Biology 30
Module 1
Lesson 2 Cell Physiology: Homeostasis
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
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Lesson 2
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Lesson 2
Lesson 2
Cell Physiology
Directions for completing the lesson:

Text References for suggested reading:
Read BSCS Biology 8th edition
Sections 5.6 - 5.11, 7-8
OR
Nelson: Biology
Chapter 1, pages 40-46
Chapter 4, page 117
Chapter 22, page 532-537, 544-545

Study the instructional portion of the lesson.

Review the vocabulary list.

Do Assignment 2.
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Lesson 2
Vocabulary
active transport
isotonic
anaphase
metaphase
cell plate
mitosis
cleavage furrow
osmosis
cytokinesis
passive transport
cytolysis
permeable
diffusion
phagocytosis
endocytosis
physiology
eukaroyotes
pinocytosis
exocytosis
plasmolysis
gradient
prokaryotes
homeostasis
prophase
hypertonic
Ringer's solution
hypotonic
selectively/differentially permeable
impermeable
telophase
interphase
turgid
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Lesson 2
Cell Physiology: Homeostasis
Introduction
In any studies relating to what life is or what actions are involved in living, it is often
difficult to give precise definitions without coming up with some exceptions. A
method of trying to overcome this has been for science to establish a number of
general characteristics which could be applicable to as many organisms as possible.
A considerable number of these characteristics have to do with specific actions
designed to maintain life. Thus, we may say a living organism is one which ingests
(food and other necessary materials), transports (these to various body parts),
assimilates, excretes, is sensitive or responsive to external as well as internal
changes, as well as carries out other important actions. In our introductory lesson,
these were seen to be performed at the multicellular level or by an entire organism as
well as by individual cells. The actions of ingestion, excretion and irritability are as
important to a cell as they are to the entire organism. Complete stoppages of such
actions at either level could mean eventual death for both cells and organism.
The actions just mentioned, in addition to others, will be examined more closely in
the following lessons. Studying how cell parts or, on a larger scale, body parts
actually function is part of physiology. When individual body parts or processes are
studied, it should be kept in mind that an organism's well-being depends on a proper
functioning of all parts or systems and not just a single one.
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Lesson 2
After completing this lesson you should be able to:
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•
explain homeostasis and discuss its importance in
maintaining life.
•
give examples of homeostasis occurring internally, or
carried out by organisms to maintain certain external
conditions.
•
understand the importance of movements of substances
in and out of bodies or cells.
•
explain diffusion and factors affecting diffusion.
•
explain osmosis and factors affecting osmosis.
•
distinguish between passive and active transport.
•
give some examples of active transport.
•
explain the general effect cell size has on homeostasis.
•
relate cell size to the cell cycle and describe the cell cycle
in prokaryotes and eukaryotes.
•
describe what generally happens to result in cancerous
developments and the effects these could have on a body.
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Lesson 2
Homeostasis
If you have ever tried, or were to try, to walk a rail fence or a
narrow plank some distance off the ground, you would know
the importance of maintaining a proper balance. The acts of
walking, running and even remaining standing upright are
other balancing actions which are not even noticed by most of
us, after learning or developing these abilities early in life.
Proper balancing in each instance is necessary in
maintaining that action. Failure to do so could result in
embarrassing or even painful consequences. Balancing of a
slightly different nature is also crucial on cellular and
organism levels.
by Robert Lawton
Homeostasis: the processes by which organisms try to maintain the most
suitable conditions possible for the functioning of its cells and body
Living organisms or cells function under certain kinds of conditions and may not be
able to do so or do so properly if those conditions are not met. Freezing temperatures
can slow down many plant processes or stop them entirely by killing plant parts or
making them go into dormancy. A lack of water can be a limitation to both plants and
animals. While temperature and water are initially part of external conditions, they
(and others) lead to internal body states in organisms which eventually affect the
functioning of individual cells.
External controls:
These examples illustrate some deliberate and controlled reactions to external
temperatures and relative humidities. Other behaviors or responses could be made to
such things as available food supplies, water conditions and (sun)light levels or
intensities.




Some plant leaves develop fine, hair-like structures to reduce wind velocities and
drying actions near their surfaces.
