Metabolism and Energetics

An Overview of Metabolism

Metabolism

•Metabolism is all the chemical reactions that occur in an organism

•Cellular metabolism

•Cells break down excess carbohydrates first, then lipids

•Cells conserve amino acids

•40% of the energy released in catabolism is captured in ATP

•Rest is released as heat

Anabolism

•Performance of structural maintenance and repairs

•Support of growth

•Production of secretions

•Building of nutrient reserves

Cells and Mitochondria

•cells provide small organic molecules for their mitochondria

•Mitochondria produce ATP used to perform cellular functions

Carbohydrate Metabolism

Most cells generate ATP through the breakdown of carbohydrates

•Glycolysis

•One molecule of glucose = two pyruvate ions, two ATP, two NADH

•Aerobic metabolism (cellular respiration)

•Two pyruvates = 34 ATP

•The chemical formula for this process is
C₆H₁₂O₆ + 6 O₂ ® 6 CO₂ + 6 H₂O

Glycolysis

•The breakdown of glucose to pyruvic acid

•This process requires:

•Glucose molecules

•Cytoplasmic enzymes

•ATP and ADP

•Inorganic phosphate

•NAD (nicotinamide adenine dinucleotide)

•The overall reaction is:
Glucose + 2 NAD + 2 ADP + 2P_i ®
2 Pyruvic acid + 2 NADH + 2 ATP

Mitochondrial ATP Production
(cellular respiration)

•Pyruvic acid molecules enter mitochondria

•Broken down completely in TCA cycle

•Decarboxylation

•Hydrogen atoms passed to coenzymes

•Oxidative phosphorylation

Oxidative phosphorylation and the ETS

•Requires coenzymes and consumes oxygen

•Key reactions take place in the electron transport system (ETS)

•Cytochromes of the ETS pass electrons to oxygen, forming water

•The basic chemical reaction is:
2 H₂ + O₂ ® 2 H₂O

•Per molecule of glucose entering these pathways

•Glycolysis – has a net yield of 2 ATP

•Electron transport system – yields approximately 28 molecules of ATP

•TCA cycle – yields 2 molecules of ATP

Synthesis of glucose and glycogen

•Gluconeogenesis

•Synthesis of glucose from noncarbohydrate precursors

•Lactic acid, glycerol, amino acids

•Liver cells synthesis glucose when carbohydrates are depleted

•Glycogenesis

•Formation of glycogen

•Glucose stored in liver and skeletal muscle as glycogen

•Important energy reserve

Lipid catabolism

•Lipolysis

•Lipids broken down into pieces that can be converted into pyruvate

•Triglycerides are split into glycerol and fatty acids

•Glycerol enters glycolytic pathways

•Fatty acids enter the mitochondrion

Lipid catabolism

•Beta-oxidation

•Breakdown of fatty acid molecules into
2-carbon fragments

•Enter the TCA

•Irreversible

•Lipids and energy production

•Cannot provide large amounts in ATP in a short amount of time

•Used when glucose reserves are limited

•Almost any organic molecule can be used to form glycerol

•Essential fatty acids cannot be synthesized and must be included in diet

•Linoleic and linolenic acid

Lipid transport and distribution

•5 types of lipoprotein

•Lipid-protein complex that contains large glycerides and cholesterol

•Chylomicrons

•Largest lipoproteins composed primarily of triglycerides

•Very low-density lipoproteins (VLDLs)

•contain triglycerides, phospholipids and cholesterol

Lipid transport and distribution

•5 types of lipoprotein (continued)

•Intermediate-density lipoproteins (IDLs)

•Contain smaller amounts of triglycerides

•Low-density lipoproteins (LDLs)

•Contain mostly cholesterol

•High-density lipoproteins (HDLs)

•Equal amounts of lipid and protein

Lipoprotein lipase

•Enzyme that breaks down complex lipids

•Found in capillary walls of liver, adipose tissue, skeletal and cardiac muscle

•Releases fatty acids and monglycerides

Protein Metabolism

Amino acid catabolism

•If other sources inadequate, mitochondria can break down amino acids

•TCA cycle

•removal of the amino group (-NH₂)

