What is energy
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What is energy?. Energy is the capacity to do work Forms of energy: mechanical, electrical, chemical, solar, thermic, and .. Two sources: food/stored on body Energy can be transformed from one form to another: Flowing water  electricity  light Glucose  ATP  muscle contraction.

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What is energy?

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What is energy?

  • Energy is the capacity to do work

  • Forms of energy: mechanical, electrical, chemical, solar, thermic, and ..

  • Two sources: food/stored on body

  • Energy can be transformed from one form to another:

    • Flowing water  electricity  light

    • Glucose  ATP  muscle contraction

Metabolism consists of two kinds of processes

  • Catabolic reactions are energy-releasing reactions in which substances are broken down/breakdown

  • Anabolic reactions are energy-requiring reactions in which simpler molecules are combined to form more complex substances/buildup or synthesis

Catabolic and Anabolic Reactions

Energy is obtained from basic food molecules through the process of aerobic respiration

Review: Metabolic furnace includes three parts

  • Glycolysis

  • Krebs cycle/Citric Acid cycle/TCA

  • Electron transport system


  • Glucose (6 C)  2 (3-C) compounds

  • End up with either pyruvate (which with oxygen can be converted to Acetyl CoA and enter the TCA) or lactic acid (in absence of oxygen)

  • Series of 10 enzymatically controlled reactions; anaerobic (without oxygen) – but can proceed in the presence of oxygen

  • Takes place in the cytosol

  • Net ATP production of 2 ATP’s

  • (potential for more ATP production H’s are carried to ETS in presence of oxygen)

Glycolysis continued

  • Key enzymes: hexokinase in all tissues and glucokinase in hepatocytes (liver)

  • Conversion of pyruvate to lactate is a reduction reaction (NADH is oxidized to NAD+); reaction is catalyzed by lactate dehydrogenase (LDH) which itself has five isoforms. LDH is composed of four subunits of either the heart (H) or muscle (M) type (MMMM, MMMH, MMHH, MHHH, HHHH), and different tissue will express different forms. (ex. Heart attack)

  • The conversion of pyruvate to acetyl CoA involves a complex of enzymatic reactions

Krebs cycle/Citric Acid Cycle

  • Acetyl CoA combines with Oxaloacetic Acid to form Citric Acid

  • Series of 9 enzymatically controlled reactions / coenzymes are B-vitamin based

  • Occurs in mitochondria

  • Aerobic

  • Endproducts include carbon dioxide, a little energy (GTP ATP), and H atoms

Electron Transport Chain/System

  • In mitochondria/aerobic

  • H atom electrons drive the phosphorylation of ADP to ATP

  • Hydrogen atoms at the end of the chain combine with oxygen to form water (called metabolic water)

Estimating the energy content of food

  • Grams of carbohydrate  4kcal/g= kcal from carbohydrate

  • Grams of protein  4 kcal/g = kcal from protein

  • Grams of fat  9 kcal/g = kcal from fat

  • Grams of alcohol  7 kcal/g = kcal from alcohol

Sources of stored energy in a 70kg man:

  • Glycogen in liver and muscle –1400 kcal

  • Glucose and lipids in body fluids –100 kcal

  • Triglycerides in adipose tissues –115,000 kcal

  • Protein in muscle –25,000 kcal

Carbohydrate, protein, fat and alcohol can be oxidized for energy

  • Carbohydrate is preferentially used for energy

  • Protein in excess of body needs can be used for energy

  • Alcohol is readily used for energy or stored as fat

  • Fat is preferentially stored as body fat

Making fat from carbohydrates is energetically costly

  • Carbohydrates must be converted into acetyl CoA, then assembled into long chains

  • These conversions use up about 20% of the energy originally present in the carbohydrate

Making fat from protein is energetically costly

  • The amino group must be removed

  • The carbon skeleton must be broken down into acetyl-CoA units

  • The acetyl-CoA units must be assembled into long chains

  • This process uses up even more energy than the conversion of glucose

Turning dietary fat into body fat is easy

  • The process of storing dietary fat uses up only 2-3% of the energy contained in the fat

The body preferentially burns carbohydrate and then protein; excess dietary fat is usually stored as body fat

What are BMR and RMR?

  • The basal metabolic rate (BMR) is the

    minimum amount of energy that an awake,

    resting body needs to maintain itself; usually measured under laboratory conditions

  • The resting metabolic rate (RMR) is

    similar to BMR, but easier to measure,

    and usually within 10% of the BMR;

    an estimate

What is the basal metabolic rate? Energy expended for/during

  • Breathing

  • Body temperature

  • Muscle tone

  • Heart beating

  • Glands/nerves/cells functioning


  • 60-70% of typical day’s energy needs

  • A function of a person’s lean body mass (LBM)

  • Liver + brain = 50% of BMR use (4% body weight)

Is RMR always the same?

