Food Science and Metabolism
4. Metabolism and Metabolic Rate
Learning outcomes
- I can define metabolism as the sum of all chemical reactions in an organism.
- I can distinguish between anabolic and catabolic processes.
- I can explain the concept of metabolic rate.
- I can identify factors that affect metabolic rate.
- I can interpret data related to metabolism and energy expenditure.
What Is Metabolism?
Every living organism is constantly carrying out thousands of chemical reactions.
Even when you are sitting still or sleeping, cells are:
- Breaking down nutrients.
- Producing ATP.
- Building proteins.
- Repairing damaged structures.
- Transporting substances across membranes.
- Producing new cells.
- Breaking down waste products.
- Maintaining stable internal conditions.
The term metabolism refers to the sum of all the chemical reactions occurring within an organism.
These reactions are organised into interconnected pathways and are controlled largely by enzymes.
Metabolic Pathways
A metabolic pathway is a sequence of chemical reactions in which the product of one reaction becomes the starting material for another.
A simplified pathway might look like:
Substance A → Substance B → Substance C → Substance D
Each step is usually controlled by a particular enzyme.
This allows cells to regulate their chemistry carefully rather than allowing reactions to occur randomly.
Metabolic pathways can generally be divided into two broad categories:
- Catabolic pathways
- Anabolic pathways
Catabolic Processes
Catabolism includes metabolic reactions that break larger molecules into smaller molecules.
These processes often release energy that can be transferred into useful forms such as ATP.
Examples include:
- Breakdown of glucose during cellular respiration.
- Breakdown of glycogen into glucose.
- Breakdown of stored lipids.
- Breakdown of proteins into amino acids.
A useful general pattern is:
Large molecules → smaller molecules + energy released
Cellular Respiration as Catabolism
Cellular respiration is an important example of a catabolic process.
During aerobic respiration:
glucose + oxygen → carbon dioxide + water
Energy is released and transferred into ATP.
Glucose is gradually broken down through a series of enzyme-controlled reactions.
Because a relatively complex molecule is being broken into simpler products, respiration is classified as catabolic.
Anabolic Processes
Anabolism includes metabolic reactions that build larger, more complex molecules from smaller molecules.
These reactions generally require an input of energy.
Examples include:
- Amino acids joining to form proteins.
- Glucose molecules being used to produce glycogen.
- Production of lipids.
- Synthesis of DNA.
- Formation of new cellular structures.
A useful general pattern is:
Small molecules + energy → larger molecules
Protein Synthesis as Anabolism
Proteins are built from smaller molecules called amino acids.
Cells join amino acids together in specific sequences to produce proteins.
This requires energy.
Therefore:
amino acids + energy → proteins
Protein synthesis is an anabolic process.
The proteins produced can become:
- Enzymes
- Antibodies
- Structural proteins
- Transport proteins
- Some hormones
- Muscle proteins
Comparing Anabolism and Catabolism
| Catabolism | Anabolism |
|---|---|
| Breaks molecules down | Builds molecules |
| Large → smaller | Small → larger |
| Often releases energy | Requires energy |
| Can produce ATP | Often uses ATP |
| Includes cellular respiration | Includes protein synthesis |
| Includes breakdown of energy stores | Includes formation of energy stores |
Anabolism and catabolism are not competing processes. Both occur continuously within living organisms.
Together, they form metabolism.
Connecting Catabolism and Anabolism
Catabolic and anabolic reactions are closely linked.
Catabolic pathways can release energy and transfer some of it to ATP.
ATP can then provide energy for anabolic reactions.
For example:
Glucose breakdown → energy transferred to ATP → ATP used for protein synthesis
The products of one pathway can also become the starting materials for another.
This means metabolism is better understood as an interconnected network rather than a collection of isolated reactions.
What Is Metabolic Rate?
Metabolic rate describes how quickly an organism uses energy through its metabolic processes.
A higher metabolic rate means energy is being used more rapidly.
A lower metabolic rate means energy is being used more slowly.
Metabolic rate can be expressed in units such as:
- kJ per hour
- kJ per day
- kcal per hour
- kcal per day
For example, a person expending 10,000 kJ during one day has a greater daily energy expenditure than someone expending 8,000 kJ during the same period.
However, comparisons between people may also need to consider factors such as body size and composition.
Basal Metabolic Rate
Even a person who remains completely inactive still requires energy.
Energy is needed to:
- Keep the heart beating.
