2.4 Exocytosis and the secretory pathways

Exocytosis is the process by which molecules are released to the outside of the cell. This includes the release of proteins to the plasma membrane and the release of secreted molecules into the extracellular fluid. All eukaryotic cells need a system to transport molecules to their plasma membrane, and many cells secrete proteins into the extracellular environment. In addition, cells in multicellular organisms communicate with each other via a variety of signalling molecules, which are
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2.3 Sorting for the basolateral and apical zones of the plasma membrane

Many cells are permanently polarised, and this means that surface proteins are selectively localised to different areas of the plasma membrane, depending on their function. For example, endothelial cells have adhesion molecules on the surface that contacts the basal lamina, but receptors that take up molecules from the blood (e.g. the transferrin receptor – see below) are located on the surface of the cell that is in contact with the blood. Cell surface molecules can normally diffuse latera
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4.1 Glucose metabolism

We are now in a position to draw together the major concepts and components of signalling, and show how they operate in one well-understood system, namely the regulation of the storage or release of glucose in the human body. From this, you will be able to recognize archetypal pathways represented in specific examples, you will be able to appreciate how the same basic pathways can be stimulated by different hormones in different tissues, and you will see how opposing hormones activate separat
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5.3.3 Phosphorylation of proteins as a means of regulating activity

Phosphorylation is an important mechanism for regulating the activity of many proteins, either switching on or switching off some activity of the protein.

  • What protein that we have already discussed is both positively and negatively regulated by phosphorylation?

  • Src kinase activity is switched on by dephosphorylation of
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4.2 Amino acid sequence homologies and why they occur

Consider two genes encoding proteins that have 50% of their amino acid sequence in common.

  • How can this sequence homology be explained in terms of evolution?

  • The most parsimonious explanation is that the similarities result from the fact that the two organisms share a common evolutionary past and that the genes encoding
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1.6 Fibrous proteins

Most of the proteins described so far have been globular proteins. There are, however, some distinctive features that characterise fibrous proteins and we present here a general overview of these. Elongated fibrous proteins frequently play a structural role in the cell. They do not readily crystallise but tend to aggregate along their long axis to form fibres. X-ray diffraction studies of these fibres, in contrast to analysis of protein crystals, provides only very limited information on the
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3 Reproduction in marsupials

The study of mammals requires you to deal with measurements, which we call numerical ‘data’, and you will get practice with compiling and analysing data if you work through all the units in this series. We assume only that you can add, subtract, multiply and divide. In this section, we ask you to use units
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1.4.5 Velocity–time and speed–time graphs

Just as we may plot the position–time graph or the displacement–time graph of a particular motion, so we may plot a velocity–time graph for that motion. By convention, velocity is plotted on the vertical axis (since velocity is the dependent variable) and time (the independent variable) is plotted on the horizontal axis. In the special case of uniform motion, the velocity–time graph takes a particularly simple form – it is just a horizontal line, i.e. the gradient is zero. Ex
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5 Summary

Domesticated organisms evolve in artificial environments under artificial selection, and opportunistic or enforced hybridisation often occurs between species that would not normally interbreed. Natural selection cannot be eliminated and continues to operate. At least two different forms of dwarfism are common in domesticated livestock and humans, but only the rarer midget type of dwarfism occurs in wild lineages. Domesticated mammals and birds have distinctive patterns of skin pigmentation th
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2.2 Size and shape

The shape of the head is determined mainly by the relative sizes of the jaws and the nose and the back of the skull containing the brain, eyes, ears and, in artiodactyls, the horns or antlers. All these structures may differ greatly between otherwise similar species.

SAQ 7


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3.4 Competition

In plants it is particularly obvious that many more potential offspring (seeds) are produced than can survive. To a very large extent it is a matter of chance as to which are the survivors. Some are eaten, others overlooked or stored away and forgotten. Those that survive to germinate might be on unsuitable soil, too dry or too wet, so that they shrivel or rot. The successful seedling could be in poor soil, deficient in minerals, or there may be many other plants that are already established
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2.6 The surface

David A. Rothery Teach Yourself Planets, Chapter 6, pp. 66–75, Hodder Education, 2000, 2003.

Copyright © David Rothery

Look at the Moon even with the unaided eye, and you will see that it has dark patches on a paler background (Figure 2). This simple observation picks out the two distinct types of crust on the Moon. The paler areas are the lunar highlands, and the darker areas are the lunar ‘seas’ or maria (singular: mare). Both the highla
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2.4 The atmosphere and polar ice

David A. Rothery Teach Yourself Planets, Chapter 6, pp. 66–75, Hodder Education, 2000, 2003.

Copyright © David Rothery

The Moon's atmosphere is almost as insubstantial as Mercury's, and probably has much the same origin. The Clementine mission returned our first clear views of the lunar poles, showing sites in particular near the south pole that are permanently in shadow, and which could therefore be places where ice might accumulate (Figure 1). Clementine'
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2.3 Missions to the Moon

David A. Rothery Teach Yourself Planets, Chapter 6, pp. 66–75, Hodder Education, 2000, 2003.

Copyright © David Rothery

The Moon was the first extraterrestrial target for space missions. Probes have been directed towards it since almost the very dawn of the space age (see below), and it was the main focus of the 1960s–1970s ‘space race’ between the USA and the then Soviet Union. In the end, only NASA attempted to put people on the Moon, and the six suc
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2.1 The Moon

David A. Rothery Teach Yourself Planets, Chapter 6, pp. 66–75, Hodder Education, 2000, 2003.

Copyright © David Rothery

In this chapter you will learn:

  • about the nearest planetary body to the Earth

  • about the long record of impact cratering on its surface, and about the ancient eruptions that flooded many low-lying areas.

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1 Observing the Moon

Activity 1

0 hours 30 minutes

Try to make out features on the surface of the Moon, even if you have no optical aid available. If you have the use of a pair of binoculars you will probably
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Learning outcomes

After studying this unit you will be able to:

  • retrieve, evaluate and interpret data and information about the Moon, so that (for example) using a close-up picture of the Moon's surface you could identify the types of feature visible and recognise the processes responsible for creating them;

  • interpret simple tables;

  • express, manipulate and compare very small numbers.


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Acknowledgements

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

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

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9 Unit summary

You have learned about the following concepts in this unit:

  • Each type of atom contains a characteristic number of protons in a central nucleus and an equal number of electrons in layers surrounding the nucleus.

  • Elements are substances that consist of only one type of atom. Compounds contain two or more elements combined together. There are two kinds of bond between atoms: covalent and ionic.

  • Molecules are the smallest
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7 Ions and ionic bonding

This section returns to bonding – the way in which atoms are joined to each other. You have already met one type of bonding involving covalent bonds, which is found in molecules. However, this is not the only bonding found in compounds. In this section you will look at ionic bonding and the ionic compounds that contain such bonding. What is the main difference between the covalent compounds you met in Author(s): The Open University