Ch. 24 Nutrition, Metabolism, & Energy Balance
PART 1 NUTRIENTS
most are used for metabolic feul, but some are for cell structures & molecular synthesis
nutrient: substance in food needed for growth, maintenance, repair
macronutrient: 3 major nutrients that make bulk of injected food (carbohydrates, lipids, proteins)
micronutrients: 2 other nutrients required in small amounts (vitamins, minerals)
essential nutrients: nutrients that must be eaten bc body can’t synthesize them (~40-50 are considered essential)
nonessential nutrients also vital to life, but if not enough available, liver can convert another into one if needed
energy value is measured in kilocalories (kcal)
calorie is amount of heat needed to raise temp of 1kg H2O by 1oC (1 dietary calorie=1kcal)
24.1 Role of Carbohydrates, Lipids, & Proteins
o Carbohydrates:
Dietary sources:
primarily from plants (starch-complex carbs) in grains & veggies
insoluble fiber: cellulose in veggies provide roughage
sugars (mono- & disaccharides) in fruits, sugarcane, milk
soluble fiber: pectin in apples & citrus reduces blood cholesterol levels
small amount in milk sugar, glycogen in meats
Uses in body:
glucose: fuel used to make ATP
some cells use fat for energy (neurons & RBCs rely on glucose)
excess glucose converted to glycogen/fat, then stored
fructose & galactose converted to glucose by liver b4 entering circulation
Dietary requirements:
rec. daily intake: 45-65% of total calories
should be mostly of complex carb (whole grains & veggies). simple ones should be limited
high amounts of sugars→obesity & nutritional deficiencies
American diet is ~46% carbs that include rice, pasta, breads
inexpensive, so higher percentages seen in low-income groups
o Lipids:
Dietary sources:
triglycerides is most abundant form
found in saturated fats in meats, dairy foods, tropical oils, or hydrogenated oils (trans fats)
unsaturated fats found in seeds, nuts, olive/veggie oils
cholesterol found in egg yolk, meats, organ meats, shellfish, & milk products (liver makes ~85%)
liver can convert some fatty acids into others
2 essential fatty acids liver can’t synthesize found in most veggie oils:
linoleic acid: omega-6 fatty acid (component of lecithin)
linolenic acid: omega-3 fatty acid
Uses in body:
adipose tissue for protection, insulation, fuel store
phospholipids vital in myelin sheaths & cell wall
cholesterol stabilizes membrane; precursor of bile salts, steroid hormones
prostaglandins, smooth muscle contraction, BP control, inflammation
major fuel of hepatocytes & skeletal muscles
Dietary requirements:
fats: should be 20-35% total caloric intake
saturated fats: limited to ≤10% total fat intake
cholesterol: not required in diet since liver makes its own, so intake should be kept low as possible especially those w/ high total blood cholesterol bc cardiovascular disease
o Proteins:
Dietary sources:
animal products (milk, eggs, fish, meats) and soybeans are complete proteins
contain all needed essential amino acids
legumes, nuts, & cereals contain incomplete proteins (lack some amino acids)
legumes & cereal grains 2gether contain all
Uses in body:
structural materials (keratin in skin, collagen/elastin in CT, muscle proteins)
functional molecules (enzymes & some hormones)
3 factors help determine whether amino acids are used to synthesize proteins/burn fuels:
all-or-none rule:
all amino acids needed must be present for protein synthesis to occur, if not, amino acids are used for energy
adequacy of caloric intake:
protein used as fuel if insufficient carb/fat
hormonal controls:
anabolic hormones (GH, sex) accelerate protein synthesis & growth
adrenal glucocorticoids promote protein breakdown & conversion of amino acid to glucose
nitrogen balance: homeostatic state where rate of PS=rate of breakdown
positive nitrogen balance: synthesis exceeds breakdown (normal in kids, pregnant, tissue repair)
negative nitrogen balance: breakdown excess synthesis (stress, infection, injury, starvation)
Dietary requirements:
needs reflect age, size, metabolic rate, nitrogen balance
rule of thumb: daily intake of 0.8g per kg body weight
American diet provides more than needed
24.2 Role of Vitamins & Minerals
o Vitamins: organic compounds crucial to use nutrients
most function as coenzymes
most must be ingested, except vit. D, some B & K synthesized by gut bacteria, & beta-carotene converted to vit. A
no one food group contains all vitamins
2 types of vitamins based on solubility:
water-soluble vitamin:
B complex & C absorbed w/ H2O
B12 absorption requires intrinsic factor
not stored in body (any not used within 1hr excreted)
fat-soluble vitamins:
A, D, E, & K absorbed w/ lipid digestion
stored in body (not K since excessive consumption can cause health probs)
dangerous free radicals generated during normal metabolism
vits C, E, A, & mineral selenium are antioxidants that neutralize these free radicals
