B1.2 What happens in cells?
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B1.2 What happens in cells (and what do cells need)?
This section introduces the learners to the key molecules of life. They will be aware of the structure of DNA and how this structure codes for the production of proteins. They will also investigate the importance of enzymes, key protein molecules, which help control cellular reactions.
B1.2a describe DNA as a polymer
B1.2b describe DNA as being made up of two strands forming a double helix
B1.2c describe experiments that can be used to investigate enzymatic reactions
B1.2d explain the mechanism of enzyme action
Life processes depend on biological molecules whose structure is related to their function. Inside every cell is genetic material and this is used as a code to make proteins. Enzymes are important proteins in biology. Metabolic processes such as photosynthesis and respiration are controlled by enzymes. Organic compounds are used as fuels in cellular respiration to allow the other chemical reactions necessary for life. Life on Earth is dependent on photosynthesis, in which green plants and algae trap light from the Sun to fix carbon dioxide and combine it with hydrogen from water to make organic compounds and oxygen.
Underlying knowledge and understanding
Learners should have a simple understanding of the double helix model of DNA. Learners should be familiar with the idea of enzymes as biological catalysts.
Common misconceptions or difficulties students may have
One misconception is that once synthesized on the ribosome, proteins remain in their folded state. Learners often believe that after a protein is released from the ribosomes, there are no further modifications that occur. We are not synthesising enzymes when we are in translation; we are using chains of amino acids to then go on and make proteins. Review the purpose and functions of proteins. Explain to learners that they are still able to undergo changes after being released from the ribosomes.
Learners commonly hold the misconception that all enzymes have an optimum temperature of 37°C (human body temperature). The range of optimum temperatures of enzymes should be introduced through the teaching of this topic and further addressed when considering homeostatic mechanisms for controlling temperature.
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 understanding of the way in which biological molecules such as DNA and enzymes are made and behave is fundamental to many topics including ‘Supplying the cell’, ‘Inheritance’, ‘Feeding the human race’ and ‘Monitoring and maintaining health’. These include cell division, describing how genetic variants may influence phenotype, genetic engineering and the detection of pathogens.
The knowledge and understanding of ‘What happens in cells’ builds on the work covered previously in Section B1.1 Cell structures sub-cellular structures. This topic involves looking at the structure and function of DNA and enzymes and links to important cellular reactions controlled by enzymes such as respiration (Section B1.3) and photosynthesis (Section B1.4).
As a starter activity use mini-whiteboards to review the structure of DNA. Get learners to write answers to the questions to gauge their level of prior learning. Example questions include:
- What are the fours nucleotide bases of DNA?
- What are the complementary base pairs?
- What does a section of DNA (a gene) code for?
Show learners a video clip of transcription of DNA, for example some teachers might like to show their classes the one provided in the resources link.
Learners complete a crossword to review all the relevant terminology and meanings related to DNA structure.
This provides a quick way to assess learners recall and understanding of keywords for the many unfamiliar words covered in this topic area.
Provide learners with materials to build a model of DNA. Learners follow instructions and apply the principles previously learnt of the structure of DNA to complete the model. A good model can be found at CSIRO.
Learners can self-evaluate their models against an exemplar model. They could then work in pairs and swap models to peer-evaluate.
This is a very quick and interactive PowerPoint that provides a ‘snappy’ visual assessment for learning opportunity.
Learners take it in turns to choose a coloured square, which reveals an answer to a question. In an interesting ‘spin’ on the usual Q&A approach, here the learner selects a coloured square on the chequerboard with an answer to specific question on enzymes and they have to then provide the question.
Approaches to teaching the content
Continuing with the experimental nature of the topic as a whole, this section again provides a wide range of practical activities, both real and virtual. It is crucial that learners are provided with practical opportunities to develop appropriate skills but importantly this will encourage a deeper understanding of the complex aspects of the cellular reactions that are to be studied.
'Reviewing DNA structure' is a starter activity that allows for a quick check on prior learning. Provide learners with mini-whiteboards for them to write the answer to questions that have quick one word/number/answers. There are many activities that can be used as starters or plenaries (such as ‘bingo’ seen in 'Transcription of DNA and translation of MRNA') and ‘chequerboard‘ in 'Enzyme starter activity') and these mini-activities can support both engagement and assessment for learning. The type of resource seen in 'Enzyme starter activity' can easily be adapted to serve as a vehicle for assessment for learning throughout the course. It could be adapted to show pictures of important structures/concepts substituted behind the numbers/colours. Learners will recognise this ‘numbers board’ concept used in many TV quiz programmes such as ‘Question of Sport’.
Interactive online software (such as found in 'Transcription of DNA and translation of MRNA') can provide an opportunity to reinforce learning of difficult concepts that are hard to visualise. Learners have the facility to manipulate structures in 3D that are not normally able to be seen moving in 2D representation. This can only serve to increase their awareness and understanding of the mechanisms involved.
A good worksheet to guide learners through the process of extracting DNA can be found by clicking on the first link - pages 12-13).
This is an easy way to get a visible quantity of DNA. The DNA precipitates in the alcohol layer and appears as a clump of long thin strands. This is an impressive way to introduce learners to DNA. Onions or leek are a very good source to extract DNA from.
Further details on methods on DNA extraction can be found at Planet Science and Wikieducator.
This activity investigates the action of catalase, a widespread enzyme, found in nearly all aerobic cells (animals, plants and microbes). Catalase enzyme catalyses the decomposition of hydrogen peroxide into molecular oxygen and water.
2H202(l) → 2H20(l) + O2 (g)
This activity is a simple, small-scale method to monitor the oxygen produced by the decomposition reaction. The enzyme is extracted then adsorbed onto paper disks. The disks are then dropped into hydrogen peroxide and sink to the bottom of the tube. The oxygen produced causes the disks to float. Timing this reaction can be used to measure the action of the catalase enzyme.
Additional details of this investigation can be found at SAPS and The Nuffield Foundation.
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