Lipids Living systems

The major classes of organic compounds common to living systems are lipids pro terns nucleic acids and carbohydrates Carbohydrates are very familiar to us— we call many of them sugars They make up a substantial portion of the food we eat and provide most of the energy that keeps the human engine running Carbohy drates are structural components of the walls of plant cells and the wood of trees Genetic information is stored and transferred by way of nucleic acids specialized derivatives of carbohydrates which we 11 examine m more detail m Chapter 28... 

Lipids in living systems are by solvents extractable compounds. Among the lipids are the fatty acids, glycerides, steroids, terpenes, and complex lipids as lipoproteins. 

F. H. Westheimer (1987) has provided a detailed survey of the multifarious ways in which phosphorus derivatives function in living systems (Table 4.7). The particular importance of phosphorus becomes clear when we remember that the daily turnover of adenosine triphosphate (ATP) in the metabolic processes of each human being amounts to several kilograms Phosphate residues bond two nucleotides or deoxynucleotides in the form of a diester, thus making possible the formation of RNA and DNA the phosphate always contains an ionic moiety, the negative charge of which stabilizes the diester towards hydrolysis and prevents transfer of these molecules across the lipid membrane. 

The molecules that form the foundation of living systems are often organized into four categories. They are the primary metabolites nucleic acids, proteins, carbohydrates, and lipids. The categories can be grouped together in different ways, based on features that they have in common. For example, nucleic acids, proteins, and polysaccharides are polymeric. Nucleic acids and proteins are further related because they are templated polymers. Other classification systems are also possible.1 Interest in the development of size-expanded versions of biomolecules has grown over the past... 

This text is divided into seven parts part l, An Overview of Biochemical Structures and Reactions That Occur in Living Systems part 2, Protein Structure and Function part 3, Catalysis part 4, Metabolism of Carbohydrates part 5, Metabolism of Lipids part 6, Metabolism of Nitrogen-Containing Compounds and part 7, Storage and Utilization of Genetic Information. 

Steroids are lipids found in living systems that all have the ring system shown in Figure 3.8 for cholesterol. Steroids occur in bile salts, which are produced by the liver and then secreted into the intestines. Their breakdown products give feces its characteristic color. Bile salts act on fats in the intestine. They suspend very tiny fat droplets in the form of colloidal emulsions. This enables the fats to be broken down chemically and digested. 

Hierarchic design of assembled structures should be important for preparation of functional mesoscopic structures. Typical examples can be seen in biological systems where tissues of organisms in living systems consist of assemblies of cells with cell membranes composed of self-assembled lipids, proteins, saccharides, etc. Therefore, tissues and organisms can be regarded as (at least) two-level assemblies lipid to... 

As a generalization, we may be allowed to state that the transition temperature for cell membranes in biological living systems is found between 0 and 40°C and the chain lengths are between 16 and 18 carbons. This is in conspicuous contrast to the lipids of the stratum corneum barrier where chain lengths up to and over 30 carbons have been demonstrated.14,19 From such facts we expect the transition temperature of the skin barrier lipids to be around 40°C, and this has also been substantiated in a number of investigations.20-22 This means that under normal conditions with a skin temperature about 30°C, the barrier will essentially be impermeable to water. 

There are four basic kinds of organic molecules that exist in large quantities in organisms or living systems carbohydrates, lipids, proteins, and nucleotides. These combine with water, which makes up between 50 and 95% of the volume of most cells. 

Branching. For example, stearic acid (18 carbons) and arachidic acid (20 carbons) are both saturated and linear. They melt at 70°C and 75°C, respectively, whereas 10-methylstearic acid melts at 10°C. However, branched fatty acids are rare in living systems and the evolution of synthetic pathways for them has not occurred as a major device for keeping lipids fluid. 

Carbon is present in all living cells as the element in carbohydrates, proteins, lipids, etc. Without carbon we would not exist. Carbon dioxide is exhaled by all animals and used by green plants to synthesize carbohydrates. There is a constant exchange of carbon in living systems... 

Biological membranes define the very existence of cells. They provide compartments for the different components of the living system interact with, transport and are permeable to substrates. They are involved in lipid and protein syntheses, energy transduction, ion and group transport, information transmission and molecular and cellular recognition. These multitude of activities are accomplished by the unique morphology of the biological membrane and by its ability to affect the transport of species by different mechanisms. 

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