- Maintain breathing.
- Support brain activity.
- Maintain ion gradients across cell membranes.
- Carry out protein synthesis.
- Maintain body temperature.
- Support kidney and liver function.
- Repair and maintain tissues.
Basal metabolic rate, or BMR, refers to the rate of energy expenditure required to maintain essential body functions under carefully defined resting conditions.
Resting Does Not Mean Zero Energy Use
Suppose someone spends an hour lying quietly on a bed.
Their skeletal muscles may be doing relatively little external work, but their cells remain highly active.
The heart continues contracting.
The brain continues functioning.
Cells maintain concentration gradients.
Proteins are continually produced and broken down.
Body temperature is regulated.
Therefore:
Resting energy expenditure is low compared with vigorous exercise, but it is never zero in a living person.
Total Energy Expenditure
A person's total daily energy expenditure comes from several components.
These include:
- Energy required for basic body functions.
- Energy used during physical activity.
- Energy associated with processing food.
- Energy required for growth or other physiological processes when applicable.
The relative contribution of each component varies between individuals and from day to day.
Physical Activity and Metabolic Rate
Physical activity can greatly increase energy expenditure.
During exercise, muscle cells require ATP at a much faster rate.
This increases metabolic activity.
For example, compare:
Sleeping → sitting → walking → jogging → sprinting
In general, energy expenditure per minute increases as the intensity of activity increases.
This happens because working muscles require more ATP.
Muscle Mass and Metabolic Rate
Body composition can influence metabolic rate.
Muscle tissue requires energy for maintenance even when it is not performing vigorous exercise.
People with greater amounts of metabolically active tissue may therefore have higher resting energy expenditure than people of the same body mass with less metabolically active tissue.
However, metabolic rate is influenced by many variables, so muscle mass is only one factor.
Body Size
Larger bodies generally require more total energy to maintain than smaller bodies.
One reason is that there is simply more living tissue requiring:
- Nutrients
- Oxygen
- Cellular maintenance
- Protein turnover
- Ion transport
This means total metabolic rate and metabolic rate relative to body mass are not necessarily the same thing.
Age and Metabolism
Metabolic rate can change throughout life.
Children and adolescents require energy for:
- Basic metabolism
- Physical activity
- Growth
- Development of new tissues
During adulthood, energy expenditure can change as body size, body composition, activity levels, and physiology change.
It is therefore too simplistic to say that age alone determines metabolic rate.
Temperature and Metabolic Rate
Temperature can influence metabolism.
Human cells rely on enzyme-controlled reactions that function effectively within a relatively narrow temperature range.
During a fever, metabolic rate may increase as body temperature rises.
Environmental temperature can also influence energy expenditure because the body must maintain its internal temperature.
For example, prolonged exposure to cold can increase energy expenditure through mechanisms that generate heat.
Hormones and Metabolism
Hormones help regulate metabolic activity.
One important example involves hormones produced by the thyroid gland.
Thyroid hormones influence metabolic activity in many tissues.
Other hormones also help regulate:
- Blood glucose
- Energy storage
- Energy mobilisation
- Growth
- Responses to exercise and stress
Metabolic rate is therefore partly controlled by the endocrine system.
Genetics and Metabolism
Genetic differences can contribute to variation in:
- Body size
- Body composition
- Hormone regulation
- Enzyme activity
- Energy expenditure
However, genes do not act alone.
Metabolism results from interactions between genetics, physiology, behaviour, diet, environment, and health.
Food and Metabolic Rate
Processing food itself requires energy.
After eating, the body uses energy to:
- Digest food.
- Absorb nutrients.
- Transport nutrients.
- Process and store nutrients.
This increase in energy expenditure associated with processing food is often called the thermic effect of food.
Different nutrients require different amounts of processing, so the effect is not identical for every meal.
Factors Affecting Metabolic Rate
Important factors include:
Body Size
Larger bodies generally have greater total energy requirements.
Body Composition
The amount and type of metabolically active tissue influence energy expenditure.
Physical Activity
Muscle activity increases ATP demand and therefore increases energy expenditure.
Age and Growth
Growth and development require additional energy.
Hormones
Hormones, particularly thyroid hormones, influence metabolic activity.
Body and Environmental Temperature
Thermoregulation can change energy expenditure.
Genetics
Inherited differences can influence metabolic characteristics.
Food Intake
Digestion and nutrient processing require energy.
Health
Illness, fever, injury, and some medical conditions can alter metabolic demands.
