IB Biology - Curriculum Notes 

​2.3 Carbohydrates and lipids  

∑ - Understandings:

∑ - Monosaccharide monomers are linked together by condensation reactions to form disaccharides and polysaccharide polymers.

Energy molecule used in aerobic respiration

Nutritive sweetener in foods

Fruit sugar
Malt sugar found in barley consists of 2 glucose molecules

Sugar found in milk

Transport sugar found in plants because of its solubility

Storage carbohydrate in plants

Storage carbohydrate in animals

Main component in plant cell walls

When two monomers combine together they form a dimer. When many monomers combine together they form a polymer.
Condensation Reactions: The building of large macromolecules (polymers) by the removal of water molecules when monomers combine. Each time two monomers combine, one water is removed.

For example, glucose is a monosaccharide that is used to build up large storage molecules (polysaccharides) in plants and animals.  In plants, many glucose molecules combine through condensation reactions to form the polysaccharide starch. In animals, glucose molecules are combined to form the polysaccharide glycogen through condensation reactions.

When a plant or an animal needs to use energy stored in polysaccharide molecules, the opposite reaction to condensation takes place. This break down of larger polysaccharides into smaller monosaccharides through the addition of water is called hydrolysis (water split or separate)
Starch and glycogen are broken down by the addition of water into glucose molecules (the energy molecule used in aerobic respiration).
In lipids, the polymer is called a triglyceride.
Hydrolysis of a triglyceride uses water to break apart the lipid into glycerol C3H5(OH)3 and 3 fatty acids.

β - Applications and skills:

β - Application: Structure and function of cellulose and starch in plants and glycogen in humans.

Polysaccharides are long chains of monosaccharides held together with glycosidic linkages made by condensation reactions
Starch, cellulose and glycogen are all polysaccharides that are made from long chains of glucose; however, they differ in their structure and the type of glucose, which leads to different functions

Α (Alpha) Glucose 
Β  (Beta) Glucose


Starch -a long chain of α (alpha) glucose molecules used as glucose storage by plants
Starch consists of two types of molecules, amylose which linear and amylopectin which is branched
Since the bonds in starch are α-glucose, the –OH groups from the glucose molecules are always pointed down, causing the starch to have a curved appearance. This makes starch a good molecule for storing glucose in plants.

The picture to the right --> 

Even though glucose is hydrophilic, starch is too large to be soluble in water at room temperature
Cellulose are unbranched straight chains of β  (beta) glucose molecules, held together with glycosidic bonds
Since the –OH groups point out in opposite directions and every other β glucose is flipped 180 degrees, cellulose forms a nice straight chain
These straight chains also allow cellulose to form bundles linked by H-bonds
This is essential for cellulose’s function, which is to provide strength for cell walls in plant cells (high tensile strength)
Notice the up and down alternating glycosidic bonds between the glucose molecules

Glycogen – Is a multi-branched energy storage polysaccharide for animals
Glycogen consists of many α (alpha) glucose molecules linked 

      by glycosidic bonds

It is highly branched, making the molecule more compact and a perfect molecule for energy storage
It is stored in the liver and some muscles of humans

β - Skill: Use of molecular visualization software to compare cellulose, starch and glycogen.

Use the following link to analyze and compare the above polysaccharides.
Method to manipulate the molecules with the mouse is on the bottom right corner of the webpage

∑ - Fatty acids can be saturated, monounsaturated or polyunsaturated.

Fatty Acids

Saturated Fatty Acid
Monounsaturated Fatty Acid 
Polyunsaturated Fatty acid
Main component of triglycerides and phospholipids

Fatty acids are non-polar and therefore hydrophobic

Chains consist of covalently bonded carbon with hydrogen

Saturated FA’s are all single bonds and are therefore saturated with hydrogen.

Unsaturated FA’s contain a double bond or double bonds.
Saturated Fatty Acid                                            
Monounsaturated Fatty Acid                    
Polyunsaturated Fatty acid


∑ - Unsaturated fatty acids can be cis or trans isomers.


If the hydrogen atoms are on the same side of the double bond then the isomer is “cis” (yellow H above) and if the hydrogens are on the opposite side of the double bond then the isomer is “trans”
“cis” fatty acids have a kink at the double bonds, causing the fatty acids to pack more loosely, lowering the melting point and making them liquid at room temperature
“trans” fatty acids do not have the kink at the double bond, can pack more tightly, have a higher melting point and are solid at room temperature.
Trans fats are partially hydrogenated oils found in some processed foods like margarine. They can cause health risks for humans.

∑ - Triglycerides are formed by condensation from three fatty acids and one glycerol.

Fatty acids have a long hydrocarbon (carbon and hydrogen) chain with a carboxyl (acid) group. The chains usually contain 16 to 18 carbons.
Glycerol contains 3 carbons and 3 hydroxyl groups. It reacts with 3 fatty acids to form a triglyceride or fat molecule through a condensation reaction, which gives off 3 water molecules and forms an ester bond

Triglyceride or Fat


Check out the interactive video on the molecular structure of fats

β - 1) Application: Scientific evidence for health risks of trans fats and saturated fatty acids.

β - 2) Application: Evaluation of evidence and the methods used to obtain the evidence for health claims made about lipids

Use the following links as examples and find one journal article and one web post on the health risks of trans or saturated fats. You need to find one article from what you would consider as a reputable source and one that is not a good source. You will present these articles to the class discussing the above two applications. Make sure you have a clear understanding of what trans fats and saturated fatty acids are and how they affect our bodies. Critically analyze the evidence for health risks and the methods used to obtain the evidence.


β - Application: Lipids are more suitable for long-term energy storage in humans than carbohydrates.

Energy Storage

One’s body requires energy to function, more specifically each cell relies on a source of energy to drive the chemical reactions involved in metabolism, growth and other physiological functions
Both carbohydrates and lipids (triglycerides) are a major source of energy in animals.
Fats contain about twice as much energy as carbohydrates. Each gram of carbohydrates stores about 4 calories of energy, whereas each gram of lipid stores about 9 calories.
Therefore, lipids serve as a more compact way to store energy, since it contains more energy per gram than carbohydrates. As a result, your body tends to use fat to store energy over long periods of time and uses carbohydrates to store energy short-term.
Glycogen (carb storage) can be quickly into glucose for energy.
Triglycerides (fats) contain glycerol and 3 fatty acids and is stored mainly in the body’s adipose tissue
Fats also provide thermal insulation, protection for organs (shock absorber) and hormones

B - Skill: Determination of body mass index by calculation or use of a nomogram.


Use the nomogram on the following quick reference guide from Health Canada to calculate your family members' BMI. Ask your parents’ permission if they are willing to share the data. 

Metric BMI Formula           BMI = weight (kg) / [height (m)]2

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