Falsification of membrane structures
The nature of science and membrane structure.
The accepted model of membrane structure today is the fluid-mosaic model but this has not always been so. Since 1900 there have been a series of hypothetical structures proposed and tested. The evidence has supported, step by step, a succession of new models each a little closer to the real structure of membranes in nature. The story of this progress illustrates many ideas about the nature of science, especially those connected with exploration and discovery. The 2017 Nobel prize for cryo-electron microscopy is included to illustrate how biological ideas are still changing as new technology helps research.
Activity 1 - Sorting the cards and explaining the nature of science
Students cut out the hexagons on the student hexagons worksheet and arrange them on their bench to explain just one of the key points in the blue box below.
For example: 5. The hypothesis gets closer and closer to the actual structure found in nature.
Students arrange the cards to show the increasing accuracy of the model and work out what you need to say to explain this. Notes could be made next to each card to explain the details of the explanation.
Key points about the nature of science
- There are a wide range of methodologies used by biologists.
- Imagination and creativity are used to work out the models.
- There are some flashes of intuition unsupported by the evidence at the time.
- Evidence from experiments supports or falsifies hypotheses.
- The hypothesis gets closer and closer to the actual structure found in nature.
- There is collaborative endeavor between biologists.
- New technology promotes new discoveries.
The Table of information may be useful for explaining the activity and also to follow the links to the original research papers linked to each card. Click the eye symbol to display the table
This is the student hexagon worksheet. Student hexagon worksheet
Activity 2 - Share ideas and record explanations
Students explain their diagrams to a partner by telling the key point they chose about the nature of science. This explanation is recorded in some way by the students as a record of their research.
- Stick the hexagons on paper and write notes around them.
- Video each other as they give the explanation.
- Take photographs and build a PowerPoint presentation / or worksheet.
Here are some examples kindly shared by students in 2014.
Read about the later work on proteins in the membrane in the Further information from Jonathan Singer himself.
Activity 3 (Alternative) - How do we know the structure of a membrane?
Use the the student hexagons worksheet and the sheet below printed on A3 size paper to answer the question,
"How do biologists know about things which we can't see?"
Arrange the hexagons on the page and write statements outlining how each experiment has contributed to out understanding of membrane structure.
These suggestions will help the explanation.
The role of new evidence
- Support evidence (providing more evidence for the idea that .... / building on the idea of ....)
- Falsification (suggesting that this explanation is incomplete / wrong that ....)
The techniques used to find out about things we cannot see
- Technology which helps extend the senses.
- Creativity used to imagine a structure which fits the observed data.
- Ingenious method to make the invisible visible, e.g. using fluorescent dyes.
Each experiment described uses a different method for data collection and analysis.
- Some of the evidence from electron microscopy led to the proposal of the Davson-Danielli model.
- Other evidence in the activity caused the falsification of the Davson-Danielli model that led to the Singer-Nicolson model
For inspiration this gallery of images may be useful:
Summary activity and some questions
Read the details on the worksheet How do we know about membrane structure? about the Daveson-Danielli and Singer-Nicholson models and answer the questions about the role of evidence to falsify or support theoretical ideas.
This lesson will address the NOS point about evidence from electron microscopy having falsified the Danielli Davson model of membranes. Better than simple factual recall the students will also deepen their understanding of other aspects of the nature of science in this activity. It is useful to point out that the ideas in these activities may also be useful in the TOK essay or presentation;
By reading each of the hexagons and rearranging them to explain the 'story' of the discovery of the structure of membranes students will see something of the creativity required in research. Key points of the Nature Of Science will become apparent and students should find the activity engaging. Talking about the information and making presentations will challenge some students.
Giving the students a choice will motivate the students, but may involve a bit of classroom management, so they don't all choose the same NOS poinr if one aim is to try to explain all the NOS ideas in a future lesson.
Some students will probably need help during this activity if they have not fully grasped the structure of membranes and the components of the membranes. For this activity no real knowledge of biochemistry is needed.
Weaker students could be asked to talk about each of the hexagons they have chosen, or to order the hexagons chronologically and identify some of the key points.
The timing is aimed to be about 1 hour. However if students present their ideas in larger groups or make more detailed notes this may require extra time.
The information is presented in a simplified way and students who choose to read the original papers will see that some of the techniques used are really quite complex. Students may easily find other examples of research as there are many of these which haven't been included for the simple reason of saving time.
An alternative sheet, which has the diagram of membrane structure has been added for teachers who wish to have more structure in the work.
The worksheet connected to this alternative activity gives a structured summary of what students need to remember from this lesson and the 2017 Chemistry Nobel prize winning work on Cryo-electron microscopy is included in a final question. (Although this is not in the syllabus, it is interesting to think how new evidence might continue to change our understanding of membrane structures - this certainly relates to NOS)
There are model answers for the summary questions here: How do we know structure - model answers