6.2 Endocytosis Fluid-phase uptake by pinocytosis can be broadly categorised according to the size of the endocytic vesicle and this also relates to how the vesicle is coated (Figure 35). The rate of internalisation is directly proportional to (i) the concentration of extracellular molecules, (ii) the volume enclosed by the vesicle and (iii) the ra
4.5 Summary Targeting sequences at the N-terminus of proteins direct translation across the ER, and act as signals for import to the nucleus, mitochondrion and chloroplasts. Sequences at the C-terminus control traffic through the ER and the Golgi and to peroxisomes. Glycosylation is directed by signal sequences that act as targets for N-linked glycosylation in the ER and O-linked glycosylation in the Golgi apparatus. Glycosylation and remodelling of polys
3.2 Formation of clathrin and COP-coated vesicles The structure of clathrin and clathrin-coated vesicles is known in some detail. Clathrin consists of a heavy chain of Mr 180,000 together with a light chain of Mr 35,000. Clathrin molecules successively assemble into a polyhedral, cage-like coat on the surface of the coated pit. The clathrin coat is made of sub-assemblies, each consisting of a three-pronged protein complex, a triskelion, each leg of which is made of one heavy and one light chain (Author(s):
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
Acknowledgements Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit: This content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 LicenceSee Terms and Conditions Figure: 1 Copyright Â
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References 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: Introduction This unit will introduce you to the wide-ranging types of mammals that live in the trees. You will learn how they thrive in this demanding environment, with the help of a range of intriguing adaptations related to their unusual life-styles. This is the eighth in a series of units about studying mammals. To get the most from these units, you will need access to a copy of The Life of Mammals (2002) by David Attenborough, BBC Books (ISBN 0563534230), and The Life of Mammals ( 3 Herbivore teeth Tables are a useful way of recording key information. The headings for Tables 1 and 2 have been prepared for you, and you can copy and complete the tables in your notebook. If you need to find any of this information again later, then it is very useful to have it summarised in a table. I 4.1 Levels of toxic substances Toxic substances are a feature of the natural world; many plants contain chemical compounds that make them anything from mildly distasteful to lethally poisonous to animals that might eat them. Some animals are equipped to deal with dangerous plants in their environment and possess detoxification mechanisms that break down harmful compounds. Humans, for example, rely on their liver to neutralise the harmful effects of alcohol. During human cultural evolution, cooking techniques have developed 2 The global water cycle The flow of water through the land, the atmosphere and the sea is shown in Figure 3. 3.2.1 Axes A graph is made using two different scales or axes, forming a right angle. The horizontal axis (x-axis) is used to represent the variable that changes in a consistent way, such as time, or in a way that you can control. The vertical axis (y-axis) is used to represent a variable that you measure but may not be able to control directly, such as a patient's temperature. Each axis should be carefully labelled to indicate what it represents. To plot a graph, you put a mark at the poin 3.2 The anatomy of a graph A graph shows how two different types of data that can take on different values (known as variables) are related, or change in relation to each other; for instance, how a patient's temperature changes over time. Each measurement consists of two variable values: the patient's temperature and the time at which the temperature was taken. Author(s): 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: 6.3 Summary of Sections 4 to 6 Hair cells do not have axons and therefore do not generate action potentials. The nerve that communicates with or innervates the hair cells along the basilar membrane is known as the vestibulocochlear nerve or VIIIth cranial nerve. The cochlear portion of the nerve contains afferent fibres that carry information in the form of action potentials from the organ of Corti to the brain, and efferent fibres that bring information from the cerebral cortex to the periphery. Most of the af 6.1 Firing-rate hypothesis Information about stimulus intensity is encoded in two ways: the firing rates of neurons and the number of active neurons. Intensity is assumed to be encoded by an increase in discharge rate of action potentials within the auditory system. As the stimulus gets more intense, the basilar membrane vibrates at a greater amplitude causing the membrane potential of activated hair cells to be more depolarised and this causes the nerve fibres that synapse onto the hair cells to fire at a greate 4 Neural processing of auditory information In this section we will look at how the frequency selectivity found along the basilar membrane is preserved or modified by the auditory nerve and how information about the intensity of the signal is encoded in the response of the auditory nerve fibres. The nerve that communicates with or innervates the hair cells along the basilar membrane is called the vestibulocochlear nerve or VIIIth cranial nerve. It enters the brainstem just under the cerebellum and conveys information from 3.8 Revision questions Discuss the two ways in which the middle ear increases the effectiveness with which sound is transmitted from the external ear to the inner ear. 2.1 Structure and function of the outer and middle ear Figure 1 is a diagram of the human ear. The outer ear consists of the visible part of the ear or pinna, the external auditory canal (meatus), and the tympanic membrane (tympanum) or eardrum. The human pinna is formed primarily of cartilage and is attached to the head by muscles and ligaments. The deep central portion of the References
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