Cell Organelles

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Cell Organelles
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Cell Membrane

• Controls chemical traffic in and out of the cell
• Selectively Permeable
• 8 nm thick

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What is Selective Permeability?

• Allows some substances to cross more easily than
  others
• Why must it be selective?

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Why Must Cells Allow Some Substances to Pass
Across the Membrane?

• To maintain HOMEOSTASIS
• the tendency to maintain stability in an organism
  amid environmental change
• (ability to adjust to changes)
• --------------------------------------------------
  -------
• cells must be able to receive info, move water
  molecules, food particles, ions across the
  membrane

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Membrane Structure

• Synthesized in ER
• sugars added in ER Golgi

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Cell Membrane Synthesis

• AP Bio Book Reference

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Membrane Structure (Cond)

• Lipid Bilayer (2 layers of phospholipids)

layer 1 layer 2
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The Phospholipid

• hydrophilic (water loving) polar heads

• hydrophobic (water fearing) nonpolar tail

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Phospholipid Bilayer

• Polar head - attracted to H2O (hydrophilic)
• Nonpolar tails - push away or repel H2O
  (hydrophobic)
• --------------------------------------------------
  ---
• In order to best interact w/ H2O inside outside
  of cell, membrane forms 2 layers of phospholipids
• hydrophobic interactions hold membrane together

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What is allowed to easily pass through?

• 1) Nonpolar (hydrophobic) molecules dissolve
  easily in membrane
• -hydrocarbons (molecules with C H)
• -oxygen
• -(smaller molecules move faster)

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What can easily pass through?

• 2) Polar (hydrophilic) uncharged molecules
• - water, carbon dioxide

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What can easily pass through?

• 3) Small, polar molecules
• - pass easilly between membrane lipids

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What cannot pass through easily?

• Ions large polar molecules
• -do not easily pass thru membrane hydrophobic
  area

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Fluid Mosaic Model

• 1) Lipid Bilayer -tough but flexible
• 2) Transport (channel or carrier) Proteins
• 3) Receptor Proteins
• 4) Marker Proteins
• http//www.virtualcell.com/

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Lipid Bilayer (cond)

• 1) Stops large polar molecules
• -cannot pass thru NONPOLAR tails
• -thus, membrane serves as protective BARRIER

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Lipid Bilayer (cond)

• 2) is FLUID
• -not rigid
• -phospholipid proteins
• can move laterally
• (rarely flip)

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Membrane Fluidity

• Dependent on composition
• -unsaturated vs. saturated fatty acid tails
• -cholesterol

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Transport Proteins

• Look like doughnuts in membrane surface
• allow many impt. molecules ions to cross
• specific for substances they transport or
  translocate
• like locked doors - will only let some thru

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Transport (Carrier) Proteins
Can be carrier proteins which do not extend all
the way thru the membrane
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How Carrier Proteins Work
Carrier Proteins bond and drag molecule thru
bilayer and release on other side
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Transport (Channel) Proteins
Can be channel proteins that span the length of
the membrane
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How Channel Proteins Work
Molecules randomly move through by a process
called diffusion
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Physical Structure of Transport Proteins

• -unilateral embedded partway thru membrane
• -transmembrane completely span membrane

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Types of Transport Proteins

• 1) uniport carries single solute
• 2) symport
• -translocates 2 different solutes
• -move simultaneously in same direction
• 3) antiport
• exchanges 2 solutes
• transports molecules in opposite directions
• ex (Na/K pump)

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Receptor Proteins

• Shaped like boulders in membrane
• convey info to inside of cell (communication)
• -hormones
• special shape holds only certain type of molecule

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How Receptor Proteins Work

• when molecule of right shape fits receptor
  protein, it causes a change at other end of
  receptor, triggering response in cell

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Receptor Proteins
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Marker Proteins

• Look like trees sticking out of membrane
• Have carbohydrates on surface
• Name Tags of cells
• Different for every individual

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Marker Proteins
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Cell-Cell Recognition

• Based on recognition of cell surface
  macromolecules
• oligosaccharides are probably important cell
  recognition markers - vary!
• glycolipids
• -sugars covalently linked to lipids
• glycoproteins
• -sugars covalently linked to proteins
• N-linked asparagine
• O-linked serine, threonine

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How can proteins which can be polar fit into
nonpolar region of membrane?

• Proteins made of amino acids
• Of 20 aa, some polar some nonpolar
• Some aa can attract neighboring aa
• folding, twisting
• unique function

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Protein Variation

• Allows for channel proteins - embed themselves
  in membrane
• receptor proteins
• marker proteins

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