Membrane Structure
Cell Membrane.
Layer inside of the cell wall (only Plant cells and prokaryotic cells have a cell wall but animal cells still have the membrane) that controls the movements of materials in and out of the cell.
It consists of phospholipids and proteins . By 1915, scientists believed it had a sandwich-like structure, the proteins being on the top and bottom surfaces and phospholipids in the middle due to the Davson-Danielli model. Then in 1972, scientists J. Singer and G. Nicolson proposed that the proteins were inserted into the phospholipid bilayer, thus forming a mosaic floating in a fluid later of phospholipids (refer to image above). That model is still present. Membranes are important because they define compartmentalization in the cell.
- Peripheral Proteins: they remain bound to the surface of the membrane. Attached to them are Glycoproteins which control the recognition of like cells and are involved in immune responses.
Membrane proteins impart different functions to different membranes. There are many different proteins but the usually have six general functions:
Since plant cells lack of cholesterol molecules, they depend on saturated or unsaturated fatty acids to maintain proper fluidity.
Applications and skills:
For those who are just beginning to learn about the plasma membrane, it is important to know about the past theories because they need to know for what reason someone came up with those theories before the actual one. Also, they would have something to compare the present theory with and maybe they can even make their own deductions.
For those who are going to teach about the plasma membrane and its structure, it is also important to know why someone came up with that theory and why it was later discredited so that they can teach more in depth about the "evolution" of the membrane.
Layer inside of the cell wall (only Plant cells and prokaryotic cells have a cell wall but animal cells still have the membrane) that controls the movements of materials in and out of the cell.
It consists of phospholipids and proteins . By 1915, scientists believed it had a sandwich-like structure, the proteins being on the top and bottom surfaces and phospholipids in the middle due to the Davson-Danielli model. Then in 1972, scientists J. Singer and G. Nicolson proposed that the proteins were inserted into the phospholipid bilayer, thus forming a mosaic floating in a fluid later of phospholipids (refer to image above). That model is still present. Membranes are important because they define compartmentalization in the cell.
- Phospholipid bilayer: contains both hydrophilic (attracted water) and hydrophobic (reject water but are attracted to each other) molecules. The hydrophilic area is also called hydrophilic phosphate head which has hydrophobic hydrocarbon tails attached. When in water, the phospholipids form double layers with the hydrophilic heads on both sides and the hydrophobic tails are away from the water in the center. This arrangement causes the membrane to be very stable.
- Proteins: Major components of the cellular membrane. There are usually two types of proteins:
- Peripheral Proteins: they remain bound to the surface of the membrane. Attached to them are Glycoproteins which control the recognition of like cells and are involved in immune responses.
Membrane proteins impart different functions to different membranes. There are many different proteins but the usually have six general functions:
- Hormone binding sites: site exposed outside of the membrane that allows an specific hormone to bind.
- Enzymes: they either catalyze reactions inside or outside of the cell, depending on whether the active site is on the inner or outer surface.
- Electron carriers: they are arranged in chains in the membrane so that electrons are able to pass from one carrier to another.
- Channels for active transport: passageways in the center of the membrane for the substances to pass through.
- Pumps for active transport: pumps release the cellular energy of ATP in order to move specific substances across the membrane.
Since plant cells lack of cholesterol molecules, they depend on saturated or unsaturated fatty acids to maintain proper fluidity.
Applications and skills:
- Application: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.
- Skill: Drawing of 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-Nicolson model.
- The explanation of the structure of the plasma membrane has changed over the years as new evidence and ways of analysis have come to light. Under what circumstances is it important to learn about theories that were later discredited?
For those who are just beginning to learn about the plasma membrane, it is important to know about the past theories because they need to know for what reason someone came up with those theories before the actual one. Also, they would have something to compare the present theory with and maybe they can even make their own deductions.
For those who are going to teach about the plasma membrane and its structure, it is also important to know why someone came up with that theory and why it was later discredited so that they can teach more in depth about the "evolution" of the membrane.