BIOL213 Lecture Notes - Lecture 7: Fluid Mosaic Model, Membrane Fluidity, Sterol
Lecture 7 – Membrane Structures and Functions
• Fluid mosaic model: the membrane mosaic is fluid because most of the interactions
of the components are non-covalent. Free movement can laterally occur in
the plane of the membrane.
• Composition of membranes can vary substantially between organisms in terms of :
1. Ratio of lipid to protein
2. Type of phospholipid
3. Abundance and type of sterols
Variation amongst membranes reflects the diversity of biological roles of
membranes.
• Prokaryotic cells (eg. E. Coli) do not have sterols in their membranes —> this is
how they can be distinguished from eukaryotes.
Membrane Asymmetry:
• Membrane composition also varies between different organelles within the same
cell.
• The 2 leaflets (aka monolayers) differ in lipid and protein composition. This is
known as sidedness.
• Outer leaflet (exposed to extracellular fluids) is generally more positively charged
than the inner leaflet. Thus sidedness affects membrane potential.
• Sidedness affects fluidity and also function
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Lecture 7 – Membrane Structures and Functions
Membrane Proteins:
• Main roles of membrane proteins is transport of substances across the membrane
(mainly integral proteins); and receptors which communicate with
the extracellular environment
• Integral: have hydrophobic domains to interact with lipids, and have hydrophilic
domain to interact with water. May span the entire membrane. Difficult to break
these proteins, as you need to break both the hydrophobic and hydrophilic bonds.
Can be removed with a detergent however.
• Peripheral: associated with hydrophilic or surface portions of membrane lipids and
proteins. Easily moved. Do not interact with polar head groups.
• Amphitropic: are sometimes associated with a membrane and sometimes are
not. Depends on what the cell is up to at the moment and also what the cell’s
function is. The protein binds to the membrane when it needs to, and unbinds
when it needs to. Some biological trigger influences when it binds or not.
Physical properties of membranes:
• membranes are dynamic and flexible structures
• Important factors involved in controlling membrane fluidity:
-lipid fatty acid composition and amount of cholesterol
-degree of fatty acid saturation and chain length
-temperature.
One of the major ways that our bodies are affected by changes in temperature is
that membranes are compromised, and what enters the cell is affected. It
essentially slows down our metabolic rate.
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Document Summary
Lecture 7 membrane structures and functions: fluid mosaic model: the membrane mosaic is fluid because most of the interactions of the components are non-covalent. Free movement can laterally occur in the plane of the membrane: composition of membranes can vary substantially between organisms in terms of , ratio of lipid to protein, type of phospholipid, abundance and type of sterols. Variation amongst membranes reflects the diversity of biological roles of membranes: prokaryotic cells (eg. e. coli) do not have sterols in their membranes > this is how they can be distinguished from eukaryotes. Membrane asymmetry: membrane composition also varies between different organelles within the same cell, the 2 leaflets (aka monolayers) differ in lipid and protein composition. This is known as sidedness: outer leaflet (exposed to extracellular fluids) is generally more positively charged than the inner leaflet. Thus sidedness affects membrane potential: sidedness affects fluidity and also function.
