1 Natural groups

Darwin made extensive observations on a great many creatures, including mammals, and noticed that species fell into natural groups, e.g. lions, tigers and leopards have many similarities, and resemble cats. On the basis of his observations, he was able to place mammals in distinct groups.

His work has continued, and we now recognise that mammals have evolved from a common ancestor, and have branched into many different groups, or ‘Orders’. The animation below shows the different O
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7.3 Regulation of secretion

Up to this point we have made a clear distinction between constitutive secretion and regulated secretion. In reality however the border is a bit more blurred. For example, many molecules are constitutively expressed on the surface of a cell, but their expression is increased in response to a particular stimulus. In other words, surface expression is determined by both constitutive and regulated secretion. Constitutive secretion is regulated primarily at the level of protein synthesis, whereas
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2.5 The endocytic pathways and lysosomes

Endocytosis is the process by which cells internalise molecules from the outside, and it includes pinocytosis, the uptake of small soluble molecules in vesicles, and phagocytosis, the internalisation of large insoluble particles. These are two ends of a spectrum as seen microscopically, but the receptors, the subsequent intracellular trafficking pathways and the fate of the internalised molecules, vary depending on the cell type and its functions. The endocytic pathway co
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1.4 Specialised intermediate filaments

Compared with other cytoskeletal elements, intermediate filaments are more like a fixed scaffolding for the cell. They have a higher tensile strength than microtubules and microfilaments. Consequently they contribute greatly to the overall integrity of the cell and preservation of its shape. Not all eukaryotic cells have cytoskeletal intermediate filaments, and of those that do, each cell type has its own distinct set of intermediate filaments. The intermediate filaments, being cell-type spec
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1.3.3 Reverse turns and loops

In compact globular proteins, a polypeptide often makes a sharp turn called a reverse turn. For instance, these turns often link adjacent strands in antiparallel β pleated sheet (as represented in Figure 12a). Also known as β bends, reverse turns involve four amino acid residues with a hydrogen bond between the C=O group of the f
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1.3.2 β pleated sheets

Another common secondary structure is the β pleated sheet, which contains extended stretches of polypeptide chain with hydrogen bonds between neighbouring strands. In parallel β pleated sheet, polypeptide strands run in the same direction (i.e. from N- to C-terminus) whereas in antiparallel β pleated sheet, neighbouring strands extend in opposite directions (Author(s): The Open University

8.5 Line spectra: Activity 7 Colours and broad lines

Activity 7: Colours and broad lines

0 hours 20 minutes

Read Peterson Sections 1.3.3 and 1.3.4 by clicking the link below.

6.2.1 The need for a reference frame for describing the Universe

The speed of the Earth in its orbit round the Sun is 29.8 km s−1, in a heliocentric frame. But to specify the velocity vector, it is not sufficient to specify the Sun as the origin of the coordinate system; fixed directions must also be identified.

Question 13


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3.2 Some general properties of galaxies

Firstly, we note that galaxies tend to occur in clusters rather than singly. The mutual gravitational attraction of galaxies naturally tends to hold them on paths that remain close to each other. Typically a cluster contains tens or hundreds of galaxies. There are, however, large clusters with thousands of galaxies, and there are some solitary galaxies. Our own Galaxy is a member of a smallish cluster of about 36 galaxies called the Local Group (see Author(s): The Open University

3.1 First steps towards a distance scale

As you will see from Table 2, when it comes to astronomy and cosmology, one is called on to deal with a wide range of distances. (Note that a light-year (ly) is the distance light travels in one year, i.e. 9.46 × 1015 m. The distances are also quoted in a very commonly used astronomical unit of distance: the megapar
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2 Radiation from the galaxies

Stars occur in great collections called galaxies. The distribution and motion of galaxies provide us with the first important experimental information on which we shall build our understanding of the type of universe we inhabit. So, what do we know about galaxies?

All the stars that can be distinguished by the naked eye – a few thousand in number – belong to one galaxy: our own Milky Way Galaxy. Sometimes it is just written Galaxy, with a capital G, to distinguish it from all
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1 Introducing cosmology

General relativity has a very different conceptual basis from that of Newtonian mechanics. Its success in accounting for the precession of Mercury's orbit, and the bending of light by massive objects like the Sun, gives us confidence that our picture of space and time should be Einstein's rather than Newton's. In this and the following units, we turn our attention to the study of the large-scale structure of spacetime. We see how spacetime as a whole is curved by the gross distribution of mas
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Learning outcomes

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

  • describe the characteristics of light emitted by stars, and hence the information of cosmological interest that can be deduced from it;

  • distinguish between true and false statements relevant to the distribution and motion of stars within galaxies, and of galaxies within clusters and superclusters;

  • outline the methods used for estimating the distances to stars and to galaxies;

  • explain and
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Introduction

In this unit, we present the three main lines of experimental evidence pointing to the big bang origin of the Universe: (i) the recession of the galaxies; (ii) the microwave remnant of the early fireball; and (iii) the comparison between the calculated primordial nuclear abundances and the present-day composition of matter in the Universe.

A data sheet of useful information is provided as a pdf for your use. You may wish to print out a copy to keep handy as you progress through the unit
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Acknowledgements

The material acknowledged below is Proprietary and used under licence (not subject to Creative Commons licence). See Terms and Conditions.

Grateful acknowledgement is made to the following for permission to reproduce:

Figure 1a: Neil Borden/Science Photo Library; Figure: 1b NOAA/Science Photo Library; Figure 1c: Max-Planck-Institute for Radio Astronomy/Science Photo Library; Figure 11: Science Photo Library; Figure 14: Science Museum.


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7 Unit summary

Section 2

The law of conservation of charge applies locally at each point and time, so any variation of the total charge within a closed surface must be due to charges that flow across the surface of the region. This principle leads to the equation of continuity:

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6 Appendix: a note on displacement current density

This appendix is optional reading. It is included for the sake of comparison with other texts.

The Ampère–Maxwell law,

is sometimes expressed in the form


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5.1.6 Pulling it all together

The electric and magnetic fields given by Equations 7.21 and 7.23 can satisfy all four of Maxwell's equations in empty space. Gauss's law and the no-monopole law are immediately satisfied because the fields are transverse. Faraday's law and the Ampère–Maxwell law will also be satisfied if we can find electric and magnetic fields that obey Equations 7.24 and 7.26.

We are looking for wave-like solutions, so it is sensible to try

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5.1.4 Getting agreement with the no-monopole law

Substituting Equation 7.23 into the no-monopole law gives immediate agreement because

The no-monopole law is analogous to Gauss's law in empty space, and it leads to a similar conclusion: the magnetic wave must be transverse. This has already been established using Farada
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1.4 Decimal places

If you have less than one unit you should put a zero before the decimal point to make it easier for yourself and others to read the value (e.g. you should write 0.4 rather than just .4, as will be explained later in this unit). However, how many zeros should you put after the last whole number in the series? For instance, is 0.4 the same as 0.40?

The short answer is that on one level, it is. However, by writing 0.40 we are saying that there are four tenths and zero hundredths, an
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