Cell Membrane Structure

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Cell Membrane Structure Cell Transport Notes
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Cell Membrane

• Fluid mosaic model
• Mosaic ? mixed composition primarily of
  phospholipids and proteins.
• Fluid ? it is not a solid layer it is
  selectively permeable.
• Phospholipids are arranged in a bilayer
• Proteins on the surface (act as markers)
• Proteins embedded in the bilayer (transport)

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Structure of Phospholipids
Hydrophilic, Water-Loving, Polar

• Phospholipids contain.
• hydrophilic phosphate head
• 2 hydrophobic fatty acid tails

Hydrophobic, Water-Fearing, Non-Polar
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Phospholipid Bilayer

• Forms due to the polarity of the phospholipids
• Phosphate heads (polar)
• Close to the water of the cytoplasm and/or water
  inside the cell
• Face outward
• Fatty acid tails
• Face inward to avoid being near water
• Creates a barrier b/w inside and outside of the
  cell

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Phospholipid Bilayer
Phosphate heads
Fatty acid Tails
Phosphate heads
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Transport Proteins

• Span the phospholipid bilayer
• Interior is able to open to both sides
• May change shape when they interact with solute
• Move water-soluble substances across a membrane
• May be passive or active

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Cell Membrane Structure
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Cell Membrane Structure
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Cell Membrane Function

• Creates a barrier b/w inside outside of cell
• Proteins that pass through the entire bilayer act
  a channel to transport large substances OR
  transport substances against their gradients

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Cell Membrane Selectively Permeability

• CAN PASS THRU MEMBRANE
• -------------------------------------
• O2 and CO2 (non-polar)
• Some polar water molecules (slip through gaps in
  the non-polar lipid bilayer that occur when the
  fatty acid tails bend)

• CANNOT PASS THRU MEMBRANE
• -------------------------------------
• Ions (charged)
• Large polar molecules

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Cell Transport

• Passive Transport
• Uses no ATP
• High ? Low Concentration
• Down the concentration gradient
• Ex.
• Simple diffusion
• Osmosis
• Facilitated Diffusion

• Active Transport
• Uses ATP
• Low ? High Concentration
• Against the concentration gradient
• Ex.
• Transport/Carrier Protein
• Endocytosis
• Exocytosis

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Types of Passive Transport

• Simple Diffusion ? any movement from high to low
  concentration
• Osmosis ? water moved from high to low
  concentration
• Facilitated Diffusion ? using a transport protein
  to move a substance from a high to a low
  concentration

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Simple Diffusion A Type of Passive Transport

• Molecules are always moving ? called Brownian
  Motion
• Constant random motion of molecules
• Dynamic Equilibrium
• Continuing movement of molecules with NO
  overall/net change in diffusion
• Substances diffuse independently of each other
• Each molecule follows their own concentration
  gradient

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Simple Diffusion A drop of dye inters a bowl of
water. Gradually, the dye molecules become
evenly dispersed through the molecules of water.
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Factors Affecting Diffusion Rate

• Steepness of concentration gradient
• Steeper gradient ? faster diffusion
• Molecular size
• Smaller molecules ?faster diffusion
• Temperature
• Higher temperature ? faster diffusion

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Concentration Gradients

• Molecules ALWAYS move down their concentration
  gradient
• From high ? low

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Osmosis A Type of Passive Trasnport

• Net movement of water from high ? low
• Between two regions separated by a selectively
  permeable membrane

Red dot H20
Blue Dot Solute
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Solute concentrations effect osmosis The
membrane side with the most solute particles has
the lower water concentration.
semipermeable membrane between two compartments
protein molecule
water molecule
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How will Water Move?

• Water will always move from where there is more
  water (less solute) to where there is less water
  (more solute)

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Water Always moves from a hypotonic to a
hypertonic solution

• Hypotonic solution
• More water
• Less solutes
• Hypertonic solution
• Less water
• More solutes
• Isotonic solution
• Same water
• Same solute

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2 sucrose solution
2 sucrose solution
10 sucrose solution
distilled water
Hypotonic Conditions
Hypertonic Conditions
Isotonic Conditions
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Turgor Pressure Plasmolysis

• Occurs in plants b/c of the cell wall
• Turgor Pressure
• Occurs when water moves from hypotonic
  environment into the hypertonic plant cell
• Cell membrane expands and pushes against the cell
  wall creating pressure
• Plasmolysis
• Occurs when water moves from hypotonic plant cell
  to a hypertonic environment
• Cell membrane shrivels (plants wilts may die)

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Turgor Pressure Plasmolysis
TURGOR PRESSURE
PLASMOLYSIS
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Facilitated Diffusion A Type of Passive
Transport

• Using a protein to move a substance from where it
  is in a high concentration to where it is in a
  low concentration

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Active Transport

• Uses energy (ATP)
• Moves from low to high concentration
• Against concentration gradient
• Several types
• Transport Protein
• Endocytosis Phagocytosis
• Exocytosis

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Transport Protein A Type of Passive Transport

• Na/K Pump

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Endocytosis A Type of Active Transport

• Moves bulk items that are larger

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Endocytosis

• A patch of cell membrane forms a vesicle that
  sinks into the cytoplasm.
• Cell takes in molecules
• A special type is called phagocytosis

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Phagocytosis or Cell Eating

• Active (requires ATP)
• Changes in microfilaments change cell shape to
  form a bulging lobe that surrounds target and
  forms a vesicle

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Exocytosis A Type of Active Transport

• A cytoplasmic vesicle fuses with the cell
  membrane and its contents are released outside.

Cell membrane
cytoplasm