1-20 of 3,455 Results

Chapter

Cover Physical Chemistry: Quanta, Matter, and Change

Ab initio methods  

Contents Configuration interaction 251 Brief illustration 29.1: Configuration interaction 252 Example 29.1: Finding the energy lowering due to CI 252 Many-body perturbation theory 253 Example 29.2: Setting up Møller–Plesset perturbation theory 254 Checklist of concepts 254 Checklist of equations 255 Why do...

Chapter

Cover Fundamentals of Plant Physiology

Abiotic Stress  

This chapter explores how plants adapt and respond to abiotic stresses in the environment. Like all living organisms, plants are complex biological systems comprising thousands of different genes, proteins, regulatory molecules, signaling agents, and chemical compounds that form hundreds of interlinked pathways and networks. When exposed to unfavourable environmental conditions, this complex interactive system adjusts homeostatically to minimize the negative impacts of stress and maintain metabolic equilibrium. The chapter begins by distinguishing between adaptation and acclimation in relation to abiotic stress. It then describes the various abiotic factors in the environment that can negatively affect plant growth and development. Finally, the chapter considers the specific metabolic, physiological, and anatomical changes that result from signaling pathways and that enable plants to adapt or acclimate to abiotic stress.

Chapter

Cover Plant Physiology and Development

Abiotic Stress  

This chapter provides an integrated view of how plants adapt and respond to abiotic stresses in the environment, beginning with a distinction between adaptation and acclimation in relation to abiotic stress. It describes the various abiotic factors in the environment that can negatively affect plant growth and development. It also analyzes plant stress-sensing mechanisms and processes that transform sensory signals into physiological responses. The chapter talks about specific metabolic, physiological, and anatomical changes that result from signaling pathways and that enable plants to adapt or acclimate to abiotic stress. It illustrates how efforts to enhance the abiotic stress tolerance of plants can improve agricultural yield and how the different physiological processes affected in plants during abiotic stress can be used for plant monitoring and treatment in the field using precision agriculture practices.

Chapter

Cover Plants, Genes & Agriculture

Abiotic Stresses and How They Affect Crop Yield  

Maarten J. Chrispeels

This chapter assesses abiotic stresses such as drought, floods, acidic soils, and soil salinity, and how they affect crop yield. Abiotic stresses cause plant cells to more highly express many genes that help the plant cope with the stress so that growth can continue, albeit at a slower rate. The formation of reactive oxygen species (ROS) is a response to many stresses. ROS damage cells but also serve as signals and cause cells to turn on stress-response genes. The chapter then considers water potential, which is the physical property of water that drives water movement from the soil through the plant and into the atmosphere. It also looks at how some common agricultural practices, other human activities, and climate change are leading to soil degradation and increased abiotic stresses.

Chapter

Cover Cytopathology

Abnormal cervical cytology  

Andrew Evered

This chapter explores the epidemiology and natural history of cervical neoplasia and describes the morphological features of cellular abnormalities in the cervix. It determines the histopathological basis of cervical abnormalities and their benign lookalikes, providing a well-rounded appreciation of the benefits and limitations of cervical cytology. It also gives a brief description of the treatment methods currently available for cervical neoplasia. The chapter covers cervical glandular intraepithelial neoplasia, which illustrates the precancerous condition of the endocervical columnar epithelium and is the precursor lesion of invasive endocervical adenocarcinoma. It mentions the cytological term dyskaryosis that is used to explain the abnormal cell changes seen in cervical samples taken from women with cervical neoplasia.

Chapter

Cover Clinical Biochemistry

Abnormal pituitary function  

Garry McDowell

This chapter reviews the general functions of hormones, their chemical nature, mechanism of action, and regulation, before focusing on disorders of hormones released by the pituitary gland. Hormones are chemical messengers produced by endocrine glands that circulate in the blood and act on target cells via receptors. The pituitary gland is influenced by release of peptides from the hypothalamus and also releases peptide hormones itself, which influence release of hormones from other endocrine glands located in the thyroid, adrenals, and gonads. The chapter then differentiates between hyperfunction and hypofunction of the anterior pituitary. Posterior pituitary dysfunction can result in low antidiuretic hormone (ADH) secretion, which presents clinically as cranial diabetes insipidus. Release of hormones from the pituitary gland can be investigated by measuring the concentration of single hormones in serum or by dynamic function testing.

