The lipid membrane is a barrier to molecular movement. This barrier property enables cells to use membranes as batteries, to store electrical and chemical energy.

As you might expect, cell membranes are not simple barriers. How difficult it is to pass through a membrane depends upon who your are chemically.

We can think of the barrier in graphic terms as a line of hydrophobic policemen.

If you are hydrophilic, this barrier is quite high.

On the other hand, if you are hydrophobic, the barrier is low.

You may even find it difficult to leave the membrane once you enter it.

The more hydrophobic a molecule, the more readily it passes through cellular membranes.

This relationship is known as Overton's law and was the basis for the first models of membrane structure.

The earliest membranes were likely to have been composed of similar, but simpler molecules with much shorter hydrophobic chains.

Based on the properties of lipids, we can map out a plausible sequence for the appearance of membranes.

Lipids with very short hydrophobic chains, 2 to 4 carbons long, can dissolve in water.

As the lengths of the hydrophobic chains increase, the molecules begin to self-assemble into micelles.

By the time the hydrophobic chains reach ~10 carbons in length, the molecules will begin to associate into semi-stable bilayers.

Bilayer stability increases further as hydrophobic chain length increases. At the same time, membrane impermeability also increases.

It is a reasonable assumption that the earliest biologic systems used short chain lipids to build their 'proto-membranes' and these membranes were relatively leaky.

Cells must exchange materials from the outside world in order to maintain their structure, grow and reproduce.

The appearance of more complex lipids, capable of forming more stable and more impermeable membranes therefore depended upon the appearance of mechanisms that enabled hydrophilic molecules to pass through the membrane.

The process of interdependence of change is known as co-evolution.

In the late 1940's Collander and colleagues studied the movement of molecules into cells.

They noticed that small water soluble molecules entered cells faster than predicted by Overton's law.

They postulated that membranes contained features that enabled them to act as molecular sieves.

These turn out to be protein pores, channels and pumps which we will discuss further on.

Diffusion and Osmosis

In solution molecules are in constant movement.

As they collide with one another, they exchange energy - they change direction and speed.

Each molecule performs a random walk. These random movements are the basis of process of diffusion.

A group of particles, initially confined to a small volume will, over time, disperse.

The opposite behavior, a group of dispersed particles concentrating back into a small volume, is never observed

This asymmetry of behavior is the basis of the second law of thermodynamics, it is described by the concept of entropy.

It is this behavior that explains why, if there is a difference between the concentration of a molecule in one region compared to another, a concentration gradient, there will be a net flux from the region of high concentration to the region of low concentration.

Both solute and solvent can diffuse. Osmosis is the net diffusion of solvent.

Cells are packed full of molecules. These molecule take up space, space no longer occupied by water.

water is white

It is therefore very common for the concentration of water outside of the cell [water]_outout to be higher than the concentration of water inside the cell [water]_in.

The presence of brackets means concentration. [A] means the concentration in moles per liter of the molecule A.

The water gradient is capable of doing work -- it can lift a fraction of the solution against the force of gravity.

At equilibrium, the force generated by osmosis, the osmotic pressure is balanced by the weight of the levitated solution.

In an important sense, the concentrated cytoplasmic organization of the cell represents stored energy.

Dealing with osmosis

The water concentration gradients leads to a net flux of water into the cell.

The volume contained within the plasma membrane can expand somewhat by the straightening out and thinning of the plasma membrane.

However, both are limited and the membrane will burst like an over-inflated balloon unless something else is done.

Organisms such as plants, fungi and bacteria use a rigid cell wall to deal with osmosis.

The cell wall is a specialized and relatively rigid extracellular matrix located outside of the plasma membrane. The cell wall is relatively porous and does not present a barrier to the diffusion of small molecules.

The space between the cell wall and the plasma membrane is called the periplasmic space.

As water enters the cell by osmosis, the plasma membrane is pressed up against the cell wall.

The force exerted by the cell wall on the membrane balances the force of water entering the cell. When the two are equal, net influx of water into the cell stops.

When the [water] outside decreases, this pressure is reduced, and the plant wilts.

This is a passive effect, built into the wall when it was assembled. Once built, a cell with a cell wall does not need to expend energy to resist osmotic effects.

Dealing with osmosis without a cell wall.

Animal cells do not have a rigid cell wall. This allows them to be active predators, moving and engulfing their prey

It also means that they must use other mechanisms to deal with the effects of osmosis.

Most free living protozoa, which are single celled eukarya without a rigid external wall, live in dilute aqueous solutions, where osmotic effects are particularly severe.

They deal with the constant in-flux of water by actively pumping this water back out using an organelle known as the contractile vacuole.

To expel the water, the vacuole connects with plasma membrane and is squeezed by cytoskeletal systems within the cytoplasm.

This squirts the water out of the cell.

The process of vacuole contraction is an active process, it involves work and requires energy.