TEACHER Question and answer packEdexcel IAL Biology UNIT 5 SCIENTIFIC ARTICLE: JUNE 2018 The nose takes the starring role Unit 5 is synoptic, so it is worth remembering that when you are preparing for the Unit 5 exam. In this scientific article there are several areas that are not from Unit 5, but are from other units. The topics covered in this article are listed below: Unit 2 Classification Unit 4 Evolution Unit 5 Respiration FMRI Organisation of the brain and nervous system Development of the nervous system Nature versus nurture Receptors and how they fire off an action potential Touch receptors (mechanoreceptors) The structure of the human retina; fovea Visual cortex of the human brain; ocular dominance columns 1 Potential questions and answers Page 2 Explain the link between the star-nosed mole’s ‘high metabolism and voracious appetite’. (P2) (3 marks) The star-nosed mole is very small weighing only 50 grams. They live in the colder northern hemisphere and do not hibernate. In addition the mole is a mammal and as such is a homiotherm, with a large surface area: volume ratio. This means that they lose a lot of heat through their body surface. This means that they must consume large amounts of food and have an increased rate of respiration to release heat to maintain a constant body temperature. The star-nosed mole is highly evolved to hunt and eat its prey; with its star nose for finding and tweezer-like front teeth for grasping tiny insect larvae and other ‘high reward’ underwater prey. The high quality of these food items offsets the metabolic cost of making the mole’s super-fast nasal ‘saccades’. Draw a diagram to show the classification of the star-nosed mole (P2) (3 marks) Domain: Eukarya Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Insectivora Family: Talpidae Genus: Condylura Species: Condylura cristata 2 Suggest how the lamellated receptor at the bottom of an Eimer’s organ fires off an action potential into the central nervous system (P3-4) (2 marks) These are mechanoreceptors and are activated by pressure or compression. When an Eimer’s organ first contacts an object/vibrations the lamellae are compressed and pressure is exerted onto the tip of the sensory neurone. This causes sodium gates to open and sodium ions to enter by facilitated diffusion. If enough sodium gates open and enough sodium ions enter then threshold level is reached and an action potential fired off along the sensory neuron. In addition to the lamellated receptor there is another nerve fibre which makes contact with a Merkel cell; suggest how these 2 cells work together to send an action potential along the sensory neuron into the CNS? (P4) (2 marks) The Merkel cell is also a mechanoreceptor; it is sensitive to sustained depression of the skin. The sodium gates will only open when there is continual depression of the skin. This will then allow sodium ions to enter by facilitated diffusion. If enough enter then threshold level is reached and an action potential fired along the sensory neuron into the CNS. Page 3 The third receptor mentioned in Paragraph 5 has a unique structure; describe this structure and suggest how this receptor fires off an action potential along the sensory neuron into the CNS. (2 marks) This consists of a series of nerve endings that form a circular pattern just below the outer skin surface. This system is unique to moles. This gives touch perception. It gives a microscopic analysis of the texture of the surface which has been touched. The nerve endings are compressed when pressed against a surface which opens sodium gates and allows sodium ions to enter by facilitated diffusion. If enough enter then threshold level will be reached and an action potential fired off along a sensory neuron into the neocortex. 3 Draw a diagram to illustrate the overall structure of the Elmer’s organ; include all the cells and their functions. (P3, 4, 5) (Pages 2, 3 and 5) (3 marks) CC = cell column: consists of nerve endings that terminate just underneath the surface of the epidermis. These are very sensitive and give a detailed microscopic analysis of the texture of the surface that has been touched MC= Merkel cell: sensitive to sustained pressure. LC= lamellae; detects vibrations or initial touch with an object. The star- nose allows the mole to detect prey with extreme sensitivity. This is a huge advantage so why don’t all moles and other species of mammals have these noses? (3 marks) The function of the skin is protection from desiccation and abrasion and as a sensory surface. The star-nose is an extension of the skin. In the Eimer’s organs in the nose the 2 functions of the skin are at odds with each other. In other species of mole the Eimer’s organs show wear and abrasion due to the repeated contact with soil. These moles live in drier tunnels and have stiff whiskers or bristles that are more resistant to scouring. The downside of this, however is that they cannot have a very elaborate nose, which reduces sensitivity. Star-nosed moles, however are the only species to live in the moist, muddy soil of the wetlands. The high humidity and soft mud are less abrasive which meant that the nose could evolve a more elaborate structure, increasing its sensitivity. In addition, the wetlands contain a large variety of small prey e.g. earthworms, small invertebrates and insect larvae. To detect these a high resolution sensory surface is necessary. It is possible therefore that there may have been a selective advantage to moles with an elaborate nose to survive in the wetlands, where the wet soil decreased abrasion but increased their ability to detect small prey. 4 Calculate the approximate area occupied by 1 Eimer’s organ (P6) (2 marks) 25,000 Eimer’s organs form the star. Surface area of the star is less than 1cm2 Therefore the area occupied by 1 Eimer’s organ is 1/ 25000 = 0.00004cm2 or 40 μM2 Calculate the number of areas that the star could touch per minute? (1 mark) The star touches 12 or more areas per second. Therefore it can touch at least 12 x 60 = 720 per minute. Calculate how many separate prey items that could be found and eaten in 1 minute. (P7) (2 marks) The star-nosed mole can find and eat 5 separate prey items per second. Therefore it could find and eat 5 x 12 = 60 per minute. What is the neocortex? (P6) (1 mark) A part of the cerebral cortex concerned with sight and hearing in mammals. Most recently evolved part of the cortex. Which area of the cerebral cortex contains the visual cortex? Label this on the diagram below: (2 marks) The Occipital lobe. Position 5. 