Integrated SequencePhysics Chemistry Organic Biology

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Special points of emphasis

Stereochemistry

Carbohydrates

Bacteria and Archaea

Fungi

Plants

Mammalian Tissues and Histology

Many polysaccharides play biologically significant rolls as structural components within the cells and tissues of many diverse types of organisms. Important polysaccharides include amylose, amylopectin, glycogen, cellulose and chitin.

The orientation of the glycosidic bond in starch and glycogen permits the polymer chains to twist into compact spirals while in cellulose, this twisting is not permitted. Cellulose chains are straight, the hydroxyl groups allowing for extensive cross-linking to occur.

Note: When plants come up in these discussions, it is almost always because the context gives us a useful field to discuss basic principles. A real key to understanding a concept is to see it play out in different contexts. In the above discussion, I want you to get ever more comfortable seeing terminology like 'glycosidic bond' in usage, or to be thinking about the role of hydroxyl groups in determining macromolecular superstructure.




Chemical Thermodynamics and the Equilibrium State

Reactions of Alcohols and Ethers

Reactions of Organic Phosphorus Compounds

Carbohydrates

Biological Membranes

Bioenergetics and Cellular Respiration

Integration of Metabolism

The Endocrine System

The existence of separate anabolic and catabolic pathways is an important motif in biochemistry. One example is the synthesis and breakdown of glycogen, which occur by different reaction pathways. The advantage of having separate anabolic and catabolic pathways is much greater control. For example, in this case, the free energy of the synthesis of glycogen, does not depend on the concentration of phosphate, which is involved in the degradation. Glycogen synthesis occurs by the activity of the glycosyl donor, uridine diphosphate glucose, or UDP-glucose (we are a step past MCAT level here). Glycogen breakdown occurs by phosphorolysis (cleavage with phosphate) rather than hydrolysis, promoted by the enzyme phosphorylase. There are several advantages of phosphorolysis over hydrolysis. The glucose produced can enter glycolysis already phosphorylated. Furthermore, the ionized phosphate group decreases the rate of diffusion of glucose through membranes.

As a control factor, cyclic AMP is centrally important in the hormonally coordinated control of glycogen synthesis and breakdown. Present significantly when ATP is depleted (a favorite MCAT fact), cyclic AMP activates phosphorylase. Low cyclic AMP leads to an increase in blood glucose by, among other effects, triggering glycogenolysis.








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