CBL  1.2

Biologist reporting to others sometimes need to represent diagrams of what they are seeing under the microscope. It is important when they do this to accurately represent size, just as cartographers do on a map. They use scaled diagrams. 

There are 3 aspects to drawing scaled diagrams of cells:

1. determining magnification used when viewing under a microscope, and distinguishing it from resolution

2. determining size of an object seen under a microscope using field of view and magnification

3. drawing diagrams to scale 

The fluid mosaic model is a way for us to see understand how cell membranes are structured and how they function.

The cell membrane consists of a phospholipid bilayer with many compounds such as proteins, glycoproteins, glycolipids and lipoproteins fitted on or sitting inside the bilayer.

This array of protein and lipid compounds gives the membrane a mosaic appearance. The ‘fluid’ component of the model’s name is derived from how the phospholipid molecules and embedded proteins can shift positions on the cell membrane. It is fluid and not static! For each phospholipid, there is a hydrophobic tail and a hydrophilic head.

The phospholipid bilayer has a unique structure: the hydrophobic tails are touching each other while their hydrophilic heads are pointing in opposite directions: one exposed into the extracellular (outside cell) environment and the other in the intracellular (inside cell) environment.

The combination of having these two hydro- properties is why phospholipids are called amphipathic molecules.

Note: the cell membrane is usually shown as a sheet, but it is a sphere that encloses the cell. 

The cell membrane is important because it acts as a physical barrier between the intracellular (inner part of the cell) and extracellular (outside) environments with selective permeability (allows some substances to pass freely but restricts/blocks others) property to regulate the movement of substances across the membrane = in and out of the cell).

The substances which pass into and out of the cell include dissolved ions, nitrogenous wastes, organic molecules, and gases such as oxygen and carbon dioxide.

The cell membrane also provides and maintain the cell’s shape (cell shape varies depending on cell type – e.g. bone cells, plant cells and red blood cells).

The cell membrane’s selective permeability, which facilitates or restricts various substances in and out of the cell, comes from the amphipathic  phospholipids.  The hydrophobic ends hinders highly soluble (high polar) molecules from entering the cell while assisting non-polar molecules to diffuse through the bilayer (in and out of the cell). 

Substances need to move through the cell membrane to keep cells alive.

For example, mitochondria break down glucose into ATP in the process of cellular respiration, which provides our bodies with the energy we need to stay alive. But glucose is not made inside the cell. Animals consume food which the digestive system breaks down to glucose. The glucose is dissolved in the blood and carried in the blood vessels. Cells surrounding the vessels access the glucose by moving the glucose across the cell membrane.

Understanding the cell membrane via the fluid mosaic model is particularly important when chemists need to design the structure of new medicines so that they can pass through the cell membrane.