Module 5
Cell Membranes
Chapters 6, 8, 10, 11
, Chapter 8
Cell Membranes
Unit 8.1 Cellular Membranes are Fluid Mosaics of Lipids and Proteins
• Plasma membrane = boundary that separates the living cell from its surroundings, a uid
structure of phospholipids with a mosaic of proteins embedded in it
• Thin + uid — 8nm thick
• Phospholipids = most abundant lipid in the plasma membrane
• Amphipathic molecules = have both a hydrophobic and a hydrophilic region
• Phospholipid bilayer can exist as a stable boundary between two aqueous compartments
• Molecular arrangement places the hydrophobic trails away from the water and
hydrophilic heads are exposed to water
• Most membrane proteins = amphipathic
• Reside in the phospholipid bilayer with hydrophilic regions protruding
• Orientation maximises contact of hydrophilic regions with water in the cytosol and
extracellular uid
• Gives hydrophobic regions a non aqueous environment
• Not randomly distributed in the membrane
• Associate in long-lasting specialised patched that carry out common functions
Fluidity of Membranes
• Membrane = primarily held together via hydrophobic interactions
• Much weaker than covalent bonds
• Phospholipids can move: laterally (10^7 times per second)
: ip op (once a month)
• Membrane remains uid as temp decreases UNTIL phospholipids settle into a closely packed
arrangement
• Membrane solidi es depending on the type of lipids it is made of
• Unsaturated fatty acids lead to a more uid membrane
• The unsaturated molecules have a kink/bend that creates a gap between the
molecules
• E ect of cholesterol on membranes…
• Di ering e ects at di erent temps
• At high temps (eg. 37°C) — makes membrane less uid by restraining phospholipid
movement
• BUT because it restrains the movement… it lowers the temp required to solidify the
membrane
• Cholesterol hinders solidi cation in a low temperature by disrupting regular packing
of phospholipids
• FLUIDITY BUFFER!! — resists changes in membrane uidity that can be caused by
changes in temperature
• Membrane uidity a ects:
• Permeability
• Ability of membrane proteins to move to where their function is needed
• Solidi cation of the membrane may denature enzymatic proteins in the membrane
• TOO uid a membrane cannot support protein function either
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, Membrane Proteins and Their Functions
eg. Red blood cell membrane has ±50 proteins
• Proteins play a key role in determining membrane functions
• Membrane is both a structural and functional mosaic
2 major populations
1. Integral proteins — penetrate hydrophobic interior of the lipid bilayer
• Transmembrane proteins - span the membrane
• Other integral proteins only extend partway into the hydrophobic interior
• Hydrophobic regions of integral proteins = one or more stretches of non-polar amino acids
coiled into ⍺ helices
• Hydrophilic regions are exposed to aqueous solutions either side of the membrane
• Some proteins have hydrophilic channels to allow hydrophilic substances through the
membrane (eg. Aquaporins allow water through the membrane)
2. Peripheral proteins
• Not embedded in the lipid bilayer but rather appendages loosely bound to the surface of
the membrane exposed to parts of integral proteins
6 major functions of membrane proteins
1. Transport
• eg. Hydrophilic channel
2. Enzymatic activity
3. Signal transduction
• A membrane protein (receptor) may have a binding site with a speci c shape that ts the
shape of a chemical messenger, such as a hormone.
