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
3.4.2 Compaction and adhesion Around the time of the 8- to 16-cell division, the conceptus undergoes a morphological (shape) change, called compaction, in which the cells fatten on each other, and the outlines of individual cells become hard to distinguish. This stage, sometimes referred to as a morula, from the Greek word for mulberry, is shown in Figure 17i. At this stage it is hard to see individual cells; in fact, unless the cells are separated by various laboratory treatments, it is not possible to see the two
2 Summary of Section 4 Gametes are special cells because they contain only one set of chromosomes instead of the more usual two sets. The chromosome number is halved by meiosis. The crossing over and random assortment of chromosomes in meiosis produces a unique set of genes in every gamete, and thus in every individual (except for identical twins, who are derived from the same conceptus). Sperm production involves many rounds of
4.6 Hormonal control of egg production As you can see from the preceeding section, hormones play a crucial role in the maturation of the oocyte. Figure 3 showed you how levels of oestogen and progestogen vary throughout the menstrual cycle, and suggested that hormone balance is important for a woman's fertility, but you can now see how subtle the control really is. Cells have to develop sensitivity to hormones at the times when the hormones are likely to be present, otherwise the entire operation will fail. 4 Summary Many rivers are fed by springs, which occur at points where groundwater reaches the surface. Springs can occur in different geological settings, forming valley springs, stratum springs or solution channel springs. The water in a river originates from overland flow, from interflow and from baseflow. Baseflow forms a higher proportion of river water in summer than in winter, and in rivers flowing over good aquifers. River discharg 3.4.4 Sediment filling The lifetime of reservoirs can vary greatly. Many reservoirs have lasted for over a hundred years, but some may be useful for only a much shorter period—fifty years or so — not because of the general deterioration of the dam as it gets older, but because sediment accumulates in the reservoir. Rivers carry large amounts of mud, silt and sand in suspension, particularly during floods, and when a river enters a reservoir it slows down and the sediment carried in suspension is deposited on th 3.4 Environmental effects of reservoir construction Reservoirs may totally alter the water resources of a country. Before the Aswan Dam was completed in Egypt, more than half of the 8 × 1010 m3 of water that flowed down the River Nile through Egypt each year ran into the sea. Most of the water can now be used in Egypt, mainly for irrigation, and instead of a single annual crop grown after seasonal flooding, more than one crop can be grown each year. However, advantages such as these must be considered in conjunction with 3.3 Dams To economize on constructional materials and costs, it is desirable to build a dam at a narrow part of a valley so that the dam can be kept as short as possible. The quantity of constructional materials needed to build dams, and their cost, can be enormous. The Aswan High Dam, built during the 1960s, cost £400 million for a 1.2 km dam. Though shorter than the Aswan High Dam, the longest dam in Britain, the Kielder Dam in Northumberland (Author(s): 2 River flow The total land area drained by a river system, including all its tributaries, is called a river catchment. The water in a river comes not only from direct precipitation, springs and overland flow (i.e. water flowing across the ground surface, excluding that in streams and rivers; this is rare in temperate vegetated areas) but also from the underground flow of water, directly to the river. Part of this underground flow is interflow, that part of infiltration which moves th Introduction We have seen that where precipitation reaches the ground, some runs off the surface into streams and rivers and some of it infiltrates, passing through the soil. Water that reaches the water table to become groundwater may eventually re-emerge at the surface as springs where the water table intersects the surface. Almost all streams and rivers have springs or seepages as their ultimate source, or are fed by them at various points along their courses. This unit is from our archive and is Acknowledgements Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit: The content acknowledged below is Proprietary and used under licence (not subject to Creative Commons licence). See Terms and Conditions. Figure 1 Copyr 4 Groundwater movement Groundwater flows underground in response to elevation differences (downwards) and pressure differences (from areas of high pressure to areas of low pressure). Near the water table, this means that groundwater usually flows ‘downhill’, i.e. from a higher level to a lower level, just as it would on the surface. The difference in energy between two points that are l metres apart horizontally on a sloping water table is determined by the difference in height (h) between them (< Introduction Many people have the impression that underground water occupies vast caverns, such as those in the Derbyshire Peak District, flowing from one cavern to another along underground rivers. This is a common misconception: underground caverns are fairly rare, but huge quantities of water exist underground, within rocks. This is because many rocks contain pores, spaces that come in all shapes and sizes. In sediments, and consequently sedimentary rocks, there are often pores between grains which can Acknowledgements Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit: >Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence 1 Wave energy The energy carried by ocean waves derives from a proportion of the wind energy transferred to the ocean surface by frictional drag. So, ultimately it stems from the proportion of incoming solar energy that drives air movement. Just how much energy is carried by a single wave depends on the wind speed and the area of ocean surface that it crosses; wave height, wavelength, and therefore wave energy, are functions of the distance or fetch over which the wind blows. Not surprisingly the ma Learning outcomes By the end of this unit you should be able to: explain the principles that underlie the ability of wave power to deliver useable energy; outline the technologies that are used to harness the power of waves; discuss the positive and negative aspects of wave energy in relation to natural and human aspects of the environment. Introduction Energy from sources other than fossil or nuclear fuels is to a large extent free of the concerns about environmental effects and renewability that characterise those two sources. Each alternative source supplies energy continually, whether or not we use it. Many alternative sources of energy have been used in simple ways for millennia, e.g. wind and water mills, sails, wood burning – but only in the last two centuries has their potential begun to be exploited on an industrial scale. Except Acknowledgements Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit: The content acknowledged below is Proprietary and used under licence (not subject to Creative Commons licence). See Terms and Conditions. Figure 1a Cour 3 Summary Power output from wind turbines is proportional to the area swept by their blades, and to the cube of wind speed. The narrow range of useable wind speeds restricts the areas where wind energy can be exploited. Wind power has great potential, but has three main drawbacks. Output depends on intermittent wind speeds, irregular distribution of suitable wind speeds, and occupancy of large areas of land. 2 The future of wind energy A great advantage of using wind energy is that, unlike power generation from combustion of fossil fuels, it produces no gas emissions. Even a small 750 kW wind turbine operating with wind speeds just above that of turbine cut-off would reduce annual emissions to the atmosphere by 1200 t of carbon dioxide, 6.9 t of sulphur dioxide and 4.3 t of nitrogen dioxide, compared with the equivalent power output from coal-fired generators. Nevertheless, wind turbines and their infrastructures are substa
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