Anatomy & Physiology Lecture Outline: Week #2

• Metabolism and Nutrition

LSc 107 Anatomy and Physiology Spring 1999

Thibodeau Ch 27

What is nutrition? Foods we eat and nutrients contained in them

What is meant by metabolism? Metabolism is the sum total of chemical reactions in all body cells

Nutrient metabolism: reactions which the nutrient molecules undergo in the cells after absorption from the intestines and circulation in the blood

Carbohydrates are either broken down, or built up and stored

Fats are either broken down or built up and stored

Amino acids are either broken down, or some can be built

In addition, fats and protein can be converted to glucose, and glucose can be converted to fat.

All of these activities take place as a series of enzyme catalyzed steps (pathways)

Metabolism and energy

• Catabolic reactions break down larger macromolecules, release energy
• Anabolic (synthetic) reactions build macromolecules, consume energy
• Metabolism can be seen in terms of energy balance - energy obtained from some reactions is used to allow others to proceed
• Nutrients contain energy in the C-H bonds, and when they are broken down, the energy of those bonds is transferred to other compounds, or -- lost as heat

See Focus on transferring chemical energy Box 27-1 p. 790

• ATP is the "currency" of energy transfer. Energy is stored in the ADP~P bond, and can be used immediately for an anabolic reaction.
• ADP is not the only compound which receives energy – NAD can receive energy in the form of H⁺ & 2e^-, and FAD can receive the same, forming NADH⁺ and FADH⁺.
• NADH⁺ and FADH⁺ are eventually able to transfer their energy to ATP

Where does metabolism take place?

In cells, but not all activities take place in all cells. What types of anabolic and catabolic reactions take place in each of the following?

• Liver cells
• Pancreas cells
• Heart cells
• Skin cells
• Bone marrow cells

 

Carbohydrate Metabolism

Complex carbohydrates (polysaccharides) – Starch, glycogen, cellulose

Simple sugars (monosaccharides) - glucose, fructose, galactose

Summary of glucose catabolism and anabolism Fig. 27-10

Each step of each pathway needs a specific enzyme.

Glucose catabolism

1. Glycolysis Fig 27-4

• 10 step breakdown of 6C glucose to two 3C pyruvic acid molecules
• Takes place in cytoplasm
• Needs no O₂
• What happens to energy when the glucose molecule is split?
• Only 5% of the available energy in glucose is obtained by this process, some lost as heat

2. Citric acid cycle (TCA cycle, Krebs' cycle) Fig 27-6

• Converts pyruvic acid (3C compound) to CO₂ and water
• Takes place inside the mitochondrion
• Can not take place without O₂
• Where is the energy? What has it been transferred to?

3. Electron transport system and oxidative phosphorylation Fig. 27-7

• Takes place on the inner membranes of the mitochondrion
• Needs O2
• All the compounds which accepted high energy electrons (& H⁺) in the Krebs' cycle transfer the energy to ADP (makes ATP) and the H⁺ to O₂ (makes water)
• This releases 90% of the energy in a glucose molecule - some of it as heat

Fig. 27-9 for total energy extracted from glucose

Can glucose be broken down in the absence of O₂?

Yes. Glycolysis does not need O₂, but energy yield small

What happens to the pyruvate in absence of O₂ – forms lactic acid

When O₂ is again available, ATP is used to reconvert lactic acid to pyruvate, and Kreb's cycle can continue - repays oxygen debt

Glucose anabolism

Glycogenesis: when glucose is in excess, alternate pathway forms glycogen Fig. 27-11 (Glycogen) Which tissues?

Glycogenolysis: when glucose levels are low, catabolic pathway to breakdown glycogen

(in liver, kidney, and intestinal mucosa, glycogen can be fully converted back to free glucose and has the potential to leave the cell); (in muscle, glycogen can be converted back to glucose-6-P which can undergo glycolysis for energy production)

Balance of glycogenesis and glycogenolysis - Fig. 27-13

Gluconeogenesis primarily occurs in the liver

Production of glucose from proteins or from glycerol Fig. 27-15

Control of glucose metabolism - Fig. 27-16, 27-17

Pancreatic hormones: insulin and glucagon

Lipid Metabolism

Lipids in diet: triglycerides, saturated vs unsaturated

Fats in the blood (role Fig. 27-18)

• chylomicra – fat droplets in blood following fat absorption (triglycerides with small amounts of cholesterol and phosopholipids)
• lipoproteins (triglycerides, cholesterol, phosopholipids, AND proteins) HDL, LDL etc
• Free fatty acids

Lipid Catabolism Fig. 27-19

• Glycerol can enter the glycolysis pathway
• Fatty acids are oxidized (b -oxidation sequentially strips 2C acetyl-CoA), which enters CAC
• If large quantities of fats are broken down, the fatty acids form ketone bodies (acetone)
• Breakdown of fat gives more energy per gram (9kcal) than protein or glucose (4kcal)

Lipid anabolism

• Synthesis of lipids, lipogenesis
• Triglycerides
• Cholesterol
• Phospholipids
• Fat storage in body

Note that lipids can be synthesized from excess glucose or protein

Essential fatty acids must be taken in diet - can not be synthesized

Control of lipid metabolism – related to control of carbohydrate metabolism, when carbo catabolism meets energy needs, fats remain stored in adipose tissue

 

Protein Metabolism

Carbohydrates and Fats are primary energy-supplying foods… proteins are primarily tissue-building, therefore, protein anabolism is primary

Protein in diet - Amino Acids Table 27-1

• essential amino acids
• non-essential

Protein anabolism (protein synthesis)

• Proteins assembled at ribosomes according to mRNA (instructions from genes)
• Most cells synthesize their own structural proteins and enzymes, some cells synthesize proteins for export (Ex: liver synthesizes plasma proteins for export and glandular cells synthesize hormones)
• Functions of proteins

Protein catabolism

In liver:

Deamination - splitting off NH₂ group - converted to urea and eventually secreted in urine

Rest of amino acid may go to glucose ( via gluconeogenesis), or fat (via liogenesis), or be broken down for energy (CAC and ETS)

Nitrogen balance - intake vs output; positive vs negative protein or nitrogen balance

Control of protein metabolism

Anabolic

• testosterone
• growth hormone

Catabolic

• cortisol

 

** Summary Chart for Metabolism Table 27-2 **

 

Vitamins and Minerals

• Essential to life
• Needed in minute quantity
• Many act as coenzymes
• Tables 27-3, 27-4

 

Metabolic Rate(rate of catabolism)

Energy expenditure under basal conditions (Basal Metabolic Rate BMR)

Influences on BMR p 810 & Fig. 27-25

Energy balance and body weight – order of catabolism: carbohydrates, fats, proteins for most cells (muscle tissue often catabolizes fat first)

Mechanism for regulating food intake: appetite center and satiety center etc. p. 813

 

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