Biological membranes (2.1.5)
Navigate to resources by choosing units within one of the unit groups shown below.
Delivery guides are designed to represent a body of knowledge about teaching a particular topic and contain:
- Content: A clear outline of the content covered by the delivery guide
- Thinking Conceptually: Expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject
- Thinking Contextually: A range of suggested teaching activities using a variety of themes so that different activities can be selected which best suit particular classes, learning styles or teaching approaches.
Content (from AS and A-level)
The content from the specification that is covered by this delivery guide is:
|2.1.5 Biological membranes|
|(a)||the roles of membranes within cells and at the surface of cells||To include the roles of membranes as,
• partially permeable barriers between the cell and its
environment, between organelles and the cytoplasm and within organelles
• sites of chemical reactions
• sites of cell communication (cell signalling).
|(b)||the fluid mosaic model of membrane structure and the roles of its components||To include phospholipids, cholesterol, glycolipids, proteins and glycoproteins
the role of membrane-bound receptors as sites where hormones and drugs can bind.
|(c)||(i) factors affecting membrane structure and permeability
(ii) practical investigations into factors affecting membrane structure and permeability
|To include the effects of temperature and solvents.
M0.1, M0.2, M1.1, M1.2, M1.3, M1.6, M1.11, M3.1, M3.2, M3.3, M3.5,M3.6
HSW1, HSW2, HSW3, HSW4, HSW5, HSW6
|(d)||(i) the movement of molecules across membranes
(ii) practical investigations into the factors affecting diffusion rates in model cells
|To include diffusion and facilitated diffusion as passive methods
active transport, endocytosis and exocytosis as processes requiring adenosine triphosphate (ATP) as an immediate source of energy.
M0.1, M0.2, M0.3, M1.1, M1.2, M1.3, M1.6, M1.11, M2.1, M3.1, M3.2, M3.3, M3.5, M3.6, M4.1
HSW1, HSW2, HSW3, HSW4, HSW5, HSW6
|(e)||(i) the movement of water across membranes by osmosis and the effects that solutions of different water potential can have on plant and animal cells
(ii) practical investigations into the effects of solutions of different water potential on plant and animal cells.
|Osmosis to be explained in terms of a water potential gradient across a partially-permeable membrane.
M0.1, M0.2, M0.3, M1.1, M1.2, M1.3, M1.6, M1.10, M1.11, M2.1, M3.1, M3.2, M4.1
HSW1, HSW2, HSW3, HSW4, HSW5, HSW6
The effect of temperature on membrane permeability (2.1.5c).
This practical uses beetroot and the release of betalains from the inside of the beetroot cells.
This practical relates to PAG5 if a colorimeter is used and could also relate to PAG8.
Mathematical content includes M1.2, M1.3, M3.3.
The effect of surface area to volume ratio on the rate of diffusion (2.1.5d).
This practical uses agar and phenolphthalein as an indicator. The agar blocks can either be stained pink and the time taken for the colour to disappear recorded or the blocks can be colourless and the time taken for the colour to appear is recorded. However, the latter can be more problematic to achieve consistent results; it is more subjective than observing the point at which the cube becomes colourless.
This practical can be extended by making different agar shapes instead of cubes and opens up discussions on organisms and evolution with links to 3.1.2 Transport in animals.
This practical relates to PAG8.
Mathematical content includes M4.1 and M0.3 for surface area, graph plotting M3.2 and potentially M1.2, M1.3.
An investigation to find the water potential of a plant cell (2.1.5e).
This practical allows for good extension work in terms of numerical processing, analysis and evaluation (links to 1.1: Practical skills assessed in a written examination). The title derives from the fact that students often like to make a face with their potatoes when the cores have been removed!
This practical relates to PAG4 and/or PAG8.
Mathematical content includes M1.2, M1.3, M3.2, M3.4.
Approaches to teaching the content
The overreaching concept for this topic is linkage; it is an umbrella subject relevant to all content that follows in the syllabus.
It is therefore crucial that students grasp the learning outcomes here in order to extend that understanding in later topics. As such, this topic can be taught in the wider biological context, with constant reference to later subject matter. The general role of membranes should be taught within the framework of relevant examples, e.g. for 2.1.5(a) the role of internal membranes as a site of chemical reactions, you could use the mitochondrial membrane and enzymes used in respiration (linking to 5.2.2g). Membrane receptors appear throughout the syllabus and this link is developed in the activity ‘How receptive are you?’
Common misconceptions or difficulties students may have
The lateral movement of the phospholipids is a concept that students find tricky to visualise and can influence their ability to grasp the role of cholesterol in the plasma membrane. The fluid nature of the membrane is often a static statement for a student as they cannot imagine a dynamically moving membrane. This movement can be shown with the ‘Lively lipids and wiggly worms’ activity and links well with the earlier learning outcome 2.1.2(h) the structure of lipids.
