It is time I stepped aside for a less experienced and less able man'

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"It is time I stepped aside for a less
experienced and less able man." - Professor
Scott Elledge on his retirement from Cornell
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Membrane structure function
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The plasma membrane separates the living cell
from its nonliving surroundings plasma membrane
is selectively permeable allowing some
substances to cross more easily than others main
macromolecules in membranes are lipids and
proteins, but include some carbohydrates
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Phospholipids are amphipathic molecules have both
hydrophobic regions and hydrophilic regions
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Two types of membrane proteins
Peripheral proteins are not embedded in the lipid
bilayer they are loosely bounded to the surface
of the protein, often connected to the other
population of membrane proteins
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may differ in lipid and proteins
composition outer surface also has carbohydrates
? impt for cell/cell recognition asymmetrical
orientation begins during synthesis in the ER
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Proteins in the plasma membrane may provide a
variety of major cell functions
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• Cell membranes semi-permeable barriers allow
  some molecules to pass through, but block others.
• Whether or not a molecule can pass through
  depends on its
• Size
• Electrical charge overall charge ( or -)
• Polarity areas of molecule are - due to
  uneven distribution of electrons

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Methods of Membrane Transport

• Passive Transport
• Active Transport
• Cytosis

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1. Passive Transport - spontaneous no work
required - releases energy - includes a)
simple diffusion b) facilitated diffusion
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Diffusion

• spreading out of molecules
• movement of molecules from areas of high
  concentration to areas of low concentration
• particles move downhill down a gradient
  (difference) in concentration
• due to random motion of atoms in nature
• no energy needed instead, energy is released
  as molecules go downhill

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• Examples of diffusion
• dye diffusing through water
• perfume diffusing through air

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Molecules that can cross cell membranes by simple
diffusion
Molecules that cant
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Osmosis simple diffusion of water from a high
water to a low water across a selectively
permeable membrane
Fig. 8.11
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Isotonic solution solution with a solute
concentration that is equal to that of another
solution
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Hypotonic solution a solution with a lower
concentration of solutes than another
hypotonic
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Hypertonic solution solution with a higher
concentration of solutes than another
hypertonic
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The H2O balance of living cells
Fig. 8.12
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contractile vacuole
Paramecium
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• Facilitated diffusion
• diffusion of molecules across cell membrane
  w/help of transport proteins embedded in the
  membrane
• molecules move down a gradient (from high to
  low) no energy needed
• a transport protein may be either

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• Molecules that cross by facilitated diffusion
• sugars (ex. glucose)
• amino acids (20 different kinds)
• ions (charged atoms)
• - sodium Na
• - potassium K
• - calcium Ca
• - chloride Cl-

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2. Active Transport the movement of molecules
across a membrane up a gradient (from low
concentration to high concentration) with the
aid of a carrier protein and energy
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Review passive vs. active transport
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3. Cytosis movement of substances in/out of
cells via vesicles
a. exocytosis vesicle in cytoplasm fuses with
cell membrane, dumping molecules out of cell b.
endocytosis region of cell membrane folds
inward pinches off to form vesicle, bringing
molecules in to cell
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• Endocytosis includes
• pinocytosis cell drinks in fluids containing
  large molecules (proteins, nucleic acids,
  polysaccharides)

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• phagocytosis cell eats (engulfs) dead
  dying cells, invading bacteria, etc.

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Pinocytosis (drinking) is common in all animal
cells, but Phagocytosis (eating) is rare
occurs only in a few specialized cells
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Receptor-mediated endocytosis is very specific in
what substances are being transported triggered
when extra-cellular substances bind to receptors,
ligands, on the membrane surface triggering the
formation of a vesicle
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Cell-to Cell Junctions

• Plants
• Plasmodesma
• Animals
• Tight Junctions
• Adhering Junctions
• desmosome
• Gap Junctions

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