Measuring Metabolic Rate
Scientists can estimate metabolic rate in several ways.
One method is direct calorimetry, which measures heat released by the body.
Another is indirect calorimetry, which estimates energy expenditure using respiratory gases.
Because aerobic respiration consumes oxygen and produces carbon dioxide, measurements of gas exchange can provide information about metabolic activity.
Oxygen Consumption as Evidence
Imagine two situations.
Situation A
A person is sitting quietly.
Oxygen consumption is relatively low.
Situation B
The same person is running.
Oxygen consumption increases substantially.
The increased oxygen consumption provides evidence that aerobic metabolism has increased to meet the greater ATP demand of working muscles.
Therefore, oxygen consumption can be used as an indicator of metabolic rate under many conditions.
Interpreting Metabolic Data
Suppose a person's energy expenditure is measured during different activities.
| Activity | Energy Expenditure |
|---|---|
| Sleeping | 280 kJ/hour |
| Sitting | 360 kJ/hour |
| Walking | 900 kJ/hour |
| Cycling | 1,600 kJ/hour |
| Running | 2,600 kJ/hour |
Several conclusions can be drawn.
The lowest energy expenditure occurs during sleep.
Sitting requires somewhat more energy.
Walking produces a substantial increase.
Cycling and running require much more energy per hour.
The general pattern is:
Greater physical activity → greater ATP demand → greater energy expenditure
Reading a Metabolic Rate Graph
Graphs are commonly used to show how metabolic rate changes with activity.
When analysing such a graph:
- Identify the independent variable on the horizontal axis.
- Identify the dependent variable on the vertical axis.
- Look for the overall trend.
- Compare specific values.
- Identify unusual values or exceptions.
- Use biological knowledge to explain the pattern.
For example, if metabolic rate increases as exercise intensity increases, the explanation should connect increased muscular activity with increased ATP demand.
Worked Example: Calculating Energy Expenditure
Suppose a student walks for 2 hours at an energy expenditure of 850 kJ/hour.
Energy used:
Energy = rate × time
Energy = 850 × 2
Energy = 1700 kJ
Now suppose the same student spends one hour running at 2500 kJ/hour.
The running session uses:
2500 × 1 = 2500 kJ
Even though the running session is shorter, it uses more energy because the rate of energy expenditure is much greater.
Worked Example: Comparing Two People
Suppose two people have the following resting energy expenditures:
Person A: 6,400 kJ/day
Person B: 7,200 kJ/day
Person B has the greater measured resting energy expenditure.
The difference is:
7200 - 6400 = 800 kJ/day
However, the data alone do not tell us why Person B has the higher value.
Possible factors could include differences in:
- Body size
- Body composition
- Age
- Hormonal activity
- Physiological state
A good scientific conclusion distinguishes between what the data actually show and possible explanations.
Interpreting Percentage Changes
Suppose metabolic rate increases from 400 kJ/hour to 1000 kJ/hour during an activity.
Increase:
1000 - 400 = 600 kJ/hour
Percentage increase:
Percentage increase = (increase ÷ original value) × 100
Percentage increase = (600 ÷ 400) × 100
Percentage increase = 150%
The metabolic rate has therefore increased by 150%.
Notice that the new value is 250% of the original value, but the increase is 150%.
Metabolism and Energy Balance
Metabolism connects directly with energy intake and expenditure.
Energy enters the body through food.
Energy is used through metabolic processes and physical activity.
Over time:
Energy intake ≈ energy expenditure → energy stores tend to remain relatively stable
Energy intake > energy expenditure → energy stores tend to increase
Energy intake < energy expenditure → energy stores tend to decrease
However, the human body is a complex biological system. Appetite, hormones, body composition, activity, growth, health, and other factors can influence both intake and expenditure.
Metabolism Changes Throughout the Day
Metabolic rate is not fixed at one value.
Consider one day:
Sleeping
Energy expenditure is relatively low.
Getting ready for school
Movement increases energy expenditure.
Sitting in class
Energy expenditure decreases compared with active movement.
Physical education
Energy expenditure rises substantially.
Eating lunch
Processing food contributes to energy expenditure.
Sports practice
Muscular activity produces another major increase.
The body's metabolic rate continuously changes according to its physiological demands.
Anabolism, Catabolism and Growth
Growing organisms provide a particularly clear example of the interaction between anabolic and catabolic processes.