5 What is meant by ‘high resolution central area of your retina, the fovea’ and how does this compare to the appendages of the star in the star-nosed mole? (P8-9) (3 marks) The retina consists of 2 different types of photoreceptor cells; the rods and the cones. Similarly the star in the star-nosed mole consists of 20 appendages (equivalent to the rods) and two 11th appendages (equivalent to the cones). In the retina: the rods are located in all areas of the retina and several rod cells synapse with a single bipolar cell. This means that rod cells are extremely sensitive to low intensity light due to summation. However, rod cells have low acuity or accuracy of vision. The rod cells are equivalent to the 20 appendages in the star. These also give low accuracy and are used to scan an area quickly. In the retina: The cones however, are densely packed in the fovea; they synapse 1:1 with a bipolar cell and give extremely detailed vision, with high acuity and high resolution. The cone cells are equivalent to the 11th appendages in the star and are used to give a detailed analysis of an object. Suggest how the sensory neurons are arranged in the 11th appendages (the short appendages) to allow detailed investigation; compare this to the fovea of the human eye (P8, 9, 14) (2 marks) It is possible that the 11th appendages synapse with the sensory neurons 1:1, equivalent to the synapsing of the cone cells of the retina with the bipolar cells. This reduces summation and increases acuity and resolution of the object being touched. Suggest the significance of the density of sensory nerve endings in the star (P9) (2 marks) The 11th appendages (pair of short appendages) above the mouth have the greatest density of sensory nerve endings. This is probably because there is a 1:1 synapse between the receptors and the sensory neurons. There are a great number of receptors in the 11th appendages in a small area, which will increase the number of sensory nerve endings. Due to this there will be less summation and so any objects touched will be analysed with a higher acuity and resolution. This enables the star-nosed mole to detect potential ‘food’ very accurately; which is very important to the mole since it has poor vision and lives in complete darkness. 6 Page 4 Suggest how cortical brain maps and visual cortex maps were obtained; give details on the techniques used and the experiments that were needed to obtain the data in paragraphs 12-14. (3 marks) FMRI may have been used to scan the brains of star-nosed moles. FMRI detects the level of oxygenation of the blood /measures changes in blood flow within brain. This increased flow of blood will supply more oxygen; this suggests increased activity of that area of the brain. This area of the brain will then light up. By placing moles on various surfaces and scanning their brains it is possible that the areas of the brain associated with various different surfaces could be determined. Suggest why some people might object to the study of star- nosed moles. (2 marks) There may be ethical issues with the use of moles: moles have rights and are not able to decide for themselves. During the experiments the moles may be hurt or the nose may be damaged. Page 5 What is meant by ‘a dramatic case of convergent evolution’? (P17) (2 marks) Unrelated organisms evolve a similar appearance as they adapt to the same conditions. This can be applied to the similarities in the star-nosed moles fovea for touch and the human retinal fovea for accurate vision. Suggest why the human brain would have to be much larger if the retina contained only fovea cells. (P17, 18, 20) (3 marks) In the fovea of the human retina there are 1:1 synapses between the cone cells and the bipolar cells; this increases the number of neurons entering the visual cortex of the brain. The area of the visual cortex would be increased to process the information. The majority of the retina however, consists of rods which have 3 rods synapsing with 1 bipolar cell; this reduces the number of neurons entering the visual cortex of the brain. This means that the area of the visual cortex processing information from the rods is smaller than that coming from the cones. If the retina only consisted of fovea cells/cones there would be an increased number of neurons compared to the present retina and so the area of the visual cortex would be increased and the overall size of the brain increased. 