The Plasma Membrane

From MyMCAT

Contents 1 Introduction 2 Structure and the Fluid Mosaic Model 3 Transport 3.1 Diffusion 3.1.1 Passive 3.1.2 Facilitated 3.2 Osmosis 3.3 Active Transport 3.4 Secondary Transport

Introduction

The plasma membrane acts as shell around prokaryotic and eukaryotic cells; it keeps the important stuff in, and junk out. While all cells and organelles have a membrane, it is important and vital to know the differences in structure and function between eukaryotic, prokaryotic, and the different organelle (i.e. mitochondrial) membranes.

Structure and the Fluid Mosaic Model

All plasma membranes are composed of a bilayer of phospholipids. The tight packing of phospholipids together all oriented in the same direction form each layer, and the whole system is stabilized by hydrophobic interactions. Various cell proteins and carbohydrates (attatched to proteins and/or lipid heads) are found throughout these layers. As a result, the bilayer is often more accurately described as a fluid mosaic, in which elements (such as proteins) are free to float around in and on the bilayer.

Transport

Diffusion

Diffusion is the natural process of molecules moving down their concentration gradient from high to low. It does not require energy and can be compared to how an object that is hot will cool down as it warms other objects nearby (ie. a heat gradient).

Passive

Because the plasma membrane has a polar outer surface and a hydrophobic inside, only certain molecules can diffuse through it efficiently. Small, nonpolar molecules can diffuse freely, however large molecules (such as glucose) can not and require different transport mechnisms.

Facilitated

To assist larger molecules in diffusing across the plasma membrane, cells often have proteins designed to allow specific molecules through. These channels thus allow large molecules to diffuse and at the same time allow the cell to be specific as to which molecules can cross.

Osmosis

Just as molecules tend to flow from high to low concentration, water also flows, but in the direction of osmolarity. Water will always flow from low to high solute concentration. For example, while the pressure inside the circulatory system is much higher than the surrounding tissue, water does not leave easily because the solute concentration inside the blood is also much higher than the surrounding tissue's solute concentration.

Active Transport

Moving molecules down their concentration gradient does not require energy as this is their preferred direction, however cells often wish to go against the gradient, and in these cases additional energy (usually in the form of ATP) is required. By coupling the drive of molecules against their gradient with the expense of "burning" ATP, a cell can work against this unfavourable direction.

Secondary Transport

An alternative to using ATP in moving molecules against their concentration gradient is to couple the movement with the movement of a different molecule. In this case, the energy of one molecule moving in its preferred direction can be used to drive the movement of another molecule in an unfavoured direction. Secondary transport can either be symport (in which both molecules move in the same direction, but in different directions with respect to concentration gradient) or antiport (in which the molecules move in different directions).