2.4 Membrane Structure

Phospholipid bilayer Flexible, adaptable, in motion Selectively permeable “Controlled entry” Small, hydrophobic Hydrophobic fatty acid tails repel water and form the middle layer Hydrophilic phosphate heads attract water and form outer layers

Why are membranes important? Membranes define compartments Membrane proteins control traffic Membranes are fluid http://learn.genetics.utah.edu/content/cells/membranes/

1.4 Membrane Transport Turn in homework Announcement: Test date

Davson-Danielli Sandwich Model (1930) Proposed layers of protein adjacent to the phospholipid bilayer Evidence Proteins appear dark in electron micrographs and phospholipids appear light

Singer-Nicolson Fluid Mosaic Model Evidence 1. Freeze etched images of the center of membranes was interpreted as trans membrane proteins 2. Improvements in biochemical techniques led to discovery of new membrane structures. Membranes were found to be varied in size and shape.

Mini Research on Transport Topics: Endocytosis & Exocytosis, Simple/facilitated diffusion & Osmosis, Active Transport In your presentation, explain: What is it? What kind of materials are transported across the membrane, and how? Give a specific example of materials transported across the membrane.

Cholesterol Lipids move laterally in a membrane, but flip-flopping across the membrane is rare. Unsaturated hydrocarbon tail of phospholipids have kinks that keep the molecules from packing together, enhancing membrane fluidity. Cholesterol reduces membrane fluidity by reducing phospholipid movement at moderate temperatures but it also hinders solidification at low temperatures.

2.4.4 Passive transport: 1. Simple Diffusion Passive movement of particles from a region of high concentration to a region of low concentration (down a concentration gradient)

2.4.5 Passive transport: 2. Facilitated diffusion Passive movement of particles from a region of high concentration to a region of low concentration (down a concentration gradient), through a selectively permeable membrane, facilitated by carrier proteins Carrier proteins are integral proteins that allow some molecules to pass through

Solute = component of solution Solvent = substance that dissolves the solute

2.4.4 Passive transport: 3. Osmosis Passive movement of water molecules from a region of low solute concentration (high water concentration) to high solute concentration (low water concentration) through a selectively permeable membrane

2.3.6 Active transport Uses energy, in the form of ATP, to move molecules against a concentration gradient, using membrane protein pumps i-biology.net

2.3.6 Active transport E.g sodium potassium pump –action potential produced by nerve cells.

How do cells take in large particles that cannot be diffused?

2.4.7 Vesicles

Membrane fluidity Unsaturated hydrocarbon tail of phospholipids have kinks that keep the molecules from packing together, enhancing membrane fluidity. Cholesterol reduces membrane fluidity by reducing phospholipid movement at moderate temperatures but it also hinders solidification at low temperatures.

Liposomes for drug delivery Artificially produced vesicles that can be used for transport of drugs around the body Good method of targeting cancer