Chapter

Cover Clinical Biochemistry

Abnormalities of lipid metabolism  

Mike France

This chapter explores the role of lipids in the development of cardiovascular disease. Different types of lipids occur in the body and include fatty acids, triacylglycerol, phospholipids, and cholesterol. Lipids are insoluble in water and associate with apoproteins in the blood to give lipoproteins, and this is the form in which they are transported in the circulation. Lipid disorders can either be genetic in origin or secondary to other diseases, drug treatment, or defective nutrition. The chapter then looks at hypercholesterolaemia, hypocholesterolaemia, and hypertriglyceridaemia. Deposition of lipids in arterial walls and the subsequent formation of an atheroma are key features of atherogenesis and coronary heart disease. Management of hyperlipidaemia involves using a combination of lifestyle changes aimed at reducing risk factors and the use of lipid-lowering drugs such as statins.

Chapter

Cover Elements of Physical Chemistry

Absolute entropy  

This chapter studies the molecular interpretation of entropy and how it relates to the third law of thermodynamics. The third law of thermodynamics states that the entropies of all perfectly crystalline substances are the same at T = 0. By convention (and as justified statistically), S(0) = 0 for all perfectly ordered crystalline materials. The standard molar entropy is the molar entropy of a substance in its standard state (pure, at 1 bar) at the temperature of interest. Meanwhile, the statistical entropy is the entropy calculated from the Boltzmann formula, as the logarithm of the weight of a configuration. The chapter then looks at the residual entropy of a solid, which is the contribution to the entropy at T = 0 from positional disorder that is frozen in.

Chapter

Cover Pharmaceutical Chemistry

Absorption, Distribution, Metabolism and Excretion  

Chris Rostron

This chapter discusses the effectiveness of a drug on the physiological processes within the body, which is determined by its ability to enter the body, reach its intended target, and remain in the body for as long as is necessary for it to achieve its desired effect. It refers to drugs that are xenobiotics, which prevents them from entering the body and removes them when they do get inside the body. The chapter examines a vital part of drug design and development which is dedicated to overcoming or exploiting these mechanisms, known as the science of pharmacokinetics. This is commonly divided into absorption, distribution, metabolism, and excretion. The chapter provides an overview of the science of pharmacokinetics in order to explain the basic concepts.

Chapter

Cover Study and Communication Skills for the Biosciences

Academic integrity and avoiding plagiarism  

Johnson Stuart and Scott Jon

This chapter explains academic integrity and plagiarism, which is a failure of academic integrity and is an increasing problem. It begins by defining in more detail what plagiarism is. It then looks at some of the reasons students give for plagiarizing in their work. The chapter then outlines a method to avoid plagiarism and highlights some of the many good reasons for doing so. It is important to note that, although most forms of plagiarism probably occur in the context of essays and practical reports, it is possible to plagiarize in any form of communication. As such, this chapter argues that the study skills required to help avoid plagiarism in writing are essential to success in every area of academic work.

Chapter

Cover Core Carbonyl Chemistry

Acetals and ketals  

This chapter examines acetals and ketals. Aldehydes and ketones react reversibly 1:1 with alcohols under general acid or general base catalysis, to give hemiacetals and hemiketals, respectively. With excess alcohol and a catalytic amount of a strong acid, further reversible reaction takes place to replace the OH group and give an acetal (from an aldehyde) or ketal (from a ketone). If arrangements are made to remove the water from the continuously equilibrating mixture, by distillation or other means, then the aldehyde or ketone is quantitatively converted to an acetal or ketal, as the case may be. The reaction proceeds more easily with aldehydes than with ketones, and is practically limited to primary alcohols. The chapter then looks at dithioacetals, dithioketals, and orthoesters. It also considers the protecting group principle.

Chapter

Cover Applied Organometallic Chemistry and Catalysis

Acetic acid and acetyl chemicals  

This chapter outlines the acetyl chemicals family which includes acetic acid, acetaldehyde, methyl acetate, acetic anhydride, and vinyl acetate. It traces the manufacture of acetic acid and its use in a wide range of end-products, such as vinyl acetate, acetate esters, as a solvent specifically for the oxidation of p-xylene to terephthalic acid, and in food and pharmaceutical applications. It also talks about processes used for the production of acetic acid and the acetyl chemicals which involve homogeneous catalysis by transition metal ions or complexes. The chapter describes the reaction of acetaldehyde with acetic anhydride. This affords ethylidene diacetate, which can be thermally de-acetylated to form vinyl acetate with the elimination of acetic acid. The chapter finally refers to alkane oxidation, which is a radical chain process and main propagation that does not need to involve the metal ion.

