facilitated diffusion Passive transport facilitated diffusion Phospholipid bi-layer hydrophilic hydrophobic, protein gate semi permeable diffusion osmosis Equilibrium

Fick’s Law solute turgor pressure hypertonic Hypotonic, Plasmolysis Isotonic Solvent Fluid Mosaic model Concentration Gradient Carrier protein

The fluid mosaic model of the membrane (shown above) shows the membrane is composed of a number of proteins which are similar to shifting tiles. The spaces between the tiles are filled with fluid-like phospholipids. The phospholipid consists of hydrophilic heads, which point towards the outside environment and the cytoplasm. The hydrophobic tails repel the water and point in.

Thus, the phospholipids form a bilayer that acts like a barrier between the cell and the environment. The phospholipid bilayer also contains cholesterol, which makes the bilayer stronger, more flexible and more permeable. There are a number of important proteins in the plasma membrane which will be discussed later.

Diffusion Diffusion is one of those really, really important concepts which is impossible to understand at first glance, but when you think about it, it's about as simple as things come! It is used in so many aspects of science, that understanding it is an incredible help. For the complete explanation go to this site. This lecture is a cut down version of the contents of this site www.blobs.org/science/diffusion/index.shtml

Diffusion Put most simply, diffusion is the even spreading of something - that is, if you have a huge amount of something in one small area, then the diffusion of that particular substance would mean that it would be spreading out to occupy a larger area. When someone farts… I mean passes gas, does the smell stay in one place? Oh heck no! It diffuses throughout the entire room so everyone can experience it. www.blobs.org/science/diffusion/index.shtml

The strict definition of diffusion is the movement of particles from an area of high density to an area of low concentration. Because the concentration is not uniform, it is called a concentration gradient.

Everything needs to be spread out evenly, and so if there's more of something on one side, there are more collisions that make the particles transfer to the side without as many particles. …until they're equal.

It's not simply a case of movement in one direction It's not simply a case of movement in one direction. Particles move in both directions, but because one side has more particles to start with, they are more likely to move in one direction than the other.

When both sides have the same number of particles, they are just as likely to move one way as the other way, so there will be no change in amounts - both sides will stay equal. Equilibrium

Clearly it takes a certain amount of time for the two to even up Clearly it takes a certain amount of time for the two to even up. This amount of time varies depending upon certain factors. Remember:” High to low and very slow” difference in concentration has an important effect on the rate at which diffusion happens. The bigger the difference in concentration the faster diffusion happens.

Diffusion also happens across a surface Diffusion also happens across a surface. Obviously because there is this barrier in the way, the process will be slower, but certain substances can still get across a membrane because it's not a completely solid barrier Ultimately what you're looking for is a state of equilibrium, where there is as much of the substance going from left to right as there is going from right to left.

A man named Adolf Eugene Fick (1831-1879) did some experiments which showed that the rate of diffusion doesn't just depend on difference in concentration (or difference in partial pressures), but also on the (surface) area of the membrane, and on the thickness of the membrane. We've already established that if there's a huge difference, it will even out more easily. However, if there is a larger area to work with, then it will be easier to find a gap - the substance will be more likely to go through. Ficks Law

Ficks Law

Osmosis

Osmosis What is osmosis?                              Unfortunately osmosis is where the concept of diffusion gets even more confusing. Rather than the simple process of one chemical evening itself out, osmosis relates to a situation where something is dissolved in something else. It is initially rather important that you understand the difference between a solvent and a solute !

Osmosis If you understand the concept of equilibria, think about the system trying to establish an equilibrium. If you have one cup of weak tea and one cup of strong tea, and you have to add water to one of them to make them the same strength, then you dilute the cup of strong tea. In the same way, if you have a strong solution (i.e. high concentration) and a weak solution (i.e. low concentration), the only way to get them in equilibrium is to move some of the solvent from the weak solution to the strong solution. !

Facilitated Diffusion Consider it a “Front of the line pass” if you have the correct credentials

The purpose of the membrane is to control what goes in and out of the cell. The items that go in are highly regulated. It also communicates with other cells for example with receptors on the surface or cell to cell adhesion. Proteins that are found in the bilayer are receptor proteins, which deal with communication, recognition proteins and transport proteins that regulate the movement of water and soluble molecules through the membrane. In order to regulate the transport of molecules, there are two types of proteins in the cell: carrier proteins and transport proteins. The two types of transport proteins are channel and carrier protein. Transport is either active or passive. Active transport is moving molecules against the concentration gradient and energy is required in the form of ATP. Passive transport is moving molecules down the concentration gradient and no energy is required. Examples of passive transport are diffusion, which moves from high concentration to low concentration and osmosis, which is the diffusion of water molecules.

Cell membrane hyperlink http://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.htm Cell membrane hyperlink