Chapter 7 The Cell Membrane AP Biology Overview

Chapter 7 The Cell Membrane AP Biology

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

Phospholipids § Fatty acid tails § are hydrophobic Phosphate group form a hydrophilic head interaction with H 2 O is complex u & very important! u AP Biology

Phospholipids in water § At surface of a cell phospholipids are arranged as a bilayer u u AP Biology 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 AP Biology

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer AP Biology

Membrane Proteins § Proteins determine most of membrane’s § § specific functions Plasma membrane & organelle membranes each have unique collections of proteins Membrane proteins: u u AP Biology 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? AP Biology

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

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Integral Proteins § Within membrane hydrophobic regions with nonpolar amino acids u often coiled into alpha helices u § Outer surfaces of membrane u AP Biology 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 u outer surface also has carbohydrates u AP Biology 2004 -200

Membranes provide a variety of cell functions AP Biology

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

Unsaturated fats § more fluid membranes u AP Biology 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 u important in organ & tissue development u basis for rejection of foreign cells by immune system u AP Biology

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer AP Biology 2004 -20

Any Questions? ? AP Biology