I. Basic Membrane Structure
A. Phospholipid Bilayer

The basic component of a biological membrane is a molecule, known as a phospholipid. It is a large phosphate group structure (hydrophilic) connected to fatty acid tails (hydrophobic). In a membrane, the phospholipids arrange themselves such that they form two layers, with the fatty acid tails facing inward and the phosphate groups located on the outer surfaces of the membrane. As there are no strong bonds between neighboring phospholipids, they are free to move past one another, making the membrane fluid. The membrane is known as semipermeable because it will only let certain molecules pass through. Small molecules freely pass through membrane as they can slip pass the mobile bilayer. However, if the particle is an ion, the charge will prevent it from getting past the nonpolar fatty acid tail. So we have a rule that if a particle is LARGE or CHARGED (ionic, not simply polar like water) it will not pass through a membrane.

B. Protein component of membranes

Membranes are not made of only phospholipids. Various proteins are also found amongst the phospholipids. These proteins serve three functions: recognition proteins (identify cells as belonging to an individual), reception proteins (detect presence of molecules such as insulin or hormones) and protein channels (allow large or charge molecules across membrane). Because the phospholipids are free to move past one another, and there are various proteins found amongst the sea of phospholipids, we describe the cell membrane by the fluid mosaic model. This model accounts for its ability to flow and self-seal while also accounting for the various portions of the composition.

II. Crossing a Semipermeable Membrane

A particle that needs to past a membrane can do so in three ways:

A. Diffusion/Osmosis

Diffusion is the movement of particles from areas of high concentration to low concentration. It doesn’t require extra energy to disperse the particles, such as an open bottle of alcohol that quickly evaporates.

Osmosis is a special case of diffusion in which molecules of water move across a semipermeable membrane. Osmosis doesn’t require extra energy, moves from high concentration to low concentration, but gets tricky when you start looking at the concentration of solute and solvent. Water is the solvent and we use the terms "hypertonic" to denote a solution with a great amount of solute when compared to a "hypotonic" solution with relatively little solute. If the liquid on both sides of a membrane are equal in amount of water and solute, they are known as "isotonic."

B. Protein Medicated Transport

 

1. Passive/Facilitated Transport – Some large/charged particles move across membranes by means of specially shaped transport protein channels. These channels are specially shaped for specific molecules, and they don’t require extra energy (passive) but they can’t move against a gradient (still move from high to low concentration).

2. Active Transport – Some large/charged particles move across membrane but the cell must dedicate ATP to the process as it requires energy to move the substance, either due to the specific particle or because the material is being moved against a gradient.

C. Bulk Transport

When the movement of a material into or out of a cell isn’t at the individual molecule or ion level, the movement is known as bulk flow. For example, if a vesicle with many enzyme molecules is released at once, this is bulk flow. In the case of bulk flow, three distinct forms are recognized:

1. Endocytosis ("enter")– When a vesicle, and it membrane, fuse with the outer cell membrane, resulting in the products being brought into the cell.

2. Exocytosis ("exit") – When a vesicle, and its membrane, fuse with the outer cell membrane, resulting in the products being exported from the cell.

3. Phagocytosis – A special form of endocytosis in which a cell engulfs a large amount of material, usually another microoganism. This is seen in amoebas and white blood cells.