Chapter

Cover Polar Rearrangements

Acid catalysed aromatic rearrangements  

This chapter reviews arbitrary distinction to create the classification of aromatic rearrangements. It considers reactions that are initiated by the generation of an arenium ion and involve migration around the aromatic ring or from a side chain onto the aromatic nucleus. It also points out that the migration terminus positioned on an aromantic ring is initiated by protonation or by a Lewis acid, which is commonly a consequence of equilibration to form thermodynamically stable products. The chapter highlights intramolecular migration, which occurs via a π-complex that leads to equilibration between arenium ion species. It talks about the mechanism of arenium ion generation which bears a close resemblance to the mechanism for electrophilic aromatic substitution.

Chapter

Cover How to Succeed in Organic Chemistry

Acid Catalysis in Organic Reactions Part 1  

Introduction We are going to have to do this one in two stages. We don’t know enough (yet) to give a full treatment of the problem, but it’s too important to not make a start! Acid catalysis is widely used to increase the rates of...

Chapter

Cover How to Succeed in Organic Chemistry

Acid Catalysis in Organic Reactions Part 2  

Answer 1 Here is the equilibrium again. [image] We know that...

Chapter

Cover How to Succeed in Organic Chemistry

Acid Catalysis in Organic Reactions Part 2  

Introduction We started looking at this problem in Basics 22. Now we have looked at the thermodynamics of reactions involving charged species (Basics 35), we can consider the implications of acid catalysis with a strong acid and with a weak acid.... This...

Chapter

Cover Human Physiology

Acid–base balance  

This chapter explains how the body continually produces CO2 and non-volatile acids as a result of metabolic activity. The chapter refers to the blood hydrogen ion concentration [H+] which is normally maintained within the relatively narrow range of 40-45 nmol of free hydrogen ions per litre. This corresponds to a blood pH between 7.35 and 7.4, and the extreme limits that are generally held to be compatible with life range from pH 6.8 to pH 7.7. The chapter talks about hydrogen ions which are absorbed by other molecules in a process known as buffering, and acid products are subsequently eliminated from the body via the lungs and kidneys. The concept of acid-base balance refers to the processes that maintain the hydrogen ion concentration of the body fluids within its normal limits.

Chapter

Cover Clinical Biochemistry

Acid–base disorders  

David Tierney

This chapter assesses homeostasis of H+ions, the causes and consequences of acid-base disorders, and their laboratory investigation. The physiological control of H+ concentration is maintained by three interrelated mechanisms: buffering systems, the respiratory system, and the renal system. Intracellular and extracellular buffering systems, such as bicarbonate and haemoglobin, provide an immediate, but limited, response to pH changes. The respiratory system, which can be activated almost immediately, controls PCO2 by changing alveolar ventilation. The renal system regulates [HCO3 -] and is the slowest to respond. The physiological response to an acid-base disturbance, which limits the change in H+ concentration, is referred to as compensation. The chapter then looks at acidosis, alkalosis, and mixed acid-base disorders. Acid-base data can be interpreted in a systematic manner, from laboratory results, by examining pH status, PCO2 results, and the compensatory response by HCO3 -.

Chapter

Cover Elements of Physical Chemistry

Acid–base equilibria of salts in water  

This chapter studies the acid–base equilibria of salt solutions. The ions that a dissolved salt provide are themselves either acids or bases, sometimes both. Acidity constants can be used to predict the pH of solutions, and that information in turn can be used to account for the variation of pH during the course of a titration. That information is also helpful as a guide to the selection of solutes that stabilize the pH of solutions. The chapter looks at acid–base titrations, explaining how the pH of a mixed solution of a weak acid and its conjugate base is given by the Henderson–Hasselbalch equation. It then considers the buffer action, examining the buffer solution and differentiating between an acid buffer and a base buffer.

Chapter

Cover Making the Transition to University Chemistry

Acid–base Equilibrium  

This chapter explains the acid-base equilibrium. This involves the transfer of protons in line with the Brønsted–Lowry theory. A strong acid or strong base is fully ionized in an aqueous solution, while weak acids or bases are only partially ionized in an aqueous solution. Acid-base titrations measure the unknown concentration of one solution by reaction with another standard solution with a familiar concentration. The chapter also notes how indicators are typically water-soluble weak organic acids with varying colours at different pH values. It explores the alternative theory of acid-base reactions proposed by Gilbert Lewis: a Lewis acid is an electron-pair acceptor, while a Lewis base is an electron-pair donor.