Membranes 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.

H O H HCCO H C CC C H H HH HOOH HOCH 2 H H H H H N O N N O O C C O CHEMICAL EVOLUTION Light energy O C H C H O H C O H H C O H Heat 1. Simple molecules in atmosphere of ancient Earth 2. Reduced carbon- containing compounds Simple Membranes and the First Cells  Biological Evolution began with the first self- replicating molecule  The first cell arose when this molecule became enclosed in a membrane 3. First carbon- carbon bonds

Membranes are Composed of Phospholipids Amphipathic lipids can form a bilayer structure in an aqueous solution

Water 62.5 nm Liposomes: Artificial membrane- bound vesicles Liposomes will form spontaneously in solution

The Fluid Mosaic model of Plasma Membranes Phospholipid Bilayer Cholesterol Glycocalyx Proteins

Evidence for the drifting of membrane proteins

Hydrophobic molecules Small, uncharged polar molecules Large, uncharged polar molecules Ions O 2, CO 2, N 2 H 2 O, glycerol Glucose, sucrose H +,Na +,NCO 3 –, Ca 2+,CL -,Mg 2+,K + Phospholipid bilayer Summary of relative permeabilities Component 1: Phospholipid bilayer

H2CH2C H2CH2C H2CH2C H2CH2C H2CH2C H2CH2C CH 2 CH CH 2 Kink Unsaturated fatty acid Saturated fatty acid Double bonds present, fewer H atoms No Double bond, maximum H atoms Phospholipids may have saturated or unsaturated fatty acid tails Double bonds cause kinks in hydrocarbons.

Lipid bilayer with no unsaturated fatty acids Lipid bilayer with many unsaturated fatty acids Low fluidity Low permeability High fluidity High permeability Kinks change the fluidity and permeability of membranes.

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 2: Cholesterol fills spaces between phospholipids.

Component 3: Membrane Proteins Integral proteins Transmembrane proteins Peripheral Proteins

Asp Ser Gly Pro Ile Pro Met Tyl Glu Non- Polar amino acids Transmembrane proteins are amphpathic Polar amino acids

Some functions of membrane proteins

Component 4: Glycocalyx 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

Movement of Substances Across Membranes 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).

Each type of molecule will travel down its concentration gradient across a permeable membrane

Water will move down (diffuse) its own concentration gradient--> osmosis solute solvent water is also moving in the direction of lower solute concentration to higher solute concentration

Start with: Hypertonic solution Hypotonic solution Isotonic solution No change Membrane swells or even bursts Membrane shrinks Result: Arrows represent direction that water moves via osmosis Water enters vesicle if internal solution is hypertonic to the external solution.

Figure 8.12 The water balance of living cells

Movement of Substances Across Membranes Movement across membranes is affected by the presence of membrane proteins. 3 types of transporter proteins: Channel Proteins Carriers Pumps

Facilitated diffusion follows the concentration gradient and requires no input of external energy Channel ProteinCarrier 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

When transport doesn’t work: Cystic Fibrosis 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