A cell’s plasma membrane defines a cell and separates it from its environment. Yet at the same time, it must allow for interaction between the cell and its environment. Membranes
Biological Evolution began with the first self-replicating molecule The first cell arose when this molecule became enclosed in a membrane CHEMICAL EVOLUTION Light energy Heat HOCH2 OH H H O H H C C H C C O O H O O H O C H H H C C C H H O C C C O H H H O N H N H O HO OH H H C N 1. Simple molecules in atmosphere of ancient Earth 2. Reduced carbon- containing compounds Simple Membranes and the First Cells 3. First carbon-carbon bonds
Amphipathic lipids can form a bilayer structure in an aqueous solution Membranes are Composed of Phospholipids
Liposomes: Artificial membrane-bound vesicles Water Water 62.5 nm Liposomes will form spontaneously in solution
Phospholipid Bilayer Cholesterol Glycocalyx Proteins The Fluid Mosaic model of Plasma Membranes
Evidence for the drifting of membrane proteins
Component 1: Phospholipid bilayer Phospholipid bilayer Summary of relative permeabilities Hydrophobic molecules O2, CO2, N2 Small, uncharged polar molecules H2O, glycerol Large, uncharged polar molecules Glucose, sucrose H+,Na+,NCO3–, Ca2+,CL-,Mg2+,K+ Ions
Phospholipids may have saturated or unsaturated fatty acid tails Double bonds cause kinks in hydrocarbons. H2C CH2 H2C CH2 H2C CH Kink CH H2C CH2 H2C Unsaturated fatty acid Saturated fatty acid CH2 H2C Double bonds present, fewer H atoms No Double bond, maximum H atoms
Kinks change the fluidity and permeability of membranes. Lipid bilayer with no unsaturated fatty acids Low fluidity Low permeability Lipid bilayer with many unsaturated fatty acids High fluidity High permeability
Component 2: Cholesterol fills spaces between phospholipids. Polar Nonpolar The more cholesterol within the membrane, the less permeable it is Cholesterol keeps the membrane fluid at low temps, prevents it from becoming too fluid at warmer temps
Component 3: Membrane Proteins • Integral proteins • Transmembrane proteins • Peripheral Proteins
Transmembrane proteins are amphpathic Polar amino acids Glu Tyl Met Pro Ile Pro Gly Ser Non- Polar amino acids Asp
Some functions of membrane proteins
Sugar molecules attached to the outer surface of the plasma membrane Glycoproteins: oligosaccharides attached to proteins Glycolipids: oligosaccharides attached to lipids Used for cell identification: antigens Component 4: Glycocalyx
Diffusion Molecules will diffuse from areas of high concentration to areas of lower concentration Diffusion is a spontaneous, passive process (doesn’t require the input of external energy) . Movement of Substances Across Membranes
Each type of molecule will travel down its concentration gradient across a permeable membrane
Water will move down (diffuse) its own concentration gradient--> osmosis solvent solute water is also moving in the direction of lower solute concentration to higher solute concentration
Water enters vesicle if internal solution is hypertonic to the external solution. Start with: Isotonic solution Hypertonicsolution Hypotonic solution Arrows represent direction that water moves via osmosis Result: No change Membrane swells or even bursts Membrane shrinks
Figure 8.12 The water balance of living cells
Movement across membranes is affected by the presence of membrane proteins. 3 types of transporter proteins: Channel Proteins Carriers Pumps Movement of Substances Across Membranes
Facilitated diffusion follows the concentration gradient and requires no input of external energy Channel Protein Carrier Protein
Active transport through pumps moves molecules against their concentration gradients and requires energy The sodium-potassium pump
Summary of Transport across membranes
Electrogenic pumps generates a membrane potential (voltage) across the membrane
The generation of a gradient can be used to do work or cotransport other molecules across a membrane
Getting Big Stuff Out of Cells Exocytosis
Getting Big Stuff into cells The three types of endocytosis in animal cells Phagocytosis Pinocytosis Receptor-mediated endocytosis
Cystic fibrosis is a defect in the Cl- transporter, CFTR About 1 baby in 2000 of Northern European racial origin is affected Symptoms thick, sticky mucus that clogs the lungs and leads to lung infections. Obstruction of the pancreas, preventing digestive enzymes from reaching the intestines When transport doesn’t work: Cystic Fibrosis
