wastes away, giving oft' as waste matter tlie various
clieniical products of the combustion, and lil)eratin<r eneri^v as
lieat and meclianical work. Tlie loss of substance is, liowever
continually made good (much as the coal is replenished) 1)V the
absorption of new substance in the form of food, wliifli mav
consist of actual protoplasm, derived from other living bein<'-s,
or of substances convertible into it. These substances are in
some unexplained way converted into protu])lasni and tlius
built into the living fabric.

To this dual process of waste (" /I'rt?^*^^^*??/.") and repair
{'''• anahoUsm^^) is applied the term metaholism^ which must be
considered as the most characteristic and fundamental propertv
of living matter. It is evident from the foregoing that meta-
bolism involves on the one hand a destructive action {katahol-
isiix) through whicli protoplasm disintegrates and energy is set
free, and on the other hand a constructive action (anaholinn)
whereby new proto23lasm is built up from the income of food and
fresh energy is stored. It is a most remarkable fact that as far
as known the constructive action resulting in the formation of
new protoplasm never takes place except through the inmiediate
agency of protoplasm already existing. In otlier words, there is
no evidence that "spontaneous generation" or the production
of living from lifeless matter without the intluence of antecedent
life ever takes place. Nor is there any evidence that any energy
can be "generated," but rather that the vital energy of living
things is only the transformed energy of their food, and that
"vital force" having an origin elsewhere than in such energy
does not exist.

Chemical Relations. We know nothing of the precise chemi-
cal composition of living protoplasm, because, as has been said
(p. 2), living protoplasm cannot be subjected to chemical analy-
sis without destroying its life. But the results of chemical ex-
aminations leave no doubt that the molecules of protoplasm are
highly complex and are probably separated from one anotlier by
layers of water.

A. Proteids. It has already been stated (}>. 3) that tlie
characteristic products of the analysis of protoplasm are the
group of closely -related substances known sls proteids. But pre^-
teids form only a small part of the total weight of any plant or



34



PROTOPLASM AND TUE CELL.



animal, being always associated with quantities of other sub-
stances. Even the white of an Qgg^ which is usually taken for
a typical proteid, contains only twelve per cent of actual proteid
matter, the remainder consisting cliielly of water. The follow-
ing table shows the j^ercentage of proteids and other matters in
a few familiar organisms and their products :



PROXIMATE PERCENTAGE COMPOSITION OF SOME COMMON

SUBSTANCES.*

Arranged according to richness in Proteids.



I



1
2

3
4
5
6
7
8
9

10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27



Apples..

Indian corn, aerial portion fresh

Oysters, shells included

Turnips

Melons. ....

Sweet potatoes

Crayfish, whole

Irish potatoes

Clams, round, shells included...

Oats, aerial part fresh

Grass, " " "

Peas, " " "

Cow's milk.

Flounder, whole.

Lobster, "■ .

Poplar and elm leaves, fresh. . . .

Crab, whole

Brook trout, whole

Hen's eggs, shells included

Mutton " chops "

Chicken, whole

Beef, heart

Beef, liver..

Beefsteak, round, lean .

Beans

Cheese..

Cheese from skimmed milk





Pro-


Carbo-




Other


Water,


teids.


hy-


Fats.


Sub-






drates.




stances.


84.8


0.4


14.3


0.0


0.5


843


0.9


13.7


0.5


0.6


15.4


1.0


0.6


0.2


82 8


91.2


1.0


69


0.2


0.7


95.2


1.1


2 5


0.6


0.6


75.8


1.5


21.1


0.4


1.2


10.0


1.9


1


0.1


87.9


75.5


2.0


21.3


0.2


1.0


27.3


2.1


1.3


0.1


69.2


81.0


2.3


15.3


0.5


0.9


75.0


3.0


19.9


0.8


0.3


81.5


3.2


13.8


0.6


0.8


87.4


3.4


4 8


3.7


0.7


27.2


5.2


0.0


0.3


67.3


33.0


5.4


0.2


0.5


60 9


70.0


6.0


22.0


1.5


0.5


34.1


7.3


0.5


0.9


56.2


40.3


9.9




1.1


48.7


65.6


11.1


0.5


10.8


12.0


41.3


12.5


• • • •


29.3


16.9


42.2


14.3




11


42.4


53.4


14.9


• • • -


^.8


6.9


69 5


20.1


3.5


5.4


15


60.0


20.7


• • ■ •


8.1


11.2


13.7


23.2


57.4


2.1


3.6


31.2


27.1


2 4


35.4


3.9


41.3


38.3


9.0


6.8


4.6



All proteids have nearly the same chemical composition and
similar physical properties, however different may be the forms
of protoplasm in which they occur. The analysis of protoplasm,
or ratlier of the proteids which are its basis, teaches us really
nothing of its vital properties, but serves only to sliow the
chemical composition of the material basis by which these are
manifested.

