B1.1 Cell structures
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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;
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B1.1 Cell Structures
This section of the delivery guide focuses on use of microscopy to examine cells and sub-cellular structures.
A clear development of the practical skills required in both the use and interpretation of images from light microscopes is required from learners. This will require learners to be practically involved in activities that encourage the acquisition of these skills.
Learners are expected to know the main sub-cellular structures in eukaryotic and prokaryotic cells and understand how these relate to their functions within the cell. The learner should be able to locate such structures on cell diagrams and pictures. Learners should be able to explain how these structures help the cell perform any specialized function it may have.
With an understanding of the limitations of the light microscope, learners should be able to identify the use of the electron microscope as a method for observing structures that are beyond the limitations of a light microscope.
B1.1a describe how light microscopes and staining can be used to view cells to include: lenses, stage, lamp, use of slides and cover slips, and the use of stains to view colourless specimens or to highlight different structures/tissues and calculation of the magnification used.
B1.1b explain how the main sub-cellular structures of eukaryotic cells (plants and animals) and prokaryotic cells are related to their functions to include: nucleus, genetic material, chromosomes, plasmids, mitochondria (contain enzymes for cellular respiration), chloroplasts (contain chlorophyll) and cell membranes (contain receptor molecules, provides a selective barrier to molecules).
B1.1c explain how electron microscopy has increased our understanding of sub-cellular structures to include: increased resolution in TEM.
Cells are the fundamental units of living organisms. Cells contain many sub-cellular structures that are essential for the functioning of the cell as a whole. Microscopy is used to examine cells and sub-cellular structures. This part of the delivery guide provides scope for developing the necessary practical skills involved in using the light microscope and should be seen as very much a ‘hands-on’ experience. These skills will also enable the mathematical learning outcomes like demonstrating an understanding of number, size and scale and the quantitative relationship between units to be developed.
Learners should be familiar with cells as the fundamental unit of living organisms, and with the use of light microscopes to view cells. They should also be familiar with some sub-cellular structures, and the similarities and differences between plant and animal cells.
Learners will develop skills in using light microscopes and use staining techniques to identify important cellular structures in animal and plant cells. Magnification calculations are included and these will allow learners to appreciate the size and scale of cells and sub-cellular structures, often a difficult concept to grasp. Using the microscopy skills developed, learners will develop an understanding of the main cellular differences between eukaryotic (plants and animals) and prokaryotic cells.
Common misconceptions or difficulties students may have
Learners are required to develop microscopy skills and this often poses challenges in that learners often lack the fine motor skills needed to focus on cellular structures. These can be encouraged and developed using virtual microscopes, of which there are a number of examples currently available on the market. The use of virtual microscopes would allow learners to develop skills and avoid the frustrations of being unable to focus on ‘live’ cellular structures. If the fine motor skills are developed the experience for learners when making stained slides of ‘live’ material will be more productive.
Learners commonly have difficulty understanding the concept of a cell as a 3D structure, so this should be addressed during the teaching of this topic. There are a number of approaches that can be used involving modelling the cell and its sub-structures such as making a 3D cell with a plastic bag, golf ball peas and water. The 3D nature of the cell can be shown, then placed on an OHT to project the image onto a whiteboard. On the 2D projection the image can then be drawn around on the board. These approaches can be both motivating and concept-developing for learners.
Conceptual links to other areas of the specification - useful ways to approach this topic to set students up for topics later in the course
The microscopy skills developed in this section of the delivery guide will be used in a number of other topics in the specification. Using a light microscope is a skill directly needed in several topics. These include observations of blood smears and the gross structure of blood vessels in animals as well as observing root hair cells and locating the position of xylem and phloem in plants.
The knowledge and understanding of sub-cellular structures will provide a precursor to the work on ‘What happens in cells’. This involves looking at the structure and function of DNA and important cellular reactions controlled by enzymes such as respiration and photosynthesis. This is covered in the next section of this delivery guide.
Learners can engage in a skill development programme to enhance their use of microscopes when using them in real life situations.
This can be productive in building confidence in using equipment and set-ups that can be quite daunting in real-life situations. It can ultimately save time and frustration when learners use actual light microscopes.
The first website provides a worksheet with examples of how to use the equation to work out the actual size of an image when you know the magnification and actual size, or be able to work out the magnification if you know the image and actual size or the actual size when you know the image size and magnification.
The video can be used to support this activity as either a starter or plenary activity. It outlines the relationship between units used to measure microscopic images.
This interactive software allows learners to track across a 3D diagram of an animal cell and focus in and click on the sub-cellular structures which then magnifies and animates the chosen structure providing information on its function.
It also allows the learner to click and convert the image to a plant cell to see the additional features that a plant cell contains.
This is a Rap/Poem covering the main sub-structures of animal and plant cells. It could be used as a starter to recap prior learning or to provide a memory aid.
An extension activity could be for students to consider writing their own rap memory aid.
