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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 + 2Pi
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 37.2oC

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