Lipids are a group of biological molecules that include fats, oils and some steroids. They are built from fatty acids bonded to a wide range of other compounds.
Their importance in the biological world is immense. They fill a number of important roles in the cells of all of Earth’s organisms. Of the four molecules of life, lipids arguably have the greatest variation in their basic structure and are far more difficult to define than proteins, carbohydrate and nucleic acids.
Almost all lipids are insoluble in water. They are known as hydrophobic molecules because they are repelled by water.
Why are lipids important?
Lipids are essential for all life on Earth. They play many important roles in maintaining the health of an organism.
Arguably the most important function lipids perform is as the building blocks of cellular membranes. Other functions include energy storage, insulation, cellular communication and protection.
Cell membranes
Cell membranes are made from a double layer of lipids known as ‘phospholipids’. The plasma membrane around a cell provides a barrier that separates the contents of a cell from the external world. It is responsible for controlling what substances enter and leave a cell.
Organelles are specialized, membrane-bound structures found in the cells of animals, plants, fungi and protists. They perform particular tasks such as photosynthesis and respiration.
Organelles are the key feature of eukaryotic cells that have made plants, animals and fungi so efficient at using resources. Organelles would not exist if it weren’t for lipid-based membranes.
Energy storage
Lipids play an important role in storing energy. If an animal eats an excessive amount of energy it is able to store the energy for later use in fat molecules.
Fat molecules can store a very high amount of energy for their size which is important for animals because of our mobile lifestyles. Plants, on the other hand, store energy less efficiently in carbohydrates because they don’t need to move.
Insulation
Fats are important for heat insulation. Marine mammals, such as seals, dolphins and whales, offer a perfect example of how fats can provide insulation. In order to prevent themselves from freezing to death in water, many marine mammals have a thick layer of fat called ‘blubber’ to keep them warm.
Blubber covers their entire bodies apart from their fins and head and prevents water from cooling their internal body temperatures. The layer of blubber also makes their bodies extremely streamlined for moving through water.
Fats also provide a protective layer around important organs in animals such as our liver and kidneys.
Cell-cell communication
Steroids are a group of lipids involved in cellular communication. A number of steroids are hormones and are important for many processes in the body including growth, sexual development, regulating metabolism and immune defence.
Structure of lipids
There is a huge variety of different lipids and the chemical structure varies between each of them. Because of this, it is difficult to outline a general structure of a lipid. All lipids do however contain at least one hydrocarbon chain (i.e. a chain of carbon and hydrogen atoms) with an acidic end.
Hydrocarbon chains are insoluble in water. The vast majority of lipids have long hydrocarbon chains which are why they are hydrophobic molecules i.e. they don’t dissolve in water.
Fatty acids
Fatty acids are a defining feature of lipids. A fatty acid is a long hydrocarbon (alkyl) chain with an acidic head. The acidic head is more correctly known as a ‘carboxylic acid’ and has the chemical structure -COOH, the same structure that makes vinegar acidic.
A fatty acid can be saturated or unsaturated. If two carbon atoms of the hydrocarbon chain share a double bond then a fatty acid is known as ‘unsaturated’.
If there are no double bonds along the alkyl chain, the fatty acid is saturated. This is because all of the carbon atoms have bonded to as many hydrogen atoms as possible. The alkyl chain is therefore saturated in hydrogen. The presence of a double bond makes a fatty acid unsaturated because it is possible for the alkyl chain to be bonded to more hydrogen atoms.
Fats and Oils
Fats are a common and well-known form of lipids. They are made by bonding fatty acids to an alcohol.
The most common fat is triacylglycerol. Triacylglycerol is a fat made from three fatty acids bonded to an alcohol called ‘glycerol’. Glycerol is a three-carbon alcohol and each of the carbons bond to one fatty acid.
The structure of the fatty acids of a fat determines if a fat is saturated or unsaturated. Double bonds in one or more alkyl chains of the fatty acids create an unsaturated fat. A fat molecule with no double bonds in any of its alkyl chains is known as a saturated fat.
A double bond creates a bend in an alkyl chain. This reduces how tightly fat molecules can be packed together. Loosely packed fats have lower melting points which is why unsaturated fats, such as vegetable oils, are commonly liquid at room temperature. Saturated fats, on the other hand, have higher melting points and are more likely to be found as solids at room temperature.
The main function of fat is to store energy. They are most common in animals because they contain a very large amount of energy for their weight.
A fat molecule will hold far more energy than a carbohydrate molecule of the same weight. For mobile animals carrying extra weight is not ideal so storing energy in lightweight molecules is beneficial. Fats are stored in tissue known as ‘adipose tissue’ and in cells known as ‘adipose cells’.
Phospholipids
Phospholipids are less well-known than fats and oils but are essential to life on Earth. They are the molecules used to build the membranes found around and inside cells. Without a plasma membrane, a cell would not be able to survive.
A phospholipid is similar in structure to a triacylglycerol. It contains TWO fatty acids plus a phosphate group bonded to the three carbons of a glycerol molecule. The sole difference between a phospholipid and a fat is the replacement of one fatty acid with a phosphate group.
A phosphate group has a negative charge so many other molecules can attach themselves to the phosphate group. This makes a large variety of different possible phospholipids.
The combination of fatty acids and a phosphate group make phospholipids ideal for making cellular membranes. The phosphate group is soluble in water and is therefore attracted to water (hydrophilic). The fatty acids are insoluble in water and are hydrophobic.
A phospholipid membrane contains two layers of phospholipids. In both layers, the hydrophobic fatty acids point inwards towards each other. The phosphate groups face outwards towards the water-based environments of the cell and the surrounding world.
This phospholipid bilayer creates a barrier for substances moving in and out of a cell. If a substance wants to pass through the membrane it must be able to move through the hydrophilic barrier of phosphate groups and the hydrophobic barrier of fatty acids. Many substances can only move in and out of cells via membrane channels created by proteins.
Steroids
Steroids are a particular type of lipid with a unique chemical structure. They are characterized by having carbon atoms arranged into four adjacent rings – three rings made from 6 carbon atoms and the final ring made from 5 carbon atoms.
Steroids are produced naturally in the body. Examples include cholesterol and the sex hormones testosterone, progesterone and estrogen. Cholesterol is the most abundant steroid in the body and is produced in the brain, blood and nerve tissue.
Summary
- Lipids are biological molecules such as fats, oils, phospholipids and steroids
- They are important for cell membranes, energy storage, insulation, cell-cell communication
- Lipids have a wide variety of structures but all include a hydrocarbon chain which is almost always in the form of a fatty acid.
- Fats are lipids made by bonding fatty acids with an alcohol – the most common fat is triacylglycerol which contains three fatty acids bonded to a 3 carbon alcohol called glycerol.
- Phospholipids are the compounds that makeup cell membranes – they have water-soluble and water-insoluble ends which form a useful barrier around cells.
- Steroids are a form of lipid with carbon atoms arranged into four rings. They are produced naturally in the body and include hormones such as cholesterol, testosterone and estrogen.
Last edited: 31 August 2020