This PowerPoint provides a suggested approach to delivering activities that encourage learning of the components and functions of plant and animal cells.
It has assessment for learning activities that includes interactive diagram labelling, Venn diagram construction for overlap of plant/animal cell structures, true/false statements and extended writing.
Students need to be able to describe how to prepare microscope slides and also how to set up and use a microscope. This activity describes how to develop these skills.
Care needs to be taken when using the slides and coverslips and also in obtaining the cheek cell swab, both of these would require a centre based risk assessment. The cheek cell slides need to be decontaminated after the practical and this is an opportunity to discuss good microbiological techniques.
The website provides a PowerPoint resource that has starter and plenary activities to support this activity along with diagrams to guide students in how to obtain the best results from the slide preparations.
Approaches to teaching the content
The topic lends itself to a wide range of practical activities both real and virtual. It is vital that learners are provided with practical opportunities to develop appropriate microscopy skills. Learners should have the opportunity to gain skills that will be used again in a number of areas of the specification. As skills are an important aspect of this section, there are a number of activities that could be set up to be used over a period of time in order to continue to develop skills ('MICRODOT kit' and 'Cell size and scale').
There are many activities that can be used as starters or plenaries (such as 'Cell structure: Rap/poem memory aid') and these mini-activities can support engagement and assessment for learning. This is particularly important in a topic that can be seen as challenging, both in terms of practical and mathematical skills required in using light microscopes and interpretation of microscopic images. The use of Venn diagrams in 'Plant and animal cells' can support an understanding of the similarities and differences between plant and animal cells and could be used in assessment situations.
Interactive software (such as 'Inside a cell') can provide a useful vehicle to reinforce learning of cell sub-structures. Learners have a 3D visual representation of the sub-structures to support their learning and also it helps them to locate the size and positioning within the cell.
Set up a display of pictures, light micrographs and diagrams of objects/cells/sub-cellular structures that can be seen by:
- the naked eye
- hand lens
- light microscope
low magnification (e.g. x100)
medium magnification (e.g. x200)
high magnification (e.g. x400)
- electron microscope
Arrange this display around the side of the room and space them out according to the correct position on a banner that has the measurement scale of:
This will familiarise students with the units of measurement required for different levels of magnification and support their understanding of scale.
Students can observe the images and complete a table that identifies the structure, size and type of magnification device required to see the structure.
This interactive webpage allows students to move a slider from images that can be seen by the naked eye to those that require an electron microscope. It provides a grid with scaled measurements of various objects/cells/sub-cellular structures.
The human eye can see objects down to approximately 100µm, e.g. a human egg cell. It is possible to detect some cells using a hand lens (e.g. snowberry berry cells Symphoricarpos albus, although care should be taken as these berries are toxic, causing vomiting, dizziness and slight sedation in children). To visualise smaller cells one needs to use a light microscope. A light microscope can be used to visualise most cells including larger bacteria, however it is not powerful enough to visualise viruses.
Light microscopes can even resolve some internal cellular structures (e.g. nucleus, mitochondria and chloroplasts), but not the smaller organelles. The problem is caused by the wavelength and scattering of light used to visualise the sample. To see smaller objects one would need the magnification and resolving power of the electron microscope.
These microscopes use a beam of electrons rather than light. Typically this electron beam is either directed through the object (transmission electron microscopes TEMs), or onto the surface of suitably prepared samples (scanning electron microscopes or SEMs). Although there are some specialist electron microscopes e.g. tunnelling electron microscopes that can even resolve molecules and atoms.
'Cell structure using sweets' is a brief description of how to make 3D models of cells using jelly and various types of sweets.
There are several suggested alternatives for sweets that can be used for sub-cellular structures. 'Build 3D models of animal and plant cells' gives two alternative methods for making 3D model cells, either using jelly and sweets or by using craft resources.
African Violets (Saintpaulia Spp.) are excellent plants for investigating specialised cells. Students investigate a variety of specialised cells using light microscopy and see cytoplasmic streaming in trichomes. The link below includes further details on this investigation.
How to see some specialised cells on the leaves:
Take thin slices from the underside of the leaf using a scalpel/razor blade. Take care! Extreme caution should be taken when using sharp instruments.
Mount on a microscope slide in a drop of water and add a cover slip.
Observe using the low magnification objective lens. Photos can be taken using a camera over the eye piece. Camera phones work particularly well for this activity.
The hairs are made of large cylindrical shaped cells.
Stomata and guard cells
Guard cells have no pink pigment unlike the cells around them. They stand out as white against the pink background.
Specialised cells in amazing african violets video
Students can also observe the video showing movement of chloroplasts by cytoplasmic streaming in Elodea.
Cytoplasmic streaming in Elodea can also be seen very well done practically, especially if there is the facility of projecting a microscope image. About 10-15 minutes after preparing a wet mount of an Elodea leaf, evaporation of some of the water makes it possible to see chloroplasts in motion inside of the cytoplasm.
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