Some animals can move into areas of shade or sunlight while others can move
into deeper or shallower waters; these actions are attempts to keep the
temperatures on their body surfaces about the same.
Ants and termites build piles or mounds having fairly uniform internal humidities
and temperatures.
Worker bees or wasps "fan" at their hive entrances to create cooling drafts in high
temperatures.
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Lesson 2


In our own situations, we can put on or take off clothing or we can turn up
heating or cooling units, to establish temperatures that we like.
If we don't like carrying out these actions or if we still don't like the conditions, we
can carry out a behavior common to some other animals and birds – migration.
Internal controls:
There are other homeostatic actions which occur without organisms being able to
control them consciously or, for that matter, without even being aware of many of
them. These commonly occur at the organ, tissue or cellular levels. The internal
human body temperature normally fluctuates two or three degrees Celsius around an
average of 37.5 C. There are several ways to maintain this balance.
 If a warmer body temperature than this is sensed by the hypothalamus of the
brain, a "command" is sent to sweat glands to produce more perspiration. The
evaporation of sweat from the skin has a cooling effect.
 Dilation of blood vessels and moving more blood closer to the skin surface results
in "flushing" and exposing that blood for faster cooling at the surface.
 If body temperature is cooler than the average, blood is moved away from surface
areas to conserve heat. Rapid muscle contractions and relaxations (or shivering)
can also occur in order to generate body heat.
Other examples of homeostasis occur during exercises or higher levels of body
actions which lead to higher heart and breathing rates as attempts are made to
balance oxygen, carbon-dioxide and glucose levels in the blood.
Homeostasis involving internal body conditions generally revolves around a number
of types of body receptors and feedback principles. That is, some part of the body is
responsible for monitoring or checking on a particular body condition and then
regulating it through hormone or nerve messages.



The hypothalamus, which was mentioned as being an important body temperature
regulator, is also sensitive to levels of glucose (sugar) in the blood. Proper glucose
levels are very important in the functioning of body cells, especially those of the
brain. A high blood glucose level will cause the hypothalamus to send a message
to the liver to cause that organ to slow down its release of sugar.
pancreatic cells called islets of Langerhans are also affected by glucose levels.
These cells secrete the hormone insulin which causes body cells to absorb more
glucose, thereby reducing its levels in the blood. If the blood sugar level falls below
normal, a third control center is activated.
The medulla of the adrenal gland will signal the liver to release larger than normal
amounts of glucose. (This becomes the source of strength or speed for some people
in unusual situations – such as lifting a heavy weight which had accidentally
pinned someone else, or running faster or jumping higher in front of spectators at
a track meet.)
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Lesson 2
The feedback mechanism for all three control centers is the amount of blood glucose.
It could be compared to temperature levels and a thermostat in a home. Low
temperatures cause a thermostat to activate furnace and fan to release more heat. At
a certain level or temperature feedback, the thermostat will then shut down furnace
and fan.
To a certain extent, homeostasis can also be more generally applied on a larger scale
than a single organism. In studies of ecology and population, observations commonly
illustrate actions which tend to promote stability. An increase in Saskatchewan's
rabbit population leads to predator increases which bring rabbit numbers down.
Population numbers could also be related to food and water supplies. Corrective
measures of some type are common occurrences when organisms or conditions go
above or below certain numbers or states. However, for the most part, homeostasis
should be regarded as any actions relating to attempts to maintain uniform internal
conditions of individual organisms.
Movements and Homeostasis
The significance of movements to homeostasis was probably noticed for both external
and internal levels of organisms. Organisms could actually move about seeking
favorable external conditions. Movements of substances or messengers also take
place within bodies between certain receptors, organs or tissues, to try and maintain
stable internal conditions. Cells are really the fundamental units of both structure
and function of living organisms. Therefore, movements of substances in and out of
cells are of prime importance in maintaining all life-sustaining actions. Proper cell
functioning is dependent on receiving nutrients or messengers from outside the cell
and sending out substances which could be used by other cells, or wastes which
could be harmful in higher concentrations. Some of the ways in which substances
move in and out of bodies or cells will be examined more closely.
