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1 Chapter 1 Cell structure Topic 1 Light microscopy 2 Measuring and drawing cells 3 Electron microscopy 4 Prokaryotes and eukaryotes 5 Tissues and organs Syllabus section Number of 40-minute periods Outline of topic content A (e) (g) 4 5 Using a light microscope; temporary preparations; comparing animal and plant cells A (a) (g) (h) A (b) (c) (d) (e) (g) (h) 2 Eyepiece graticule and stage micrometer; calculating magnification and actual sizes (light microscopy); units used in cell studies 9 Resolution and magnification; electron micrographs; animal and plant cells; measuring and drawing cells Resources in coursebook Pages 1 5; SAQ 1.1 EOCQ 1, 3 Pages 4 7; Box 1A, page 4; Measuring cells, page 7; Magnification worked examples 1 3, pages 8 10; SAQ 1.2a EOCQ 10 Pages 8 13; SAQ 1.2b, EOCQ 2 4, 6 9, 11 Resources in Teacher s Resource A (i) 1 Comparing prokaryotic and eukaryotic cells A (f) 1 Low power plans of tissues and organs Pages 13 20; SAQ 1.6 EOCQ 5, 6viii Pages 20 28; EOCQ 6ix xii Practical 1.1B: Making a temporary preparation of onion epidermal cells Practical 1.2: human cheek cells Practical 1.3: epidermal cells from a lettuce leaf Practical 1.4: Elodea leaf cells Practical 1.1A: Calibrating an eyepiece graticule Practical 1.1B: Making a temporary preparation of onion epidermal cells Practical 1.2: human cheek cells Practical 1.3: epidermal cells from a lettuce leaf Practical 1.4: Elodea leaf cells Worksheet 1.2: Interpreting an electron micrograph of an animal cell Worksheet 1.1: Interpreting an electron micrograph of a plant cell Worksheet 1.4: Microscopy Worksheet 1.5: Sizes Animation How cells make and secrete proteins Worksheet 1.3: Prokaryotic and eukaryotic cells The material on plant tissues and organs is covered in Chapter 7, Practical 7.2 N.B.: any animation available on this Teacher s Resource CD-ROM is also available on the Coursebook CD-ROM; EOCQ = end-of-chapter question; SAQ = self-assessment question Cambridge University Press

2 Topic 1 Light microscopy There may not be time to carry out all four practicals. Practicals 1.1B and 1.2 are probably the best for comparing animal and plant cells. Practical 1.4 has the advantage that chloroplasts are seen as a typical feature of plant cells, but the technique required is a little more challenging. Practicals also include measurements of cells. Some teachers prefer to use Practicals 1.1 and 1.2 for students to gain experience and confidence in using a microscope and comparing animal and plant cells, before using Practical 1.3 and/or 1.4 to introduce measurement of cells. See circus activity below. Common misunderstandings and misconceptions Cell walls and cell surface membranes are sometimes confused. Sometimes students imagine that plant cells have walls, whereas animal cells have cell surface membranes (implying that plant cells lack a cell surface membrane). Cell membrane is often used instead of cell surface membrane or plasma membrane. It is worth stressing the need for precision. Sometimes the term cytoplasm is thought to include the nucleus. The term protoplasm should be distinguished from the term cytoplasm. Homework ideas SAQ 1.1 EOCQ 1, 3 Topic 2 Measuring and drawing cells Once the eyepiece graticule of a given microscope has been calibrated, there is no need to repeat the process providing the eyepiece is kept in the same microscope. It will save time if students keep a record of their initial calibrations and use the same microscope for every lesson involving cell measurement. Calibrating an eyepiece graticule is an exercise that many students find difficult. Although they need to understand the term stage micrometer, it is helpful at the beginning to explain that it is simply a ruler and perhaps to refer to it as such for the first exercise. It is useful to collect and display class results for measuring cells on a board or screen. Anomalies can soon be picked up and addressed. Discussion of themes such as variability of cell size and appropriate sample sizes can also be facilitated. See circus activity below. Common misunderstandings and misconceptions Students often fail to convert mm to µm when carrying out calculations involving measurements. Cambridge International AS and A Level Biology Teacher s Resource 2

