Membrane Structure and Function

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Membrane Structure and Function
Topic 3
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Plasma Membrane Functions
Maintain a high concentration of materials in the
cell.
Keep harmful materials out.
Control the movement of materials into and out of
the cell.
Let the cell sense its environment.
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Membrane Structure

• Phospholipid bilayer
• Amphipathic
• liquid-crystal properties/behavior allow for
  constant movement of HC tails

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The Plasma Membrane is Semipermeable
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Davson-Danielli vs. Fluid Mosaic
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Membrane Characteristics

• self-sealing
• Flexible
• Tend to form vesicles
• (ENDOCYTOSIS)
• Tend to fuse w/ other bilayers
• (EXOCYTOSIS)
  

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LIPID MOBILITY
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http//www.d.umn.edu/sdowning/Membranes/phospholi
pidlateralmovanim.html
http//www.d.umn.edu/sdowning/Membranes/phospholi
pidrotationalanim.html
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Relationships w/ other intracellular membranes

• Nuclear Rough
• Memb. ER
• vesicles
• Golgi Bodies
• lysosomes vesicles
  cell memb
• vacuoles

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MEMBRANE PROTEINS

• Integral Proteins
• Anchored into membrane
• Transmembrane proteins

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• Peripheral Proteins
• no anchoring to membrane
• often bound to exposed regions of integral
  proteins

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PROTEIN MOBILITY
http//www.d.umn.edu/sdowning/Membranes/proteinmo
bilityanim.html
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Proteins Are Critical to Membrane Function
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Membrane Protein Functions

• Cell Adhesion
• Communication Channels
• Transport Channels
• Signal Receptors btwn. Ext/Int cell
• Attachment Sites
• Pumps
• Enzymes

• Peripheral
• Integral
• Integral
• Integral
• Peripheral
• Integral
• Both

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Protein Signaling
http//www.d.umn.edu/sdowning/Membranes/signaling
anim.html
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Membrane Components
Phospholipids
Proteins (peripheral and integral)
Cholesterol
Carbohydrates
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The Plasma Membrane Gateway to the Cell
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MOVEMENT ACROSS THE CELL MEMBRANE
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Transport Processes - Diffusion
Solutes move down a concentration gradient until
they are evenly distributed. This is diffusion.
Another way of saying this is that solutes move
from a region of higher concentration to a region
of lower concentration until there is no
difference in concentration.
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Methods of Movement

• PASSIVE TRANSPORT
• DIFFUSION
• SIMPLE DIFFUSION
• OSMOSIS
• FACILITATED DIFFUSION

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Three Forms of Transport Across the Membrane
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• ACTIVE TRANSPORT
• PUMPS
• EXOCYTOSIS
• ENDOCYTOSIS
• PHAGOCYTOSIS
• PINOCYTOSIS
• RECEPTOR-MEDIATED

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DIFFUSION

• Cell obtains / releases substances by taking
  advantage of natural properties of movement
  (kinetic E)!

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Diffusion is

• NET movement of mcls from
• Down the gradient . NO Energy required.

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Rate of Diffusion influenced by

• Size / shape of molecules moving
• Electrical charges across membrane
• Temperature ( movement,
• . Diffusion )

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• Substances diffuse down their own gradient
• NOT affected by gradient of other
  substances !!

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Diffusion continues until

• Dynamic
• Equilibrium
• is
• reached!

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OSMOSIS

• Diffusion of water across the semi-permeable
  plasma membrane.
• Water moves down its own gradient.
• Zones of Hydration

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Spheres of Hydration
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Osmotic Pressure

• tendency of water to move into a solution by
  osmosis.
• High solute low water
• High Osmotic Pressure

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Comparing Solutions

• HYPOTONIC
• refers to the less concentrated
• (more dilute) of two solutions.

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• HYPERTONIC
• refers to the more concentrated
• (less dilute) of two solutions.

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• If a cell is placed in a hypotonic solution, the
  cell will.
• S W E L L and
• B U R S T !!
• Turgidity issues

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• If a cell is placed in a hypertonic
• solution, the cell will
• S H R I N K !
• (CRENATION)

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• ISOTONIC
• refers to two solutions where there is no net
  movement of water between the two.

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• a cell is placed in an isotonic solution, there
  will be.
• no net movement of water
• into / out of the cell.

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The Problem of Osmotic Pressure

• Whats the risk?
• How can osmotic pressure be minimized?

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• Live in an isotonic enviroment
• Contractile vacuoles
• plasmolysis

• Marine invertebrates
• Human RBCs
• Unicellular protists
• Plant cells

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Contractile Vacuoles
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Plasmolysis Cell Wall Action
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Transport Problems ?

• CYSTIC FIBROSIS
• LIDDLES SYNDROME