• The external messenger (signaling molecule) may cause the protein to change shape,
allowing it to relay the message to the inside of the cell, usually by binding to a
cytoplasmic protein
4. Cell-cell recognition
• eg. Glyco-proteins that act as identi cation tags
• Usually short lived cell-binding
5. Intercellular joining
• Hook together in various junctions to bind in a long term way
6. Attachment to the cytoskeleton and extracellular matrix
• eg. Integrins
• Micro laments or other cytoskletal elements bind noncovalently
• Allows for maintenance of cell shape and stabilisation of membrane proteins
• Proteins that bind to ECM molecules can coordinate extracellular and intracellular
changes
• Membranes have distinct inside and outside faces — determined as the membrane is built by
the ER and Golgi apparatus
• Lipid bilayer composition may di er
• Each protein has directional orientation in the membrane
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Cell Membranes
Chapters 6, 8, 10, 11
, Chapter 8
Cell Membranes
Unit 8.1 Cellular Membranes are Fluid Mosaics of Lipids and Proteins
• Plasma membrane = boundary that separates the living cell from its surroundings, a uid
structure of phospholipids with a mosaic of proteins embedded in it
• Thin + uid — 8nm thick
• Phospholipids = most abundant lipid in the plasma membrane
• Amphipathic molecules = have both a hydrophobic and a hydrophilic region
• Phospholipid bilayer can exist as a stable boundary between two aqueous compartments
• Molecular arrangement places the hydrophobic trails away from the water and
hydrophilic heads are exposed to water
• Most membrane proteins = amphipathic
• Reside in the phospholipid bilayer with hydrophilic regions protruding
• Orientation maximises contact of hydrophilic regions with water in the cytosol and
extracellular uid
• Gives hydrophobic regions a non aqueous environment
• Not randomly distributed in the membrane
• Associate in long-lasting specialised patched that carry out common functions
Fluidity of Membranes
• Membrane = primarily held together via hydrophobic interactions
• Much weaker than covalent bonds
• Phospholipids can move: laterally (10^7 times per second)
: ip op (once a month)
• Membrane remains uid as temp decreases UNTIL phospholipids settle into a closely packed
arrangement
• Membrane solidi es depending on the type of lipids it is made of
• Unsaturated fatty acids lead to a more uid membrane
• The unsaturated molecules have a kink/bend that creates a gap between the
molecules
• E ect of cholesterol on membranes…
• Di ering e ects at di erent temps
• At high temps (eg. 37°C) — makes membrane less uid by restraining phospholipid
movement
• BUT because it restrains the movement… it lowers the temp required to solidify the
membrane
• Cholesterol hinders solidi cation in a low temperature by disrupting regular packing
of phospholipids
• FLUIDITY BUFFER!! — resists changes in membrane uidity that can be caused by
changes in temperature
• Membrane uidity a ects:
• Permeability
• Ability of membrane proteins to move to where their function is needed
• Solidi cation of the membrane may denature enzymatic proteins in the membrane
• TOO uid a membrane cannot support protein function either
ffff flfi fl fffl fl fi ffflff fi fl fl fl flfl fl
, Membrane Proteins and Their Functions
eg. Red blood cell membrane has ±50 proteins
• Proteins play a key role in determining membrane functions
• Membrane is both a structural and functional mosaic
2 major populations
1. Integral proteins — penetrate hydrophobic interior of the lipid bilayer
• Transmembrane proteins - span the membrane
• Other integral proteins only extend partway into the hydrophobic interior
• Hydrophobic regions of integral proteins = one or more stretches of non-polar amino acids
coiled into ⍺ helices
• Hydrophilic regions are exposed to aqueous solutions either side of the membrane
• Some proteins have hydrophilic channels to allow hydrophilic substances through the
membrane (eg. Aquaporins allow water through the membrane)
2. Peripheral proteins
• Not embedded in the lipid bilayer but rather appendages loosely bound to the surface of
the membrane exposed to parts of integral proteins
6 major functions of membrane proteins
1. Transport
• eg. Hydrophilic channel
2. Enzymatic activity
3. Signal transduction
• A membrane protein (receptor) may have a binding site with a speci c shape that ts the
shape of a chemical messenger, such as a hormone.
• The external messenger (signaling molecule) may cause the protein to change shape,
allowing it to relay the message to the inside of the cell, usually by binding to a
cytoplasmic protein
4. Cell-cell recognition
• eg. Glyco-proteins that act as identi cation tags
• Usually short lived cell-binding
5. Intercellular joining
• Hook together in various junctions to bind in a long term way
6. Attachment to the cytoskeleton and extracellular matrix
• eg. Integrins
• Micro laments or other cytoskletal elements bind noncovalently
• Allows for maintenance of cell shape and stabilisation of membrane proteins
• Proteins that bind to ECM molecules can coordinate extracellular and intracellular
changes
• Membranes have distinct inside and outside faces — determined as the membrane is built by
the ER and Golgi apparatus
• Lipid bilayer composition may di er
• Each protein has directional orientation in the membrane
fi ff fi fi fi
