BILD 1 Chapter Notes - Chapter 6: Fluid Mosaic Model, Unsaturated Hydrocarbon, Lipid Bilayer
18 Dec 2018
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7.1 Cellular membranes are fluid mosaics of lipids and proteins
● Selective permeability: allow some substances to cross more easily than others
● Phospholipids are most abundant lipids in membranes
● Amphipathic: both hydrophilic region and hydrophobic region
○ Phospholipid bilayer exist as stable boundary → hydrophobic tails are hidden
and hydrophilic heads are to water
○ Most membrane proteins are like this
● Fluid mosaic model: membrane is a mosaic of protein molecules bobbing in fluid bilayer
of phospholipids
○ Specialized patches of protein to carry out specific functions
The Fluidity of Membranes
● Membrane held together by hydrophobic interactions → weaker than covalent bonds
● Lipids and some proteins can shift on membranes
● Proteins are larger than lipids → move slower; some may drift
● Membrane remain fluid until temp decreases → phospholipids settle into closely
packed arrangement → membrane solidies
○ Temp which membrane solidifies depends on types of lipids it is made of
○ Temp decrease → membrane remain fluid if made up of phospholipids w/
unsaturated hydrocarbon tails
■ Kink in tails where double bonds are located, unsaturated hydrocarbon
tails cannot be close together → more fluid
○ Ex: steroid cholesterol → high temperature → cholesterol makes membrane less
fluid → however hinders close packing of phospholipids → lowers temperature
required for membrane to solidify → buffer
● Membrane solidifies → permeability changes → proteins in membrane become
inactive
● Membrane too fluid does not support protein function
Evolution of Differences in Membrane Lipid Composition
● Fish live in extreme cold → membranes w/ high unsaturated hydrocarbon tails
● Bacteria and archaea thrive at high temp → prevent excessive fluidity at such high
temp
Membrane Proteins and their functions
● Phospholipids form main fabric of membrane → proteins determine most of
membrane’s function → diff cells = diff membrane proteins
● Integral proteins: penetrate hydrophobic interior of lipid bilayer
○ Transmembrane proteins: span entire membrane
● Hydrophilic parts of molecule are exposed to aqueous solution on either side of
membrane
● Peripheral proteins: not embedded in lipid layer; loosely bound to surface of membrane
● Single cell may have cell surface membrane proteins that carry out several diff functions
● Single membrane protein may carry out multi functions
● Cells recognize other cells by binding to molecules on the surface of plasma membrane
● Glycolipids: covalently bound to lipids
● Glycoproteins: covalently bonded to proteins
Synthesis and Sidedness of Membranes
● Proteins on the plasma membrane is determined as membrane being built by the
endoplasmic reticulum and Golgi apparatus
7.2 Membrane structure results in selective permeability
● Fluid mosaic model helps explain how membranes regulate the cell’s molecular traffic
● Cell membranes are selectively permeable and substances do not cross barrier without
obstacles
The Permeability of the Lipid Bilayer
● CO2 and O2 are hydrophobic → lipids → dissolve in lipid bilayer of membrane and
cross it
○ Hydrophobic interior of membrane impedes direct passage through membrane of
ions and polar molecules
● Polar molecules such glucose and other sugars pass only slowly through lipid bilayer
● Water does not cross rapidly relative to nonpolar molecules
● Charged atom or molecule and surrounding shell of water are less likely to penetrate
hydrophobic interior of membrane
Transport Proteins
● Transport proteins: specific ions and variety of polar molecules can’t move through cell
membrane so these help pass through
○ Allow only certain substance to cross membrane
● Channel proteins: hydrophilic channel that certain molecules or atomic ions use as tunnel
through membrane
● Aquaporins: passage of water molecules through membrane in certain cells
○ Without these only a fraction of water molecules would pass through same area of
the cell membrane in a second
● Carrier proteins: hold on to passengers and change shape in a way that shuttle them
across membrane
7.3 Passive Transport is diffusion of a substance across a membrane with no energy investment
● Diffusion: movement of particles of any substance so they space out into available space
● Once point is reached → dynamic equilibrium with roughly many dye molecules
crossing membrane each second in one direction as in the other
Document Summary
7. 1 cellular membranes are fluid mosaics of lipids and proteins. Selective permeability: allow some substances to cross more easily than others. Phospholipids are most abundant lipids in membranes. Amphipathic: both hydrophilic region and hydrophobic region. Phospholipid bilayer exist as stable boundary hydrophobic tails are hidden and hydrophilic heads are to water. Fluid mosaic model: membrane is a mosaic of protein molecules bobbing in fluid bilayer of phospholipids. Specialized patches of protein to carry out specific functions. Membrane held together by hydrophobic interactions weaker than covalent bonds. Lipids and some proteins can shift on membranes. Proteins are larger than lipids move slower; some may drift. Membrane remain fluid until temp decreases phospholipids settle into closely packed arrangement membrane solidies. Temp which membrane solidifies depends on types of lipids it is made of. Temp decrease membrane remain fluid if made up of phospholipids w/ unsaturated hydrocarbon tails.
