membranes
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MembranesMembranes
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The Plasma MembraneThe Plasma Membrane The Plasma membrane is approximately 8-nm The Plasma membrane is approximately 8-nm
thick and is selectively permeable.thick and is selectively permeable.
Selective Selective MembraneMembrane
PorePore
SubstanceSubstancess
PermeablePermeable
PermeablePermeable
Not Not permeablepermeable
Selectively permeable means that the membrane Selectively permeable means that the membrane is selective as to which substances can pass is selective as to which substances can pass through itthrough it
The following example will demonstrate this point:The following example will demonstrate this point:
Larger molecules, such as polysaccharides, proteins, Larger molecules, such as polysaccharides, proteins, and lipids cannot fit, because they are too big.and lipids cannot fit, because they are too big.
The Plasma MembraneThe Plasma Membrane
Cellular membranes are composed primarily of…Cellular membranes are composed primarily of…
LipidLipidss
(fats(fats))
ProteinProteinss
Carbohydrates Carbohydrates (sugars)(sugars)
In a cell’s plasma membrane, back to back In a cell’s plasma membrane, back to back phospholipid molecules form two distinct layers.phospholipid molecules form two distinct layers.
Outer Outer layerlayer
Inner Inner layerlayer
Water is the main component inside and outside Water is the main component inside and outside the cellthe cell
AqueousAqueous inside the inside the
cellcell
AqueousAqueous environmenenvironmen
t outside t outside the cellthe cell
A descriptive word meaning “watery” is the term A descriptive word meaning “watery” is the term aqueousaqueous..
The outside of the cell is known as the extracellular The outside of the cell is known as the extracellular side.side.
CytoplasmiCytoplasmic sidec side
ExtracellulExtracellular sidear side
The inside of the cell is the cytoplasmic side.The inside of the cell is the cytoplasmic side.
The two layers together are known as a phospholipid The two layers together are known as a phospholipid bilayer.bilayer.
Extracellular Side Extracellular Side (aqueous)(aqueous)
Cytoplasmic Side Cytoplasmic Side (aqueous)(aqueous)
PhospholipiPhospholipid bilayerd bilayer
The yellow phosphate heads of the lipids are The yellow phosphate heads of the lipids are hydrophilichydrophilic..
Extracellular Side Extracellular Side (aqueous)(aqueous)
Cytoplasmic Side Cytoplasmic Side (aqueous)(aqueous)
Phosphate Phosphate head head ((hydrophilichydrophilic))
HydrophilicHydrophilic means “water means “water loving.”loving.”
Phosphate Phosphate head head ((hydrophilichydrophilic))
The blue colored lipid tails are The blue colored lipid tails are hydrophobichydrophobic. .
Extracellular Side Extracellular Side (aqueous)(aqueous)
Cytoplasmic Side Cytoplasmic Side (aqueous)(aqueous)
Phosphate Phosphate head head ((hydrophilichydrophilic))
Lipid Tails Lipid Tails ((hydrophobihydrophobicc))
HydrophobicHydrophobic means “water means “water fearing.”fearing.”
Phosphate Phosphate head head ((hydrophilichydrophilic))
A Phospholipid bilayer is an A Phospholipid bilayer is an amphipathicamphipathic structure, structure, meaning it has both a meaning it has both a hydrophilichydrophilic region and a region and a hydrophobichydrophobic region. region.
HydrophilicHydrophilic““water water loving”loving”
HydrophilicHydrophilic““water water loving”loving”
HydrophobiHydrophobicc““water fearing”water fearing”
The picture on the right is a tunneling electron The picture on the right is a tunneling electron micrograph of a cell’s plasma membrane.micrograph of a cell’s plasma membrane.
PhospholipidsPhospholipids are the most abundant lipids are the most abundant lipids in most membranes.in most membranes.
