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4.3 Gamete production in men

A sexually mature man is producing sperm all the time at a rate of around 300–600 per gram of testis per second. This provides the 500 million or so which are released at each ejaculation. But the formation of an individual sperm takes about nine weeks (64 days). Sperm are produced in the testes, and production is most efficient at a temperature several degrees lower than the normal body temperature of 371°C. For this reason the testes (plural of testis) are suspended outside the body cavi
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8 Summary

  1. The rate at which water infiltrates into the ground depends on the permeability of the rocks and the state of the ground surface. Below the ground surface there is an unsaturated zone which has air in the pore spaces, and a saturated zone which has all the pores filled with water. The water table is the boundary between the unsaturated zone and the saturated zone, and is the level at which water stands in wells. Water below the water table is called groundwa
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6 Permeability

It is important to distinguish clearly between porosity and permeability. Porosity is a measure of how much water can be stored in a rock, whereas permeability is a measure of the properties of a rock which determine how easily water and other fluids can flow through it (see Section 4). Permeability depends on the exte
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2.6 Summary

  1. Receptors comprise a limited number of structural motifs, which determine binding affinity and specificity of receptor–ligand complexes. Some ligands bind to several receptors and some receptors bind to several ligands.

  2. Acetylcholine is a good example of a ligand with two structurally different kinds of receptor. Nicotinic receptors are ion channels, which are found predominantly in skeletal muscle, and are stimulated by nicotine. Nicotinic
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2.4 Receptor inactivation

As with all signalling components, receptors need to be switched off as well as on. Receptor inactivation can operate in several ways including removal of the ligand by degradation or sequestration, and desensitization of the target cell.

Binding of a ligand to its receptor is a reversible process, as the ligand will ultimately dissociate from the receptor and may be degraded. Acetylcholine is a good example of a signal regulated in this way; it is degraded by the enzyme cholinesterase
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1.5 Quaternary structure

This level of protein structure applies only to those proteins that consist of more than one polypeptide chain, termed subunits. In such proteins, sometimes referred to as multisubunit proteins, the same kinds of non-covalent interaction that stabilise the folded polypeptides also specify the assembly of complexes of subunits. Quaternary structure refers to the way in which the subunits of such proteins are assembled in the finished protein.

Multisubunit proteins can have a numbe
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1.3 Pre-existing human TSEs

Before BSE was recognised, several distinct human TSEs were already known, the most significant of which are outlined below. Human TSEs can have incubation periods of several decades between initiation of the disease and recognition of its early symptoms. Following provisional diagnosis, all these TSEs inevitably culminate in the patient's death after varying periods of decline. Definitive diagnosis required post-mortem examination of the brain. Although in all TSEs brain tissue has a patholo
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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:

Text

Figures
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1.7.1 Unit summary

1. A coordinate system provides a systematic means of specifying the position of a particle. A system in one dimension involves choosing an origin and a positive direction in which values of the position coordinate increase. Values of the position coordinate are positive or negative numbers multiplied by an appropriate unit of length, usually the SI unit of length, the metre (m).

2. The movement of a particle along a line can be described graphically by plotting values of the particle's
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1.6.4 Drop-towers revisited

In Section 1 we described how research into near weightless conditions can be carried out on Earth by using a drop-tower or a drop-shaft (Figure 41). We are now in a position to examine drop-shafts in more detail (Example 3).

Figure 41
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1.6.3 The acceleration due to gravity

In the absence of air resistance, an object falling freely under the influence of the Earth's gravity, close to the surface of the Earth, experiences an acceleration of about 9.81 m s−2 in the downward direction. The precise value of the magnitude is indicated by the symbol g and varies slightly from place to place due to variations in surface altitude, the effect of the Earth's rotation and variations in the internal composition of the Earth. Some typical values f
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1.6.2 The equations of uniformly accelerated motion

Equations 22, 23 and 24 provide a complete description of uniformly accelerated motion. By combining them appropriately, it is possible to solve a wide class of problems concerning the kinematics of uniformly accelerated motion. Nonetheless, those particular equations are not always the best starting point for the most common problems. For example, it is often the case that we want to know the displacement from the initial position after some specified period of constant acceleration, rather
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1.5.3 A note on functions and derivatives

This subsection introduces two crucially important mathematical ideas, functions and derivatives, both of which are used throughout physics.


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1.5.2 Instantaneous acceleration

The procedure of Question 15 for determining the instantaneous velocity of the car can be carried out for a whole set of different times and the resulting values of vx can be plotted against t to form a graph. This has been done in Figure 28, which shows how the velocity varies with time. At time t = 0 s, the car has zero velocity because it starts from rest. At later times, the velocity is positive because the car moves in the direction of in
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1.5.1 Instantaneous velocity

Uniform motion is simple to describe, but is rarely achieved in practice. Most objects do not move at a precisely constant velocity. If you drop an apple it will fall downwards, but it will pick up speed as it does so (Figure 24), and if you drive along a straight road you are likely to encounter some traffic that will force you to vary your speed from time to time. For the most part, real motions are non-uniform motions.

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6.2.1 Injecting the radioactive substance

The injection may be given immediately before the imaging process, or there may, for certain procedures, be a delay of several hours.

The patient's details and the dose being administered are carefully checked by two people before the injection is given. A lead-screened syringe is used to protect the staff from unnecessary radiation dose (see
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6 Modern Homo sapiens

Modern Homo sapiens evolved in Africa about 200 000 years ago, migrating out of Africa over a long period of time. In doing so they colonised much of the world, displacing populations of Homo erectus and other Homo species. The term ‘modern’ implies that the people were similar to Homo sapiens living today, in both appearance and behaviour. Evidence for an African origin includes the find in Ethiopia of the oldest known H. sapiens fossil, the Omo Kibish
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3.3 Variation

Fossil rodents are first found in rocks that date from around 65 million years ago (from the Eocene) and are thought to have evolved from insectivore/omnivore-type mammals that lived 100 million years ago (in the Cretaceous period). To say that they evolved from simply means that there probably is a direct line of descent but that the descendants have changed from their forebears. One of the most significant ways that evolutionary change can be brought about is by a process known as na
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3.2 Adaptation

If you are working through the units in this series in sequence, you have already been introduced to the idea that many features of an animal's behaviour and structure are adaptations to their way of life. Unit S182_2 looked at the oily fur and the flipper-like feet of the water shrew, comparing the water shrew to the common shrew, a close relative that does not have these features and that does not chase prey under water. We also thought very carefully about the way that adaptations are desc
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2 The global water cycle

The flow of water through the land, the atmosphere and the sea is shown in Figure 3.


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