3 Mistakes in calculating actual sizes and magnifications are often made by measuring in centimetres and not converting to micrometres correctly. It is better if students always measure in millimetres and not in centimetres. Homework ideas SAQ 1.2 EOCQ 10 Mistakes in calculating actual sizes and magnifications are often made by measuring in centimetres and not converting to micrometres correctly. It is better if students always measure in millimetres and not in centimetres. Topic 3 Electron microscopy If possible, it is stimulating and informative for students to be able to visit a working electron microscope suite, perhaps in a University or in a research institute. Contact with such facilities is useful because they may be able to provide examples of electron micrographs (EMs) or some of the materials used in preparation of specimens for electron microscopy, such as glass knives, copper grids for mounting specimens and resin-embedded specimens. If a student visit is not possible, a visit by the teacher may still be useful, especially if the teacher has no previous experience of practical electron microscopy. There is a danger that students learn a lot about individual organelles but lose sight of how the activities of the organelles combine to achieve the overall functions of the cell, such as secretion. Secretion by the pancreatic acinar cell is a good example to use towards the end of this topic to take a holistic view of organelle activity (EOCQ 9). Show the Animation How cells make and secrete proteins. A lot can be achieved with books, slides, the internet, etc. but there is no substitute for working with, handling and discussing real EMs. Plasticine models of some organelles are useful. Suitable examples are mitochondria and the nucleus. The presence of nuclear pores in the latter gives students a better three-dimensional understanding than EMs. See circus activity below. Common misunderstandings and misconceptions Students often find it difficult to appreciate how the two-dimensional images seen in sections (e.g. the circular transverse section (TS) of a mitochondrion) relate to the three-dimensional structure (e.g. mitochondria often sausage-shaped). A good exercise is to use Plasticine modelling to construct simple three-dimensional shapes (e.g. a sausage shape) and to cut the model with a knife at various angles. The cut surfaces will reveal the variation to be expected in sections. This can be done fairly quickly by a group of students, or the teacher could pre-prepare some examples for class demonstration and discussion. (EOCQ 9c). Another possibility is to enclose a piece of Plasticine of one colour inside a ball of a different colour. A section may or may not reveal the internal colour, mimicking, for example, the fact that sectioning a nucleus may or may not reveal a nucleolus. Understanding of the electromagnetic spectrum requires discussion of short and long wavelengths. Students often use the terms small and large for short and long. This is a very good opportunity to Cambridge University Press

4 stress the need for scientists to use language precisely. Try to make sure the students use the correct terms in discussion. Students sometimes find it difficult to understand that a magnet can act as a lens the concept of a lens is associated with glass. What is a lens? Homework ideas Worksheets 1.4 and 1.5 are particularly suitable as homework exercises. Mark schemes are provided. Q1 on Worksheet 1.1 and Q1 on Worksheet 1.2 may already have been used in lessons, but Worksheet 1.1 Q2 and Worksheet 1.2 Q2 and Q3 could be used for homework. SAQ EOCQ 2 4, 6i vii EOCQ 7 9, 11 Topic 4 Prokaryotes and eukaryotes Worksheet 1.3 is an activity-based approach to this topic, with extension work for more able students. A philosophical point is often raised in discussion, namely which type of cell is more successful, prokaryote or eukaryote. How is success judged? Total biomass? Potential to survive environmental change? Make a loop activity by typing some or all of the terms used in this chapter with their definitions into a spreadsheet. Now, move the all terms down by one cell so that term and definition no longer match. Type in START against the first definition and END against the last term (see below). START Chloroplast Nucleus Ribosomes Cell wall Photosynthesis occurs in this organelle Chromosomes are found in this structure in eukaryotic cells These are found on rough endoplasmic reticulum (RER) This structure is made of cellulose END Print the spreadsheet onto stiff card and cut up so that each piece of card has a term and a definition. Shuffle the cards and then hand out to the class. The student with the START card reads out the definition and the person with the matching term identifies themself, reads out the term and then reads the definition on their card. This continues until it reaches the END card. The cards can be taken in, shuffled and the loop activity repeated to see if the students can do it faster the second time. The teacher should have a master copy of the loop to correct any mistakes that might happen. Common misunderstandings and misconceptions Students are likely to come across the structures in prokaryotes known as mesosomes. It is now known that these are artefacts and do not exist in living cells. Cambridge International AS and A Level Biology Teacher s Resource 4

5 Homework ideas SAQ 1.6 EOCQ 5, 6viii Worksheet 1.3 if not used as an activity in a lesson. Calculate the ratio of volume of prokaryote cell to volume of eukaryote cell by assuming spherical cells of 1 µm and 20 µm diameter, respectively. Topic 5 Tissues and organs The tissues and organs referred to in this section of the syllabus are dealt with in more detail in Chapter 7 where further ideas for lessons will be found. Apart from the practical skills of microscopy and drawing, this section focuses on levels of organisation. See circus activity below. The concept of a system could usefully be introduced at the same time as tissue and organ, as in the coursebook. If xylem and phloem together form the vascular system, why are they regarded as tissues and not organs? Common misunderstandings and misconceptions Students need to be careful when using the terms xylem and phloem. These are tissues that contain a variety of cells. In Chapter 7, students will meet the terms xylem vessel, xylem vessel element, sieve tube and sieve tube element. Homework ideas EOCQ 6ix xii When students have completed syllabus section A, answer the 10 multiple-choice questions from the coursebook CD-ROM. They could also be used for a test covering Topics 1 to 5. Topics 1, 2, 3 and 5 Microscopy, drawing and measurement One way of approaching this section of the syllabus is to set up a circus of activities. A circus requires a number of different work stations to be set up in the teaching room. A single student or small group works at each station, but rotates to the next station after a given time interval. Eventually, students will have carried out a piece of work at each station. Advantages of this approach include: it is helpful if resources are limited it encourages efficient use of time since students have to move on at fixed time intervals it provides a stimulating variety of activities and can be interactive, encouraging group discussion it can be adapted as a revision exercise at the end of this section of the syllabus, with fewer examples provided to reduce the time required. Cambridge University Press

