Biology A level revision resource: Testing cell membrane permeability

How do we study cell membranes?

Cell membranes are phospholipid bilayers that are usually visualised by the 'fluid mosaic model' consisting of proteins, carbohydrate polymers and glycoproteins that are able to move around relatively freely amongst the phospholipids. These molecules have many functions, including acting as channels to allow movement of molecules into and out of the cell, as cell signalling apparatus or even enzymes involved in metabolism. The phospholipids have polar heads which are hydrophilic, and nonpolar tails which are hydrophobic. The hydrophobic tails mean that it is difficult for polar molecules (which includes many substances that easily dissolve in water) to pass through the membrane without the help of channels or other cell membrane machinery. There are factors that affect the fluidity of the cell membrane, and these can affect membrane permeability making it easier for us to influence what substances can go into or out of cells.

A typical A Level membrane permeability experiment involves investigating the influence of a named variable on the membrane permeability of a vegetable such as beetroot (Beta vulgaris). Common variables to investigate are the effect of solvents or temperature because both of these factors can change the fluidity of the membrane. Beetroot is a useful subject for this experiment because of the distinctive betalains pigment that the stem tuber contains. These pigments are a useful indicator of membrane fluidity as they are typically contained within the vacuole of intact beetroot cells. An increase in membrane fluidity will cause the pigment to leak out of the cell, and the amount of pigment can be measured simply by using a colorimeter.

Don't forget to wash your cores thoroughly before the experiment, as we are interested in the amount of pigment that will leave intact cells, not those that have been damaged by coring. For more information about avoiding systematic and random errors, see the enzyme reaction rate resource.

What variables should we consider?

Variables are something that you should always be conscious of when conducting any experiment. Some are common to most experiments like temperature, pH, concentration and surface area to volume ratio. Others can be quite specific, such as the impact of solvents on membrane permeability or the intensity of light when using a potometer to measure transpiration rates. Here, we will discuss the potential variables that could impact results in the membrane permeability experiment, but every experiment that you do has a range of variables to consider.

So, what are the variables that could affect cell membrane experiments? We already know that the two named variables that we are likely to investigate are the impact of solvents or temperature on membrane permeability, so whichever of these you are investigating, be sure to control the for the other. For example, if you are measuring solvent concentrations then there might be potential for your solvent to react exothermically with a part of your experiment, therefore increasing the temperature. You could control for this by monitoring the temperature of your samples as you perform each test, or by setting up the practical in a fixed temperature water bath.

Surface area is a variable that you are unlikely to be investigating in this experiment, but is something that needs to be controlled during your experimental procedure. All of the beetroot cores should be cut with the same corer and cut precisely to the desired length to ensure that they have the same mass and same surface area. The power of the surface area affect can be massive. Consider how much paint you would need to fully cover an entire football pitch. Probably a fair few tins? If your paint particles were only 1 nanometre thick (and nanotechnology is starting to become available in paints) then you would only need 1 cubic centimetre (about a teaspoon) of paint to cover your football pitch entirely!

In some practicals there may be a pH difference between the reactant and the product. For example, when measuring the effect of catalase concentration on hydrogen peroxide decomposition, the reactant, hydrogen peroxide, is acidic and the product, water (plus oxygen gas), is neutral. Therefore this pH change could be affecting the reaction in addition to your experimental variable. A buffer solution can be used in such experiments to control small changes in pH, which will help to ensure the variable of interest is having the measured effect, rather than the pH.

Why are membranes so important?

The results of this experiment helps us to better understand the function and structure of cell membranes. Once we understand the factors that can affect membrane permeability, we are then able to better understand the transport of molecules into and out of the cell. Membrane permeability will have a big impact on both passive (diffusion, including osmosis, and facilitated diffusion) and active forms (active transport and endo/exocytosis) of transport. Cell membranes can also be highly specialised for very particular tasks, such as the villi of the epithelial cells in the small intestine, or the axon of a neurone, so understanding their typical function allows us to appreciate the way that these specialised cells are adept at carrying out their particular roles.

Currently, the most important aspect of transport across cell membranes for researchers is the rise of antibiotic resistance. There are two main strategies that bacteria have when they develop resistance - either to prevent the antibiotic reaching its target, or to modify or bypass the target. There can be implications to membrane structure and function as part of both of these strategies. Efflux pumps (a kind of active transport) that remove antibiotics and enzymes that modify or destroy them (which can be extrinsic membrane proteins) are ways that the membrane can be involved in resistance. Membranes can also be the target for antibiotics, for example penicillin acts on the bacterial outer membrane preventing the structural peptidoglycan from cross linking. Resistance can be developed by the bacteria altering this peptidoglycan layer or the penicillin binding protein. Some bacteria are also able to make their cell membranes less permeable to antibiotics as a method of resistance.