1.3: Membrane Structure
Understandings:
1. Phospholipids form bilayers in water due to amphipathic properties of phospholipid molecules. 2. Membrane proteins are diverse in terms of structure, position in the membrane, and function. 3. Cholesterol is a component of animal cell membranes. |
Skills:
Apply: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes. Skill: Draw, label and annotate the Fluid Mosaic Model. Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model. Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicholson model. |
Mission 1: In Da Club, Part I
Mission Objectives. You should be able to...
1. Draw and label a diagram of the plasma membrane.
2. Identify components of the plasma membrane.
3. Explain the role of cholesterol in the plasma membrane.
4. Explain why the Davson-Danielli model was discarded in favor of the Singer-Nicholson model.
Mission Objectives. You should be able to...
1. Draw and label a diagram of the plasma membrane.
2. Identify components of the plasma membrane.
3. Explain the role of cholesterol in the plasma membrane.
4. Explain why the Davson-Danielli model was discarded in favor of the Singer-Nicholson model.
Structure
The plasma membrane surrounds the cell. It is semi-permeable (meaning it is selective about what it allows into the cell and out of the cell). Examples of substances that can enter the cell are sugars, ions, and water. Examples of substances that can exit the cell are carbon dioxide and waste. The membrane is made up of a phospholipid bilayer. There are two layers of phospholipids (hence the "bilayer") that contain a hydrophilic phosphate head and a hydrophobic fatty acid tail. See the images to the right. Hydrophilic means "water loving" and hydrophobic means "water hating." The hydrophilic head is polar (so that it can interact with water) and the hydrophobic head is nonpolar (so it cannot). Because these molecules contain polar and nonpolar ends, they are called amphipathic. The phospholipids are in a bilayer structure to hide the hydrophobic fatty acids and expose the hydrophilic phosphate heads. This is a very stable, semi-permeable structure that allows water and gases easy access, but larger molecules and small polar molecules cannot. |
Cholesterol At various locations in the hydrophobic (water-hating fatty acid) layer are cholesterol molecules. They play a part in determining membrane fluidity, which fluctuates with temperature. The cholesterol allows the membrane to function effectively within a wider temperature range. Cholesterol can reduce fluidity of the membrane and make it less permeable to to small water-soluble particles. It also plays a role in determining the shape of the molecule.
Proteins The proteins in the phospholipid bilayer are what give it diversity and the mosaic effect. There are a variety of protein types embedded within the membrane. There are two major kinds: integral proteins and peripheral proteins. Integral proteins are amphipathic as well and extend into the hydrophobic region and peripheral proteins are attached to the hydrophilic heads, or are attached to the external surface. They are typically anchored to an integral protein.
Proteins The proteins in the phospholipid bilayer are what give it diversity and the mosaic effect. There are a variety of protein types embedded within the membrane. There are two major kinds: integral proteins and peripheral proteins. Integral proteins are amphipathic as well and extend into the hydrophobic region and peripheral proteins are attached to the hydrophilic heads, or are attached to the external surface. They are typically anchored to an integral protein.
Davson-Danielli's Lipid Bilayer. Basically, in 1935, these scientists developed a model of the plasma membrane and called it the lipid bilayer. Lipids, of course, are fats (and oils) and bilayer means "two layers." Their model suggested that the membrane was constructed like an ice cream sandwich, with the lipid bilayer acting as the cream and being surrounded by globular proteins (aka the cookies).
Singer & Nicolson's Fluid Mosaic. In 1972, much of what was known about the then-accepted lipid bilayer model had to be discarded, as the model couldn't explain the lack of symmetry some membranes have, and the fact that a protein layer simply doesn't work because it is mostly non-polar and wouldn't interface with water, which is essential. This, along with other evidence, was gathered by electron microscopes and observing cell cultures in solutions. As a result, scientists Seymour Singer & Garth Nicolson came up with the currently accepted model of the plasma membrane, the Fluid Mosaic.
All cellular membranes, whether plasmic or organelle, have the same general structure. It is a phospholipid bilayer. A phospholipid has a phosphate head and a fatty acid tail. The phosphate head is polar and hydrophilic, meaning it interfaces with water extremely well. The fatty acid tail is nonpolar and hydrophobic, meaning it does not interface with water. Similar in idea to the lipid bilayer, the Fluid Mosaic forms a sandwich with the phosphate heads on the outside and the fatty acid tails tucked in. However, there are cholesterol and protein molecules embedded throughout the phospholipid bilayer, giving it a mosaic appearance
All cellular membranes, whether plasmic or organelle, have the same general structure. It is a phospholipid bilayer. A phospholipid has a phosphate head and a fatty acid tail. The phosphate head is polar and hydrophilic, meaning it interfaces with water extremely well. The fatty acid tail is nonpolar and hydrophobic, meaning it does not interface with water. Similar in idea to the lipid bilayer, the Fluid Mosaic forms a sandwich with the phosphate heads on the outside and the fatty acid tails tucked in. However, there are cholesterol and protein molecules embedded throughout the phospholipid bilayer, giving it a mosaic appearance
You should be able to complete a basic sketch of the plasma membrane and describe it using the following terms: phospholipid bilayer, amphipathic, phosphate head, fatty acid tail, hydrophilic, hydrophobic, polar, nonpolar, cholesterol function and protein functions. Additional information about membrane structure is of course found here.