1.3 Membrane structure
Understandings:
• * Phospholipids form bilayers in water
due to amphipathic properties
• * Membrane proteins are diverse
• * Cholesterol is a component of
animal cell membranes
Nature of science:
* Using models as representations of the
real world; there are alternative models of
membrane structure
* Falsification of theories with one theory
being superseded by another: evidence
falsified the Davson-Danielli model
Applications:
• Cholesterol in mammalian
membranes reduces
membrane fluidity and
permeability
Skills:
• Drawing the fluid mosaic model
• Analysis of evidence from electron microscopy
that led to proposal of Davson-Danielli model
• Analysis of falsification of the Davson-Danielli
model that led to the Singer-Nicolson model
Understanding: Phospholipids form bilayers in water due to
the amphipathic properties of phospholipid molecules.
Hydrophilic = water-loving (attracted to water)
Hydrophobic = water-fearing (not attracted to water)
Amphipathic = both (part of molecule attracted to water, other part not)
Ambi- and Amphi- = two or both
Name a word that starts with ambi- or amphi-….???
Phospholipid = phosphate head + hydrocarbon tail
http://www.uic.edu/classes/bios/bios100/lectures/membranes
01.htm
http://telstar.ote.cmu.edu/biology/MembranePage/index2.htm
l
• Nature of Science: Using models as representations of the real world:
there are alternative models of membrane structure
1920s – Gorter & Grendel = bilayer model
1935 - Davson-Danielli model = Sandwich model
• 1970s – Danielli = protein lining the pores in the membrane model
• 1970s – Singer & Nicolson = Fluid Mosaic Model
Fluid Mosaic model
• Fluid = phospholipids & proteins can move laterally within each layer
• Mosaic = protruding proteins appear dark in electron microscope &
are like tiles in a mosaic.
Nature of Science: Falsification of theories: evidence falsified
the Davson-Danielli model
• Freeze-etched electron micrographs – showed transmembrane
proteins
• Structure of membrane proteins – found proteins that were globular
in shape and hydrophobic (attracted to hydrocarbon tails inside
membranes)
• Fluorescent antibody tagging – mixed throughout membrane,
showing proteins can move within membrane; not fixed on top
Oliver Cromwell’s Rule
• “Think it possible that you might be mistaken.”
• http://en.wikipedia.org/wiki/Cromwell's_rule
Skill: Evidence for and against the DavsonDanielli model of membrane structure
• For homework!
• P 28, purple box, #1-4
• Due Monday!
Understanding:
Membrane proteins are diverse in terms of structure,
position in the membrane & function.
1.
2.
3.
4.
Hormone-binding sites (hormone receptors, e.g. insulin receptor)
Immobilized enzymes with active site on outside
Cell adhesion to form tight junctions between cells
Cell-to-cell communication (e.g. receptors for neurotransmitters at
synapses)
5. Channels for passive transport (to allow hydrophilic molecules
across membrane by facilitated diffusion)
6. Pumps for active transport (use ATP to move molecules across
membrane)
Understanding:
Cholesterol is a component of animal cell
membranes.
• In animal cell membranes
• Cholesterol = lipid, but not fat or oil… it is a STEROID.
• Mostly hydrophobic – attracted to tails of phospholipid
• Hydroxyl group is hydrophilic – attracted to head
• Located between phospholipids
Application:
Cholesterol in mammalian membranes reduces
membrane fluidity & permeability to some solutes.
• Membrane is fluid – it’s free to move
• Fluidity is controlled
• Cholesterol disrupts regular packing of tails,
so prevents them crystallizing and being solid
• Restricts motion and controls fluidity
• Reduces permeability to hydrophilic molecules such as sodium and
hydrogen ions
• Helps membranes curve, to form vesicles during endocytosis
• 4 fused rings; yellow
• Cell Membrane 3-D Pop-Up Model
• Create a 3-D model (sort of like a page in a kids’ pop-up book) of the phospholipid bilayer
of an animal cell. Use any materials available.
• Label each of these structures:
•
•
•
•
•
•
•
•
Phospholipid bilayer
Hydrophilic head
Hydrophobic tail
Integral protein
Peripheral protein
Glycoprotein
Cholesterol
Protein channel