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4.3.4 Removal of NO

Laboratory experiments have shown that, under the conditions in the catalytic converter, the decomposition of NO to O2 and N2 over noble metal catalysts is too slow to be significant. When the A/F ratio is stoichiometric (or below stoichiometry), NO can be removed by reduction with CO and/or hydrocarbons. For simplicity we shall consider only reduction with CO, as with the oxidation reaction, the situation with hydrocarbons is considerably more complicated.

In prin
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4.3 Exhaust emission characteristics

Before we consider how the three-way catalyst functions in any detail, it is important to understand how the emissions of CO, HC and NOx, from the engine depend on the ratio of air (A) to fuel (F) – the air/fuel ratio (or A/F ratio). The significance of this will become clear when we see that the ratio at which the three-way catalytic converter operates is crucial for its success.

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4.1 Exhaust pollutants

The most important chemical reaction in a petrol engine – that is, the one that provides the energy to drive the vehicle – is the combustion of fuel in air. In an ‘ideal’ system, combustion would be complete so that the only exhaust products would be carbon dioxide and steam. In practice, the complete oxidation of the fuel depends on a number of factors: first, there must be sufficient oxygen present; second, there must be adequate mixing of the petrol and air; and finally, there must
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References

Blakemore, C. and Cooper, A. (1970) Development of the brain depends on visual environment, Nature, 228, pp. 477–8.
Caspi, A., McClay, J., Moffitt, T. E., Mill, J., Martin, J., Craig, I. W., Taylor, A. and Poulton, R. (2002) Role of genotype in the cycle of violence in maltreated children, Science, 297, pp. 851–4.
Caspi, A., Sugden, K., Moffitt, T. E., Taylor
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End of unit questions

Question 1

Explain your reasons for agreeing or disagreeing with each of the following sentences.

(a) If a disease has a genetic basis and someone has the (abnormal) alleles for the disease, then that person will have the symptoms of the d
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10 Unit summary

The unit began by considering what factors contribute to individual differences. The case was made, with the spiders, and later with genetic diseases, that the genome was very important. Subsequent sections revealed that the external environment (e.g. maternal care, the presence of light) and the internal environment (e.g. hormones and drugs) were very important and that they can both shape and determine the development of the organism. Environmental factors, in the form of hormones and drugs
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9.4 Summary of Section 9

This section has illustrated what has to be done, by way of a long-term study, to yield meaningful information on the relationship between genes and development and the behaviour of the organism. It also illustrates the hugely complex nature of the relationship between genes and development and the behaviour of the organism. Yet this complexity is not the exception, it is the rule.


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9.2 Antisocial behaviour disease

The psychological arena is hugely complex because there are additional issues of responsibility and treatment. Briefly, society takes a more lenient attitude towards the behaviour of someone who is ill (diseased) compared to someone who is well. The diseased person is not fully responsible for their actions (‘They can't help it’). Therefore any individual with antisocial (aggressive) behaviour who is diagnosed as having a disease is largely absolved of blame. Having a disease, means, at l
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8.4 Fragile X syndrome

Fragile X syndrome is the final example of a genetic disease considered here.

Activity 21

What does the term ‘genetic disease’ mean?

Answer

Genetic disea
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8.3 Lissencephaly

Lissencephaly, literally meaning ‘smooth brain’, is characterised by the absence of sulci and gyri, and by a four-layered cortex, instead of the usual six layers, with the majority of cortical neurons in layer four (Figure 22). Babies born with lissencephaly have a very poor prognosis; the disease proving lethal be
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8.2 Wilson's disease

The effects of a protein that is absent, or present but not doing its job, may not be evident for many years. This is called late onset, and is exemplified by Wilson's disease. Many molecules within the body require small amounts of minerals such as iron, magnesium or copper to function properly. There are mechanisms for absorbing these minerals from the diet. However, in excess, these same minerals can be toxic, as is the case with copper. So there are also mechanisms for getting rid
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7.8 Summary of Section 7

This section has sought to illustrate the formation of connections between neurons and their targets by exploring a few examples. The picture that emerges is one of cells at different stages of development subjected to a vast array of signals. These signals are the medium through which environmental factors exert their effects. To some of these signals, some cells respond; to other signals, other cells respond. What a cell, a neuroblast, a growth cone actually does is dependent on the combina
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7.6 Synaptogenesis

The formation of synaptic connections is an essential property of nervous system development. Synapses are formed between neurons and also with targets that are not part of the nervous system, e.g. muscle. Axon terminals, under the direction of a variety of extracellular cues, grow towards particular targets. Once they arrive at the target, they stop growing and the growth cone changes to form a synapse. As with axon growth, the formation of the synapse is dependent on an interaction between
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7.4 Elixirs of the nervous system: neurotrophins

According to Section 7.2 axons obtain an elixir from targets at their synapses.

Confirmation that there is indeed an elixir came from a series of events that reveals how much of science really works. Elmer Bucker, working with Hamburger in the mid-1940s, had removed a limb bud from a chick and replaced it with a tumour from
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7.2 Selected to survive: studies of the PNS

Viktor Hamburger carried out a series of classic embryologieal experiments over a period of about 30 years. He investigated the relationship between the size of target tissue in chick embryos and the size of the pool of neurons that innervated it. His technique was to remove or add target tissue to the tissue which would eventually form a limb, usually the hind limb, and is called the limb bud. A few days later he observed the effect of the tissue addition or removal on the pool of neurons de
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7.1 Neuron proliferation

There is a huge proliferation of neurons in early life. Even whilst that proliferation continues, some cells, e.g. neuroblasts, stop being able to divide. At some later stage the proliferation itself virtually ceases. It follows that cells switch from being able to divide, to being unable to divide, and that they switch at the appropriate time: the process of cell proliferation is controlled. The details of the control of proliferation are not yet understood and are not considered here. But o
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5.4 Retinoic acid

The retinoic acid story is both distressing and illuminating. It is distressing because with hindsight it is possible to see how the suffering of many people could have been averted. It is illuminating because we now understand much about how retinoic acid works.

Retinoic acid is a natural product of vitamin A. It had been known since the 1930s that a lack of vitamin A, a vitamin A deficiency, led to fetal abnormalities. Subsequent studies in animals showed that an excess of vitamin A a
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4.2 Licking/grooming-arched back nursing

Rat mothers perform a number behaviours towards their pups: they build a nest for their pups, keep them in it and occasionally lick them and nurse them. (Rat fathers have a parental role too but it is not essential and the experimental set-up is simplified by his absence.) Licking occurs predominantly at the time when the dam arches her back and nurses her young, allowing a composite behaviour of licking/grooming-arched back nursing to be identified and recorded. If licking/grooming-arched ba
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3.4 Sensitive periods

The steroid hormone testosterone plays a major role in the development of mammals. In particular it is instrumental in causing differences between males and females. One well explored difference concerns play-fighting in young rodents. In the rat, play-fighting is a sequence which begins when one animal pounces on another. The pounce is followed by wrestling and/or boxing and the play-fight usually finishes with one animal on top of the other. A similar sequence of play-fighting is seen in yo
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Learning outcomes

After studying this unit you should be able to:

  • recognise definitions and applications of each of the terms printed in bold in the text;

  • critically evaluate statements about the influence of the genome on behaviour;

  • explain the ways in which genetic and environmental factors influence the development of the nervous system;

  • provide examples of the influence of genetic and environmental factors on the development of the nervous s
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