Chapter 25: Metabolism and Nutrition – Mesa Community College

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Chapter 25: Metabolism and Nutrition

Chapter Objectives
      1. Generalize the way in which nutrients are processed through the three major
         metabolic fates in order to perform various energetic and structural functions in the
      2. Review carbohydrate metabolism in the GI tract, liver and body cells.
      3. Describe the controlled, facilitated entry of glucose into different cell types, and the
         means to capture and/or release it from specific cells.
      4. Describe the processes of glycolysis (review in text), glycogenesis, glycogenolysis,
         and gluconeogenesis. Describe starting materials and end products and which
         hormones control each process.
      5. Explain the formation and removal of chylomicrons from blood.
      6. Discuss how lipids are transported in blood.
      7. Discuss the sources of cholesterol and indicate the concentrations that are considered
         as potential problems for coronary artery disease.
      8. Describe how lipolysis breaks down the triglycerides into glycerol and fatty acids.
         Explain where glycerol enters carbohydrate metabolism. Describe how fatty acids
         undergo beta oxidation and where those products enter the carbohydrate metabolism
      9. Describe ketogenesis and lipogenesis.
      10. Trace the general path of amino acid absorption and use by the liver and the body as a
         whole. List the hormones that have a role in controlling protein metabolism.
      11. Describe the difference between essential and non-essential amino acids.
      12. Distinguish between the major characters of the absorptive and postabsorptive state in
         term of the conditions that establish the states and hormones that control the
         associated activities.
      13. Discuss the metabolic reactions that occur for glucose, amino acids, and lipids that
         predominate during the absorptive state.
      14. Describe how metabolism during the absorptive state is regulated.
       15. Describe the reactions of the postabsorptive state.
       16. Describe how metabolism during the postabsorptive state is regulated.
       17. Discuss the seven guidelines for healthy eating.
       18. Discuss the importance of minerals in the body.
       19. Discuss the importance of vitamins in the body. Distinguish between water soluble
             and lipid soluble vitamins.
       20. Discuss when vitamin and mineral supplements should be taken.

Chapter Lecture Notes
                                     Introduction to Metabolism

The food we eat is our only source of energy for performing biological work (Fig 25.1)

Three major metabolic destinations for the principle nutrients from food

   used for energy for active processes

   synthesized into structural or functional molecules

   stored as fat or glycogen for later use

Don't have time to go over the central pathway of metabolism; the processes of glycolysis, Krebs

       (TCA) cycle, and the electron transport chain/oxidative phosphorylation

Please review on your own to recall the pathways

Will not be tested on those pathways

Most metabolism based on the central pathway which involves glucose

Digestive system brings carbohydrate nutrients into body

   polysaccharides broken down into simple sugars

   absorption of simple sugars (glucose, fructose & galactose)

Liver gets first chance at processing the incoming nutrients

   fructose & galactose transformed into glucose

   excess glucose can be stored as glycogen (also in muscle)

Body cells

   can oxidize glucose to produce energy
   can store glucose as glycogen (liver & muscle)

   can convert glucose into fats or amino acids

   can store energy as triglycerides in adipose tissue (limited amount in muscle)

                                    Glucose Movement into Cells

In GI tract and kidney tubules

   Na+/glucose symporters

Most other cells

   GluT facilitated diffusion transporters

   insulin increases the insertion of GluT transporters in the membrane of most cells

   in liver & brain, always lots of GluT transporters

Glucose 6-phosphate forms immediately inside cell (requires ATP) thus, glucose is "hidden"

       when it is in the cell

   concentration gradient remains favorable for more glucose to enter

                                    Other Carbohydrate Pathways

Glycogenesis (Fig 25.11)

   production of glycogen, a polysaccharide, for glucose storage

   4 steps to glycogen formation in liver or

   skeletal muscle

   stimulated by insulin

Glycogenolysis (Fig 25.11)

   breakdown of glycogen for glucose release to the bloodstream

   not a simple reversal of steps

   one of the enzymes of glycogenolysis, phosphorylase, is activated by glucagon (pancreas) or

           epinephrine (adrenal gland)

   the final enzyme of the glycogenolysis pathway, glucose-6-phosphatase, is only in

       liver has the ability to release glucose to the bloodstream

       muscle cannot release glucose but uses the glucose released from glycogen as an energy

               source for ATP production

Gluconeogenesis - the conversion of protein or fat molecules into glucose (Fig 25.12)

   glycerol (from fats) may be converted to glyceraldehyde-3-phosphate

   some amino acids may be converted to pyruvic acid

   stimulated by cortisol, thyroid hormone, epinephrine, glucagon, and human growth hormone

                                          Lipid Metabolism

Lipid transport - most lipids are transported in the blood in combination with proteins as


   Chylomicrons (2 % protein)

       form in intestinal mucosal cells

       transport exogenous (dietary) fat

   VLDLs (10% protein) (Fig 25.13)

       transport endogenous triglycerides (from liver) to fat cells

       converted to LDLs

   LDLs (25% protein) - "bad cholesterol"

       carry 75% of blood cholesterol to body cells

   HDLs (40% protein) - "good cholesterol"

       carry cholesterol from cells to liver for elimination


   Two sources of cholesterol

       food we eat

       liver synthesis

   For adults, desirable levels of blood cholesterol

       TC (total cholesterol) under 200 mg/dl
       LDL under 130 mg/dl

       HDL over 40 mg/dl

       Normally, triglycerides are in the range of 10-190 mg/dl

Lipid storage

   lipids are stored in the body as triglycerides in adipose tissue

Lipid catabolism  lipolysis (Fig 25.14)

   triglycerides are split into fatty acids and glycerol

       stimulated by epinephrine, norepinephrine, or glucocorticoids

   glycerol can be converted into glucose by conversion into glyceraldehyde-3-phosphate

   beta oxidation  fatty acids are broken down as carbon atom pairs

       the resulting pairs are converted to acetyl-coA and enter the Krebs cycle

Lipid catabolism  ketogenesis (Fig 25.14)

   two acetyl-CoA (from beta oxidation) bond to form acetoacetic acid which can then be

           converted to beta-hydroxybutyric acid and acetone (ketone bodies)

       occurs in liver

       diffuses to other tissues through bloodstream

   heart muscle & kidney cortex prefer to use acetoacetic acid for ATP production

Lipid anabolism  lipogenesis (Fig 25.14)

   lipogenesis - conversion of glucose or amino acids into lipids

       stimulated by insulin

       occurs in liver and adipose cells

                                        Protein Metabolism

Digestion, absorption and transport

   proteins are hydrolyzed into amino acids in small intestine

   amino acids are absorbed by the capillaries of villi and enter the liver via the hepatic portal

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