Food molecules can be broken down through catabolic pathways.
Some of the released energy is transferred to ATP.
ATP can then support anabolic processes that build:
- Proteins
- Cell membranes
- DNA
- Glycogen
- New cells and tissues
Therefore:
Catabolism helps provide energy and materials for anabolism.
Growth requires both.
Common Mistakes
Saying Metabolism Means Digestion
Digestion is only one part of the body's chemistry.
Metabolism includes all chemical reactions occurring in the organism.
Saying Metabolism Only Breaks Things Down
Metabolism includes both:
- Catabolism
- Anabolism
Saying Catabolism Always Means Digestion
Digestion involves catabolic reactions, but catabolism also occurs inside cells.
Cellular respiration is an example.
Saying Anabolism Releases Energy
Anabolic reactions generally require an input of energy to build larger molecules.
Saying Metabolic Rate Is Zero During Sleep
Metabolism continues continuously while an organism is alive.
Assuming a Higher Metabolic Rate Is Automatically Better
Metabolic rate reflects energy expenditure. A higher or lower rate is not inherently "good" or "bad."
Assuming Metabolic Rate Is Controlled by One Factor
Metabolic rate is influenced by many interacting factors, including body size, body composition, activity, hormones, temperature, genetics, food intake, and health.
Assuming Data Explain Their Own Cause
A graph may show that one person has a higher metabolic rate than another, but additional evidence is required to determine why.
Check Your Understanding
1. Define metabolism.
2. What is a metabolic pathway?
3. Define catabolism.
4. Give two examples of catabolic processes.
5. Define anabolism.
6. Give two examples of anabolic processes.
7. Explain how catabolic and anabolic processes are connected through ATP.
8. What is meant by metabolic rate?
9. Why does a sleeping person still require energy?
10. Identify four factors that can influence metabolic rate.
11. Explain why physical activity increases metabolic rate.
12. How can oxygen consumption provide information about metabolic rate?
13. A person walks for 3 hours at an average energy expenditure of 700 kJ/hour. Calculate the total energy expenditure.
14. Metabolic rate increases from 500 kJ/hour to 800 kJ/hour. Calculate the percentage increase.
15. Two people have different resting metabolic rates. Explain why it would be inappropriate to conclude that physical activity is responsible without additional evidence.
Key Terms
- Metabolism – the sum of all chemical reactions occurring within an organism.
- Metabolic pathway – a sequence of linked chemical reactions within a cell or organism.
- Catabolism – metabolic reactions that break larger molecules into smaller molecules, often releasing energy.
- Anabolism – metabolic reactions that build larger molecules from smaller molecules and require energy.
- Metabolic rate – the rate at which an organism uses energy through metabolic processes.
- Basal metabolic rate (BMR) – energy expenditure required to maintain essential body functions under defined resting conditions.
- ATP – a molecule used to transfer energy for cellular processes.
- Energy expenditure – the amount of energy used by the body.
- Energy intake – energy obtained from food and drink.
- Cellular respiration – a catabolic process that releases energy from nutrient molecules.
- Protein synthesis – an anabolic process in which amino acids are joined to produce proteins.
- Glycogen – a carbohydrate used for glucose storage in animals.
- Thermic effect of food – energy expenditure associated with digesting, absorbing, and processing nutrients.
- Calorimetry – measurement of energy transfer.
- Indirect calorimetry – estimation of energy expenditure using measurements such as oxygen consumption and carbon dioxide production.
Key Takeaways
- Metabolism is the sum of all chemical reactions in an organism.
- Metabolic reactions are organised into enzyme-controlled pathways.
- Catabolic reactions break molecules down and often release energy.
- Cellular respiration is an important catabolic process.
- Anabolic reactions build larger molecules and require energy.
- Protein synthesis is an example of an anabolic process.
- Catabolism and anabolism are connected through energy-transfer molecules such as ATP.
- Metabolic rate describes how rapidly an organism uses energy.
- The body continues using energy even during complete rest.
- Basal metabolic rate represents the energy required to maintain essential functions under defined resting conditions.
- Physical activity increases metabolic rate because working muscles require more ATP.
- Body size, body composition, age, growth, hormones, temperature, genetics, food intake, and health can influence metabolic rate.
- Oxygen consumption can be used to estimate metabolic activity.
- Metabolic data can be analysed using rates, comparisons, graphs, and percentage changes.
- A measured difference in metabolic rate does not by itself explain the cause of that difference.