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1. PURPOSE

  1. This chapter focuses on the main pathways and enzymes that regulate processing of glucose into energy

  2. Think about it as a highway from glucose to ATP, with many entrances and exits to different pathways depending on what the cell needs

  3. Step 1 also likes to ask about how fed/fasting state affects the balance of different pathways, so understanding how pathways connect is key

2. OVERVIEW OF METABOLIC PATHWAYS

  1. On the “highway” of glucose to ATP, the central pathways are the focus of this chapter

  2. Pathways can be generally categorized as “fed-state” or “fasting-state” based on whether they build energy reserves or release energy for later use

  3. We can further break down the processes into 4 main categories

    1. Breakdown of energy sources: Glycolysis; fructose/galactose metabolism; the tricarboxylic acid (TCA) cycle; and oxidative phosphorylation

    2. Storage of energy: Glycogenesis and lipid synthesis

    3. Fasting state metabolism: Gluconeogenesis, lipolysis, glycogenolysis, protein catabolism

    4. Side pathways (other functions): Pentose phosphate pathway (HMP shunt), urea cycle, cholesterol synthesis

  4. Side pathways contribute energy from other sources and/or create energy stores and will be discussed in subsequent chapters

Figure 5-1.

Overview of metabolic pathways.

Figure 5-2.

Pathways in fed and fasting states.

3. BREAKDOWN OF GLUCOSE

  1. Glucose may enter the cell from outside or be released from intracellular stores (glycogen)

  2. Glycolysis occurs in the cytosol

    1. Glucose is broken down into smaller components

    2. Only 2 molecules of ATP per glucose are produced

    3. Other carbohydrates, glycogen, and lipid metabolites (glycerol) enter the glycolysis pathway

  3. The TCA cycle takes over in the mitochondria when oxygen is present

    1. This produces the intermediates NADH (derived from vitamin B2) and FADH2 (derived from vitamin B3)

    2. Amino acids can also enter into the TCA cycle to supply energy

  4. Oxidative phosphorylation occurs on the inner mitochondrial membrane to turn NADH and FADH2 into ATP

Table 5-1.Glucose facilitators/transporters.

4. GLUCOSE ENTRY INTO THE CELL

  1. Glucose enters the cell through facilitators (GLUT or sometimes SGLT proteins), and the cell needs a way to trap it there so it can't exit the cell

    1. Different cell ...

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