Chapter 7.2 and 7.4, The Cell's Plasma
Membrane and Cellular Transport
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In Chapter 1 we discussed one of the seven
characteristics of life is the ability to maintain
homeostasis, or a stable internal environment.
One of the key roles of the plasma membrane is to
maintain homeostasis within a cell.
An important property of a plasma membrane is
it's ability to be selectively permeable
Selective permeability is a property of cells that it
allows certain substances in and keeps other
substances out.
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Remember from chapter 6 that one of the
important lipids in biology is a phospholipid.
A phospholipid is two fatty acid tails that are
non-polar (repelled by water) and a phosphate
head that is polar (attracted to water).
The plasma membrane is made up of two
layers of phospholipids that separate the cell's
interior from the cell’s exterior.
This double-layered structure is known as a
phospholipid bilayer.
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Other important substances present in the
plasma membrane are cholesterol, proteins,
and carbohydrates.
One important type of protein that transports
substances across the plasma membrane are
called transport proteins.
Cholesterol keeps the fatty acid tails in the
plasma membrane from sticking together.
Carbohydrates can identify chemical signals
on the plasma membrane.
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A good analogy that is used to describe how the
substances in the plasma membrane behave is to
think of the plasma membrane as a bath tub full of
water with apples floating on top. The apples
would represent the phospholipids, and the water
would represent the cytoplasm.
This analogy describes what is known in biology as
the fluid mosaic model.
It is the way in which substances behave and
interact on the plasma membrane.
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One of the ways in which cells receive the
substances and nutrients they need is by simple
diffusion.
Diffusion is the movement of particles from areas
of high concentration to areas of low
concentration until the concentration is equal and
equilibrium in achieved.
Think of a pitcher of water and some food
coloring. If a drop of food coloring was added to
the water, the dye would spread out evenly until
the color of the water was all uniform.
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Diffusion occurs until equilibrium is achieved
and the concentration gradient is zero
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The three main factors affecting the rate of
diffusion in cells are concentration gradient,
temperature, and pressure.
The higher the concentration, temperature and
pressure, the faster the rate of diffusion.
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Some substances are small enough to move across
the plasma membrane by simple diffusion.
Other particles that are much larger, require
assistance from transport proteins.
When objects require transport proteins in order
to move across the plasma membrane, but energy
is not required, this movement is called facilitated
diffusion. This particular type of transport is
achieved without the input of energy, therefore it
is considered a form of passive transport.
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There are two ways that transport proteins
accomplish facilitated diffusion.
One way is by channel proteins. Channel proteins
allow only certain size particles to move by passive
transport. (works like a sieve)
Another type of transport protein that does not
require energy, is referred to as carrier proteins.
Carrier proteins can change shape to move the
particles across the plasma membrane
In both cases, the substance is still moving from
areas of high to low concentration.
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If a dissolved solute substance can’t move
across the cell membrane to achieve
equilibrium, then water can move in the
direction of the higher solute concentration to
achieve equilibrium.
This process is called osmosis.
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If a cell is in a solution (for instance, blood) that has the
same concentration of a solute as the solution, the cell
will be isotonic to the solution. In this case no osmosis
will occur and the cell will not change.
If a cell is in a solution that has a lower concentration of a
solute than the cell, then the solution is hypotonic to the
cell. In this case, osmosis will move water inside the cell
and the cell could swell and burst (lyse).
If a cell is in a solution that has a higher concentration of a
solute than the cell, then the solution is hypertonic to the
cell. In this case, osmosis will move water out of the cell
into the solution and the cell will shrivel up.
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How does a cell move a substance from a lower
concentration to a higher concentration?
In order to do this, a cell must use energy.
When a cell uses energy to move substances across
the cell membrane it is called active transport.
To do this, special carrier proteins called pumps
move substances against the concentration gradient
from areas of low to high concentration.
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One common type of active transport is a pump called the
sodium potassium ATPase pump.
The role of the sodium potassium ATPase pump is to carry
sodium out of the cell and bring potassium into the cell.
In order to do this an enzyme called ATPase uses energy
from ATP to pump 3 sodium ions out of the cell and pump
2 potassium ions into the cell.
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Some substances are too big to move by simple diffusion,
facilitated diffusion, or active transport.
These substances require a different method to get across
the plasma membrane
If a large object needs to leave the cell, the plasma
membrane can engulf the substance and eject the
substance by exocytosis.
If a large object needs to enter the cell, the plasma
membrane can engulf the substance and draw it into the
cytoplasm by endocytosis.