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2.2 Using A. tumefaciens to genetically modify plant cells

Genetic engineers have capitalised on the fact that part of the DNA from the Ti plasmid of A. tumefaciens is integrated into the plant genome during the infection process. Ti plasmids can be isolated and a foreign gene spliced in at an appropriate point, making it possible to transfer the novel gene into the plant.

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Preamble

Your answers to Activity 1 will have revealed that the initial development of commercial GM crops has focused on the introduction of two traits: herbicide tolerance and insect resistance. However, many other traits have been introduced into crops that have yet to be grown commercially on any scale. These traits include characteristics such as resistance to viral, bacterial and fungal infections, stress tolerance (for example to high levels of salt in the soil), changes to flower pigmentation,
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Learning outcomes

By the end of this unit you should be able to:

  • understand more about the science that underlies the development of genetically modified organisms and in particular how gene transfer is brought about;

  • know something of the potential benefits and uncertainties associated with gene transfer and the high levels of technical ingenuity involved;

  • be better able to understand the science that underpins the development of Golden Rice and understand why the u
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Introduction

In recent years, scientists have made huge gains in their understanding of how genes can be altered and transferred from one organism to another – but that knowledge has been acquired amidst controversy and concern. The deep ethical concerns that have resulted from the emergence of genetic manipulation are explored in this unit. We begin with an examination of the basic structure and function of genes. A number of pioneering examples and techniques are explored, helping to explain why our p
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3 Summary

Hydropower was the earliest means of commercial electricity generation, and currently dominates alternative electricity supply. However, its global capacity for large-scale exploitation is less than six times that currently installed.

Growth of hydropower is slow and its contribution to global electricity supply is falling. Both are due to economic factors, the slow pace of large-scale project construction, the remoteness of high-potential sites, and increasing resistance to the social
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2 The future of hydropower

Hydroelectric power generators produce no emissions — except indirectly during their construction — and the water used is renewable. Generation can be started and stopped almost instantaneously, simply by opening or closing the inlets to the turbines. The generating technology is well-established. Other positive aspects of hydroelectric schemes are river regulation, and provision of irrigation and drinking water supplies, fisheries and recreational facilities.

These advantages have
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1 Hydropower

Hydroelectric energy is ultimately solar energy converted through evaporation of water, movement of air masses and precipitation to gravitational potential energy and then to the kinetic energy of water flowing down a slope. That energy was harnessed for centuries through the use of water wheels to drive mills, forges and textile works, before being supplanted by coal-fired steam energy. Electricity generation using water turbines, although first centred on constricted streams, has increasing
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Acknowledgements

The content acknowledged below is Proprietary (see terms and conditions) and is used under licence.

Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

Figures

Figure 3 Courtesy of Biophoto Associates;

Figu
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5 Summary of unit

We have seen in this unit that, despite having a high natural abundance, iron is in very short supply because of the insolubility of its oxides and hydroxides. A result of this is that organisms have developed methods for the uptake, transport and storage of iron. Bacteria, in particular, secrete very powerful iron chelators known as siderophores. Of all the iron–siderophore complexes, the iron(III)–enterobactin complex has the exceptionally high stability constant of 1049 mol<
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4.3 Iron storage

In humans, iron is stored mainly in the bone marrow, spleen and liver. About 10 per cent of all the iron in the body is in storage. Two proteins are involved in iron storage; these are called ferritin and haemosiderin (they also occur in other organisms). We shall only study the better characterised (and simpler!) ferritin.

Each ferritin molecule can store iron up to about 20 per cent of its total mass. This is a very high percentage, considering that less than 0.2 per cen
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4.2 Iron transport

It is obvious that iron must be transported around the human body. Firstly, it must be transported from the food in the gut to the places where it is required. Mostly, iron is required in the bone marrow, where red blood cells are formed. Red blood cells have a finite lifetime of about only four months, and old cells are destroyed, usually in the spleen. Iron from the destruction of these cells is then transported from the spleen back to the bone marrow to be recycled.

Iron cannot be tr
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3.1 How do organisms take up iron?

Nearly all organisms are able to take up iron. However, only a handful of organisms have had their iron-uptake chemistry studied. The organism that has received most attention (other than human) is a single-cell, prokaryotic bacterium (found in the human large intestine and elsewhere), called Escherichia coli (abbreviated to E.coli), a high-resolution image of which is shown in Author(s): The Open University

2.2 Summary of iron chemistry

Try to summarise the main points of iron chemistry for yourself, and then compare it with the list below.

  1. From a bioinorganic point of view, we can conclude that despite the high natural abundance of iron, it is in scarce supply in water.

  2. The more stable form of iron in oxygenated water is iron(III).

  3. The low concentration of Fe3+ in water is because of the extreme insolubility of Fe(OH)3.


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2.1 The problems of iron uptake

Iron has a high natural abundance. It is the second most abundant metallic element by mass in the Earth's crust (7.1 per cent).

Activity 1

What are the main oxidation states of iron?


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1 How do organisms acquire iron?

Metals are an essential part of biological chemistry. Of all the trace elements, iron is the most important, especially as it is present in many essential enzymes and proteins. But how do organisms acquire the iron from their surroundings? Clearly, organisms need to absorb iron biochemically before it can be used in proteins. Also, some method of replacing lost iron quickly is needed: for instance, how is blood replaced once it has been lost through a cut? This prompts the question: what bioc
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Learning outcomes

By the end of this unit you should be able to:

  • describe some of the biochemical methods by which organisms uptake iron;

  • describe some of the biochemical processes by which organisms store and transfer iron;

  • explain why iron is present only in very low concentrations in aqueous solution;

  • use aspects of iron(III) chemistry to explain the role of macrocyclic ligands in iron uptake and transfer.


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References

Green, S. (1971) The Curious History of Contraception, Ebury press, London.

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3.9 Questions for Unit

Question 1 (Objective 2)

Figure 19 is a graph showing how the viscosity (thickness and stickiness) of a woman's cervical mucus changes with time. Day 0 is the start of her menstrual period.


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3.8 Review

In this unit we have looked at some of the factors which infuence a couple's decision to have a child. For people who choose not to do so, at least for some part of their lives, a range of contraceptive practices is available. We have looked at the biological processes involved in producing gametes, and at the precise sequence of events necessary to produce a healthy, fertilized egg. We have seen how this egg develops, first as a pre-programmed, free-living embryo, then becomes buried within
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3.6.2 Experiencing the pregnancy

If a woman does find herself pregnant, what can she expect? Pregnancy is a time of enormous physical and emotional changes, and these are often difficult to cope with. To begin with, the physical effects of early pregnancy can be extremely unpleasant. The nausea and vomiting of morning sickness can be very severe, and although in many women the symptoms abate after a while, in others they persist right through the pregnancy. Sickness is thought to be due to the high levels of progestogen circ
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