What you'll learn
- How simple sugars join to form larger carbohydrates such as starch and glycogen.
- Why condensation and hydrolysis reactions matter in digestion and storage.
- How carbohydrate intake links to respiration, ATP and energy balance.
- How to handle common quantitative ideas: food energy, energy balance and BMI.
Why carbohydrates matter
Carbohydrates are biological molecules used mainly for energy supply, energy storage and sometimes structural support. In this topic, they are especially important because diet, energy intake and long-term energy balance can affect risk factors for conditions such as cardiovascular disease.
Carbohydrate
A carbohydrate is an organic molecule made from carbon, hydrogen and oxygen. Many carbohydrates are sugars or polymers of sugars, and many have the approximate empirical formula CH2O\mathrm{CH_2O}CH2O.
Carbohydrates are not automatically “bad”. Your cells need glucose for respiration, but problems can arise when total energy intake repeatedly exceeds total energy expenditure.
From monomers to polymers
A monomer is a small molecule that can join to similar molecules. A polymer is a large molecule made from many repeating monomers.
For carbohydrates, the key monomers are monosaccharides.
Monosaccharide
A monosaccharide is a single sugar unit. Examples include glucose, fructose and galactose.
Glucose is especially important because it is a major respiratory substrate — a molecule that can be broken down in respiration to transfer energy to ATP.
A disaccharide is made from two monosaccharides. For example:
- maltose = glucose + glucose
- sucrose = glucose + fructose
- lactose = glucose + galactose
A polysaccharide is made from many monosaccharides joined together. Examples include starch, glycogen and cellulose.
The diagram below shows how carbohydrate molecules build up from monosaccharides to disaccharides and polysaccharides.
Condensation and hydrolysis
When two monosaccharides join, they form a glycosidic bond.
Glycosidic bond
A glycosidic bond is the covalent bond formed between two monosaccharides in a carbohydrate.
The reaction that forms the bond is a condensation reaction. This means that two molecules join together and one molecule of water is released.
The reverse reaction is hydrolysis. This means a bond is broken using water. In digestion, enzymes catalyse hydrolysis reactions to break carbohydrates into smaller sugars that can be absorbed.
Condensation and hydrolysis
Condensation builds larger carbohydrates and releases water. Hydrolysis breaks carbohydrates down and uses water.
Counting water molecules released
If 120 glucose molecules join to form one unbranched polysaccharide chain, how many glycosidic bonds form and how many water molecules are released?
- In a chain of monomers, each join between neighbouring glucose molecules forms one glycosidic bond, so the number of joins is one fewer than the number of monomers: 120−1=119120 - 1 = 119120−1=119.
- Each glycosidic bond forms by one condensation reaction.
- Each condensation reaction releases one water molecule, so 119 water molecules are released.
Storage carbohydrates: starch and glycogen
Glucose is useful, but storing lots of free glucose would affect the water potential of cells because glucose is soluble and osmotically active.
Instead, organisms store glucose as insoluble polysaccharides.
Starch in plants
Starch is the main storage carbohydrate in plants. It is made from alpha-glucose units and occurs as:
- amylose — mostly unbranched and coiled
- amylopectin — branched
Starch is compact, insoluble and can be hydrolysed to release glucose when needed.
Glycogen in animals
Glycogen is the main storage carbohydrate in animals and fungi. In humans, it is stored mainly in the liver and muscles.
Glycogen is highly branched, which means enzymes can act on many chain ends at once. This allows glucose to be released quickly, useful during exercise or between meals.
Cellulose as fibre
Cellulose is a structural polysaccharide in plant cell walls. It is made from beta-glucose, forming straight chains that hydrogen bond together into strong microfibrils.
Humans cannot digest cellulose because we do not produce the enzyme needed to hydrolyse its beta-glycosidic bonds. It still matters in the diet as fibre, helping gut movement.
Mixing up starch and cellulose
Do not describe cellulose as a glucose storage molecule in plants. Starch stores glucose; cellulose provides structural support in cell walls.
Carbohydrates and respiration
Glucose can be used in aerobic respiration. Respiration transfers energy from glucose into ATP, the immediate energy currency used by cells.
ATP
ATP, or adenosine triphosphate, is the molecule cells use as an immediate source of energy for processes such as muscle contraction, active transport and biosynthesis.
The overall aerobic respiration equation is:
C6H12O6+6O2→6CO2+6H2O\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \to 6\mathrm{CO_2} + 6\mathrm{H_2O}C6H12O6+6O2→6CO2+6H2OEnergy is not simply “released by breaking bonds”. Respiration involves a series of enzyme-controlled oxidation reactions that transfer energy to ATP.
Saying energy is stored in ATP forever
ATP is an immediate energy carrier, not a long-term energy store. Long-term energy is stored mainly in molecules such as glycogen and triglycerides.
Energy content of food
Energy in food is measured in joules, usually kilojoules. Approximate values are:
- carbohydrate: about 17 kJ g⁻¹
- protein: about 17 kJ g⁻¹
- lipid: about 39 kJ g⁻¹
Lipids contain more energy per gram than carbohydrates because they are more reduced and contain more carbon-hydrogen bonds that can be oxidised during respiration.
A simple practical method to estimate food energy is to burn food beneath a known mass of water and measure the temperature rise. The energy transferred to the water is estimated using:
E=mcΔTE = mc\Delta TE=mcΔTwhere EEE is energy in joules, mmm is mass of water in grams, ccc is the specific heat capacity of water, 4.18 J g⁻¹ °C⁻¹, and ΔT\Delta TΔT is the temperature change in °C.
