To provide an accurate representation (model) of the molecular structure of the cell membrane for students who are studying Biology, and to build this structure utilizing the model.
A foundational theme in biology is the connection between structure and function. This is true both at the molecular and cellular level. The molecules which make up the cell membrane are phospholipids which do not dissolve in water. This property of phospholipids is vital in the formation of the cell membrane as the membrane is the structure which surrounds the cell.
The structure of the cell membrane is a double layer of phospholipids called a lipid bilayer.
Each phospholipid is composed of a polar “head” and 2 nonpolar “tails”. Polar molecules have a positively and a negatively charged end, while nonpolar molecules do not. The polar heads are attracted to water and the nonpolar tails are repelled by water. An example of a nonpolar substance is oil. As water inside and outside of the cell repels the nonpolar tails, they are pushed to the inside of the lipid bilayer (see picture). The polar “heads” however are attracted to water and are therefore located on either side of the bilayer.
Transport proteins are molecules which allow substances to move into or out of the cell, which are unable to move directly through the cell membrane.
The simple model below can be quickly prepared to present the student with information that other students will find in detailed pictures of the cell membrane found in their biology books. This model is accessible to students both tactually and visually.
16 – 2.5” diameter white Styrofoam balls – the polar “heads”
16 – white pipe cleaners – the nonpolar “tails”
2 cardboard tubes from paper towel rolls – The transport proteins
Cut all pipe cleaners in half.
Make 2 marks about 1” apart on each Styrofoam ball with a pencil.
Insert 2 cut pipe cleaners into each Styrofoam ball at the pencil marks. It is easiest to do so if you hold the pipe cleaner firmly close to the end. If it is difficult to insert, make a small hole at each mark with a pencil point first.
The cardboard tube and marble will be discussed below.
After discussing the structure of the cell membrane, provide students with 16 model phospholipids and inform them that they will build the structure of the cell membrane out of the phospholipid molecules provided. It may be helpful for students to work in groups of two.
Remind students that the polar portion of the molecule is the head and the nonpolar portion are the tails. Students should quickly understand which portion of the model is the head and which the tails are.
Ask students – What do we know about water? (It is polar) Remind students that the liquid inside and outside of the cell is an aqueous (water-based) solution and therefore polar.
Ask students – Which portion of the phospholipid molecules will be repelled by water and which attracted?
Heads (because they are polar) – attracted
Tails (because they are nonpolar) – repelled
Have students build the cell membrane. Guide students only if necessary.
After students seem comfortable with the structure of the cell membrane, have them add the transport protein
Ask students – What do you think a transport protein does? (Transports) – It transports substances through the cell membrane which can’t travel directly through.
Have students place the transport protein through the cell membrane and use the marble to represent substances moving through the transport protein.
Students can be asked to build the cell membrane as part of the assessment on cell structure.
MS-LS1-2. Develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function.