Cell Transport

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Cell Transport

• Passive Transport

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A. Introduction

• 1. When organisms adjust internally to changing
  external conditions, this is called homeostasis.
• 2. One way cells maintain homeostasis is by
  controlling the movement of substances across
  their cell membrane.
• 3. There are two main types of cell transport
  passive transport active transport.

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B. Passive Transport

• 1. Passive transport movement across the cell
  membrane that does NOT require energy
• 2. 3 types include diffusion, osmosis, and
  protein-aided diffusion

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• Concentration gradient A difference in the
  concentration of a substance across a space.
• a. In all forms of passive transport, substances
  move from an area of high concentration to an
  area of low concentration.
• b. This is called moving DOWN the concentration
  gradient.

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• Draw in movement down a concentration gradient

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I. Diffusion NO energy required!

• a. Diffusion depends on random particle movements
• b. Movement continues until equilibrium occurs

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• c. Equilibrium a condition in which the
  concentration of a substance is equal throughout
  a space.
• i. Even at equilibrium, particles continue to
  move back and forth

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II. Osmosis NO energy required!

• a. Osmosis refers to the diffusion of water
  through a selectively permeable membrane.
• b. Water will always move from an area of high
  concentration to an area of low concentration.
• c. There are 3 types of solutions in osmosis
  hypertonic, hypotonic, and isotonic.

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1. Hypertonic solution water moves out of a
cell, the cell shrinks

• a. The fluid outside the cell has a higher
  concentration of dissolved particles thus it has
  a lower concentration of free water molecules.
• b. There is a higher water concentration inside
  the cell

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• c. Water will move DOWN the concentration
  gradient (out of the cell) causing the cell to
  shrink.
• i. plant cells will go flaccid, plant will wilt
• ii. animal cells will shrink and die if too much
  water is lost

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Elodea in normal water Elodea after adding
salt solution
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1. Hypotonic solution water moves into of a
cell, the cell swells

• a. The fluid outside the cell has a lower
  concentration of dissolved particles thus it has
  a higher concentration of free water molecules.
• b. There is a lower water concentration inside
  the cell

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• c. Water will move DOWN the concentration
  gradient (into of the cell) causing the cell to
  swell.
• i. plant cells will become turgid due to water
  pressing outward against cell wall, helps plant
  cells keep their shape (and plant standing
  upright!)
• ii. animal cells will burst if they take in too
  much water

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• Red blood cells in a hypotonic solution

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1. Isotonic solution no NET water movement in
or out of cell

• a. The fluid outside the cell has the same
  concentration of dissolved particles as inside
  the cell.
• b. State of equilibrium exists.

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• c. Water moves in and out of cell at equal rates,
  so the cell size stays the same.
• i. Animal cells do best in isotonic solutions
• ii. Plants cells will survive but prefer a
  hypotonic solution

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Hypertonic, Isotonic, Hypotonic?

• Isotonic
• Hypotonic
• Hypertonic

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III. Protein-aided diffusion across membrane

• a. Certain molecules (ions and polar molecules)
  cannot pass through the non-polar interior of
  cell membrane (i.e. Na, K, Ca2, Cl-)
• b. Transport proteins can provide a passageway
  for ions polar molecules across the cell
  membrane

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1. Ion Channels

• -Doughnut shaped transport protein with polar
  pore through which ions can pass

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• a. The pores of some ion channels will always be
  open Other channels may be closed off by a gate.
• b. The opening and closing of the channel gates
  may be stimulated by
• i. Stretching of the cell membrane
• ii. Change in electrical charge
• iii. Binding of specific molecules

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2. Facilitated Diffusion

• -Carrier proteins transport substances (amino
  acids sugars) down the concentration gradient
• a. Specific molecules outside the cell bind to
  carrier proteins
• b. Carrier protein transports the molecule across
  membrane
• c. The molecule is released inside, and the
  carrier protein reverts back to normal shape

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C. Rate of Diffusion

• 1. How fast or how slow diffusion occurs depends
  on the temperature, size of molecules,
  concentration of molecules, and type of molecules
  (polor vs. non-polar).