  • No, some factors decrease RMR:

    • Increasing age(increases with growth spurts)

    • Fasting and starvation

  • Some factors increase RMR:

    • Growth

    • Pregnancy and lactation

    • Fever

    • Physical activity

    • Leaner bodies and thinner bodies

      Plus…individual variation +/- 20%

What is the thermic effect of food?

  • The thermic effect of food (TEF) is the energy required for the digestion of food and the absorption, metabolism, and storage of nutrients.

  • TEF is approximately 10% of daily energy intake.

  • Measured by heat produced after eating

  • CHO/PRO ‘cost’ more to process than FAT

How does physical activity affect energy expenditures?

  • Physical activity accounts for 15-30% of energy expenditure for most people

  • You have the greatest control over how big or how small this component is

  • Total energy expenditure depends on the type of activity, how long you do it, body weight

  • Varies from day to day

How does physical activity affect energy expenditures?

  • Different activities require different energy expenditures; for a 150 pound person:

    • Billiards: 136 kcal/hr

    • Walking (20 min/ 1 mile): 238 kcal/hr

    • Aerobics (medium): 340 kcal/hr

    • Jogging (10 min/ 1 mile): 680 kcal/hr

    • Running (5 min/ 1 mile): 1224 kcal/hr

Resting metabolic rate is based on age, gender and weight

  • For females:

    • 18-30 years, 125 lbs: RMR = 1334 kcal/day

    • 18-30 years, 155 lbs: RMR = 1525 kcal/day

    • 60 years, 155 lbs: RMR = 1331 kcal/day

  • For males:

    • 18-30 years, 125 lbs: RMR = 1551 kcal/day

    • 18-30 years, 155 lbs: RMR = 1750 kcal/day

    • 60 years, 155 lbs: RMR = 1423 kcal/day

Multiply by the activity factor to get total energy use:

  • Sleeping, reclining: RMR  1.0

  • Driving, light activities: RMR  1.5

  • Walking, housecleaning: RMR  2.5

  • Bicycling, tennis: RMR  5

  • Basketball, soccer: RMR  7

What constitutes total energy expenditure?

  • The sum of:

    • RMR

    • TEF

    • Physical activity

Total energy expenditure



# hrs at very light activity level  RMR/hr  1.5


  • # hrs at other activity level  RMR/hr  the appropriate factor, for a total of 24 hours


  • TEF (10% of the above total)

Calculating total energy expenditures

# hrs at resting metabolism  RMR/hr  1.0

The recommended energy intake is set at the average for the population

<Adaptive Thermogenesis>

  • Energy used for environmental stress

  • Could be 0-10% of typical day’s energy needs

How to measure energy in the body

  • Estimate

  • Direct calorimetry: person in a small metabolic chamber/research – measure the heat produced over time/ in theory it is like putting someone in a bomb calorimeter

  • Indirect calorimetry/measure oxygen use or carbon dioxide production

Energy macronutrient storage

  • Carbohydrates: digested, absorbed, processed glucose

    glucose is the major energy source in the body; it is transported by blood and can be stored as glygogen in the liver and muscle; can also be converted to fat and stored in adipose tissue

  • Fats  mostly triglycerides; fats are the most predictable energy sources; stored in adipose tissue; can later be used for energy

Energy macronutrient storage

  • Proteins  amino acids; most versatile; best use: restructured to body structural/functional proteins; can be used as energy/stored as energy compounds

  • Alcohol  alcohol; used directly as energy; converted to fat (stored or energy)


  • All energy macronutrients can be stored as fat or used as energy; body stores must be broken down before used

  • Alcohol fat directly

Mass Index (BMI)?

  • Body Mass Index (BMI) is an index of weight in relation to height that is used to compare body size with a standard

  • BMI= weight in pounds/(height in inches)2 704.5

  • Or BMI= weight in kg/height in m)2

Body Mass Index serves asan indicator desirable weight levels

When energy expended equals energy consumed, weight will remain stable

Why is it so hard to lose weight?

  • The set point theory suggests that body weight is genetically determined and that there are internal mechanisms that defend against weight change.

  • After over-eating and then returning to normal intakes, body weight will fall back to its set point

  • After dieting, and then returning to normal intakes, body weight will likewise increase back to previous levels

Leptin is one factor that may be involved in maintaining body weight

  • Leptin is a protein-hormone that is produced by adipocytes (fat cells) that signals information about the amount of body fat

  • When fat stores are high, leptin levels are high, energy expenditures tend to increase and food intake decreases

  • When fat stores shrink, less leptin is released, energy expenditures decrease and food intake increases

Leptin influences the storage of fat

High kcalorie foods plus a less active lifestyle has tipped the average American toward weight gain

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