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WJEC GCSE Additional Science Teacher’s Notes 2 Cells and cell processes _What are cells (pages 12–13)___________________ Note that the structure of the electron microscope does not need to be learnt. _Are cells the same in plants and animals___________ (pages 13–15) PRACTICAL Can you find cells (page 15) Note that sampling your own cheek cells is perfectly acceptable on health and safety grounds. Long ago, when HIV first became a problem, some local authorities asked schools not to do this, but the risk is virtually insignificant and there is no reason why this technique should not be practised. Pupils should be allowed free investigation of the celery to discover what they can. Other plant material may be added or substituted. Celery is particularly good for looking at xylem cells, but is not so good for leaf epidermis (Zebrinapendula and Rhoeo discolour are good for this). The specification expects pupils to investigate specialisation of cells. Point 4 of the procedure is therefore important, as the chapter text does not deal with this. No specific specialised cells are mentioned in the specification, so it does not matter which cells the pupils investigate. _Can you call viruses living organisms (page 16)_____ Question 1. The cell theory says that all living organisms are made of cells. Viruses are not made of cells, yet the cell theory is still accepted. Suggest why. Viruses are not considered to be fully ‘living’ organisms and so the cell theory does not apply to them. _How are the activities of a cell controlled__________ (pages 17–18) PRACTICAL What is the best temperature to wash your clothes (page 18) The answers to questions 1–3 depend upon the pupil’s experimental design and results. 4
WJEC GCSE Additional Science Teacher’s Notes 4. What other factors, apart from the effectiveness of stain removal, might influence a decision about what temperature to use for your wash High temperatures are more costly because more energy is used; high temperatures cause some materials to shrink. 5. Explain why enzymes allow washing at a lower temperature than non-biological detergents. The enzymes catalyse the chemical breakdown of the stains. The end products are soluble and so can be removed by water, even at low temperatures. Breakdown of the stains by heat alone requires higher temperatures. _ How does the nucleus control the cell (pages 19–20) Note that the specification does not require candidates to know the names of the bases in DNA, referring to them only as A, C, G and T. The names are given here in order to indicate WHY they are called A, C, G and T. TASK Discovery of DNA structure (page 20) This activity is very important. The discovery of DNA is not covered elsewhere in the book but is required by the specification. _ How do new cells form (pages 20–23)____________ Questions 2. Cats have 38 chromosomes, dogs have 78 and wheat has 42. How many chromosomes would you expect to find in: a an egg cell of a dog 39 b a kidney cell of a cat 38 c a pollen cell of wheat 21 3. Why would meiosis not work as the ‘normal’ method of cell division in the body The new cells would only have a half set of chromosomes, and further divisions would constantly halve that number. PRACTICAL Observing cell division (pages 22–23) This is quite a complex, but rewarding, technique. There are risks associated with both the stain and hydrochloric acid, and ethnoic alcohol and ethano-orcein are both hazardous chemicals. See Hazcards 38 and 38A. Ethanoic alcohol is made of 3 parts absolute ethanol to1 part glacial ethanoic acid. Mix just before use, adding the acid to the alcohol. www.practicalbiology.org suggests using 95% ethanol instead of absolute, but states that chromosomes may not be as clearly defined. 5
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