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Lesson 2
Movements of Molecules
Passive Transport
In passive transport, no energy is expended by the cell. Substances are moved down
a concentration gradient, that is, from areas of high to low concentration. There are
two forms of passive transport: diffusion and osmosis.
Diffusion
Diffusion – the movement of molecules from an area of high
concentration to one of low concentration.
The random motions and collisions, especially of liquids and gases, eventually tend to
distribute molecules fairly evenly within any confining space. For example, a drop of
ink placed in water, a sugar cube in water, an air freshener hung somewhere, will all
eventually result in an even distribution of molecules in the containers or room.
The above diagram illustrates a small cube of sugar, as it dissolves in water. As the
sugar cube dissolves, it will experience more collisions between its molecules at its
initial source. This causes the sugar molecules to spread outward, into areas where
they are not as concentrated.Eventually, the sugar molecules will become evenly
distributed in the water. After this occurs, it does not mean that the molecules will
stop moving. Random collisions and movements still take place, but net movements
in various directions are about the same. One molecule leaving an area would soon
be replaced by one entering that area.
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Lesson 2
Factors Affecting Rates of Diffusion
Movements or rates of diffusion can vary and a number of factors can be responsible
for this.
1.
Concentration: The higher the concentration of molecules in an area, the
greater the frequency of collisions between them. This causes them to spread
out more.
The difference in molecule concentrations between two areas is sometimes
known as a concentration gradient or a diffusion gradient. A large difference
in molecule concentrations between two areas means that there is a high or a
steep diffusion gradient. A higher or steeper gradient will increase the rate of
diffusion.
2.
Temperature: Molecular motion increases as temperature rises.
Faster movements of molecules increase the frequency of collisions and also
the forces of the collisions, increasing diffusion rates.
3.
Size of molecules: Larger molecules move more slowly and diffusion rates will
be slower.
4.
Pressure: Increasing the pressure on molecules, particularly gases, moves
them closer together and, in effect, increases their concentration and tendency
to move out.
Barriers to Molecular Movement
Diffusion of substances or their molecules can occur up to a point with little
hindrance other than collisions with other molecules, which is really part of the
process anyway. Many molecular movements, however, eventually encounter some
forms of barriers. These barriers can be classified into three categories: permeable,
impermeable and selectively permeable.
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Lesson 2
Permeable membrane: A membrane which allows a
substance to pass through.
The above illustration shows a membrane that is permeable to both sizes of molecule.
After a time, diffusion occurring in both directions will establish fairly equal
concentrations of sugar and water.
Impermeable membrane: A membrane which prevents the passage of a
substance. The pore size of the membrane is not large enough to permit
the passage of a particle.
The above illustration shows a membrane which is permeable to the smaller sized
molecules but impermeable to the larger particles. The smaller molecules move from
left to right, since their concentration is greater on the left. The larger molecules,
despite their higher concentration on the right, remain there. The initial net
movement of smaller particles to the right creates a pressure against the membrane
from the right side. Eventually, this pressure becomes too great for any additional net
movements to the right.
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Lesson 2
Selectively permeable or a differentially permeable membrane: A
membrane that allows some substances to pass through but not others.
It could be called semi-permeable as well.
Cell walls, which are just on the outside of cell membranes of plants, tend to be
permeable as they usually allow for relatively free movements or diffusions of most
substances. Other than for movements of oxygen, carbon-dioxide and a few other
substances, cell membranes do not allow such easy passages for all molecules.
Selectively permeable membranes around cells or their organelles such as nuclei,
mitochondria and others, play key roles in what passes through them.
Particles must be dissolved before they can move through plant and animal
membranes. The usual dissolving agent or solvent is water, but there could be other
solvents such as ether, chloroform and alcohol. Once substances are dissolved, they
can then attempt to move through the membranes.
Selective absorption is the process whereby some cell membranes have an ability to
regulate movements of some substances through them.
Factors affecting membrane permeability
1. The size of membranes' pores
2. Electric charges on some dissolved particles could attract or repel them to or from
membranes, since membranes are often charged themselves.
