Biology » The Chemistry of Life » Organic Compounds

Lipids

Lipids

Molecular make-up

Lipids contain carbon (C), hydrogen (H) and oxygen (O) but have less oxygen than carbohydrates. Examples of lipids in diet include cooking oils such as sunflower and olive oil, butter, margarine and lard. Many nuts and seeds also contain a high proportion of lipids.

Structural composition

Triglycerides are one of the most common types of lipids. Triglyceride molecules are made up of glycerol and three fatty acids (see figure below). The fatty acid tails are made up of many carbons joined together. The number of carbons in the fatty acid chains can differ.

Note:

When drawing organic molecules, it can easily get confusing writing out all of the ‘C’s and ‘H’s for carbon and hydrogen respectively. Scientists overcome this by drawing the carbon backbone, and leaving out the hydrogens. Carbon will always make 4 bonds with other atoms, so it is easy to figure out how many hydrogens there must be. The carbon is indicated by a point, and the bonds between carbon molecules are indicated by lines joining the points.

Image credit: Siyavula

Role in animals and plants

Lipids are an important energy reserve and contain 37.8 kilojoules (kJ) of energy per gram. Triglyceride lipids are broken down to release glycerol and fatty acids. Glycerol can be converted to glucose and used as a source of energy, however the majority of energy provided by lipids comes from the breakdown of the fatty acid chains. Some fatty acids are essential nutrients that cannot be produced by the body and need to be consumed in small amounts. Non-essential fatty acids can be produced in the body from other compounds.

Lipids are important for the digestion and transport of essential vitamins, help insulate body organs against shock and help to maintain body temperature. Lipids also play an important role in cell membranes.

Note:

You will learn about the important role that lipids play in cell membranes in the following chapter on the basic units of life.

Saturated and unsaturated fats

Carbon can form four bonds with other atoms. Most carbons in a fatty acid chain are bonded to two adjacent carbons, and to two hydrogen atoms. Fatty acids which form four single bonds, and have the maximum number of hydrogen atoms are called saturated fatty acids because they are “saturated” with hydrogen atoms. However, sometimes two adjacent carbons will from a double bond. In this case the carbons taking part in the double bond are each joined to only one hydrogen.

Fatty acids that have carbon-carbon double bonds are known as unsaturated, because the double bond can be ‘broken’ and an additional bond with hydrogen can be made. Double bonds are stronger than single bonds and they give the fatty acid chain a ‘kink’. These kinks mean that the molecules cannot pack together tightly, and the lipids are more fluid. This is why unsaturated fats tend to be liquid at room temperature, while saturated fats tend to be solid. Fatty acid chains with many double bonds are called poly-unsaturated fatty acids.

Image credit: Siyavula

Cholesterol

Cholesterol is an organic chemical substance known as a sterol. You are not required to understand its molecular makeup or its structural composition. It is an important component in cell membranes. The major dietary sources of cholesterol include cheese, egg, pork, poultry, fish and shrimp. Cholesterol is carried through the body by proteins in the blood known as lipoproteins. A lipoprotein is any combination of lipid and protein.

Cholesterol is carried in the blood through the body by high density lipoproteinlow density lipoprotein and through triglycerides.

  1. Low density lipoprotein (LDL): Low density lipoprotein transports cholesterol around the body. It has a higher proportion of cholesterol relative to protein. It is often known as “bad” cholesterol because higher levels of LDL are associated with heart disease.
  2. High density lipoprotein (HDL): High density lipoprotein is the smallest of the lipoproteins. It has a high proportion of protein relative to cholesterol and is therefore often known as the “good” cholesterol. HDL transports cholesterol away from cells and to the liver where it is broken down or removed from the body as waste. 

You will learn more about how cholesterol can clog arteries and lead to heart disease in a later tutorial lesson on transport systems in animals.

High levels of LDL can cause heart disease. Cholesterol builds up in blood vessels that carry blood from the heart to the tissues and organs of the body, called arteries. This leads to a hardening and narrowing of these vessels, which interferes with the transport of blood, and can potentially lead to a heart attack. The biggest contributor to the amount of cholesterol in your blood is the type of fats you eat. Saturated fats are less healthy than unsaturated fats as they increase the amount of LDL cholesterol in your blood.

Test for lipids

The test for lipids relies on the fact that lipids leave a translucent `grease spot’ on brown paper bags, while non-lipid substances do not.

Investigation: Test for the Presence of Lipids

Aim

To test for the presence of lipids

Apparatus

  • piece of paper or “fish-and-chips” paper bag
  • food item e.g fries, piece of cooked meat, etc
  • 10 ml of cooking oil (positive control)
  • 10 ml water (negative control)

Method

  1. Positive control: add cooking oil to brown paper bag until it is soaked up. The part of the paper that soaks up oil should be translucent compared to the part that does not.
  2. Negative control: wet the paper with water. The paper may become wet and soggy, but should not become translucent.
  3. Experimental samples: stain the brown paper bag with the food item to be tested and hold it up to the light. If it is translucent, similar to the positive control, the food item contains lipid.

Observations

Record your observations, noting any key differences between the controls and the experimental sample.

 

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