Chapter 7

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Chapter 7 Cell Transport
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I. Cell (Plasma) Membrane

• Recall from Chapter 7 that the cell membrane is
  composed of lipids (because they can separate
  substances.)

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Cell Membrane (contd)

• The membrane is made of many phospholipids which
  have two parts hydrophilic (water loving) heads
  and hydrophobic (water fearing) tails.

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Cell Membrane (contd)

• Notice also that the membrane is made of two
  layers of lipids called a lipid bilayer.

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Cell Membrane (contd)

• Also found in the cell membrane are certain
  proteins, which play an important part in the
  cell by
• 1) Coordinating cell-to-cell behaviors.
• 2) Adjusting the cells metabolism rates.
• 3) Initiating growth division times.

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Summary of Cell Membrane Structure

• Membrane is semi permeable
• Membrane is composed of a lipid bilayer (2 layers
  of lipids and phosphates)
• Membrane has many different types of proteins
  embedded into it
• These 2 things together make the fluid mosaic
  model

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Cell Membrane (contd)
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A. Marker Proteins

• Helps cells recognize similar cells.
• This is helpful for when our defensive cells
  encounter bacteria with unknown marker proteins,
  they can then destroy them.

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B. Receptor Proteins

• Helps cells receive materials/signals outside of
  the cell.
• When cells need stored energy from the pancreas,
  they release signal molecules, which find and
  bind with the pancreas cells to let them know to
  release some energy.

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C. Channel/Carrier Proteins

• Helps substances move across the cell membrane.
• If substances are too large to get through the
  lipid bilayer channel proteins (doughnut shaped)
  act as passage ways.

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II. How Substances Move

• There are 2 types of transport across cell
  membranes
• Passive transport that does not need any
  energy.
• Active transport that requires energy.

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Passive Transport

• Passive transport (no energy required) can occur
  in 4 different ways
• i) Simple Diffusion
• ii) Osmosis
• iii) Diffusion through Ion Channels
• iv) Facilitated Diffusion (Carrier Proteins)

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Passive Transport (contd)

• During passive transport substances always move
  from high concentration to low concentration
  (this is known as a concentration gradient).
• After passive transport when the concentration of
  molecules is the same we say equilibrium exists.

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i) Diffusion

• Diffusion is the movement of molecules down a
  concentration gradient, from high to low
  concentrations.
• Examples food coloring in water, perfume
  spreading throughout a room, etc.

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ii) Osmosis

• Osmosis is the diffusion of water across a
  semipermeable (cell) membrane.
• It is the same process as diffusion except it
  specifically means water movement.

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a) Solutions

• A solution is a mixture of solutes (things being
  dissolved) and a solvent (things doing the
  dissolving).
• Water is typically the solvent. (i.e. sugar cube
  in water, the sugar is the solute)

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b) Types of Solutions

• 1. Hypertonic solution with a higher
  concentration of dissolved particles than its
  surroundings.
• 2. Hypotonic solution with a lower concentration
  of dissolved particles than its surroundings.
• 3. Isotonic solution with an equal concentration
  of dissolved particles to its surroundings.

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c) Water Balance in a Cell

• 1. Cells placed in a hypertonic environment (salt
  water) will plasmolysis. SHRINK!

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c) Water Balance in a Cell

• 2. Cells placed in a hypotonic environment (fresh
  water) will cytolysis. BURST!

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c) Water Balance in a Cell

• 3. Cells placed in a isotonic environment will be
  balanced.

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Water Movement Problems
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iii) Diffusion through Ion Channels

• An ion channel is a doughnut shaped channel
  protein that is hydrophilic (polar) inside.
• Ions can pass through the channels without
  contacting the hydrophobic (nonpolar) part of the
  cell membrane.

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Ion Channel (B)
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iv) Facilitated Diffusion

• Facilitated diffusion is a passive form of
  transport caused by carrier proteins specific for
  the molecules they transport.
• Remember movement is always down the
  concentration gradient. (i.e. high to low)
• Also facilitated diffusion needs no energy.

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a) Steps in Facilitated Diffusion

• 1. A molecule outside the cell binds to the
  carrier protein.
• 2. The carrier protein transports the molecule
  across the cell membrane.
• 3. The molecules are released inside the cell.

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B) Active Transport

• There are many types of active transport, but the
  2 we will learn are
• i) The Sodium-Potassium (Na-K) Pump
• ii) Movement by Vesicles

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Active Transport (contd)

• Active transport is the movement of molecules up
  (against) the concentration gradient, from low to
  high.
• Remember this time the cell must use its own
  energy (ATP) to move substances.

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i) Sodium Potassium Pump

• It takes 3 sodium ions out for every 2 potassium
  ions put in the cell.
• There is more sodium inside the cell and more
  potassium outside the cell so the cell is working
  against the concentration gradient.
• Energy for the pump comes from ATP made by the
  mitochondria.

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ii) Movement by Vesicles

• There are 2 types of vesicles
• Endocytosis into the cell.
• Exocytosis out of the cell.

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Any Questions?

• Knowledge is not simply another commodity. On
  the contrary. Knowledge is never used up. It
  increases by diffusion and grows by dispersion.
  
• --Daniel J. Boorstin