Cell Membrane

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Cell Membrane Cellular Transport
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HOMEOSTASIS AND TRANSPORT

• Cell membranes help organisms maintain
  homeostasis by controlling what substances may
  enter or leave cells
• Some substances can cross the cell membrane
  without any input of energy by the cell
• The movement of such substances across the
  membrane is known as passive transport
• To stay alive, a cell must exchange materials
  such as food, water, wastes with its
  environment
• These materials must cross the cell or plasma
  membrane

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Plasma Membrane

• Boundary that separates the living cell from its
  non-living surroundings.
• Phospholipid bilayer (2 parts)
• hydrophilic heads
• hydrophobic tails

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Phospholipid Bilayer
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Plasma Membrane - cont.

• Controls traffic into and out of the cell with
  phospholipids and transport proteins.
• Selectively permeable
• (allows some molecules to move across cell
  membrane)

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• Small molecules like water, oxygen, carbon
  dioxide can move in and out freely
• Large molecules like proteins carbohydrates
  cannot move easily across the plasma membrane
• The Cell Membrane is semi permeable or
  selectively permeable only allowing certain
  molecules to pass through

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Diffusion

• The net movement of a substance (molecules) down
  a concentration gradient from an area of high
  concentration to an area of low concentration.
• When the molecules are even throughout a space-
  it is called EQUILIBRIUM
• Passive transport NO energy is required.
• Facilitated diffusion Type of passive transport
  which uses transport proteins.

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Diffusion

• Diffusion is the movement of molecules from an
  area of higher concentration to an area of lower
  concentration
• Small molecules can pass through the cell
  membrane by a process called diffusion
• Diffusion across a membrane is a type of passive
  transport because it does not require energy
• Diffusion continues until the concentration of
  the molecules is the same on both sides of a
  membrane

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• Diffusion is driven by the kinetic energy of the
  molecules
• Kinetic energy keeps molecules in constant motion
  causing the molecules to move randomly away from
  each other in a liquid or a gas
• The rate of diffusion depends on temperature,
  size of the molecules, type of molecules
  diffusing
• Molecules diffuse faster at higher temperatures
  than at lower temperatures
• Smaller molecules diffuse faster than larger
  molecules

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Osmosis

• The diffusion of water across a semipermeable
  membrane is called osmosis
• The water moves from a high concentration to low
  concentration.
• 3 kinds of Osmosis in cells
• Hypotonic, Hypertonic, Isotonic

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OSMOSIS

• This diagram shows water molecules moving across
  a selectively permeable membrane. Water molecules
  are the small blue shapes, and the solute is the
  green.
• What's happening? The solute (green blobs) is
  more concentrated on the right side, which pulls
  the water molecules toward that side. The green
  blobs would move to the left to spread out
  evenly, but the membrane won't let those pass

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Solutions

• Solutions have two parts --- the solute which is
  being dissolved in the solvent
• Water serves as the main solvent in living things

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Hypertonic Solution

• Solute concentration outside the cell is higher
  (less water)
• Water diffuses out of the cell until equilibrium
  is reached
• Cells will shrink die if too much water is
  lost
• Plant cells become flaccid (wilt) called
  plasmolysis

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Hypotonic Solution

• Solute concentration is more inside the cell.
• Water moves into the cell until equilibrium is
  reached
• Animal cells swell burst (lysis) if they take
  in too much water
• Cytolysis is the bursting of cells
• Plant cells become turgid due to water pressing
  outward against cell wall
• . Turgor pressure in plant cells helps them keep
  their shape

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Isotonic Solutions

• Concentration of solutes same inside outside
  the cell
• Water moves into out of cell at an equal rate
  so there is no net movement of water

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Animal Cells

• Animal cells placed into a hypotonic solution
  will HEMOLYSIS (EXPLODE).
• Animal cells placed into a hypertonic solution
  will CRENATE (SHRIVEL).

Hemolysis
Crenation
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Plant Cells

• Firmness or tension (vacuole full) that is found
  in plant cells (cell wall) that are in a
  hypotonic environment is called TURGID.
• This process is called TURGOR PRESSURE.

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Plant Cells

• When the plasma membrane pulls away from the cell
  wall (vacuole empty) in a hypertonic environment
  (loss of water) is called PLASMOLYSIS.

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Effect of  Solutions on Cells
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                             
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Facilitated Diffusion

• Faster than simple diffusion
• Considered passive transport because extra energy
  not used
• Occurs down a concentration gradient
• Involves carrier proteins embedded in a cells
  membrane to help move across certain solutes such
  as glucose
• Carrier molecules change shape when solute
  attaches to them
• Change in carrier protein shape helps move solute
  across the membrane

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Active Transport

• The movement of molecules (small or large) across
  the plasma membrane in which energy (ATP) is
  required.
• Moves materials against their concentration
  gradient from an area of lower to higher
  concentration

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Sodium-Potassium Pump

• The mechanism that uses energy (active transport)
  released from splitting ATP to transport Sodium
  (Na) out of and Potassium (K) into cells.

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Bulk Transport

• Moves large, complex molecules such as proteins
  across the cell membrane
• Large molecules, food, or fluid droplets are
  packaged in membrane-bound sacs called vesicles
• Endocytosis moves large particles into a cell

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• There are two forms of endocytosis
• a) Pinocytosis -  a form of endocytosis that
  transports liquids into the cell.
• b) Phagocytosis - a form of endocytosis that
  transports solids into the cell
• Exocytosis is used to remove large products from
  the cell such as wastes, mucus, cell products
• ? Exocytosis is the movement of a substance out
  of a cell via vesicles.  Exocytosis is the
  reverse of endocytosis

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Animation of endocytosis exocytosis