A Closer Look at Cell Membranes

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A Closer Look at Cell Membranes

• Starr/Taggarts
• Biology
• The Unity and Diversity of Life, 9e
• Chapter 5

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Key Concepts

• Cell membrane consists of a bilayer of
  phospholipids and embedded proteins
• Transport proteins span the bilayer
• Open channels, gated channels, carriers, and
  pumps
• Receptor proteins receive chemical signals
• Recognition proteins are used for identification

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Key Concepts

• Diffusion is the movement of ions from a region
  of higher concentration to one of lesser
  concentration
• Osmosis is the movement of water across a
  selectively permeable membrane to a region where
  its concentration is lower
• Some membrane proteins function in passive
  transport whereas some function in active
  transport

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Plasma Membrane

• Bilayer of phospholipids
• Hydrophilic phosphate head
• Hydrophobic tails of fatty acids
• Fluid-Mosaic Model
• Phospholipids
• Glycolipids
• Sterols
• Proteins

lipid bilayer
water
water
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Fluid Mosaic Model

• Specialized proteins and enzymes embedded in the
  membrane

oligosaccharide groups
phospholipid
cholesteral
EXTRACELLULAR ENVIRONMENT
(cytoskeletal proteins beneath the plasma
membrane)
RECEPTOR PROTEIN
RECOGNITION PROTEIN
LIPID LAYER
ADHESION PROTEIN
(area of enlargement)
TRANSPORT PROTEINS
CYTOPLASM
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How Substances CrossCell Membranes

• Diffusion
• Movement of substance from a region where it is
  more concentrated to a region where it is less
  concentrated

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O2, CO2, other small nonpolar molecules, as well
as H2O
C6H12O6, other large, polar water-soluble
molecules, ions (such as H, Na, K, Ca, CI-)
along with H2O
X
Fig. 5.7, p. 84
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How Substances CrossCell Membranes

• Water soluble substances and water diffuse
  through channel proteins
• Passive and Active Transport

high
concentration gradient
P
energy input
low
DIFFUSION ACROSS LIPID BILAYERS lipid-soluble subs
tances as well as water diffuse across
PASSIVE TRANSPORT Water-soluble substances, and
water, diffuse through interior of
transport proteins. No energy boost
required. Also called facilitated diffusion
ACTIVE TRANSPORT Specific solutes are pumped
through interior of transport proteins. Requires
energy boost
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Factors Influencing the Rate and Direction of
Diffusion

• Concentration gradient
• Until equilibrium is reached
• Molecular size
• Small molecules move faster
• Temperature
• Faster at higher temperatures
• Electric or Pressure gradient
• Electrical charge difference across membrane
• Pressure differences

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Mechanisms By Which Solutes Cross Cell Membranes

• Diffusion of small non-polar molecules and water
• Passive transport
• Facilitated diffusion
• Polar substance transport through proteins
• Active transport
• Movement of substance against the concentration
  gradient
• Requires ATP

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Directional Movement of Water Across Membranes

• Osmosis
• Diffusion of water due to a water concentration
  gradient between two regions that are separated
  by a selectively permeable membrane
• Osmotic movement
• Dependent on concentration of solutes in the
  water
• Side with more solutes has a lower concentration
  of water

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Osmosis
selectively permeable membrane between two
compartments
water molecules
protein molecules
Effect of solute concentration on water movement
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Effects of Tonicity
2M sucrose solution

• The solute concentrations of two fluids
• Hypotonic
• Water diffuses in
• Cell swells
• Hypertonic
• Water diffuses out
• Cell shrinks
• Isotonic
• No net change

1 liter of distilled water
10M sucrose solution
2M sucrose solution
HYPOTONIC CONDITIONS
HYPERTONIC CONDITIONS
ISOTONIC CONDITIONS
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Effects of Fluid Pressure

• Volume of fluid exerts hydrostatic pressure
• Force against a wall or membrane

compartment 1
compartment 2
membrane permeable to water but not to solutes
fluid volume increases In compartment 2
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Protein Mediated Transport

• Passive transport
• Allows polar molecules to move from one side of
  membrane to the other

open channel proteins
gated channel proteins
lipid bilayer
transport protein
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Passive Transport

• Solute transport through transport protein
• Movement
• From higher to lower concentration
• No ATP required

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Active Transport

• ATP required
• Movement is against the concentration gradient
• Sodium-potassium pump
• Calcium pump

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EXOCYTOSIS Vesicle in cytoplasm moves to plasma
membrane, fuses with it contents released to the
outside
Fig. 5.17, p. 90
ENDOCYTOSIS Vesicle forms from a patch of
inward-sinking plasma membrane, enters cytoplasm
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indentation on surface of plasma membrane facing
extracellular fluid
lipoprotein particles bound to membrane receptors
self-sealing behavior of plasma membrane
fully formed vesicle moving deeper into cytoplasm
0.1 µm
clathrin filaments of coated pit
Fig. 5.18, p. 90
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Phagocytosis

• Form of endocytosis
• Cell engulfs microbes, large particles, and
  cellular debris
• Amoebas and white blood cells

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contractile vacuole (emptied)
contractile vacuole (filled)
Fig. 5.22, p. 93
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Membrane Cycling
exocytic vesicle
clathrin

• Cycling of membrane by endocytosis and exocytosis
  
• Vesicles from ER and Golgi bodies contribute
• Receptor proteins and lipids recycled

endocytin vesicle
Golgi body
lysosome
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In Conclusion

• Plasma membrane is structural and functional
• Cell membrane is a bilayer of lipids with
  proteins embedded
• Fluid mosaic model of cell membrane with diverse
  lipids and proteins within
• Transport proteins allow water-soluble
  substances to pass through membranes

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In Conclusion

• Receptor proteins bind substances
• Recognition proteins allow cells to be recognized
  
• Adhesion proteins allow cells to adhere to each
  other
• Molecules tend to move from regions of higher to
  lower concentration

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In Conclusion

• Diffusion rates are influenced by concentration
  gradients, temperature, molecular size,
  electrical charge and pressure
• Osmosis is the diffusion of water across a
  selectively permeable membrane in response to
  concentration gradients
• Water moves from hypotonic solution to a
  hypertonic solution

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In Conclusion

• Small non-polar molecules diffuse across the
  membranes bilayer
• Ions and polar substances move across by passive
  or active transport
• Transport proteins shunt solutes across membrane
• Passive transport allows movement down a
  concentration gradient

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In Conclusion

• Active transport pumps a solute across the
  membrane against the concentration gradient using
  ATP
• Exocytosis entails movement of a vesicle to the
  plasma membrane and release of particles
• Endocytosis entails infolding of the plasma
  membrane to engulf particles
• developed by M. Roig

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glacier
seawater diluted with glacial meltwater
saltier seawater
Fig. 5.1, p. 78
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Fig. 5.16, p. 89