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UNIT 1: CELLULAR
FUNCTIONS
The Movement of
Cellular Material
Every living cell is bathed in a fluid
environment called extra cellular fluid (ECF). The ECF has both a life-giving
and a “sewer” function. The concentration of dissolved gases, nutrient and
waste molecules, important ions such as sodium and potassium, chemical
messengers such as proteins, and other essential components is constantly
changing. This is because the cell membrane is continuously regulating what
enters and exits the body of a cell. The membrane ensures that the cell is
receiving a non-stop supply of nutrients from its surroundings; and, at the same
time, it steadily allows waste products to pass through it in order to exit the
cell. The membrane goes as far as to modify its own structure when it engulfs
materials to form a vesicle, and transports them to various parts of the cell’s
interior.
A. The Cell
Membrane
·
has a major role in
regulating what exits and enters a cell
·
since some substances can
pass through the cell membrane with ease, it is said to be permeable
·
most living membranes
control what passes through them, which makes them selectively-permeable
·
non-living membranes where
only certain substances pass through them while others cannot, are called
semi permeable
·
within a cell membrane,
both the phospholipid bilayer and the protein molecules help to control the
passage of materials through the cell membrane
·
the general structure of
living cell membranes consists of the hydrophobic fatty acid chains making up
the middle of the bilayer, and the hydrophilic phosphate heads pointing toward
the liquid environments inside and outside the cell (see Figure 3.3, p. 66)
·
since the phospholipid
heads are densely packed together forming the bilayer, large molecules cannot
penetrate the membrane
·
protein molecules,
embedded within the cell membrane, provide an entryway for certain small
molecules that cannot enter through the bilayer portion of the membrane
·
other functions of
membrane proteins include:
·
for an overview of cell
membrane structure and function, to
www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.htm
Homework:
1-4, p. 67.
PRE-LESSON LAB ACTIVITIES
B. The Movement
of Dissolved Substances
·
a “solution” basically
consists of two or more substances that are dissolved in a fluid
·
the substance of a
solution that exists in the larger amount is called the solvent
·
any material that is to
enter a cell must do so in the dissolved form, and is therefore considered to be
a solute
·
Figure 3.4, p. 68
illustrates a solution – keep in mind, however, that the salt cannot be seen
when it is dissolved
The Principle of Diffusion
·
all particles are
randomnly moving – even in a solid
·
therefore, it is a natural
phenomenon that over time, particles tend to spread themselves out evenly
throughout any matter
·
the tendency of particles
to move from an area of high concentration and more random collisions, to an
area of low concentration and fewer collisions is called diffusion (see
Figure 3.5, p. 69)
·
equilibrium
is established when the distribution of the particles is completely even
·
when particles move from
areas of high to low concentration they are moving down a concentration
gradient
·
movement down the gradient
is referred to as passive transport
·
the driving force of the
movement of many molecules through the cell membrane, like oxygen, carbon
dioxide, alcohol, small lipids, is diffusion
·
four factors determines
whether or not a substance can enter or exit a cell by diffusion:
1.
particle size – small go
through phospholipid bilayer, large do not
2.
lipid solubility – the
less soluble a molecule is with the inner fatty acid chain part of the membrane,
the less likely it will pass through it
3.
size of the concentration
gradient – the greater the difference across the barrier, faster the diffusion
4.
the distance the molecule
has to travel – if the distance is large, diffusion is slow
The Principle of Osmosis
·
the diffusion of water
through a selectively permeable membrane is called osmosis
·
since the phospholipids
are constantly moving because of the fluid nature of the membrane, and since
water is relatively small enough to fit through the neighbouring phospholipids,
diffusion of water is passive
·
regions of high water
pressure, or high water content/low salt content, are referred to as high
osmotic potential
·
regions of low water
pressure, or low water content/high salt content, are referred to as low osmotic
potential regions
·
naturally, water moves
across membranes from regions of high osmotic potential to regions of low
osmotic potential
·
Figure 3.6, p. 70,
demonstrates this principle
·
the osmotic nature of
solutions surrounding a cell are:
·
HYPERTONIC – when the
fluid surrounding the cell is higher in dissolved ion concentration than what is
in a cell
·
HYPOTONIC – when the fluid
surrounding the cell is lower in dissolved ion concentration than what is in a
cell
·
ISOTONIC – when the
surrounding fluid of the cell has the same amount of dissolved ions as the
inside does
·
for an excellent animation
demonstrating these three principles, click on
http://www2.nl.edu/jste/osmosis.htm#Osmosis
·
for a detailed view of the
movement of dissolved substances click on
www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html
·
water rushes out of a cell
in hypertonic environments, into a cell that is in hypotonic environments, and
undergoes no net movement across membranes of cells that are in isotonic
environments
·
to see the effects of
solute concentration on cells, see Figure 3.7, p. 71
·
the principle of osmosis
is very important to living systems – for example, the cells of freshwater
organisms are hypertonic with respect to their external environment, which means
that water naturally rushes into the cells
·
these organisms have
evolved mechanisms within their membranes to help expel excess water out of
their systems
·
eating, and drinking too
much causes an increase in water in the blood – if humans did not have a means
of regulating or controlling the osmotic potential in blood, then all the cells
of the body would be stressed
·
the kidneys make sure that
the osmotic potential of blood does not exceed tolerable levels – they either
excrete more water into the urine, or they aid in the re-absorption of water
from the tubules back into the blood
Homework:
p. 71, 1-6
C.
