Cell structure (2.1.1)
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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)
|2.1.1 Cell structure|
|(a)||the use of microscopy to observe and investigate different types of cell and cell structure in a range of eukaryotic organisms||To include an appreciation of the images produced by a range of microscopes: light microscope, transmission electron microscope, scanning electron microscope and laser scanning confocal microscope.
|(b)||the preparation and examination of microscope slides for use in light microscopy||Including the use of an eyepiece graticule and stage micrometer.
|(c)||the use of staining in light microscopy||To include the use of differential staining to identify different cellular components and cell types.
|(d)||the representation of cell structure as seen under the light microscope using drawings and annotated diagrams of whole cells or cells in sections of tissue||PAG1|
|(e)||the use and manipulation of the magnification formula||magnification = image size/object size
M0.1, M0.2, M0.3, M1.1, M1.8, M2.2, M2.3, M2.4
|(f)||the difference between magnification and resolution||To include an appreciation of the differences in resolution and magnification that can be achieved by a light microscope, a transmission electron microscope and a scanning electron microscope.
|(g)||the ultrastructure of eukaryotic cells and the functions of the different cellular components||To include the following cellular components and an outline of their functions: nucleus, nucleolus, nuclear envelope, rough and smooth endoplasmic reticulum (ER), Golgi apparatus, ribosomes, mitochondria, lysosomes, chloroplasts, plasma membrane, centrioles, cell wall, flagella and cilia.
|(h)||photomicrographs of cellular components in a range of eukaryotic cells||To include interpretation of transmission and scanning electron microscope images.|
|(i)||the interrelationship between the organelles involved in the production and secretion of proteins||No detail of protein synthesis is required.|
|(j)||the importance of the cytoskeleton||To include providing mechanical strength to cells, aiding transport within cells and enabling cell movement.
|(k)||the similarities and differences in the structure and ultrastructure of prokaryotic and eukaryotic cells.||PAG1|
Starting the course with this topic provides an ideal opportunity to do the following:
- Provide a gentle transition from GCSE to A Level with plenty of practical work
- Teach important practical skills such as use of the light microscope (PAG1), preparation of temporary mounts and the use of stains to identify biochemical constituents of specimens
- Teach, monitor and develop practical drawing skills
- Encourage student choice and build up awareness of biodiversity and cell diversity by providing a range of microscopy options for students to experiment with.
Resources for this sort of practical-led approach are listed under ‘Thinking Contextually’. Online resources to help teach the theory of microscopy and cell structure are listed here.
This is a worksheet explaining how to calibrate an eyepiece graticule using a stage micrometer in order to measure specimens under the microscope. It also gives practice questions and answers.
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Approaches to teaching the content
Students meet some significant mathematical conceptual challenges in this module, including measuring under the microscope 2.1.1(b), using the magnification formula 2.1.1(e) and using and converting to microscopic units. To allow students to gain a real understanding of magnification and the terms object and image, it might be necessary to go back to basics and begin with hand lenses rather than the light microscope itself, as in the first two activities.
Common misconceptions or difficulties students may have
Students may find it hard to cope with the use and spelling of the extensive new biological vocabulary of sub-cellular structure. Keeping a glossary list of new words and their definitions is one approach. The 2.1.1 glossary sheet provided below also lists difficult plurals (e.g. mitochondria) and adjectival forms. It could act as a model for students to develop their own glossary of terms as they progress through the course. A link to an online Biology dictionary is provided for students to check up on definitions of unfamiliar words.
Another difficulty is in students being able to conceptually relate to, or imagine, structures and events at a tiny scale. The ‘Nanosense’ and ‘Secret Worlds’ resources help to build familiarity with the logarithmic scale of measurement and the relationship of different SI units.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
These learning outcomes require knowledge and skills from this unit and can also be used to reinforce microscopy skills (PAG1), mathematical calculation skills (e.g. M0.1, M1.8, M2.2) and drawing skills:
- 2.1.5(e) osmosis e.g. in red onion bulb epidermis
- 2.1.6(d) plant cell mitosis e.g. onion root tip squash
- 2.1.6(h) specialised cells
- 2.1.6(i) tissues
- 3.1.1(c), (g) and (h) lung, gill and insect tracheal structure
- 3.1.2(c) blood vessel histology
- 3.1.3(b) plant vascular tissue
- 4.1.1(e) phagocytes and blood smears
- 4.2.2(c) cell structure of five kingdoms
- 5.1.2(b) liver histology
- 5.1.2(c) kidney histology
- 5.1.4(c) pancreas histology
- 5.1.5(l) muscle structure
- 5.2.1(b) chloroplast structure
- 5.2.2(b) mitochondrion structure.
This is another preliminary task that helps students understand the terms ‘object’ and ‘image’ by working with a magnifying glass or hand lens. Students find it easier to distinguish between these terms when they are presented with this scenario away from the microscope and when the object is in the normal size range of things that they can see and handle.
Suggested biological specimens are seeds, leaves and shells but anything small enough to fit in the first box on the sheet can be used. The binomial name of the specimen should be available to give to students (linking to 4.2.2). The magnification formula is introduced here (linking to maths skill M2.2) without the need for students to convert units (maths skill M0.1).
The onion root mitosis sample can be selected and viewed at six magnifications (x25 to x1000). The large range of magnifications and the fact that all students are accessing the same images lends itself to setting homework using this web link. Students could practise using the magnification formula to check on the real object size of a selected cell at two or three different magnifications. They could also calculate the mean cell length from three or more cells in each image (maths skills M1.2, M1.6) and be asked to give the answer to an appropriate number of significant figures (maths skill M1.1).
As the specimen is microscopic, students will need to combine understanding of the magnification formula with an understanding of micrometres and the ability to convert mm to µm (maths skill M0.1).
This is an interactive Java tutorial with a sequence of images going from the milky way to a single carbon atom nucleus and beyond in orders of magnitude. In the automated play form it could be shown to a class. In manual form clicking up through the images could be used to highlight the normal range of sizes of interest in biology.
The labelling of each picture in metres in standard form on the left and units appropriate to the size of the subject matter on the right could be used to teach or class-test standard form and the SI units in the range metre, millimetre, micrometre and nanometre (maths skill M0.1).
A list of suggested contexts for introductory practical work with the light microscope is provided in the first resource, including a hay infusion to culture protoctists, which is detailed in a separate resource. A checklist of drawing skills is also included here.
After exploring a range of biological materials using prepared and temporary slides with the light microscope, students need to gain practice with a range of electron micrographs. Websites supplying useful images are shown in the activities. Asking students to put together their own collection of images of various cell types imaged in different ways, which they then annotate, gives them the chance to be creative. In the form of a PowerPoint this resource can be shared with the class and will give the student or students who created it a chance to talk through the vocabulary of cells and microscopes. While the false colour scanning electron microscope images have instant appeal, it is important to provide print-outs of a selection of black and white transmission electron micrographs to give students practice in identifying organelles, measuring them, and calculating real object sizes.
The importance of the cytoskeleton can be enlivened by the use of videos showing its role in the movement of organelles (e.g. the Harvard video) and of specific cells, such as slime mould cells.
This is a short checklist for students to use when producing biological drawings. The 8-point checklist also provides a quick way of marking student drawings. Simply give a tick or cross for each point and tot up the total out of 8. If the order of ticks and crosses corresponds to points 1-8, students can see what aspects they are doing well and where they are making mistakes.
This provides preparation for the A Level Practical Endorsement. (For more guidance and good examples of biological drawings see the OCR Biological Drawing Skills Handbook).
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