Diffusion across the membrane (2.1.5d) is integral to the understanding of the need for mass transport (3.1.2a). Students find it difficult to relate small organisms to a large surface area to volume ratio and a faster rate of diffusion. This can be addressed at this early stage and the activity ‘Let’s diffuse’ can be expanded to anticipate this.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
Endocytosis could be described in terms of pathogen entry which links with 4.1.1(e) and the role of phagocytes. Many students fail to grasp the role of the cytoskeleton in endocytosis and exocytosis (linking back to 2.1.1j). This omission is targeted in the activity ‘Say it with balloons’.
Osmosis underpins later transport topics 3.1.3 Transport in plants and kidney function 5.1.2(d). Students often struggle with the negative numbers involved with water potential and the ‘Get negative’ activity assists with connecting a low water potential value with a high solute concentration.
Learner resource 2 (the set of 32 cards)
This activity involves 2 sets of cards. One set with the receptor and its cell (either written or written with a picture of the receptor) and one set with the name of the molecule/ligand that binds to the receptor.
These cards can be given out in pairs and shuffled. Students can either play a snap-like game or a few cards can be dealt, the rest placed face down and cards taken in turn until all pairs are found. The winner has the most pairs. Alternatively, one set of cards could be placed around the room and the other set divided out amongst the class. The students then have to find their matching pairs around the room.
This can be combined with the fluid mosaic model activity under Curriculum Content.
Pipe cleaners are used to represent the fatty acids and Velcro™ dots are used for the phosphate heads. The Velcro™ dots can be stuck back to back to form a Velcro™ ball for the phosphates. The pipe cleaners are zigzagged and then stuck to the Velcro™ ball. The phospholipids should stick to each other at the Velcro™ head with the tails next to each other. This model can be used to show the effect of temperature, the folding for a micelle, liposome and bilayer. Modelling clay could be added to represent cholesterol. One pipe cleaner and ball could be coloured to represent phosphatidylserine and the model used again when teaching apoptosis (6.1.1d).
This activity is a fun way of describing the stages of endo/exocytosis whilst highlighting the role of the cytoskeleton.
Students (or the teacher) can fill a balloon (before being blown up) with confetti, sweets, strips of paper with revision questions, or anything else you can think of! The balloon is blown up and tied. For exocytosis, the students are representing the packaging process of the Golgi Body (linking with 2.1.1g) and the balloon is the vesicle with the contents being a named protein. The rest of the class can put their hands in the air and act as the cytoskeleton. The balloon is passed across the hands to the back of the room, which would represent the plasma membrane. One or more students could role play the bilayer and burst the balloon ‘behind their backs’ to show the fusion of the vesicle and exocytosis of the contents.
2.1.5 Biological membranes is practical based and wherever possible should be linked to Module 1: Development of practical skills in biology and to practical activities. The practical work can be extended, as suggested in the activities ‘Betaling around’, ‘Let’s diffuse’ and ‘Potato heads’. Activity 2 ‘Betaling around’ provides an opportunity to use a core technique (linking to PAG5) and all the activities could relate to PAG8 and could be used to move to PAG12. As illustrated within the activities, this topic includes many mathematical requirements, which can provide ample openings for teaching mathematical skills and differentiating between students.
Linking to different aspects of the syllabus can be either taught, summarised or revised using card games (for example the ‘How receptive are you?’ activity). This could be provided for pair work or displayed around the room for individual work with good kinaesthetic movement to spot the matching pair.
An artistic approach can be used to look at the movement of phospholipids in the membrane (2.1.5c) as shown in the activity ‘Lively lipids and wiggly worms’. Chemical models can be used or crafted as outlined in the activity.
Fun elements can be introduced with the ‘Say it with balloons’ activity. The balloons can be filled with questions for revision, treats, confetti, and/or themed if near to a festivity.
A sense of intrigue can be used to introduce diffusion/osmosis with the changing colours of iodine and starch solutions as outlined in the ‘Magic colours and magic bullets’ activity. This links back to 2.1.2(q) and the test for starch. Water potential can be thought about using the ‘Get negative!’ activity.
2.1.5(d) and (e) diffusion and osmosis using the iodine test for starch 2.1.2(q).
Visking or dialysis tubing can be filled with starch solution. The tubing can be placed in a boiling tube of iodine solution. The boiling tube (with bung) is inverted a few times and students watch the contents of the visking tubing change colour. The visking tubing can be tied into different shapes to increase the ‘fun’ element of the activity, although a beaker may have to be used instead of a boiling tube and the content stirred instead of inverted.
Alternatively, you could put starch solution in the boiling tube and iodine in the visking tubing. The visking tubing can be described as a ‘magic bullet’ as it will change the colour of the solution.
OCR’s resources are provided to support the teaching of OCR specifications, but in no way constitute an endorsed teaching method that is required by the Board and the decision to use them lies with the individual teacher. Whilst every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions within these resources. We update our resources on a regular basis, so please check the OCR website to ensure you have the most up to date version.
© OCR 2015 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work.
OCR acknowledges the use of the following content:
Get negative! Activity: plant cell: BlueRingMedia/Shutterstock.com