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Metabolism and NutritionChapter 17

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Cellular metabolism> Metabolism:

• metabole: change• “…all of the chemical reactions that occur in the body” (Martini and

Bartholomew, 554).

> Cellular metabolism:• Chemical reactions only in the cells• Provides energy

• Energy needed to maintain homeostasis

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metabolism4 functions:

• Maintenance & Repairs• Growth• Secretion• Store reserves

Involves:• Catabolism – breaking down• Anabolism – building up

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catabolismCYTOSOL

• Enzymes break down large organic molecules into smaller ones:• Carbohydrates short carbon chains• Triglycerides fatty acids and glycerol• Proteins amino acids

• 2 ATP produced

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catabolismMITOCHONDRIA

• Primary ATP production site for the cell

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ANABOLISM• Synthesis of organic components serves those 4 primary

purposes:• Maintenance & Repairs• Growth• Secretion• Store reserves

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ANABOLISM• Maintenance & Repair

• Metabolic turnover

• Continual replacement requires (in order of amount)• Amino acids• Lipids• Carbohydrates

• Catabolism occurs in reverse order: carbs, fats/oils, a. acids

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Carbohydrate metabolismC6H12O6 + 6O2 6CO2 + 6H2O

• Glycolysis (glucose to pyruvic acid) occurs in the cytoplasm, then this in the mitochondria.

• Converts ADP ATP

• “During the complete catabolism of glucose, a typical cell gains 36 ATP molecules” (Martini & Bartholemew, 555).

• Mitochondria’s reactions are aerobic, cellular respiration.

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glycolysis1. Glucose

2. Cytoplasmic enzymes

3. ATP, ADP

4. NAD

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LIPID METABOLISM• Lipids have the same elements as carbohydrates, but in

different proportions.

• Triglycerides are the most abundant, so our focus will be in triglyceride metabolism.

• Lipid catabolism: lipolysis• Lipids are broken down into pyruvic acid then are transported into

acetyl-CoA, just like glucose.• In hydrolysis, the triglyceride is split into its component parts:

• 1 glycerol molecule• 3 fatty acid molecules

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Lipid metabolism• Sugars are not only able to be catabolized, but anabolized

(gluconeogenesis). It’s the same deal with lipids (lipogenesis).

• The key difference is that lipids are catabolized slower.

• Lipids are generally catabolized during rest periods and sugars in high-activity periods, due to the speed of catabolism for each type of molecule and the immediate energy needs of the body.

• Some lipids cannot be made (e.g. linoleic acid, shown).

• These are essential fatty acids. • These are produced by plants.

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Lipid catabolism• Also, lipolysis or β-oxidation (β = beta)

• Happens in the mitochondria

• Involves a different set of enzymes

• This process breaks down fatty acids into 2-carbon fragments called ketone bodies, or keto acids• Ketone comes from the German “aketon” for

acetone, generated by the body• This is easily diffused into the alveoli of the

lungs, giving the breath a distinct smell.

• β-oxidation produces 144 ATP from an 18-carbon fatty acid• Versus only 36 ATP from glucose.

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Compare acetone to glycerol…

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Saturated, Unsaturated Fats• “Saturated” refers to the amount of hydrogens on the carbon

atoms in a lipid.• A carbon atom will generally have the capacity for 4 total bonds.• A hydrogen atom can only form 1 covalent bond.• So, if all the bonds around a carbon are SINGLE covalent bonds, the

carbon atom can hold 3 hydrogen atoms AND connect to another carbon.• 3 + 1 = 4

• Since a carbon atom can also form DOUBLE covalent bonds, the carbon will not be able to hold as many hydrogen atoms. The presence of DOUBLE covalent bonds with any of the carbons will cause the lipid to be UNSATURATED. (i.e. the lipid cannot be as “filled” with hydrogens)• Saturated fats are normally solids at room temperature (e.g. butter)• Unsaturated fats are normally liquid at room temperature (e.g. oils)

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Saturated, Unsaturated Fats• “Saturated” refers to the number of hydrogens on the carbon

atoms in a lipid.• If we take another look at linoleic acid, we can see it has a few double

bonds:

•Carbon•Hydrogen•Oxygen

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Saturated, Unsaturated Fats• “Saturated” refers to the number of hydrogens on the carbon

atoms in a lipid.

