Membranes

Surrounding all cells and many organelles (tiny cellular organs living inside the cells) is a plasma membrane which acts like a barrier preventing Free Movement of cells and keeping them protected.

The basic structure of ALL membranes is the same.

Membranes

separate the inside of the cell from the outside, separates the insides of the organelles from the cytosol (a substance that is part of the cytoplasm of the cell).

Membranes

are used in cell recognition, recognising which cells are ok/allowed, or which are foreign bodys/dangerous. They can also help with cell signalling.

Membranes

Regulate what is allow in or out of the cells or their organelles, they hold components of some metabolic pathways which can be needed for respirations or reactions.

What Are Membranes Made Of?

To answer this we meet the Phospholipids.

A Phospholipid Molecule has a HYDROPHILIC Head (Water Loving), and a HYDROPHOBIC Tail (Water Hating).

When they are mixed with water they form a MONOlayer at the water surface.

If phospholipid molecules are completely surrounded by water a Bilayer forms.

The Phospholipid bilayer moves freely, a bit like wobbley jelly, just like fluid molecules do.
They are NOT bonded together but the bilayer is made stable by the Hydrophilic Head group being held into place by the Hydrophobic region.

But a simple phospholipid bilayer cannot carry out all the functions of biological membranes as it is too fragile. Therefore other components are needed in order to make a fully functioning biological membrane.

Phospholipid Bilayers will allow non-water, non-charged products through. Anything with a charge will have to use the Protein Ion Channels. These channels act like doors or gateways into the cells via the membrane).

The Fluid Mosaic Model

https://en.wikipedia.org/wiki/Fluid_mosaic_model

In 1972 a man, a singer, called Garth Nicolson, proposed a model of a cell membrane called The Fluid Mosaic Model.
It's fluid because the individual Phospholipids and protein molecules move around within their layers.
It's mosaic because it describes the pattern produced by scattered protein molecules when the surface of the membrane is viewed from above.

Fluid Mosaic Facts

1. It is a bilayer of Phospholipid molecules which are approximately 7-10 nanometres (or 0.000007 millimetres) wide.
2. Phospholipid tails are visible using an Electron Microscope (https://en.wikipedia.org/wiki/Electron_microscope) at X100,000 magnification.
3. Phospholipid tails point inwards facing each other giving a Hydrophobic Interior. The heads face the aquius (waterbased) medium that surrounds the membrane.
4. Most of the protein molecules float like mobile iceburgs but some are fixed.
5. Some proteins are embedded in the outer layer, some in the inner layer, and some proteins span the entire membrane.
6. Many proteins and lipids (fats) have short branching carbohydrate chains attached to the external surface of the membrane forming Glycoproteins (https://en.wikipedia.org/wiki/Glycoprotein) and Glycolipids (https://en.wikipedia.org/wiki/Glycolipid )
7. Cholesteral molecules are also found in the membrane for mechanical stability.

The Roles of Membrane Components

Phospholipids

act as a barrier to most water-soluble substances.

Cholesterol

regulates the fluidity of the membrane - Important for mechanical stability.

Proteins

Act as a transportation service for different kinds of ions or molecules. Some proteins play important roles in membranes of organelles and some proteins may be enzymes.
(Note: All Enzymes are proteins but not all proteins are enzymes)

Glycolipids and Glycoproteins

stabilise the membrane structure whilst acting as receptor molecules which bind with things like hormones and neurotransmitters. They may also be involved in cell signalling and allow self-recognition by the immune system.