Estimating energy transferred to water
A 0.80 g piece of food is burned under 50.0 g of water. The water temperature rises from 21.0 °C to 34.5 °C. Estimate the energy transferred per gram of food.
- Calculate the temperature change: ΔT=34.5−21.0=13.5 °C\Delta T = 34.5 - 21.0 = 13.5\ \text{°C}ΔT=34.5−21.0=13.5 °C.
- Substitute into E=mcΔTE = mc\Delta TE=mcΔT:
E=50.0 g×4.18 J g−1 °C−1×13.5 °C=2821.5 JE = 50.0\ \text{g} \times 4.18\ \text{J g}^{-1}\ \text{°C}^{-1} \times 13.5\ \text{°C} = 2821.5\ \text{J}E=50.0 g×4.18 J g−1 °C−1×13.5 °C=2821.5 J. - Convert to kilojoules: 2821.5 J=2.82 kJ2821.5\ \text{J} = 2.82\ \text{kJ}2821.5 J=2.82 kJ.
- Divide by the mass of food burned: 2.82 kJ0.80 g=3.53 kJ g−1\frac{2.82\ \text{kJ}}{0.80\ \text{g}} = 3.53\ \text{kJ g}^{-1}0.80 g2.82 kJ=3.53 kJ g−1.
Evaluating food calorimetry
This method usually underestimates the true energy content because heat is lost to the surroundings, combustion may be incomplete, and not all heat transfers to the water.
Energy balance
Energy balance compares energy intake with energy expenditure over time.
Energy balance
Energy balance is the difference between energy taken in from food and energy used by the body.
You can think of it as:
energy balance=energy intake−energy expenditure\text{energy balance} = \text{energy intake} - \text{energy expenditure}energy balance=energy intake−energy expenditureEnergy expenditure includes:
- basal metabolic rate — energy used at rest to keep cells and organs functioning
- physical activity
- thermoregulation
- growth, repair and biosynthesis
If energy intake is greater than expenditure over a long period, the body stores the surplus, mainly as triglycerides in adipose tissue. This is a positive energy balance.
If energy intake is less than expenditure, stored molecules are broken down to provide energy. This is a negative energy balance.
Calculating daily energy balance
A student takes in 9400 kJ day⁻¹. Their total daily energy expenditure is 8700 kJ day⁻¹. Calculate and interpret their energy balance.
- Use the energy balance relationship: energy balance=intake−expenditure\text{energy balance} = \text{intake} - \text{expenditure}energy balance=intake−expenditure.
- Substitute the values with units: 9400 kJ day−1−8700 kJ day−1=700 kJ day−19400\ \text{kJ day}^{-1} - 8700\ \text{kJ day}^{-1} = 700\ \text{kJ day}^{-1}9400 kJ day−1−8700 kJ day−1=700 kJ day−1.
- The value is positive, so the student is in positive energy balance for that day. If this pattern continued, surplus energy would tend to increase body energy stores.
Blaming one nutrient only
Body fat increases when total energy intake exceeds total energy expenditure over time. Excess energy can come from carbohydrates, lipids, proteins or alcohol.
BMI as a simple risk indicator
Body mass index, or BMI, is a simple way to compare body mass with height. It is often used in population studies of obesity and health risk.
BMI=mass in kg(height in m)2\text{BMI} = \frac{\text{mass in kg}}{(\text{height in m})^2}BMI=(height in m)2mass in kgAdult categories are usually:
- underweight: below 18.5 kg m⁻²
- healthy range: 18.5 to 24.9 kg m⁻²
- overweight: 25.0 to 29.9 kg m⁻²
- obese: 30.0 kg m⁻² or above
Calculating BMI
A person has a mass of 82 kg and a height of 1.70 m. Calculate their BMI.
- Square the height: (1.70 m)2=2.89 m2(1.70\ \text{m})^2 = 2.89\ \text{m}^2(1.70 m)2=2.89 m2.
- Substitute into the formula: BMI=82 kg2.89 m2=28.4 kg m−2\text{BMI} = \frac{82\ \text{kg}}{2.89\ \text{m}^2} = 28.4\ \text{kg m}^{-2}BMI=2.89 m282 kg=28.4 kg m−2.
- Compare with the adult categories: 28.4 kg m⁻² is in the overweight range.
BMI has limitations
BMI does not directly measure body fat. It can misclassify people with high muscle mass, and risk also depends on factors such as age, sex, ethnicity, fat distribution, smoking and blood pressure.
Linking carbohydrates to health risk
In this topic, the safest biological chain of reasoning is:
- Carbohydrates can be digested to glucose.
- Glucose can be respired to transfer energy to ATP.
- If total energy intake repeatedly exceeds expenditure, surplus energy is stored, mainly as fat.
- Increased adipose tissue can contribute to obesity.
- Obesity is associated with increased risk of conditions such as cardiovascular disease and type 2 diabetes.
The big link
Carbohydrates provide energy, but long-term health risk depends on overall energy balance, lifestyle and other risk factors — not on carbohydrate intake alone.
In the exam
- Use precise reaction language: condensation forms glycosidic bonds and releases water; hydrolysis breaks glycosidic bonds using water.
- In calculations, write the formula, substitute values with units, convert J to kJ when needed, and give the final unit.
- When linking diet to disease risk, explain the chain through energy balance and obesity rather than writing “sugar causes heart disease”.
Check yourself
- Why is glycogen more useful than free glucose as a storage molecule in animal cells?
- What is the difference between condensation and hydrolysis?
- How would a long-term positive energy balance affect body energy stores?