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Omega-n fatty acids• The last letter of the Greek alphabet is

omega:• Ω OR ω; upper vs. lower-case.• (Think about Revelation, when Christ calls

himself the Alpha and the Omega, the beginning and the end…)

• The number at the end of the omega just refers

to the first carbon DOUBLE bond from the end.

• So an omega-3 fatty acid has its 3rd to the last

carbon bond a DOUBLE bond.

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cis- and trans- fats• This just refers to the presence of a DOUBLE bond between

carbons.

• If there is a flip in orientation (see below), then it’s a trans-fat. These kinds of fats can happen naturally, but in very small amounts. They occur much more frequently during hydrogenation in food processing. The rationale for hydrogenation is to preserve shelf life for the foods. The only downside is that it reduces the shelf life of humans…

A trans- double bond forms a very slight s-curve, but doesn’t form sharp bend like a cis- double bond.

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lipoproteins• Most lipids circulate in the bloodstream as lipoproteins: lipid-protein complexes

• Chylomicrons (in the intestine)• chylo (Gr. “juice”) + micron (Gr. “little”)• Largest lipoproteins• Transport triglycerides absorbed from the intestine to the bloodstream

• High-density lipoproteins• HDL• Transports excess cholesterol from peripheral tissues to the liver for storage/excretion in

the bile• “good cholesterol”

• Low-density lipoproteins• LDL• May end up in arterial plaques• “bad cholesterol”

• Free fatty acids• FFA• Can diffuse easily across cell membranes

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Protein Metabolism• Remember, the cells metabolize in this order:

• Carbohydrates (sugars), lipids, proteins

Amino Acid metabolism:• First step is the removal of the amino group (nitrogen and other stuff)

• Requires Vitamin B6

• Occurs via:• trans-amin-ation

• Puts an amino group from one amino acid to another• Enables a cell to create amino acids needed for protein synthesis

• de-amin-ation• Preparation for breakdown in the TCA (Krebs/Citric acid) cycle• Generates highly toxic ammonia (NH3)• In the liver, NH3 + CO2 = CO(NH2)2,urea, excreted with the urine

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Protein MetabolismAmino Acid metabolism:

• Side-effects:• Proteins are more difficult to catabolize• Ammonia is a toxic byproduct• Because of the structural significance of proteins in any cell,

extensive protein catabolism threatens homeostasis

• Essential amino acids:• Body cannot produce these

• Non-essential amino acids:• Body can produce these

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Protein metabolism• DNA is NEVER metabolized

• RNA catabolism:• Nucleotides are broken down

• U and C enter the TCA cycle• A and G are excreted as uric acid

• Hyperuricemia: too much uric acid in the blood forms uric acid crystals (gout)

• Nucleic acid synthesis• Happens

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Cellular metabolism• No one cell performs all the metabolic processes outlined.

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Nutrition• 5 food groups:

• Grains• Vegetables• Fruits (non-magical)• Milk• Meat + beans (from the magical fruit group)

• Vegetarianism• Proteins from plants lack 1 or more essential amino acids• B12 only comes from animal products

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Vitamins, minerals, and water• Vitamins

• Vita: Latin for life• 2 types:

• Lipids (fat-soluble)• Carbohydrates (water-soluble)

• Minerals• All are metallic cations, except chlorine and phosphorus• Na, K, Cl, P, Mg

• Water• Need 2.5L per day• Or, 18.2mL/lb of body weight

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bioenergetics• 1kcal or Cal is the amount of energy need to raise 1kg of water

1°C.• 1 Cal = 1 kcal = 1000 cal (watch where the capital C is)• 150lb is about 68kg

• Rough Calorie equivalents per gram of:• Carbs: 4.18 Cal/g• Proteins: 4.32 Cal/g• Lipids: 9.46 Cal/g

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Metabolic rate, thermoregulation• Basal metabolic rate (BMR)

• Average person: 70 Cal per hour, 1680 Cal per day• Of course, this is very generalized, and will depend on:

• Age• Sex• Physical condition (shape, activity, etc.)• Body weight• Genetics

• Thermoregulation• Normal range is around 98°F• Regulated by hypothalamus’ heat-loss and heat-gain centers