3. The nature of the plasma membrane itself may be another determining factor.
Osmosis
Osmosis: The movement of water (or solvent) through a selectively
permeable membrane from an area of higher to lower concentration
Movements of any particles, whether they are dissolved in a solvent or not, are
usually regarded as diffusions. Therefore, it could be said that the diffusion of water
into or out of organisms' cells is osmosis. The movements of other molecules such as
gases, salts and minerals are said to occur by diffusion (although selective absorption
may be involved as well.)
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Lesson 2
Osmosis and Solution Concentrations
The direction in which water molecules move through a selective membrane depends
on water molecule concentrations on either side of that membrane. If a cell is placed
into a solution, that solution is classified according to a comparison between the
concentrations of water molecules in the cell and water molecules in the solution.
The presence of other molecules or solutes really determines the water molecule
concentrations of cells or solutions. The presence of more solutes lowers water
molecule concentrations. There are three commonly used classifications of solutions:
hypotonic, hypertonic and isotonic.
Diagrams of Osmosis
Hypotonic solutions
This is a solution that has a higher water
concentration than the cell. Distilled water is usually
a good example of such a solution. The higher water
molecule concentration in a hypotonic solution would
result in water moving into cells. Intake of water by
plant cells results in their cytoplasm pressing against
the cell walls. The pressure against cell walls is
commonly referred to as turgor pressure and a cell
which is firm is said to be turgid. Animal cells,
surrounded only by a plasma membrane (which is not
as strong as a cell wall), could actually burst from an
intake of water. The bursting of a cell, or cytolysis,
kills it.
A hypotonic solution has a higher water
molecule concentration, so the net movement
of the water molecules is into the cell. This
would cause an animal cell to swell –
eventually to burst.
Hypertonic solutions
This is a solution that has a lower water molecule
concentration than the cell. Sugars or salts, for
example, added to water could create hypertonic
solutions. The sugar or salt molecules (solutes) occupy
areas that would normally be occupied by water so the
water molecule concentration is lower. Plant cells in
hypertonic solutions would experience plasmolysis
where, as water molecules leave the cells, cytoplasm
shrinks away from the cell walls. Loss of turgor
pressure causes drooping or wilting of stems and
leaves and, if prolonged, can cause death. Loss of
water from animal cells causes the entire cells to
shrink and shrivel and could be fatal as well.
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A hypertonic solution has a lower
concentration of water molecules compared to
the cell. The net movement of the water
would be out of the cell. An animal cell would
shrink. In plant cells plasmolysis would occur.
Lesson 2
Isotonic solutions
These are solutions which have the same
concentrations of water molecules as inside the cells.
The usual salt concentration of cells is about 0.9%.
Therefore, water which has a salt-mineral
concentration of approximately 0.9% should be about
isotonic with most cells. Immersing cells in isotonic
Isotonic solutions have approximately the
solutions would not mean that there would be no
same water molecule concentration. The net
osmosis or diffusion. Molecules still move through the
movements in both directions is about equal.
membranes, but net movements in both directions
are about equal. Cells in such solutions would retain their firmness.
In laboratories and
hospitals, isotonic
solutions are prepared
to match the
water-salt
concentrations in
animal and human
tissues. Amphibian
Ringer's solution is
isotonic with
amphibian (such as
frogs') cells. Human
Ringer's solution
matches the water-salt
concentration in human cells and tissues. Bathing tissues or organs with these
solutions or immersing them right in the solutions keeps cells alive longer enough to
perhaps complete laboratory studies or hospital operations.
Osmosis and Homeostasis
The process of osmosis could upset the homeostasis or balance within cells,
depending on the external conditions
Lower external water molecule concentrations could result in plasmolysis and
possible deaths of cells. To try and prevent this:



animals (such as ourselves) have slightly salty or isotonic solutions surrounding
their body cells.
The external surfaces of the bodies also have fairly water-resistant coverings of
scales or skins.
Plants also have outer coverings of closely spaced epidermal cells and sometimes
waxy cuticles over these.
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Lesson 2
Higher external water molecule concentrations could cause the cells to explode as
water enters. To try and prevent this:


Some one-celled organisms have special contractile vacuoles which collect excess
water and then force it out.
Plants have fairly rigid cell walls which limit the outward expansion of cell
membranes as turgidities increase.
