Chapter 7 The Cell Membrane

Overview Plasma membrane separates living cell from nonliving surroundings thin barrier = 8nm thick Controls traffic into & out of the cell selectively permeable allowing some substances to cross more easily than others Made of phospholipids, proteins & other macromolecules

Phospholipids Fatty acid tails are hydrophobic Phosphate group form a hydrophilic head interaction with H2O is complex & very important!

Phospholipids in water At surface of a cell phospholipids are arranged as a bilayer hydrophilic heads on the outside in contact with aqueous solution & hydrophobic tails form core phospholipid bilayer forms a barrier between cell & external environment

Fluid Mosaic Model In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.

Membrane Proteins Proteins determine most of membrane’s specific functions Plasma membrane & organelle membranes each have unique collections of proteins Membrane proteins: peripheral proteins = loosely bound to surface of membrane integral proteins = penetrate into lipid bilayer, often completely spanning the membrane (a transmembrane protein)

Why are proteins the perfect molecule to function embedded in the cell membrane?

Why proteins? Proteins are mixed molecules hydrophobic amino acids stick in the lipid membrane anchors the protein in membrane hydrophilic amino acids stick out in the watery fluid in & around cell specialized “receptor” for specific molecules 2005-2006

Integral Proteins Within membrane Outer surfaces of membrane hydrophobic regions with nonpolar amino acids often coiled into alpha helices Outer surfaces of membrane hydrophilic regions with polar amino acids 2004-200

Inside & outside surfaces are different 2 layers may differ in lipid composition proteins in membrane have a direction outer surface also has carbohydrates 2004-200

Membranes provide a variety of cell functions Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made.

Fluid membranes Permeability Changing temperatures membrane must be fluid, about as fluid as salad oil Changing temperatures cells alter lipid composition of membranes cold-adapted organisms winter wheat increases % unsaturated phospholipids in autumn prevent membranes from solidifying during winter

Unsaturated fats more fluid membranes kinks in unsaturated fatty acid tails prevent tight packing

Membrane carbohydrates Play a key role in cell-cell recognition ability of a cell to distinguish neighboring cells from another important in organ & tissue development basis for rejection of foreign cells by immune system The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins. 2004-20

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