Membrane Structure and Function

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Membrane Structure and Function
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CELL STRUCTURE AND FUNCTION

• 1. WHAT IS THE STRUCTURE OF THE PLASMA MEMBRANE?
• 2. WHAT ARE SOME CELL SURFACE MODIFICATIONS IN
  PLANTS AND ANIMALS?
• 3. HOW AND WHY DO SUBSTANCES CROSS THE PLASMA
  MEMBRANE?

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PLASMA MEMBRANE GATEWAY TO THE CELL

• THE MEMBRANE IS CONSTRUCTED OF LIPIDS AND PROTEINS

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LIPIDS

• THERE ARE 3 KINDS OF LIPIDS IN THE CELL MEMBRANE
• 1. PHOSPHOLIPIDS
• 2. CHOLESTEROL
• 3. GLYCOLIPIDS

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• PHOSPHOLIPIDS

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Phospholipids
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Membrane Models

• Fluid-Mosaic Model
• Membrane is a fluid phospholipid bilayer in which
  protein molecules are either partially or wholly
  embedded.

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• CHOLESTEROL

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CHOLESTEROL

• CHOLESTEROL STIFFENS THE MEMBRANE AND IS
  PERIODICALLY INSERTED IN BOTH THE INNER AND OUTER
  LAYER OF THE PHOSPHOLIPIDS.

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• GLYCOLIPIDS

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GLYCOLIPID

• PROVIDES PROTECTION THE LONG CARBOHYDRATECHAIN
  EXTENDS ABOVE THE CELL SURFACE TO CREATE A
  SLIME COAT
• AIDES IN CELL IDENTIFICATION
• TISSUE SPECIFIC
• INDIVIDUAL SPECFIC

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PROTEIN IN THE PLASMA MEMBRANE

• THERE ARE 5 TYPES OF PROTEIN
• 1. CHANNEL
• 2. CARRIER
• 3. CELL RECOGNITION
• 4. RECEPTOR
• 5. ENZYMATIC

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PROTEINS ARE EMBEDDED IN BILIPID LAYER
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CHANNEL PROTEIN

• LARGE STABLE MOLECULES THAT PASS THROUGH BOTH
  PHOSPHOLIPID LAYERS
• ALLOWS PARTICULAR MOLECULES OR IONS TO CROSS THE
  MEMBRANE FREELY

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Fig. 5.4a
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CARRIER PROTEIN

• SELECTIVELY INTERACTS WITH A SPECIFIC MOLECULE OR
  ION TO FACILITATE CROSSING THE MEMBRANE
• REQUIRES THE USE OF ENERGY

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Fig. 5.4b
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CELL RECOGNITION PROTEIN

• PROVIDES PROTECTION THE LONG CARBOHYDRATECHAIN
  EXTENDS ABOVE THE CELL SURFACE TO CREATE A
  SLIME COAT
• AIDES IN CELL IDENTIFICATION
• TISSUE SPECIFIC
• INDIVIDUAL SPECFIC

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RECEPTOR PROTEIN

• SHAPED IN SUCH A WAY AS TO ALLOW A SPECIFIC
  MOLECULE TO ATTACH TO THE CELL SURFACE.
• THIS MIGHT SIGNAL THE CELL TO REACT IN A CERTAIN
  WAY

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RECEPTOR PROTEINS HAVE A LOCK AND KEY MECHANISM
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ENZYAMATIC PROTEIN

• ACTIVE SITE IS LOCATED ON THE INNER SURFACE OF
  THE CELL
• CATALIZES SPECIFIC CHEMICAL REACTIONS
• OFTEN REQUIRES COENZYME

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Fluid-Mosaic Model
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Plasma Membrane Structure and Function

• Proteins may be peripheral or integral.
• Peripheral proteins are found on the inner
  membrane surface.
• Integral proteins are embedded in the membrane.

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Cell Surface Modifications

• Extracellular Matrix
• Meshwork of polysaccharides and proteins in close
  association with the cell that produced them.
• Plant Cell Walls
• Plants have freely permeable cell wall, with
  cellulose as the main component.
• Joined by plasmodesmata that spans cell wall and
  contains strands of cytoplasm that allow passage
  of material between cells.

