Topic 2.3: Carbohydrates and Lipids

2.3.1 Monosaccharides are linked together by condensation reactions to form disaccharides and polysaccharide polymers

How monosaccharides bond was already explained in 2.2.12 it has been copied and pasted here for convenience. This process can be repeated for long polymer chains

A monosaccharide is a single sugar molecule whereas a disaccharide is formed when two sugar molecules are bonded together. When a monosaccharide bonds with another monosaccharide to form a disaccharide the reaction that takes place is anabolic and is a condensation reaction. During the reaction, the Hydroxide bonded to the Carbon on the far right of one molecule bonds with the Hydroxide bonded to the Carbon on the far right of another. The two Hydroxides form water and then an Oxygen is left over which bonds with both Carbons.
OH + OH = H2O + O


2.3.2 Fatty acids can be saturated, monounsaturated, and polyunsaturated

The word saturated means that something is "holding as much as it can". Below the structure of a saturated fatty acid is shown. Every single Carbon is bonded to at least one other carbon through a single bond in a saturated lipid molecule. This means that Carbon is bonded to the maximum number of hydrogens it can. The structure shown below is saturated with Hydrogens.


A monounsaturated fatty acid will have one double or triple bond between two Carbons resulting in fewer bonds with Hydrogens. A polyunsaturated fatty acid will have many such double or triple bonds between Carbons. Considering the structure of unsaturated fatty acids wherever there is a double or triple bond the entire molecule bends. This is shown in the image below

2.3.3 Unsaturated fatty acids can be Cis or trans isomers

An isomer is a different structure for the same molecule. In other words, the number of each element remains the same but the way in which they bond changes. In an unsaturated fatty acid, there are at least two Carbons bonded to each other through a double bond. This means that they are additionally only bonded to a single Hydrogen. 

In a cis-unsaturated fatty acid, the Hydrogens bonded to both Carbons are on the same side. Whereas in a                      trans-unsaturated fatty acid they are on different sides. As you can see in the diagram this also changes the molecule's structure


2.3.4 Triglycerides are formed by condensation from three fatty acids and one glycerol

Again remember condensation reactions take smaller molecules and make larger ones expending energy in the process. Dehydration synthesis is another name for a condensation reaction



2.3.5 Know the structure and function of cellulose and starch in plants and glycogen in humans.

Carbohydrates are soluble in water and not good ways to store energy for long term use. So in animals that energy is stored as glycogen, but in plants it is stored as starch. Cellulose is found in the plant cell wall and is used to make the cell wall tough and rigid.

The structure here is simplified, the IB does not mention whether you will have to draw these, but they say you should know the structure. This should be sufficient if that is true.



2.3.6 There is scientific evidence for health risks of trans fats and saturated fatty acids.

Both trans fatty acids and saturated fatty acids have a linear structure. While they travel around the body through the blood some of them might stick to the side of a blood vessel. Because of their flat structure once they are there it is difficult to pick them up again. Other trans and saturated fatty acids will also then get caught in the area resulting in a clot of fat in the blood vessel. The buildup of plaque results in a higher blood pressure and can cause atherosclerosis or coronary heart disease. 


2.3.7 Lipids are more suitable for long-term energy storage in humans than carbohydrates.

Lipids are insoluble and are much more energy dense than carbohydrates


2.3.8 Evaluate the evidence and methods used to obtain the evidence for health claims made about lipids


2.3.9 Use of molecular visualization software to compare cellulose, starch, and glycogen

2.3.10 Determination of Body Mass Index (BMI) by calculation or using a nomogram

To use a nomogram use a ruler to connect the value you find for the weight and the height. Wherever the ruler intersects the center line in the nomogram is the BMI.


2.3.11 The structure of starch should include amylose and amylopectin

Amylose has a linear structure while amylopectin is more branched



2.3.12 Named examples of fatty acids are not required


2.3.13 Sucrose, lactose, and maltose are examples of disaccharides produced by combining monosaccharides.

The Guidance is not specific on whether the structure is needed so its included. But know at the very least that they are examples of disaccharides






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