•Transamination – attaches removed amino group to a keto acid

•Deamination – removes amino group generating NH₄⁺

•Proteins are an impractical source of ATP production

Protein synthesis

•Essential amino acids

•Cannot be synthesized by the body in adequate supply

•Nonessential amino acids

•Can be synthesized by the body via amination

•Addition of the amino group to a carbon framework

Nucleic Acid Metabolism

Nucleic acid metabolism

•Nuclear DNA is never catabolized for energy

•RNA catabolism

•RNA molecules are routinely broken down and replaced

•Generally recycled as nucleic acids

•Can be catabolized to simple sugars and nitrogenous bases

•Do not contribute significantly to energy reserves

Nucleic acid synthesis

•Most cells synthesis RNA

•DNA synthesized only when preparing for division

Metabolic Interactions

Homeostasis

•No one cell of the human body can perform all necessary homeostatic functions

•Metabolic activities must be coordinated

Body has five metabolic components

•Liver

•The focal point for metabolic regulation and control

•Adipose tissue

•Stores lipids primarily as triglycerides

•Skeletal muscle

•Substantial glycogen reserves

Body has five metabolic components

•Neural tissue

•Must be supplied with a reliable supply of glucose

•Other peripheral tissues

•Able to metabolize substrates under endocrine control

The absorptive state

•The period following a meal

•Nutrients enter the blood as intestinal absorption proceeds

•Liver closely regulates glucose content of blood

•Lipemia commonly marks the absorptive state

•Adipocytes remove fatty acids and glycerol from bloodstream

•Glucose molecule are catabolized and amino acids are used to build proteins

The Postabsorptive state

•From the end of the absorptive state to the next meal

•Body relies on reserves for energy

•Liver cells break down glycogen, releasing glucose into blood

•Liver cells synthesize glucose

•Lipolysis increases and fatty acids released into blood stream

•Fatty acids undergo beta oxidation and enter TCA

The Postabsorptive state

•Amino acids either converted to pyruvate or acetyl-CoA

•Skeletal muscles metabolize ketone bodies and fatty acids

•Skeletal muscle glycogen reserves broken down to lactic acid

•Neural tissue continues to be supplied with glucose

Diet and Nutrition

•Nutrition

•Absorption of nutrients from food

•Balanced diet

•Contains all the ingredients necessary to maintain homeostasis

•Prevents malnutrition

Food

•Food groups and food pyramids

•Used as guides to avoid malnutrition

Food Groups

•Six basic food groups of a balance diet arranged in a food pyramid

•Milk, yogurt and cheese

•Meat, poultry, fish, dry beans, eggs, and nuts

•Vegetables

•Fruits

•Bread, cereal, rice and pasta

•Base of pyramid

•Fats, oils and sweets

•Top of pyramid

Nitrogen balance

•N compounds contain nitrogen

•Amino acids, purines, pyrimidines, creatine, porphyrins

•Body does not maintain large nitrogen reserves

•Dietary nitrogen is essential

•Nitrogen balance is an equalization of absorbed and excreted nitrogen

Minerals

•Act as co-factors in enzymatic reactions

•Contribute to osmotic concentrations of body fluids

•Play a role in transmembrane potentials, action potentials

•Aid in release of neurotransmitters and muscle contraction

•Assist in skeletal construction and maintenance

•Important in gas transport and buffer systems

•Aid in fluid absorption and waste removal

Vitamins

•Are needed in very small amounts for a variety of vital body activities

•Fat soluble

•Vitamins A, D, E, K

•Taken in excess can lead to hypervitaminosis

•Water soluble

•Not stored in the body

•Lack of adequate dietary intake = avitaminosis

Bioenergetics

•The study of acquisition and use of energy by organisms

•Energy content of food expressed in Calories per gram (C/g)

Food and energy

•Catabolism of lipids yields 9.46 C/g

•Catabolism of proteins and carbohydrates yields ~4.7 C/g

Metabolic rate

•Total of all anabolic and catabolic processes underway

•Basal metabolic rate (BMR) is the rate of energy used by a person at rest

Thermoregulation

•Homeostatic regulation of body temperature

•Heat exchange with the environment involves four processes:

•Radiation

•Conduction

•Convection

•Evaporation

Regulation of heat gain and loss

•Preoptic area of hypothalamus acts as thermostat

•Heat-loss center

•Heat-gain center

•Mechanisms for increasing heat loss include:

•Peripheral vasodilation

•Increase perspiration

•Increase respiration

•Behavioral modifications

Mechanisms promoting heat gain

•Decreased blood flow to the dermis

•Countercurrent heat exchange

•Shivering thermogenesis and nonshivering thermogenesis

•Differs by individuals due to acclimatization

Thermoregulation

•Problems in infants

•Lose heat quickly due to their small size

•Do not shiver

•Use brown fat to accelerate lipolysis - energy escapes as heat

•Variations in adults

•Use subcutaneous fat as an insulator

•Different hypothalamic thermostatic settings

Pyrexia is elevated body temperature

•Fever is body temperature greater than 37.2^oC

•Can result from a variety of situations including:

•Heat exhaustion or heat stroke

•Congestive heart failure

•Impaired sweat gland activity

•Resetting of the hypothalamic thermostat by circulating pyrogens