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Fig. 5.16
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Cell Surface Modifications

• Cell Surfaces in Animals
• Junctions Between Cells
• Adhesion Junctions -Desmosomes
• Intercellular filaments between cells. Allows
  stretching and still holds cells together (heart)
• Tight Junctions
• Form impermeable barriers.(intestine)
• Gap Junctions
• Plasma membrane channels are joined allows
  communication. (nerve-muscle junction)

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CELLULAR TRANSPORT

• MOVING SUBSTANCES IN AND OUT OF THE CELL
• PASSIVE TRANSPORT- REQUIRES NO ENERGY IMPUT
• ACTIVE TRANSPORT- REQUIRES ENERGY

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Plasma Membrane Permeability

• Plasma membrane is differentially permeable.
• Plasma membrane is selectively permeable.
• Plasma membrane is semi-permeable.
• FOR OUR PURPOSES THESE TERMS ARE INTERCHANGEABLE
  SOME SUBSTANCES CAN PASS THROUGH AND OTHERS
  CANT

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CELLULAR TRANSPORT

• PHYSICAL- does not require a living cell and
  requires no outside energy
• DIFFUSION
• OSMOSIS

• BIOLOGICAL-does require a living cell and be
  either active or passive
• FACILITATED DIFFUSION
• ACTIVE TRANSPORT
• ENDOCYTOSIS
• EXOCYTOSIS

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Crossing Plasma Membrane
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Diffusion

• Diffusion - Movement of molecules from a higher
  to a lower concentration until equilibrium is
  reached.
• Down concentration gradient
• A solution contains a solute (solid) and a
  solvent (liquid).

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Diffusion
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Osmosis

• Osmosis - Diffusion of water across a
  differentially (selectively) permeable membrane
  due to concentration differences.
• Osmotic pressure is the pressure that develops
  due to osmosis.
• The greater the osmotic pressure, the more likely
  water will diffuse in that direction.

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Osmosis
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Osmosis

• Isotonic Solution - Solute and water
  concentrations both inside and outside the
  membrane are equal.
• Hypotonic Solution - Solution with a lower
  concentration of solute than the solution on the
  other side of the membrane.
• Cells placed in a hypotonic solution will swell.
• Lysis

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Osmosis

• Hypertonic Solution - Solution with a higher
  concentration of solute than the solution on the
  other side of the membrane.
• Cells placed in a hypertonic solution will
  shrink.
• Plasmolysis

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Fig. 5.9
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Transport through channel proteins

• Channel proteins allow certain molecules to be
  transported through the membrane.
• Facilitated Diffusion
• Small molecules follow concentration gradient by
  passing through channel proteins.

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Transport by Carrier Proteins

• Active Transport
• Small molecules move against their concentration
  gradient by combining with carrier proteins.
• This requires that energy in the form of ATP be
  supplied

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Membrane-Assisted Transport

• Endocytosis - Cells take in substances by vesicle
  formation.
• Phagocytosis - Large, solid material.
• Pinocytosis - Liquid or small, solid particles.
• Receptor-Mediated - Specific form of pinocytosis
  using a coated pit.

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Membrane-Assisted Transport

• Large marcomolecules are transported out of the
  cell by vesicle formation.
• Exocytosis - Vesicles fuse with plasma membrane
  as secretion occurs.

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REVIEW

• DRAW AND LABEL THE CELL MEMBRANE

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REVIEW

• A seaweed cell is placed in seawater. The cell
  has the following ion concentrations
• Ca 1.7, Mg .005, Na 490, and Cl 500-600.
• The seawater has the following ion
  concentrations
• Ca 12.o, Mg 57, Na 500, K 12, and Cl 520.
• Using a diagram with arrows, show which way ions
  will move, distinguish diffusion, active
  transport and osmosis.

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Seaweed cell in seawater
Cell
Seawater Ca 12 Mg 57 Na 500 K 12 Cl 520
Ca 1.7 Mg .005 Na 90 K 490 Cl 500- 600

• 0
• K 490Mg .005
• Cl 500-
• 600

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Seaweed cell in seawater
Active

• Diffusion

Cell
Seawater Ca 12 Mg 57 Na 500 K 12 Cl 520
Ca 1.7 Mg .005 Na 90 K 490 Cl 500- 600
Diffusion
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