Transport of Molecules
Passive Transport
Osmosis or diffusion occur as a result of the motions or energies of the molecules
themselves. Cells or cell membranes have nothing to do in actually creating
molecular motions or movements (although they can regulate them to an extent). For
this reason, osmosis and diffusion are regarded as forms of passive transport, where
cells themselves do not expend energy. Pores or spaces in the membranes are
sometimes large enough to allow some molecules (water molecules) through by
simple diffusion. Channel proteins allow small, dissolved particles (ions) to diffuse
through their openings. There also appear to be certain pore-like protein molecules at
various points in a membrane which allow facilitated diffusion. This is a process in
which carrier proteins aid large molecules in passing through.
Note: the movement of molecules is from high concentration on the exterior to low
concentration in the interior. The cell does not use energy for this process to occur.
Active Transport
There are situations where movements occur in directions opposite to normal
diffusion. This is done by active transport.
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Lesson 2
Active transport: movement of molecules against a diffusion gradient or
from an area of lower concentration to one of higher concentration. The
cells are actually involved and must use energy.
Following are two examples of active transport:
1. Plants growing in prairie soils may show concentrations of manganese many times
that which is found in the soil.
2. Salt water organisms, such as some forms of algae, show levels of iodine
sometimes a million times higher than that in the water.
More evidence that the cells are using energy is indicated by their high respiration
rates, as respiration is the source of energy release in organisms. (Respiration rates
themselves can be measured by such things as temperatures or movements of
oxygen and carbon-dioxide.)
Movements of Molecules by Active Transport
By means of active transport, substances can move across a membrane against the
concentration gradient. Energy in the form of ATP is needed to do this.
Active transport involves special transporter protein molecules scattered across a
membrane's surface. Other molecules which become attached to these, either on the
inside or on the outside of the membrane, can then be transported through the
protein molecules. A protein molecule uses energy to do this and commonly gets the
energy from ATP, which will be described later.
Note: the movement of molecules is from low concentration on the exterior to
high concentration in the interior. Energy must be used by the cell to move
molecules against the concentration gradient. This is Active Transport.
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Lesson 2
Active transport has an important survival value for many organisms. Prairie plants
or salt water organisms having concentrations of substances in their cells many
times that of their external environments probably point this out. Ordinarily, some
essential nutrients or substances would not be able to enter bodies by diffusion or
osmosis against a diffusion gradient.
The following diagram illustrates the differences between passive and active
transport.
A
T
P
P
A
D
P
(b) active transport
(a) passive transport
In passive transport (a), substances move through membrane proteins down their
concentration gradient. Active transport (b) requires the use of ATP energy as well as
membrane proteins.
Endocytosis
Endocytosis is the process of substances entering cells. Two other transport
mechanisms are pinocytosis and phagocytosis. These processes require energy.
1. Pinocytosis - the idea of special carrier molecules which attach to lipids, proteins
or other organic molecules too large to pass through a membrane's pores. These
then attach to a membrane's surface. A pocket or infolding of the membrane
causes a particle to become embedded in the membrane with the "pocket"
eventually pinching off and being released into the cytoplasm. This is commonly
referred to as cell drinking.
2. Phagocytosis – the process by which organisms or cells (such as Amoeba and
white blood cells) will flow around particles and enclose them in vacuoles. This is
a more active process of engulfing food particles and is often referred to as cell
eating.
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Lesson 2
The following diagrams illustrate the process of pinocytosis and phagocytosis.
Pinocytosis by a cell
Phagocytosis in Amoeba
Exocytosis
Exocytosis is the process by which cells release substances. These can be particles
which cannot be digested, waste matter or cell secretions intended for other areas of
the body. This process works in reverse to pinocytosis.
Cell Size and Homeostasis
Movements of substances in and out of cells must be able to keep up to cells'
metabolic rates. Shortages of nutrients or excesses of wastes could lead to cellular
deaths. Since materials must go through the cell membrane, the size or area of the
membrane in relation to a cell's volume determines just how well the cytoplasm is
served. When a cell increases in size, the area of its surface membrane and its
internal volume also increase. However, the increase in the internal volume is much
greater than the increase in surface area. This may be easier to see if you can
imagine a cell having 1 cm dimensions doubling in size.