Membrane-Bound Proteins and the Movement of Cellular Material
·
there are two types of
movement across cellular membranes – one involves energy, the other is
spontaneous
A. Passive Diffusion
·
there are two kinds of
passive movement of substances across a cellular membrane
1. Simple diffusion – the
movement of a small enough substance that can directly pass through the lipid
bilayer from the side
of
high concentration to the other side of the membrane of lower concentration
2. Facilitated diffusion --
when molecules are too large to penetrate the phospholipid bilayer, or if they
are hydrophilic, they
enter a cell through protein complexes
that are embedded in the cellular membrane and span across the entire membrane
(Figure 3.8, p. 72) -- this means that the movement of these large molecules is
“helped” or “facilitated” across the membrane
·
both simple and passive
refer to the movement down a concentration gradient – from high to low
·
for an animation of
passive transport click on
www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html and
www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.htm
B. Active Transport
·
when the cell needs to
move substances across a membrane against the concentration gradient, energy is
required since the movement must be opposite to the natural, spontaneous flow of
particles
·
the energy necessary to
facilitate this movement comes from ATP (the “fuel” molecule made by cellular
respiration)
·
one important example of
this kind of transport is the Na+/K+ pump in the
cell membrane of nerve cells (neurons)
·
in order to function
properly, neurons must maintain a higher concentration of sodium ions outside
the cell compared to inside the cell, while at the same time maintain a higher
concentration of potassium inside the cell compared to the outside
·
in order to maintain this
condition, specialized transport proteins in the neural membranes pump sodium
out of the cell and potassium in until the desired concentrations are achieved
(see Figure 3.9, p. 73)
·
for an animation of active
transport click on
www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html and
www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.htm
Homework:
p. 75 (1-4)
D. Vesicles and
The Movement of Cellular Material
·
when the molecules that
enter and exit the cell are very large, like proteins and polysaccharides (i.e.
they can be seen with a transmission electron microscope), a protein
facilitator cannot make a channel large enough to allow them to pass through
·
examples: - the
uptake of large foreign material into the cell, like the engulfing of a bacteria
by a white blood cell (macrophage)
- the secretion of large molecules out
of a specialized cell such as insulin secreted by pancreatic cells
·
the process that brings
large molecules into cells is called endocytosis
·
involves the pinching in
of a portion of the cell membrane around the material to be transported into the
cell
·
the pinched-in portion
eventually breaks free from the cell membrane and forms a vesicle in the
cytoplasm
·
the vesicle is then
transported to the desired location within the cell
·
to rearrange the cell
membrane in order to make a vesicle requires a sufficient amount of energy in
the form of ATP
·
there are three types of
endocytosis: (see Figures 3.11 and 3.12, pp. 76 and 77.
1. phagocytosis - means
“cell eating” -- involves the uptake of large molecules, and sometimes whole
cells
- click
on
www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/phagocyt.html
2. pinocytosis - means
“cell drinking” – involves the transport of liquids into vesicles
- click
on
www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/pinocyt.html
3. receptor-mediated endocytosis
- a method that is utilized to take in
nutrients and proteins, such as cholesterol and insulin
- when a certain amount of molecules
accumulate at a receptor site on the outside of a target cell, the “captured”
molecules collected and the cell membrane pinches inward, forming a vesicle that
will transport them to various locations in the cell
- an example of this type of
endocytosis is the removal of excess cholesterol from the blood as it is taken
in by liver cells – individuals with the disease called hypercholesterolemia,
the receptor sites on the liver cells that are to receive and collect excess
cholesterol are either absent or too few in number – this results in the
inability to remove excess cholesterol from the blood, causes it to accumulate
and would ultimately result in death due to heart disease and failure – people
with lacking cholesterol receptor sites die before adulthood – people with
reduced numbers of receptor sites are at risk, but can be treated with a low-fat
diet and drugs
·
for an animation of
endocytosis click on
www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html
or
·
the process that expels
large molecules out of cells is called exocytosis
·
this process has similar
energy requirements to those of endocytosis, in terms of energy, however it
occurs in reverse
·
it is the process that
exports large molecules out of the cell (see Figure 3.14, . 77)
·
for example, large protein
molecules that are made inside specialized cells are packaged at the Golgi
complex and a vesicle is formed, which makes its way to the cell membrane where
it joins with it and the large molecules are expelled from the cell
·
for an animation of
exocytosis click on
www.bio.winona.msus.edu/berg/ANIMTNS/Secrtion.htm or
Homework:
p. 78 (1-4)