1.3 Mr. McGee, IB Biology (HL) Membrane Structure 1.3 Mr. McGee, IB Biology (HL)

By the End, You Will Understand: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules. Membrane proteins are diverse in terms of structure, position in the membrane and function. Cholesterol is a component of animal cells membranes. Amphipathic phospholipids have hydrophilic and hydrophobic properties.

Essential Skills That Will Be Learned: Drawing a fluid mosaic membrane, using circles with two parallel lines. A range of membrane proteins should be included. Analysis of the evidence from the electron microscope, which lead to the proposal of the Davson-Danielli model of the membrane. Analysis of the falsification of the Davson-Danielli model, which lead to the Singer-Nicolson “fluid mosaic” model.

Thickness of Cell Membrane 0.005 – 0.01μm or 5 – 10nm -How does this compare to a eukaryote or prokaryote?

Cell Membrane The cell membrane is made of a double-layer of a type of lipid called phospholipids. They are amphipathic – soluble in both water and lipids (fat). Phospholipid Bilayer: Head are hydrophilic – “affinity for water” Tails are hydrophobic – “repulsed by water” Hydrophilic Head Hydrophobic Tail

In a glass of water, phospholipids would go crazy. - Heads like water, but the tails dislike water. Oh no! My tails don’t like the water! Mr. Phospholipid Phat-Kid Phospholipid

Phospholipids would orient themselves so the tails would be out of the water.

The Phospholipid Bilayer Hydrophilic Heads Hydrophobic Tails

The Fluid Mosaic Model of the Membrane

Davson-Danielli Model Proposed, 1935 – Also known as “Protein-Sandwich Model”. Globular proteins “sandwiching” two layers of phospholipids (bi-layer of phospholipids had already been proposed in 1925). They felt the protein “coatings” explained observations in their data. Membrane is “static” – not much movement.

Fluid-Mosaic Model Proposed, 1972 (replaced Davson-Danielli Model) Currently it is the accepted model of the cell membrane. Differs in that the proteins are integrated through the membrane (how does this differ from Davson-Danielli Model?) The integrated proteins solved problems that Davson-Danielli Model couldn’t explain. Membrane is very “dynamic” – hence the name “fluid-mosaic”.

The Plasma Membrane The plasma membrane is represented by the “fluid mosaic model” Your labeled diagram of the fluid mosaic model should include the following parts: Phospholipid bilayer Cholesterol Glycoproteins (proteins with a short polysaccharide attached) Integral and peripheral proteins

IB Student Drawing: Labeled Cell Membrane

Membrane Fluidity Phospholipids rapidly move about laterally in the membrane, (billions of times a second) but they rarely flip sides. Phospholipids with saturated fatty acid tails pack together more tightly, making the membrane more viscous. Phospholipids with unsaturated (kinky) fatty acid tails make the membrane more fluid. Cholesterol, embedded in the cell membrane can decrease fluidity at moderate temperatures, but at very low temperatures it prevents excessive solidification of membranes by impeding tight packing. This is an important adaptation for organisms that must live in very cold climates. Lipid type and cholesterol concentration can change seasonally to prevent membrane solidification in winter.

Membrane Proteins The types of proteins embedded within the membrane help determine its function. Integral proteins span the width of the membrane. They must have hydrophilic ends and a hydrophobic core, determined by the placement of the hydrophilic (polar or charged) and hydrophobic (non-polar) amino acids. Peripheral proteins are attached to either surface of the membrane, and sometimes an integral protein Proteins may also move laterally in the membrane, but they are slower than phospholipids because they are larger. Proteins may be anchored in place by the cytoskeleton (interior) or fibers of the extra-cellular matrix (exterior).

Hydrophobic proteins are touching the tails of phospholipids. Hydrophilic proteins are touching water. Examine this integral protein: Which parts of the protein are hydrophobic and hydrophilic?

Assignment Draw and label a scaled diagram of a plasma (cell) membrane. Draw a scale next to the diagram for reference. Label the glycoproteins, cholesterol, integral proteins, peripheral proteins and parts of a phospholipid. Text summary – Read and outline the following: p.28 – 32 (part of 2.3 & 2.4 in book)