Proteids are so called from tlieir resemblance to protein
(Trpcyrob, first)^ a hypothetical substance first described and



* Compiled chiefly from tables of food-composition prepared by W. O. Atwater
for the Smithsonian Institution, though a few examples have been added— viz.-
numbers 2, 10, 11, 12, 16— from Johnson's How Crops &row, N. Y., 1883.



PROTEIDS.



35



named by ^rulJer. According to Iloppe-Seyler they liave ap-
proximately the following percentage composition :



From.
To....



C.



51.5
54.5



H.



6.9
7.3



N.



15.2
17.0



O.



20 9
Zi 5



S.



03
2.0



A small quantity of phosphorus is also very frerpiently present.
Associated with these elements are always small quantities of
various mineral substances wdiicli remain as the ash when proto-
plasm is burned ; but the nature of their relations to the other
elements is uncertain. The ash varies both in quantity and
chemical composition in different animals and plants. In tho
white-of-egg the chief constituents of the ash are potassium chlo-
ride (KCl) and sodium chloride (XaCl), the former being much
in excess. The remainder consists of phosphates, sulphates, and
carbonates of sodium and potassium, \\i\\\ minute quantities of
calcium, magnesium, and iron, and a trace of silicon. Many
other mineral substances occur in association witli other- kinds of
proteids, but always in very small proportion. These salts are in
some way essential to the activity of protoplasm, as we know by
familiar experience. Man, like other animals and the plants,
requires certain mineral substances (e.g. common salt), but we
have no knowledge of the part these play in protoplasm.

It is important to note the close chemical similarity of animal
and vegetal proteids, because this is one reason for regarding
vegetal and animal protoplasm as essentially similar in other re-
spects. The following table, from Johnson after Gorup-Besanez
and Ritthausen, shows the percentage composition of various pro-
teids, and proves that the difference between vegetal and animal
proteids is chemically no greater than that between different
kinds of vegetal or different kinds of animal proteids :



PERCENTAGE COMPOSITION


OF PROTEIDS.






C.


H.


N. O.


S.


Animal albumen


53.5
53.4
5:16
50 5
.>t.l
,>4.3
52.6


7.0
7.1
7.1
6.H
7.3
7.2
7.0


15.5
15.6
15.7
18.0
16.0
16. '1
17.4


22.4

22.6
24.2
21.5
20.6
21.8


16


1.0
0.5
1.1
1.0

...


Vegetal "


Animal casein


Vejjetal "


Animal (flesh) fibrin

Vegetal (wheat) "

Animal (blood) "



36 PROTOPLASM AND THE CELL.

There is a corresponding likeness in the general properties and reactions
of proteids. They are colloidal or non-diffusible, i.e., they will not pass
through the membrane of a dialyser, or only with great difficulty ; they
are rarely crystalline ; they rotate the plane of polarized light to the left.
Though not all soluble in water, they may be dissolved by the aid of heat
in strong acetic acid and in caustic alkalies, but are insoluble in cold ab-
solute alcohol and in ether. They may be precipitated from solution by
strong mineral acids, etc. Many proteids are precipitated by heat (a pro-
cess which is called coagulation) ; and it is worthy of note that tempera-
tures which produce coagulation of proteids (40° — 75° C.) produce also the
death of most organisms. "Amongst the organic proximate principles
which enter into the composition of the tissues and organs of living beings,
those belonging to the class of proteid or albuminous bodies occupy quite
a peculiar place and require an exceptional treatment, for they alone are
never absent from the active living cells which we recognize as the pri-
mordial structures of animal and vegetable organisms. In the plant, whilst
we recognize the wide distribution of such constituents as cellulose and
chlorophyl, and acknowledge their remarkable physiological importance,
we at the same time are forced to admit that they occupy altogether a
different position from that of the proteids of the protoplasm out of which
they were evolved. We may have a plant without chlorojDhyl, and a vege-
table cell without a cellulose wall, but our very conception of a living,
functionally active, cell, whether vegetable or animal, is necessarily asso-
ciated with the integrity of its protoplasm, of which the invariable organic
constituents are proteids.