broccoli, cauliflower, brussel sprouts good sources for vit A & C
mega doses of vits are useless & may cause serious problems depending on the vit
o Minerals: Ca, P, K, S, Na, Cl, Mg
other minerals are needed in trace amounts
work w/ nutrients to ensure proper body functions
uptake & excretion balances to prevent toxic overload
uses in the body:
Ca, P, & Mg salts harden bones
Fe essential or O2 binding to hemoglobin
I necessary for thyroid hormone synthesis
Na & Cl major electrolytes in blood
mineral-rich foods: veggies, legumes, milk, some meats
PART 2 METABOLISM
sum of all biochemical reactions inside cell involving nutrients
24.3 Role of Metabolism
o Anabolism & Catabolism:
anabolism: synthesis of large molecules from small ones
ex. synthesis of proteins from amino acids
catabolism: hydrolysis of complex structures to simple ones
ex. breakdown of proteins into amino acids
cellular respiration: catabolic breakdown of food fuels where energy from foods is caught to form ATP. consists of stage 2 & 3 of processing nutrients
goal is to trap chemical energy in ATP
phosphorylation: enzymes shift high-energy phosphate groups of ATP to other molecules
phosphorylated molecules become activate to perform cell functions
3 stages in processing nutrients:
digestion, absorption, & transport to tissues
cellular processing (into cytoplasm)
synthesis of lipids, proteins, & glycogen, or
catabolism (glycolysis) into pyruvic acid & acetyl CoA
oxidative breakdown of intermediates into CO2, H2O, & ATP (occurs in mitochondria)
o Oxidation-Reduction Reactions & Role of Coenzymes:
oxidation reactions: involve gain of O2 or loss of H atoms
oxidation-reduction (redox) reactions:
oxidized substances lose electrons & energy
reduced substances gain electrons & energy
redox reactions catalyzed by enzymes that require vit B coenzyme
dehydrogenases catalyze removal of H atoms
oxidases catalyze transfer of O2
2 important coenzymes act as H/electron acceptors in oxidative pathways: nicotinamide adenine dinucleotide (NAD+) & flavin adenine dinucleotide (FAD)
o ATP Synthesis: 2 mechanisms
substrate-level phosphorylation:
high-energy phosphate groups directly transferred from phosphorylated substrates to ADP
occurs twice in glycolysis & once in Krebs cycle
necessary enzymes in cytosol for glycolysis & in mitochondria for Krebs cycle
oxidative phosphorylation:
more complex, but makes most ATP
chemiosmotic process: couples movement of substances across membranes to chem reactions
energy released from oxidation of food is used to pump H+ across mitochondrial membrane, creating steep H+ con. gradient
as H+ flows back thru ATP synthase membrane channel protein, energy from flow used to phosphorylate ADP
24.4 Carbohydrate Metabolism
o Oxidation of Glucose:
catabolized via: C6H12O6 + 6O2 → 6H2O + 6CO2 + 32ATP + heat
complete glucose catabolism requires 3 pathways:
Glycolysis: an anaerobic glycolytic pathway occurring in cytosol
glucose is broken down into 2 pyruvic acid molecules in 3 major phases:
sugar activation: glucose is phosphorylated by ATP to fructose-6-bisphosphate & again by a 2nd ATP to be fructose-1,6-biphosphate
referred to as energy investment phase bc energy needed
sugar cleavage: fructose1,6-biphsophate is split into two 3-carbon fragments
fragments can be interconvert into dihydroxyacetone phosphate or glyceraldehyde 3-phosphate
sugar oxidation & ATP formation:
each 3-carbon fragment oxidised by removal of pair of H that was picked up by NAD+, forming reduced NAHD+H+
inorganic phosphate group then attached to each oxidized fragment
cleavage of phosphate groups from both fragments will result in formation of 2 pyruvic acid & 2 ATPs
final products of glycolysis are 2 pyruvic acids, 2 reduced NAD+, & net gain of 2 ATP
for the process to continue, more NAD+ must be present to accept more H+ & since supply is limited:
NADH must donate its accepted H+ to become NAD+ again to be free to pick up more H+
if O2 present, NADH will transfer H to proteins in electron transport chain (in mitochondria)
if no O2, NADH will give H+ back to pyruvic acid, making it lactic acid
Citric Acid Cycle: Krebs Cycle
in mitochondrial matrix fueled by pyruvic acid from glucose breakdown & fatty acids from fat breakdown
pyruvic acid myst be actively transported into mitochondria bc it’s a changed molecule & once inside, it enters transitional phase
transitional phase: where each pyruvic acid is made into acetyl coenzyme A in 3 steps
decarboxylation: 1 carbon from pyruvic is removed, making CO2 gas that diffuses into blood
oxidation: remaining 2-C fragment is oxidized to acetic acid by removal of H+ picked up by NAD+
formation of acetyl CoA: acetic acid combines w/ coenzyme A to form it
coenzyme A shuttles acetic acid to an enzyme that joins to 4-carbon oxaloacetic acid (produces 6-carbon citric acid)