The Cell Membrane

Studying Membranes – The Electron Microscope Understanding the structure and the function of the phospholipid bilayer cell membrane depended on the invention of the electron microscope. Beams of electrons have a very short wavelength and can resolve cell parts to 0.2 m in size. U of T graduate students James Hillier and Albert Prebus built the first electron microscope in 1938.

EM Photo of a Membrane

Cell Membrane and Homeostasis The phospholipid bilayer is responsible for maintaining homeostasis. Homeostasis is the maintenance of a steady state inside the cell regardless of external changes, to ensure survival. Liver cells - hepatocytes

Cell Membrane Functions In order to maintain homeostasis the cell must regulate the movement of molecules: The transport of raw materials into the cell. The removal of wastes from the cell. Transfer materials produced in the cell to outside the cell. Prevent entry of unwanted molecules. Prevent the escape of needed molecules. All cell organelles are also surrounded by the same phospholipid bilayer membrane.

Cell Membrane Structure There are two phospholipid layers with the hydrophobic ends facing each other on the inside of the bilayer farthest away from the fluids. Outside the cell is the extracellular (EC) fluid. Inside the cell is the intracellular (IC) fluid. The fatty acid bilayer is selectively permeable.

Phospholipids in bilayer Intracellular Fluid Hydrophilic head Hydrophobic tail Extracellular Fluid

The Membrane is Flexible and Fluid More Fluid More Viscous Unsaturated hydrocarbon tails with kinks Saturated hydrocarbon tails When the membrane is colder it is less fluid and a warmer membrane is more fluid. This is due to cholesterol molecules in the membrane.

The Membrane is Dynamic Lateral movement (~107 times per second) Flip-flop (~ once per month) Movement of phospholipids

Fluid Mosaic Model Our current understanding of cell membranes is called the Fluid Mosaic Model. There are many molecules within the membrane and other molecules connected to the membrane. Some small non-polar molecules, like oxygen and carbon dioxide, can pass through the hydrophobic part of the cell membrane. Even water, which is slightly polar, can pass through the hydrophobic cell membrane at a low rate.

Structure of the Cell Membrane

Cell Membrane Proteins Carrier proteins are transmembrane proteins that help move neutral water soluble molecules, like glucose, from outside to inside the cell. Channel proteins are also transmembrane and allow ions to pass through the membrane. Aquaporins are special proteins that help water pass through the cell membrane.

Cell Membrane Carbohydrates The lipids and the proteins often have unique carbohydrates attached to them. These carbohydrates are used as recognition sites for molecules attaching to the cell membrane. Glycoproteins also serve as markers for anchors to other internal structures like the cytoskeleton. Glycolipids also serve as recognition sites for other molecules as well as joining cells to form tissues.

Review Questions Complete in notebook List 2 (of the 4) roles of membrane proteins. What is the fluid mosaic model?

Review Questions List 2 (of the many) roles of membrane proteins Transport Enzymatic activity Triggering signals Attachment and recognition

Review Questions 2) What is the fluid mosaic model? The idea that a biological membrane consists of a fluid phospholipid bilayer, in which proteins are embedded and float freely

Mini-Test 1 Monday, September 16th Cell organelles: physical descriptions, functions, plant vs. animal Cell Membrane: function, cell membrane & homeostasis, structure, phospholipids, dynamic nature, fluid-mosaic model, cell membrane proteins, all types of passive transport methods, cell membrane diagram

Homework: Nanotechnology Nanotechnology: the manipulation of matter on an atomic and molecular scale Discuss Challenges? Interested? University of Toronto & Waterloo both offer programs!

2 transport methods Passive Transport Active Transport

Passive Transport Consists of osmosis, simple diffusion & facilitated diffusion Osmosis  movement of water Simple diffusion  ability of small and non-polar substances to move across a membrane unassisted. Ex: O2, CO2, water, glycerol

Passive Transport Facilitated Diffusion: for ions, polar and charged molecules. Transport proteins AND channel proteins are used

Passive transport 3 types: Diffusion, Osmosis & Facilitated Diffusion No expenditure

Diffusion Molecular collision Uses energy from moving particles Oxygen & carbon dioxide Concentration, temperature

Diffusion - concentration Increase in concentration  increase in molecular collisions Ex: perfume bottle

Diffusion - temperature Increase in temperature  increase in molecular collision Molecules move faster

Osmosis Diffusion of water Concentration gradient  until eventual equilibrium Isotonic  solutions being compared have equal conc. Solutes  water movement out = water movement in

OSMOSIS CONTINUED … Hypotonic  lower conc. of solute outside  cell gains water. Ex: Turgor pressure for plants Hypertonic solution  higher conc. of solute outside, water moves out of cell  cell shrinks. Ex: food preservation http://www.phschool.com/science/biology_place/biocoach/biomembrane1/isotonic.html

Facilitated diffusion Transmembrane proteins form ion channels Carrier proteins & channel proteins Speed up movement of molecules across cell membrane Specialized

Video http://www.youtube.com/watch?v=JShwXBWGMyY