"In the animal, the proteids claim even more strikingly our attention
than in the vegetable, in that they form a very much larger proportion of
the whole organism, and of each of its tissues and organs. We may indeed
say that the material substratum of the animal organism is proteid, and
that it is through the agency of structures essentially proteid in nature
that the chemical and mechanical processes of the body are effected. It is
true that the proteids are not the only organic constituents of the tissues
and organs, and that there are others, present in minute quantities, which
probably are almost as widely distributed, such as for instance phosphorus-
containing fatty bodies, and glycogen, yet avowedly we can (at the most)
only say probably, and cannot, in reference to these, affirm that which we
may confidently affirm of the proteids — that they are indispensable constit-
uents of every living, active, animal tissue, and indissolubly connected
with every manifestation of animal activity." (Gamgee, Physiological
Chemistry, Chap. I.)

The molecular instability of proteids is proved bj tlie ease
with which they may be decomposed into simpler compounds ;
their complex constitution by the numerous compounds, them-
selves often highly complex, which may thus be derived or
split off from them.



CARBOHYDRATES AND FATS. 37

Amongst tlie otlier matters fomul in protoplasm or closely
associated with it those of most frequent occurrence and gicatcHt
physiological importance are two groups of less complex sul)-
stances, viz., carbohydrates and fats. These contain carhon, hy-
drogen, and oxygen, but no nitrogen; they do not ai)j»car to he
closely related to proteids in chemical constitution, l)ut they
occur to some extent almost everywhere in living organisms, and
in many instances are known to he of great importance, es])e-
cially in nutrition. They are rich in potential energy and mo-
bile in molecular arrangement; hence it is not strange that tliey
figure largely in food, and are often laid by as reserve ft m Mi-
materials in the organism.

J^. Carbohydrates. These substances are so called because,
besides carbon, they contain hydrogen and oxygen united in the
same proportions as in water. They include stai'cli, various
kinds of sugar, cellulose, and glycogen. Starch (C JIjoOJ is of
very frequent occurrence in plant-cells, where it appears in the
form of granules embedded in the protoplasm (Fig. 9). Cel-
lulose, having the same chemical formula as starch, but quite
different in physical properties, almost invariably forms the basis
of the cell-membrane in plants.

C. Fats. These are of especial importance as reserves of
food-materials (e.g., in adipose tissue and in seeds). They con-
tain much less oxygen than the carbohydrates; are therefore
more oxidizable, and richer in potential energy.* They com-
monly occur in the form of drops suspended in the protoplasm
(Fig. 17), and are especially common in animal cells, though by
no means confined to them.

Physical Relations. The appearance, consistency, etc., of
protoplasm have already been described ; but it still remains to
speak of certain of its other physical properties, and especially
of the manner in which its activity is conditioned by various
physical agents.

lielations of Vital Action to Temperature, It is a general
law that within certain limits heat accelerates, and cold dimin-
ishes, the activity of protoplasm. We know that cold tends to

* According to careful researches, one pound of butter contains 5654 foot-
tons, and a pound of sugar 2755 foot-tons, of energy. A pound of proteid is
nearly equivalent in this respect to a pound of carbohydrate.



38 PROTOPLASM AND THE CELL.

benumb our own bodies (provided the j become really cliilled), and '
in lower animals the heart beats more slowly, the movements be-
come sluggish or cease, breathing becomes slow and heavy, — in
a word, all of the vital actions become depressed, — whenever
the ordinary temperature is sufficiently lowered. If we chill
the rotating protoplasm of Chara or Nitella^ the vibrating cilia
of ciliated cells, or an actively flowing A^noebay the movements
become slower, and finally cease altogetlier.

On the other hand, moderate warmth favors protoplasmic
action. Benumbed fingers become once more nimble before the
warmth of the fire. In a hot room the frog's heart beats more
rapidly, cilia lash more energetically, the Amoeba flows more
rapidly, and the protoplasm of Cliara courses more swiftly. In
the winter months the protoplasm of j)lants and of many animals
is in a state of comparative inactivity. Most plants lose their
leaves and stop growing ; many animals bury themselves m the
mud or in burrows, and pass the winter in a deep sleej) {Jiiberna'
tion)^ during which the vital fires burn low and seem well-nigh
extinguished. The warmth of spring re-estabhshes the activity


1 2 4 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Online LibraryW. T. (William Thompson) SedgwickAn introduction to general biology → online text (page 4 of 20)