7 Page 6 What is meant by ‘nature versus nurture’? (P22) (2 marks) Nature is due the genes that are inherited from the parents, whereas nurture is the impacts of the environment on an organism. What is meant by ‘critical periods of development’? Explain the role of nature versus nurture in the development of vision in humans. (P23) (3 marks) Critical periods of development are specific time periods where exposure to environmental stimuli is essential to mature a particular sense. This is due to nurture and not to nature. The role of nature versus nurture has been studied extensively in the development of vision in humans. Nature: the ocular dominance columns are present at birth and are determined only due to the genes. Nurture: a critical period of development where light is needed to strengthen certain synapses and neural connections to enable mature vision. If newborn babies have congenital cataracts that are not removed in the first few weeks of birth then they have permanently impaired vision. Suggest a reason why appendage 11 captures the largest area of the cortex during development. (P23-26) (3 marks) This is an example of nature and nurture. Nature: This is determined by the genes and is independent of the environment. In the embryo the Eimer’s organs on the 11th appendages develop first. Also the neural connections mature first. This results in an increased area in the cortex as seen in the cortical maps of the brain. Nurture: The final size of the area of the cortex due to the 11th appendages is due to environmental factors; these may mature the final neural connections, strengthen the synapses. This may be caused by a stimulus e.g. from suckling after birth; this would take place in a critical period of development; resulting in an enlarged area of the cortex due to the 11th appendages. If the mole were not suckled would this cause a loss of sensitivity of the 11th appendages? 8 Suggest what is meant by ‘markers for metabolic activity’. (P26) (3 marks) There are many markers for metabolic activity. These tend to be markers for aerobic respiration. Using a marker for respiration will determine the activity of the cells in an area of the cortex of the brain. One of the most used is the respiratory enzyme cytochrome oxidase. Cytochrome oxidase is the last enzyme in the electron transport chain in the inner mitochondrial membrane. It receives electrons from the previous electron transport proteins and transfers them to oxygen, converting oxygen into water which is exhaled. The more active a cell the more ATP that is needed and so there will be an increase in the rate of aerobic respiration. This will result in the movement of more electrons through the electron transport chain and so an increased activity of cytochrome oxidase and an increased uptake of oxygen to release more water. Which ultimately will result in the synthesis of more ATP. It is possible that the visual cortex of moles at various stages of development were obtained and then sliced into thin sections before being incubated with cytochrome oxidase and a coloured reagent e.g. tetramethyl-p-phenylenediamine. If there is a high activity of cytochrome oxidase then the coloured reagent changes to a purple colour. The higher the activity of the cytochrome oxidase the higher the rate of respiration and the darker the purple colour on the stain. Suggest how experiments could be undertaken to prove a critical period of development for the development of the neocortex in the star-nosed mole. (P22) (3 marks) Immediately after birth separate the moles into 2 groups; place 1 group onto a smooth surface and the other onto a rough surface. Allow the moles to live in these environments for 8 weeks. Ideally the moles should be from the same mother, to eliminate genetic differences. Feed the moles the same food types and allow them to be exposed to the same level of light and moisture. After 8 weeks place the moles onto identical rough surfaces and using FMRI scanning scan their brains. Record the areas of their neocortex that light up. If there is a critical period of development for touch then the moles that had experienced a rough surface from birth will show a greater activity of the neocortex; a greater area of the neocortex will light up as compared to the moles which had been exposed only to a smooth surface. Another method would be to take brain sections of their neocortex after 8 weeks and use a stain to detect a metabolic marker e.g. cytochrome oxidase. This would stain the areas of the neocortex which had the greatest uptake of oxygen. 9 If there were a critical period of development then the moles exposed to a rough surface would show a greater uptake of oxygen and would have a greater stained area of the neocortex, compared to moles exposed only to a smooth surface. Suggest the potential use of the research on the star-nosed mole. (2 marks) Information from this research may provide extra understanding into how mammalian sensory systems are organised. It may also increase our understanding into how sensory systems develop and the importance of environmental factors in their development. By understanding the link between touch receptors and the neocortex could also be applied to learning and be used to aid recovery from brain injury or strokes. Page 7: The coast mole and Townsend’s mole are intermediate forms of moles. Suggest how the star-nosed mole may have evolved. (P27-28) (3 marks) It is possible that geographical isolation may have occurred. There may have been a mutation, which resulted in new alleles which gave an advantage to selection pressure; enabling the moles to live in colder, wetter environments. The allele for their specialist nose would have also given them an advantage when hunting and detecting food. The moles with the more specialist nose would have been more successful at hunting and would be more likely to survive, reproduce and pass on their specialist nose allele. Over many generations the frequency of this allele in the mole population would increase. The reference is written incorrectly; rewrite the reference. (2 marks) Catania, K. (2017).The nose takes a starring role. Scientific American, 26(2), 82-87 10