HOMEOSTASIS AND TRANSPORT

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CHAPTER 5

• HOMEOSTASIS AND TRANSPORT

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PASSIVE TRANSPORT

• Particles move into and out of a cell through its
  plasma membrane.
• Passive transport is the movement of a substance
  throughout a medium or across a cells membrane
  without the expenditure of energy.
• Molecules tend to move from an area of high
  concentration to low concentration (down their
  concentration gradient).

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PASSIVE TRANSPORT

• Types of passive transport
• Diffusion is the movement of particles from an
  area of high concentration to an area of low
  concentration.
• Osmosis is the diffusion of water molecules
  across a membrane.
• Facilitated diffusion is the movement of
  particles across a cell membrane with the help of
  carrier proteins

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DIFFUSION

• Diffusion is accomplished by the random movement
  of molecules, driven entirely by the kinetic
  energy the molecules possess.
• In order to understand diffusion, you must
  understand the terms solution, solute, solvent
  and net movement.
• Rate of diffusion is affected by
• concentration gradient
• temperature
• pressure

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FACTORS AFFECTING DIFFUSION

• The difference in the concentration of molecules
  across a space is called concentration gradient.
  As the concentration gradient ?, the rate of
  diffusion ?
• As temperature ?, rate of diffusion ?
• increase in kinetic motion of molecules
• As pressure ?, rate of diffusion ?
• smaller space to cover

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DIFFUSION AND EQUILIBRIUM

• In the absence of other influences, diffusion
  will eventually cause the concentration of
  molecules to be the same throughout the space.
• When the concentration is the same, a state of
  equilibrium exists, and net movement stops
  (random movement continues)

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Diffusion Across Membranes

• Remember, the plasma membrane is selectively
  permeable because it allows the passage of some
  solutes but not others.
• If a molecule can pass though the membrane, it
  will move from area of high concentration to area
  of low concentration.
• The ability of a molecule to diffuse across a
  membrane depends upon size and type of molecule
  (polar/nonpolar, lipid soluble/ or not) and the
  chemical nature of the membrane.

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OSMOSIS

• Osmosis is the diffusion of water molecules
  through a membrane.
• The direction of osmosis depends upon the
  relative concentration of solutes on each side of
  the membrane.
• Water moves from high water concentration to low
  water concentration (in the direction of higher
  solute concentration).
• Osmotic pressure is the increased water pressure
  inside a cell that results from osmosis.
• Eventually the concentration of free water
  molecules will equalize on both sides of the
  membrane equilibrium.

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Osmosis Terms

• If the environment that a cell is immersed in
  contains a higher concentration of solutes than
  the interior of the cell, the environment is
  hypertonic to the cell.
• If the environment that a cell is immersed in
  contains a lower concentration of solutes than
  the interior of the cell, the environment is
  hypotonic to the cell.
• If the environment that a cell is immersed in
  contains equal solutes to the interior of the
  cell, the environment is isotonic to the cell.
• Since these terms are used to compare the cell
  and its environment, they will always be used as
  pairs. Ex. Cell is hypertonic to the env env.
  is hypotonic to cell

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Insert table p. 97
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How Cells Deal with Osmosis

• Cells that are exposed to an isotonic external
  environment have no problem keeping their water
  balance. ex. most marine organisms
• Many cells live in a hypotonic environment. ex.
  unicellular freshwater organisms
• Water constantly enters the cell
• Cell is in danger of exploding
• Most spend large amounts of energy constantly
  pumping water back out! (contractile vacuole)

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Plants and Osmosis

• Most plants cells exist in a hypotonic
  environment.
• Water enters the cells and they swell as they
  fill with water.
• The swelling stops when the cell membrane is
  pressed against the cell wall ? cell will not
  explode.
• The pressure these water molecules exert is
  called turgor pressure.

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Plants and Osmosis

• In a hypertonic environment, water will leave
  cells.
• In plants, cells shrink away from cell walls and
  turgor pressure is lost.
• Loss of turgor pressure results in wilting.
• This condition is called plasmolysis.

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Animals and Osmosis

• Some cells cannot compensate for changes in the
  solute concentration in their environment.
• Red blood cells in humans have no contractile
  vacuoles, solute pumps or cell walls.
• The bursting of cells is called cytolysis.

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FACILITATED DIFFUSION

• Another type of passive transport is called
  facilitated diffusion.
• Molecules that cannot move through the membrane
  on their own are assisted by specific proteins
  called carrier proteins.
• Glucose molecules are too large to move easily
  through the plasma membrane.
• They are carried through the membrane (in either
  direction) by facilitated diffusion.

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Facilitated Diffusion
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Diffusion Through Ion Channels

• Another type of passive transport involves
  membrane proteins known as ion channels.
• Some ion channels are always open others have
  gates.
• The gates may open or close in response to
• stretching of the cell membrane
• electrical signals
• chemicals in the internal or external environment
  
• Na, K, Ca and Cl- are carried this way.

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Diffusion Through Gated Channels
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ACTIVE TRANPSORT

• Often cells need to move materials from areas of
  low concentration to areas of high concentration.
• Active transport is the use of cellular energy to
  transport particles through a membrane against a
  concentration gradient.
• For example, cells must expend energy to maintain
  their internal level of amino acids and sugars.

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Cell Membrane Pumps

• Carrier proteins also assist with some types of
  active transport.
• These carrier proteins are called cell membrane
  pumps.
• Proton pumps cause the production of ATP
  molecules in chloroplasts and mitochondria
• Sodium-potassium pumps use ATP molecules to
  create a high concentration of sodium ions
  outside the cell. Coupled channels carry the
  sodium ions along with food molecules back inside
  the cell.

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Membrane Pumps
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ENDOCYTOSIS AND EXOCYTOSIS

• Some substances are too large to enter a cell
  through protein channels.
• Endocytosis and exocytosis are forms of active
  transport used to move such molecules.
• they are engulfed and brought in by endocytosis.
  
• Waste materials and excretions are dumped outside
  by exocytosis.

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Endocytosis

• Endocytosis is the process by which cells ingest
  external fluids, macromolecules and large
  particles (including other cells!!!)
• These external materials are enclosed by a
  portion of the cell known as a vesicle.
• Biologists recognize two types of endocytosis
• phagocytosis movement of solids into the cell
• pinocytosis movement of liquids into the cell

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Endocytosis
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Exocytosis

• Exocytosis moves materials out of the cell
  (reverse of endocytosis).
• Vesicles in the cytoplasm
• fuse with the cell membrane
• releasing their contents
• into the external
• environment.

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Exocytosis