Chapter 7.2 and 7.4, The Cell's Plasma Membrane and Cellular Transport

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Chapter 7.2 and 7.4, The Cell's Plasma Membrane
and Cellular Transport
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Function of the Plasma Membrane

• 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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Selective Permeability
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Structure of the Plasma Membrane

• 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 cells exterior.
• This double-layered structure is known as a
  phospholipid bilayer.

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Phospholipid Bilayer
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Other Structural Components of the Plasma Membrane

• 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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Transport Proteins
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The Fluid Mosaic Model

• 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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Fluid Mosaic Model
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Diffusion

• 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 and Equilibrium

• Diffusion occurs until equilibrium is achieved
  and the concentration gradient is zero

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Rate of Diffusion

• 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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Movement of larger particles across the plasma
membrane

• 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 also called passive transport.

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Differences in Simple Diffusion and Facilitated
Diffusion
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Methods of Facilitated Diffusion

• 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 operates
  passively, 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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Channel and Carrier Proteins
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The Movement of Water across the Plasma Membrane

• Remember that we just said some substances are
  too big to cross the plasma membrane.
• Water is not too big to move across the plasma
  membrane. It can move by simple diffusion.
• So, if equilibrium of a substance that is too big
  to move across the membrane can not occur, then
  water can move in the direction with the higher
  concentration to dilute the substance
• This process is called osmosis.

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Osmosis
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How does osmosis affect cells?

• If a cell is in a solution (for instance, blood)
  that has the same concentration of a substance as
  the solution, the cell is 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 substance 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 substance 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 Osmosis Affects Cells
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How does a cell move a substance from a low to a
high concentration?

• So far, we have only discussed ways cells move
  substances by passive transport from areas of
  high to low concentration.
• So 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.
• This 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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Sodium Potassium Pump

• One common type of active transport is a pump
  called the sodium potassium ATPase pump.
• The role of the sodium potassium 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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How do objects too large to move by diffusion or
transport proteins move across the plasma
membrane?

• Some substances are simply too big to move by
  simple diffusion or by transport proteins.
• 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.

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Endocytosis and Exocytosis