6 Example of a circus of activities Total time 2 hours (Times can be modified slightly to suit lesson times.) Student numbers 24 (8 groups of 3) Allocate each group a letter from A to H. Rotate groups according to a prepared chart, such as the one below. Time in minutes No. of Station Task students A B C D E F G H 3 2 2a B C D A F G H E 3 3 2b C D A B G H E F 3 4 2c D A B C H E F G 6 5, 6, 7 3 E, F 2 students per G, H 2 students per A, B 2 students per C, D 2 students per station station station station G E C A H F D B Task 1 Task 2 Task 3 15 minutes: compare animal and plant cells. 45 minutes: interpret up to 12 EMs (2a, 2b, 2c: up to 4 micrographs each). 30 minutes: draw a low-power plan using a light microscope. Task 4 30 minutes: calculate dimensions and magnifications of cells and organelles from photographs or drawings. Task 1 Station 1: one EM of a typical animal cell (e.g. rat liver cell) and one EM of a typical plant cell (e.g. mesophyll cell). Magnification about to allows a range of organelles to be seen. Students can discuss differences between the cells and begin to recognise organelles. Chapter 1 EOCQ 3 is also relevant. A written table of observations could be required (e.g. syllabus PDO layout (e) page 49). The EMs in Worksheets 1.1 and 1.2 could be used. Task 2 Stations 2, 3 and 4: up to four different transmission electron micrographs (TEMs) or scanning electron micrographs (SEMs) per station. The micrograph for Chapter 1 EOCQ 9 could be included. Students in each group discuss and try to identify structures seen. It is useful to put a pack of EM images together so that students can explore functions of cells and organelles in tissues and organs. The variety and abundance of organelles, and any specialised structures can be related to function (e.g. microvilli in gut epithelial cells, mitochondria in muscle). Task 3 Stations 5, 6 and 7: students work individually. Two students per station, one microscope per student. One prepared slide for each student (e.g. TS stem, TS root, TS leaf). Slides may all be of the same structure or different structures. One sheet of plain paper per student. Each student draws a lowpower plan, following guidance in the coursebook (Box 1A, page 4). Students will need a sharp HB pencil, pencil sharpener and rubber. Task 4 Stations 8 and 9: students work individually. Three rulers and three suitable micrographs (TEMs, SEMs or light micrographs (LMs)) per station. Chapter 1 EOCQ 9a and EOCQ 10 could be used. Alternatively, one micrograph per student to be rotated every 5 minutes. Micrographs in the coursebook could be used. Cambridge International AS and A Level Biology Teacher s Resource 6

7 24 Practical guidance These practicals are included to give ideas for activities to support teaching of the Cambridge International AS and A Level Biology syllabus. The practicals chosen relate closely to the learning outcomes, and may be used to develop students practical skills in preparation for practical assessment. However, they are not intended to form a complete practical course. Safety Although great care has been taken in checking the accuracy of the information provided, Cambridge University Press shall not be responsible for any errors, omissions or inaccuracies. Teachers and technicians should always follow their school and departmental safety policies. You must ensure that you consult your employer s model risk assessments and modify them as appropriate to meet local circumstances before starting any practical work. Risk assessments will depend on your own skills and experience, and the facilities available to you. Everyone has a responsibility for his or her own safety and for the safety of others. The practicals should be carried out by teachers themselves before they are presented to students. Additional notes relating to each activity in this chapter are given below, but should not be regarded as risk assessments. Practical 24.1 Flower structure In addition to the normal precautions involved with the use of sharps, care should be taken in using mains-operated microscopes with water/solutions. Students should wash their hands after handling biological material. Some students may be affected by hay fever when handing the flowers; students who are likely to suffer severely from an allergic response will not be able to do the practical. Teachers can supply flowers from whatever species are available locally. It is up to the teacher to decide how many different species to use in the time available, but it is best to start with one simple radially symmetrical insect-pollinated flower and one simple grass or cereal with large easily identifiable flower parts. Other types of flower, such as simple bilaterally symmetrical ones could be looked at later. Avoid using complex flowers such as those from composites. Teachers may wish to provide students with a diagram of a typical grass flower to supplement the photos of maize flowers from in Biology. The completed table in part B should look like the table on the next page. Cambridge University Press

