1.6 Signalling proteins can act as molecular switches

How does a signalling molecule actually convey a signal? With second messengers, it is easy to understand: they are produced or released in large quantities, diffuse to their target, to which they usually bind, bringing about a functional change, after which they are degraded or stored within a subcellular compartment (such as endoplasmic reticulum). With signalling proteins it is less obvious. Protein concentrations cannot fluctuate rapidly, and protein molecules cannot easily move within th
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1.3 Most receptors are on the cell surface

Water-soluble signalling molecules cannot cross the membrane lipid bilayer, but bind to specific receptors embedded in the plasma membrane. The receptors have an extracellular domain that binds the signalling molecule, a hydrophobic transmembrane domain and an intracellular domain.

Binding of a ligand induces a conformational change in the receptor, in particular that of its intracellular region. It is this conformational change that activates a relay of intracellular signalling molecul
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1.2.2 Cell–cell signalling via secreted molecules

Extracellular signalling molecules are all fairly small, and are easily conveyed to the site of action; they are structurally very diverse. The classification and individual names of these mainly water-soluble mediators often reflect their first discovered action rather than their structure. So, for example, growth factors direct cell survival, growth and proliferation, and interleukins stimulate immune cells (leukocytes). However, to complicate matters further, they often have different effe
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1.2.1 Cell–cell contact-dependent signalling

In some instances, cells may communicate directly with their immediate neighbour through gap junctions (Figure 3a). Communication via gap junctions partially bypasses the signalling model we have outlined above in Figure 2. Gap junctions connect the cytoplasm of neighbouring cells via protein channels, which allow the passage of ions and small molecules (s
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1.1 Introduction

The fundamental principles of signalling can be illustrated by a simple example in the yeast S. cerevisiae (Figure 1). In order to sexually reproduce, a yeast cell needs to be able to make physical contact with another yeast cell. First, it has to ‘call’ to yeast cells of the opposite mating type. It does this by secreting a ‘mating factor’
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Introduction

Even the simplest organisms can detect and respond to events in their ever-changing environment. Similarly, within a multicellular organism, cells are surrounded by an extracellular environment from which signals are received and responded to. Extracellular events are decoded and transmitted to relevant parts of individual cells by way of a series of activation/deactivation steps involving many intracellular molecules. This relay of information along molecular pathways is called signal tra
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4.1 The Milky Way

Figure 1 showed two spiral galaxies: NGC 5548, which has an active nucleus, and NGC 3277 which does not. If we accept that AGN are the result of accretion on to supermassive black holes at the centres of the galaxies which harbour them, it is natural to ask the question whether galaxies like NGC 3277, which do not
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3.5 Example 1

(a) A gravitationally bound uniform density sphere, of radius r, is composed of a large number of subelements, with total mass M. Use the virial theorem,

The gravitational energy is given byAuthor(s): The Open University

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3.4 The Eddington Limit

Thus the observations require that a luminosity of around 100 times that of the entire Milky Way Galaxy be generated within a region with a diameter only about 1000 times that of the Earth's orbit! (A truly amazing statement.)

The most obvious mechanism for generating such enormous luminosity within such a tiny region of space is an accretion process, but instead of perhaps more familiar compact stars with masses ~M
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2 Black holes: a reminder

You may have previously met the formation of a black hole at the end of the life of a massive star. Accreting black holes, which were formed in this way, are members of close binary star systems.

A black hole is formed when self-gravity causes material to collapse to such high densities that the escape speed (or escape velocity) reaches the speed of light. Using Newtonian dynamics we can calculate the magnitude of the escape velocity from planet Earth (mass M E
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1 Evolution versus creation: science and non-science

Science aims to extend our understanding of natural phenomena through testing of explanatory hypotheses by reference to hard evidence. It is not concerned with ideas that cannot be tested in this way, such as subjective opinions (for example, what is good or evil, beautiful or ugly) or religious beliefs (about, say, ‘the meaning of life’ and the existence of gods or spirits), though we will return to ideas like this at the end of this unit. The remit of science was eloquently summarised b
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1.3.3 A breeding experiment: stage two

We now turn to the second stage of the breeding experiment, but this time we will follow the phenotypes and genotypes simultaneously. The purple (Gg) grains of the F1 generation are planted and when these have developed into mature F1 plants they produce male and female flowers. These F1 plants are crossed with each other, as shown in Figure 8. The fertilised ovules develop into grains borne on cobs, and these grains are the beginning of the second f
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3.5 Muscle

There are different sorts of muscle in the body and they have different functions. Skeletal muscles are the muscles that, for example, are used for movement in your arms and legs.

Skeletal muscles store glucose as glycogen (Figure 4) and are able to use glucose as a fuel. Insulin stimulates muscles to take up glucose, and w
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3.2 Pancreas

The pancreas is a structure (an organ) that lies towards the back of the abdomen, the part of the body between the chest and the pelvis (hips). The abdomen contains the stomach, liver, spleen, pancreas, intestines and other structures. The pancreas is near the liver and the spleen (Figure 1) and opens into the small intestine.
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3.1 Introduction

The main role of glucose within the body is as a fuel but it also contributes to the fabric (tissue) by attaching to proteins. In people without diabetes, the blood glucose levels are kept within very narrow limits. The body does not allow them to become too high or too low. Several parts of the body are involved in this process. Some are large, for example the liver, and some are very small, such as the cells within the pancreas. Cells are small building blocks of the body and cannot
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2 What is diabetes?

Diabetes mellitus is a condition in which the glucose level in the blood is higher than it should be. The word ‘diabetes’ comes from the Greek word for ‘siphon’. A siphon is a way of removing liquid, and diabetes is used to describe disorders that remove liquid from the body, resulting in excessive thirst and the production of large amounts of urine. There are two forms of diabetes, diabetes mellitus and diabetes insipidus, of which diabetes mellitus is the more common. The wor
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1 Defining diabetes

This unit introduces the parts of the body and processes involved in the development of diabetes. Type 1 and Type 2 diabetes are similar but distinct conditions and, for doctors, it is not always easy to decide which type of diabetes someone has. Does this matter, and is one type of diabetes worse than the other? There are many misconceptions about diabetes among health care professionals and the population in general. We hope this unit will help you to explore and clarify your ideas about di
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Introduction

This unit introduces parts of the body and processes involved in the development of diabetes.

This unit is from our archive and is an adapted extract from Diabetes care (SK120) which is no longer taught by The Open University. If you want to study formally with us, you may wish to explore other courses we offer in this subject area.


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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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14 Unit questions and answers

Note: Question 1 is included in Section 3.

Question 2


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