1.3 Membrane Structure (IB Biology SSaQ)

Explain how the amphipathic properties of phospholipids help them to form bilayers in water.

Amphipathic refers to the characteristic where something contains both hydrophobic and hydrophilic elements. Phospholipids contains a phosphate group, which is hydrophilic, connected to a fatty acids, which are hydrophopic. The amphipathic nature of the phosolipids allow them to form the phospholipid bilayer surrounding cells, as the hydrophobic fatty acids naturally face move away from the water, and the hydrophilic phosphate groups are attracted to water.

Outline the location and function of cholesterol in animal cell membranes.

Cholesterol is found (as seen in the Fluid Mosaic model of Membrane Scructure) between phospholipids within the membrane due to its hydrophobic body and one hydophilic end. Its function is to restrict the movement of phospholipid molecules, which leads to the reduction of fluidity of the membrane. Additionally, it reduces the permeability of the membrane, making it harder for sodium ions to transport accross the membrane.

Draw a diagram of the fluid mosaic model of the plasma membrane. Use the following guidelines: phospholipids draw as a circle with two parallel lines, show a range of membrane proteins, including glycoproteins

Draw a horizontal line of circles, with two lines connected to each circle on the bottom facing downwards. Reflect what you just drew below. Insert blobs through the phospholipid bilayer to represent integral proteins, and attach blobs on top to represent peripheral proteins. Include a blob on top with a chain of diamonds that represent the glycoprotein.

Outline different types of membrane proteins, including their structure, position in the membrane, and function.

Integral protein: it is hydrophobic on at least part of their surface and they are therefore embedded in the hydrocarbon chains in the centre of the membrane, Peripheral protein: is hydrophilic on their surface so are not embedded in the membrane. most of them are attached to the surface of integral proteins and this attachment is often reversible. There are several types of membrane proteins, such as protein channels, pumps, and enzymes. Protein channels are membrane proteins used as channels to transport molecules across the membrane by simple diffusion. Protein pumps are used for active transport, in which substances are "pumped" from an area of low concentration to an area of high concentration. Some membrane proteins may function as enzymes as well.

Explain the falsification of the Davson-Danielli model that led to the acceptance of the Singer-Nicolson model of the plasma membrane.

The Davson-Danielli model depicts a layer of globular proteins over the phospholipid bilayer. It was falsified by fusing two cells together, dyed different colors. The colors mixed, which proved that the structure was fluid rather than rigid, that of which was proposed by the Davson-Danielli model.

Explain the evidence from electron microscopy that led to the proposal of the Davson-Danielli model of the plasma membrane.

The Davson-Danielli model of the plasma membrane dictated that the phospholipid bilayer is surrounded by a layer of globluar proteins. However, the electron microscopy of the plasma membrane showed that rather than surrounding the bilayer, proteins were imbedded within the bilayer (integral membrane proteins). Therefore, the Davson-Danielli model was discredited and the Singer-Nicholson model was adopted instead.