8 24 Practical guidance Feature of flower Type of flower Size and colour of petals Insect-pollinated Large and brightly coloured to attract insects Wind-pollinated Small green petals Position of anthers Position of stigma Structure of stigma Nectaries Pollen grains Enclosed within flower on stiff filaments so that insect makes contact with pollen Enclosed within flower so that insect makes contact with pollen Sticky so pollen grains from insects attach to it Present make nectar to attract insects Spiked or sticky grains to help them stick to the bodies of insects Dangle outside the flower on long flexible filaments Exposed to catch pollen grains carried by the wind Long and feathery to catch pollen Absent Scent Sweet smell to attract insects No scent Practical 24.2 Observing maize flowers and fruits Small, light grains to allow them to be carried by wind The normal precautions involved with the use of chemicals and sharps should be taken. Students should wash their hands after handling biological material. If fresh maize flowers are not available, good quality colour photographs could be obtained from the Internet to supplement the photos in the book. Try to obtain maize cobs that have leaves and silks still attached. Practical 24.3 Some adaptations of cereal plants Students should wash their hands after handling biological material. If teachers have access to fresh sorghum plants, provide intact specimens with roots. Otherwise good quality colour photographs could be obtained from the Internet to supplement the photos and diagram in the book. Prepared slides of leaf sections of maize and sorghum are available from suppliers such as Philip Harris Ltd., as well as slides with two sections, comparing leaves of C3 and C4 plants. It is difficult to obtain slides of rice aerenchyma, but leaves or stems of other hydrophytes such water lily (Nymphaea sp.) can be used. Cambridge International AS and A Level Biology Teacher s Resource 2

9 Practical 18.1 Investigating the structure of the pancreas Safety There are no special safety issues. Apparatus and materials eyepiece with calibrated graticule microscope Introduction prepared slide of pancreas, stained with haematoxylin and eosin The pancreas is both an exocrine and an endocrine organ. An exocrine organ produces substances that leave through a tube called a duct and travel to the place where they are needed. The pancreas makes pancreatic juice, which contains enzymes involved in digestion. These enzymes pass down the pancreatic duct into the duodenum where they act on food molecules. An endocrine organ produces hormones, which are secreted into the bloodstream. The pancreas produces the hormones insulin and glucagon. In this practical, you will: use a light microscope to make annotated drawings of pancreatic tissue refer to Figures and on page 331 of the Coursebook, which show a LM of an islet of Langerhans and diagrams of the two cell types that secrete hormones. Procedure 1 Examine the slide of the section of pancreas under the low- and high-power objectives of the microscope. Use Figure on page 331 of the Coursebook to help you to locate some islets of Langerhans. Within the stained section you should see the following structures. Lobes joined together by connective tissue. This tissue tends to pull apart during the preparation of the slide, so that you may see large areas of white space. Exocrine glandular tissue, stained blue. This tissue secretes pancreatic juice. Each group of glandular cells is called an acinus (plural = acini). Islets of Langerhans, stained pink. These areas secrete hormones. Within each islet are α-cells, which secrete glucagon, and β-cells, which secrete insulin. Pancreatic ducts, which carry pancreatic juice to the duodenum. Blood vessels. 2 Draw a labelled low-power plan diagram to show the distribution of tissues in a representative area of the slide. Use a calibrated eyepiece graticule to give your drawing a scale. 3 Examine an islet under the high-power objective of your microscope. Draw the outline of an islet. Within this outline, draw four or five cells to show their structure. Label your drawing. Again, use the calibrated eyepiece graticule to give your drawing a scale. 4 Add annotations detailing the functions of the islet cells and the colours they are stained. Cambridge University Press

10 Worksheet 13.1 Phagocytosis The diagram on the next page shows stages in the phagocytosis of a bacterium by a neutrophil. Match the labels and headings in the boxes below to the numbered labels (1 12) and headings (A E) on the diagram. The headings are in the shaded boxes. Write the correct number or letter next to the box. contents of lysosome empty into phagocytic vacuole bacteria digested by enzymes such as proteases Endocytosis bacterium receptor for antibody marker Attraction (chemotaxis) Recognition and attachment pseudopodium lysosome bacterium marked by an antibody Killing and digestion neutrophil bacterium attached directly to neutrophil membrane nucleus cell surface membrane of neutrophil Bacteria within a phagocytic vacuole, with lysosomes fusing to form secondary lysosome antibody, attached to bacterium, binds to receptor Cambridge University Press

11 Worksheet 13.1 Total: 17 Score: % Cambridge International AS and A Level Biology Teacher s Resource 2