Total Surface Area
6 cm2
24 cm2 (Area increased four times)
Total Volume
1 cm3
8 cm3 (Volume increased eight times)
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Lesson 2
In the illustration, you can see that if a cell was to double in size, its total surface
area would increase by four times and the internal volume has increased by eight
times. The differences between surface areas and internal volumes become even more
pronounced in continuing size increases. Disadvantages of large cell size:



makes it more difficult for enough substances to enter or to leave
its inner area may not receive enough nutrients or get rid of all wastes.
the cell becomes less efficient.
Generally, the most efficient animal cells are those which are small in size, such as
liver cells. Muscle cells and nerve fibers, although they may be longer than most
cells, are thinner than normal so that substances can leave or enter more efficiently.
A cell will grow in size until
there is too great an internal
volume for the external surface
area. At this point, two things
can happen. The cell will stop
growing while continuing to
function; or, the cell can divide
into two smaller cells. With the
exception of some major nerve
and muscle cells, most body
cells are capable of going
through repeated divisions. In
the last lesson, it was briefly mentioned that in order for division of cells to be
successful, two conditions had to be met. A cell's genetic material had to be
duplicated and then the genetic material and the original cell contents fairly equally
divided among daughter cells. The process of going through these actions form part
of a cell's cycle.
The Cell Cycle
There are three common factors which initiate a division process in a particular cell.
1. nutrient shortages and waste excesses within the cell.
2. removal of contact inhibition between cells. Whenever the protein molecules of two
adjacent cell membranes contact each other, the cells seem to be inhibited in
dividing. If some cells, of a group of cells, are separated from each other by
physical forces, injuries or deaths, that point contact could be lost. The remaining
cells then have a tendency to begin dividing.
3. chemical growth factors within plant and animal bodies can inhibit or promote cell
divisions.
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Lesson 2
Cell Division
In Prokaryotes
Cells without distinct nuclei still have DNA or nuclear material scattered throughout
the cell matter. Some of this nuclear material is attached to the cell membranes at
various points. Division of a cell follows an action where invaginations occur at
opposite sides of the cell and continue across until they meet. The entire process
could be called binary fission. Each "new" cell will end up with some of the original
cell's nuclear and cytoplasmic content. Sometime before the next divisions, cells will
duplicate or replicate their inner contents.
In Eukaryotes
All cells come from existing cells. If new cells are to have similar characteristics and
function in much the same manner as the original cells, they must have similar DNA
and cytoplasmic contents. DNA, or deoxyribonucleic acid, makes up the
chromosomes and genes of cells. These transmit the genetic information from parent
to daughter cell. The genetic "instructions" are largely responsible for the manners in
which proteins are synthesized. This is very important to a cell. Types of proteins not
only affect the general structure of a cell, but also the nature of its enzymes and
hormones. These affect the cell's functioning.
The division of a eukaryotic cell appears to be a more visibly
complex process than that of a prokaryote. Certain actions occur
with respect to the nucleus and its chromosomes; other actions
involve the rest of the cell. The life of a eukaryotic cell is a
continuous sequence of events, primarily cell division and
interphase.
Cell division can be subdivided into two stages: Mitosis and Cytokinesis.
Mitosis: the division or reproduction of the nucleus
which is followed by
Cytokinesis: the division of the rest of the cell or its cytoplasm
and organelles.
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Lesson 2
After cell division, that is, mitosis-cytokinesis is complete, the
cell is in a stage termed interphase. Although the cell may
appear inactive during interphase, many activities necessary for
maintaining life within the cell that are occurring. These
activities during interphase can be broken down.
Interphase
G1 (Growth) Phase
- the cell grows
-proteins are synthesized
S (Synthesis) Phase
-DNA replicates in
preparation for mitosis. This
ensures that a complete and
identical set of genes are
available for each new offspring cell.
G2 (Growth) Phase
-cell growth continues, involving organelles and cytoplasm
-proteins are synthesized
-cell synthesizes a substance to trigger the beginning of cell division
Some cells may not go beyond the G1 phase (if they are not genetically "programmed"
to go further, or if factors mentioned earlier do not initiate division). Other cells spend
varying amounts of time in this phase – from several hours to days, weeks or months.