The diagram on the left is a model which interprets The diagram on the left is a model which interprets the two lines in the micrograph as a bilayer of the two lines in the micrograph as a bilayer of
phospholipidsphospholipids..Plasma membrane
Membrane DynamicsMembrane DynamicsPhospholipids are very dynamic. They constantly Phospholipids are very dynamic. They constantly vibrate and make rapid lateral movements many vibrate and make rapid lateral movements many times a second.times a second.
Phospholipids move laterally at a distance of about 2 Phospholipids move laterally at a distance of about 2 μμm per second, which is about the length of a typical m per second, which is about the length of a typical bacterial cell.bacterial cell.
Phospholipids also make transmembrane Phospholipids also make transmembrane flip-flops at a rate of about once per flip-flops at a rate of about once per month.month.
Cell plasma membranes are Cell plasma membranes are usually about as fluid as salad usually about as fluid as salad oil.oil.
Extracellular Side Extracellular Side (aqueous)(aqueous)
Cytoplasmic Side Cytoplasmic Side (aqueous)(aqueous)
This bilayer also contains many This bilayer also contains many molecules of the steroid molecules of the steroid cholesterolcholesterol..
CholesterolCholesterol
Cholesterol slows Cholesterol slows the movement of the movement of phospholipids in phospholipids in cell membranes. cell membranes.
At lower temperatures At lower temperatures however, cholesterol however, cholesterol helps to keep the helps to keep the membrane from membrane from solidifying.solidifying.
The fewer the number of cholesterol The fewer the number of cholesterol molecules, the more fluid the molecules, the more fluid the membrane.membrane.
Unsaturated hydrocarbon tails Unsaturated hydrocarbon tails cause kinks in the lipid bilayercause kinks in the lipid bilayer
Bilayers with kinks have fewer Bilayers with kinks have fewer cholesterol molecules than those cholesterol molecules than those without kinks.without kinks.
The greater the number of cholesterol The greater the number of cholesterol molecules, the more viscous the molecules, the more viscous the membrane.membrane.
Saturated hydrocarbons Saturated hydrocarbons have straight tailshave straight tails
Bilayers without kinks have more Bilayers without kinks have more cholesterol molecules than those with cholesterol molecules than those with kinks.kinks.
The phospholipids, steroids and proteins of the The phospholipids, steroids and proteins of the membrane form a mosaic of substances that float membrane form a mosaic of substances that float about.about.
This is known as the This is known as the Fluid Mosaic Fluid Mosaic ModelModel..
The Fluid Mosaic Model was proposed by Singer The Fluid Mosaic Model was proposed by Singer and Nicolson in 1972.and Nicolson in 1972.
This is a 3-D model of what This is a 3-D model of what the membrane might look the membrane might look like.like.
http://www.goldiesroom.org/Note%20Packets/06%20Transport/00%20Transport--WHOLE.htm
The website below has an animation The website below has an animation which demonstrates how the which demonstrates how the
phospholipids and proteins move phospholipids and proteins move about in the membrane.about in the membrane.
http://home.earthlink.net/~shalpine/anim/Life/http://home.earthlink.net/~shalpine/anim/Life/memb.htmmemb.htm
Parting ShotsParting Shots Lipids have about twice the energy content of Lipids have about twice the energy content of
both proteins and carbohydrates. both proteins and carbohydrates. Some lipid derivatives act as hormones Some lipid derivatives act as hormones
(testosterone, estrogen, and progesterone).(testosterone, estrogen, and progesterone). Fat under the skin insulates the body against Fat under the skin insulates the body against
temperature changes (e.g. seal blubber)temperature changes (e.g. seal blubber) Fat is a hydrophobic storage molecule and thus Fat is a hydrophobic storage molecule and thus
is lightweight.is lightweight. Fat acts as a shock absorbent for some organs Fat acts as a shock absorbent for some organs
of the body (e.g. kidneys)of the body (e.g. kidneys)
Fluid Mosaic ModelFluid Mosaic Model
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