CHAPTER 5 THE STRUCTURE AND FUNCTION OF MACROMOLECULES MEMBRANE STRUCTURE AND FUNCTION How things get into and out of the cell

Cell membrane  The plasma membrane functions as a selective barrier that allows passage of oxygen, nutrients, and wastes for the whole volume of the cell.  Prevents the entry of unwanted matter and the escape of needed materials.  Maintain a steady environment: Homeostasis.

Membrane Structure  The plasma membrane separates the living cell from its nonliving surroundings.  This thin barrier, 8 nm thick, controls traffic into and out of the cell.  Like other membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others.  The main macromolecules in membranes are lipids and proteins, but include some carbohydrates.

phospholipids.  Membrane is made of special kind of lipid; phospholipids.  Phospholipids and most other membrane constituents are amphipathic molecules. Amphipathic molecules have both hydrophobic regions and hydrophilic regions. Amphipathic molecules have both hydrophobic regions and hydrophilic regions. phosphate “attracted to water” hydrophilic hydrophobic “repelled by water”

Hydrophilic head - water loving Hydrophobic tail - water hating 4.6

The Fluid-Mosaic Membrane Model The phospholipids and proteins in membranes create a fluid mosaic model. The phospholipids and proteins in membranes create a fluid mosaic model.  In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in contact with water and the hydrophobic regions are in a nonaqueous environment Fig. 8.2b

A- Membrane is a collage مجموعة مختلفة من of different proteins embedded in the fluid matrix of the lipid bilayer. A- Membrane is a collage مجموعة مختلفة من of different proteins embedded in the fluid matrix of the lipid bilayer.

 Membranes are not static ساكن ; they have a fluid consistency اتساق. Most membrane lipids and proteins can drift تنجرف about laterally in the plane مستوى of the membrane.  Cholesterol enhances membrane fluidity, allows animal membranes to function in a wide range of temperatures and also makes the membrane less permeable to biological molecules.

Peripheral proteins Integral proteins  There are two populations of membrane proteins. 1. Integral proteins مُندَمج penetrate the hydrophobic core of the lipid bilayer (transmembrane protein).  The integral proteins has a middle area, hydrophobic regions with surface area, in contact with the nonpolar amino acids. And aqueous environment, they have hydrophilic regions of amino acids 2-Peripheral proteins are not embedded in the lipid bilayer at all. Instead, they are loosely bounded to the surface of the integral proteins

Function of membrane protein The proteins in the plasma membrane may provide a variety of major cell functions:- Transportation Enzymes Receptor sites Cell adhesion Attachment to the cytoskeleton

 Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip- flop from one layer to the other.  The lateral movements of phospholipids are rapid, about 2 µm/second.  Many larger membrane proteins move more slowly but do drift.

 Membranes rich in unsaturated fatty acids are more fluid than those dominated by saturated fatty acids because the kinks منعطفات in the unsaturated fatty acid tails prevent tight packing.

B- Membrane carbohydrates are important for cell-cell recognition  Usually branched molecules of 15 or less sugar units. covalently  Some are covalently bonded to lipids: Glycolipids. covalently  Most are covalently bonded to proteins: Glycoproteins.  Function : Cell-cell recognition.  It is the basis for rejection of foreign cells by the immune system.  Cells recognize other cells by recognizing the surface molecules, often carbohydrates, on the plasma membrane. The four human blood groups ( A, B, AB, and O ) differ in the external carbohydrates on red blood cells.

Carbohydrate chains Hydrophilic مُحب للماء Phospholipid Hydrophobic كاره للماء Proteins