However, once it seems to have committed itself to synthesizing additional proteins, a
cell will eventually undergo mitosis.
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Lesson 2
Stages of Mitosis
The following graphic shows a very simplified version of mitosis-cytokinesis.
Although mitosis is a continuous process, it can be broken down into a series of
stages.
The process of mitosis is a continuous one and occurs in all cells. We can take a
“freeze frame” of each stage to see what is happening. Each stage is briefly described
below.
Interphase
-
growth stage. Cell makes new molecules, increasing
volume and mass
consists of 3 phases: G1, S, G2
DNA is duplicated in the S phase.
In an animal cell, the centriole pair duplicates in the
G2 phase of Interphase. They begin to move to
opposite
sides of the cell and prepare to form the mitotic
spindle.
Prophase
-
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Spindle fibers (microtubules) develop near
centrioles and extend across the cell.
Nuclear membrane begins disintegrating.
Chromosomes become more distinct.
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Lesson 2
Metaphase
-
Chromosomes (each actually 2 chromatids) line up
near the middle and become attached to spindle.
this ensures each daughter cell will contain one of
each of the chromatids
Anaphase
-
Paired chromatids break apart and are “pulled”
to opposite sides by the spindle fibers.
Cytokinesis usually occurring simultaneously.
Telophase
-
Reverse actions to prophase: nuclei re-form,
chromosomes disperse into nucleoplasm.
Plant and animal cells follow relatively similar patterns during mitotic actions with
two exceptions.
1. The presence of centrioles (centrosomes) in animal
cells and their apparent absence in plants. The
mitotic spindle seems to be associated with the
centrioles of animal cells. However, despite the
centrioles' absence in plant cells, spindles still form
and still function.
2. Cytokinesis: In an animal cell, indentations or
cleavage furrows begin at the sides and gradually
move across the cell to meet one another. This
results in complete separation into two cells. In
plant cells, a cell plate gradually forms across the
original cell. Completion of this plate and the
addition of cell wall material completely around each
cell results in two new cells which are still joined
together.
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Lesson 2
Cancer
Normal mitosis or cell divisions are initiated and regulated by one or more body
conditions or controls. Occasionally, bodies seem to lose control over the regulation
of the rates of divisions and how cells eventually differentiate or specialize. Cells can
begin to be formed which no longer perform any useful function. This usually
continuous reproduction of abnormal cells is cancer. Such cells not only use up
nutrients, but their continuing growths or tumors can be harmful in other ways.
They can displace normally functioning cells or they can eventually begin pressing
against the normal cells with enough force to impede proper functioning of organs.
Possible treatments can include removing these cancerous cells completely or killing
them with radiation and chemotherapy.
Summary
Cell physiology involves studies of how cells and their parts function. Proper
functioning of individual cells is extremely important to the proper functioning and
survival of entire organisms. Maintaining balanced conditions which are best for an
organism or its cells is part of homeostasis. One aspect of homeostasis involves
movements of substances in and out of living cells. Passive transport, or the purely
physical process of diffusion or osmosis, is responsible for many of these movements,
but doesn't account for all of them. Active transport is necessary to carry certain
molecules across selectively permeable membranes against diffusion or
concentration gradients. A point which had not been mentioned in the lesson
pertains to the selectivity of membranes and the living condition. When organisms or
cells die, cell membranes lose much of their selective ability. Molecules which are
normally unable to enter or leave a living cell, may be able to do so after its death.
The nature of membranes and the movements of substances are therefore quite
important to the processes of homeostasis and continued survival of cells and
organisms.
Continued survival also requires that cells operate in effective or efficient manners. In
order to do this, cells or cell parts must be repaired or replaced on a continuous
basis. In addition, cell sizes must not become so large as to make movements of
substances in and out difficult. These factors make it necessary that cells establish
and maintain cycles, where a series of actions results in the formation of new cells. It
is important during these reproductive actions that fairly accurate duplication of
genetic and cytoplasmic matter takes place, followed by relatively equal
apportionment of this matter into new cells. This ensures new cells functioning in
similar manners as the "parent" cells, maintaining the well-being of an entire
organism.
Biology 30
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Lesson 2