Biological Membranes

Lipid Bilayer

• Phospholipids form bilayers in water

• Fluid mosaic model of membrane structure

• Membrane is only 10 nm thick

• Proteins move around laterally like icebergs in the ocean

Hydrophilic

Hydrophobic

AmphipathicAmphipathicAmphipathicAmphipathic

Bilayer

• Phospholipids form bilayers because the molecules….– Have two distinct regions, one

strongly hydrophobic and the other strongly hydrophilic

– Have cylindrical shapes that allow them to associate with water most easily as a bilayer structure

Fluid Mosaic Model

• Singer and Nicholson in 1972

• Phospholipid bilayer with proteins embedded within the membrane or otherwise associated with the membrane

• Dynamic layer- molecules move about laterally in the membrane

Membranes are two-dimensional fluids

Membranes are two-dimensional fluids

Molecules move laterallyPhospholipids can rotateBut stay within their layer

Frye and Edidin (1970)Fused and labeled human and mouse cells and noted that the some of the molecules in the cell membrane had indeed moved

Michael Edidin

Graduate Professor at Johns Hopkins University in Baltimore, Maryland

Cell Membrane Fluidity

• To maintain proper function, a membrane’s lipids must be in optimum fluidity

• Too cold– a problem- could turn solid

• Too hot- a problem- could make membrane weak

Hydrocarbon Chains

• Chains can twist at the single bond sites

• This twisting increases as T increases

Solutions??• Develop some more unsaturated fats

– How would this help?

unsaturated

saturated

Solutions??• Get some cholesterol!

– How would this help?

cholesterol

phospholipid

Plant Cells

In plant cells, other steroids perform the same function as cholesterol

In plant cells, other steroids perform the same function as cholesterol

Exocytosis

The cell can create a vesicle that fuses with the membrane and then releases its contents outside of the cytosol

Endocytosis

The cell can take in nutrients by enclosing them in a vesicle and releasing the vesicle to the cytosol

Membrane Proteins• Integral Proteins- firmly bound to

the membrane• Amphipathic

– Hydrophilic regions extend outside the cell

– Hydrophobic regions intermingle with the fatty acid chains of the phospholipids

Transmembrane Protein

• Extend all the way through the cell membrane

• Some go through only once and some go through as many as 24 times

• The most common is -helix with hydrophobic amino acid side chains

Peripheral Proteins

• Located on the inner or outer surface of the membrane

• Usually bound to exposed parts of the integral proteins

Placement of Proteins in the Membrane

• As we know, there are proteins embedded in the membrane

• These proteins are not randomly placed in this location– They have a specific orientation with the

membrane

• There may be more proteins on the inside than the outside and vice versa

• As we know, there are proteins embedded in the membrane

• These proteins are not randomly placed in this location– They have a specific orientation with the

membrane

• There may be more proteins on the inside than the outside and vice versa

Membrane Protein Formation

• Inner surface proteins– Manufactured by free ribosomes– Move to the membrane through cytoplasm

and fuse with the membrane

Membrane Protein Formation

• Outer Surface Proteins– Formed by ribosomes on the

rough ER

Membrane Protein Formation• Outer Surface Proteins

– Become associated in ER membrane and push through into the lumen of the ER

• Here sugars are added that are found only in the lumen of the ER

• This makes them glycoproteins proteincarbohydrate

Membrane Protein Formation• Outer Surface

Proteins– Transport vesicles

move the newly formed gylcoprotein to the golgi complex

Golgi complex

Membrane Protein Formation• Outer Surface Proteins

– Glycoprotein becomes associated with the golgi complex in the same orientation as it did with the ER

• Further modification is made to the carbohydrate while it is in the lumen of the the golgi complex

• GC may also sort glycoproteins together

Membrane Protein Formation

• Outer Surface Proteins– GC buds and

sends vesicle to the plasma membrane where it fuses with the membrane

Membrane Protein Formation

• Outer Surface Proteins– The carbohydrate part of the glycoprotein

extends to the outside of the membrane

• Outer Surface Proteins– Formed by ribosomes on the rough ER– Become associated in ER membrane and push through into the

lumen of the ER• Here sugars are added that are found only in the lumen of the

ER• This makes them glycoproteins

– Transport vesicles move the newly formed gylcoprotein to the golgi complex

– Glycoprotein becomes associated with the golgi complex in the same orientation as it did with the ER

• Further modification is made to the carbohydrate while it is in the lumen of the the golgi complex

• GC may also sort glycoproteins together– GC buds and sends vesicle to the plasma membrane where it fuses

with the membrane– The carbohydrate part of the glycoprotein extends to the outside of

the membrane

How do Membranes Fuse?

• Golgi complex membrane is of same structure as the plasma membrane

• Fusion is like an air bubble with a speck of dirt in it rising to the surface of a glass of water and depositing the speck on the surface of the water as the water surrounding the bubble becomes part of the water’s surface.

Not

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Membrane Protein Functions