Video Transcript
In this video, we will learn about
the composition, structure, and function of different types of lipids. We will examine triglycerides and
phospholipids and how their structure is related to their function. And we’ll learn how lipids are
classified based on their structures.
Lipids are molecules generally
composed of carbon, hydrogen, and few oxygen atoms. They’re characterized by their
possession of hydrocarbon chains and also by the fact that lipids are insoluble in
water but soluble in organic solvents such as alcohols. In organisms, lipids are a dietary
source of energy and a method of long-term energy storage. They provide waterproof coverings,
and lipids make up the membranes of cells and most organelles. They serve as chemical messengers,
and they provide thermal insulation for the bodies and organs of animals. And like everything else in
biology, the structure of lipids is what allows them to carry out their specific
functions.
In order to understand the basic
structure of lipids, we’ll start by taking a look at a molecule called a fatty
acid. A fatty acid is composed of what’s
called a carboxyl group bonded to a long chain of hydrogen and carbon atoms. Each carbon atom is able to form a
bond with four other atoms. In the hydrocarbon chain, each
carbon atom bonds with two other carbon atoms, leaving two spaces free to bond with
hydrogen atoms. Carbon–hydrogen bonds store lots of
energy that organisms can transfer to other processes later. When every carbon atom is bonded
with two hydrogen atoms, the chain consists of only single bonds. This is called saturated because
the molecule has as many hydrogen atoms as it can possibly hold.
Sometimes, two carbon atoms will
form a double bond instead. This means that each of these
carbon atoms can only bond to one hydrogen atom instead of two. Since they’re less than the maximum
number of hydrogen atoms in this molecule, it’s called unsaturated. You can see that saturated fatty
acid molecules have a straight shape, and this shape allows them to naturally pack
closer together. For this reason, saturated fatty
acids tend to be solid at room temperature. In contrast, unsaturated fatty
acids generally have a bent shape. This causes them to naturally
possess more space between their molecules. And for this reason, unsaturated
fatty acids tend to be liquids at room temperature.
This is the difference between a
food like butter, which contains over 60 percent saturated fat and is solid at room
temperature, and a food like olive oil, which contains over 80 percent unsaturated
fat and is liquid at room temperature. These long chains of carbon and
hydrogen atoms bonded together are also what give lipids their nonpolar qualities
and make them insoluble in water.
Next, let’s take a look at two more
complex lipids, phospholipids and triglycerides. Triglycerides are composed of three
fatty acids and a glycerol molecule. The fatty acids bond with the
glycerol in a chemical process called a condensation reaction. A condensation reaction occurs when
the hydroxyl groups of two different molecules join. The atoms rearrange so that, at the
end, both molecules are sharing one oxygen atom. The two hydrogen and one oxygen
atoms that are released in the process bond together to form one molecule of
water. Each of the three fatty acids bonds
with one of the hydroxyl groups on the glycerol molecule. The bond that’s formed as a result
of these condensation reactions is called an ester bond.
Triglycerides can eventually be
broken apart again. This process, which is the opposite
of a condensation reaction, is called a hydrolysis reaction. In a hydrolysis reaction, one
molecule of water is consumed, and instead of sharing an atom of oxygen, the
molecules break apart and form their own hydroxyl groups. Once the triglyceride has been
separated into its components, the glycerol and fatty acids can be used as
substrates for cellular respiration, just like glucose. The energy stored in the many
high-energy carbon–hydrogen bonds is transferred and used to make ATP. This is what makes triglycerides
extremely efficient molecules for energy storage.
Phospholipids have a structure
similar to that of triglycerides. The main difference is that the
glycerol alcohol is bonded to a phosphate group, which means that there’s only space
for two fatty acid chains instead of three. This structural difference gives
phospholipids some important properties. We’re already aware that
hydrocarbon chains are nonpolar. Another way that we describe this
part of the phospholipid is hydrophobic. Hydro- is a word part that refers
to water and phobic means fearing. In contrast, the part of the
phospholipid that includes the phosphate group and the glycerol molecule is polar in
nature. Another way to describe it is
hydrophilic or water loving.
Here, we have a simplified diagram
of a phospholipid that you’re likely familiar with. It shows the hydrophilic head
region and the hydrophobic tail. When phospholipids are placed in an
aqueous solution, they have an interesting reaction. The hydrophobic tails orient
themselves towards each other to avoid interacting with the water, while the
hydrophilic heads attracted to the water and to each other form an outer layer. This spontaneous arrangement is
known as a phospholipid bilayer and it’s why phospholipids make up the membranes of
cells and most organelles. The specific structures of
phospholipids and triglycerides are what make them specially adapted to their
functions.
Next, let’s take a look at how
lipids are classified. Lipids can be divided into three
main categories: simple lipids, compound lipids, and derived lipids. Simple lipids yield fatty acids and
alcohols when hydrolyzed or broken apart. One example of a simple lipid is a
triglyceride, which we learned about a little bit earlier. Another example of a simple lipid
is wax. Waxes are made of a fatty acid
bonded to a long chain alcohol. They form the waterproof outer
layer known as the cuticle on most leaves, and they’re also made by some animals,
such as honey bees. Compound lipids yield fatty acids,
alcohols, and at least one other molecule when hydrolyzed.
When you break apart a phospholipid
by hydrolysis, you’ll get a phosphate group, a glycerol molecule, and fatty
acids. Derived lipids are compounds with
the properties of lipids that are derived from the hydrolysis products of simple and
compound lipids. One example that we’re already
familiar with is a fatty acid. Derived lipids also include a group
of compounds known as steroids, an example of which is cholesterol. Cholesterol functions to provide
strength and structure to the cell membranes of animal cells. Now that we’ve learned about the
structure of lipids, their functions, and how they’re classified, let’s try a
practice question.
Examples of biological molecules
are provided: triglycerides, nucleic acids, phospholipids, cholesterol,
cellulose.
Which molecules from the examples
given are classified as types of lipids?
This question provides us with a
list of biological molecules. Then it asks us to determine which
of these molecules will be considered types of lipids. So first, we’ll recall some facts
about lipids. And then we’ll look at the
structures of the molecules in the list provided, and we’ll use what we know to
determine which of these are lipids. Let’s recall that lipids are
molecules that are insoluble in water. They’re generally composed of
carbon atoms, hydrogen atoms, and few oxygen atoms. And they’re characterized by their
possession of what’s known as a hydrocarbon chain. Here, I’ve drawn simplified
diagrams of each of the biological molecules listed. Can you identify them based on
their structure?
If you said triglyceride, nucleic
acid, phospholipids, cholesterol, and cellulose, then you know your biological
molecules. In order to answer our question,
let’s take note of two things here. First, we’re looking for lipids,
which is a type of biological macromolecule. Well, nucleic acids are their own
category of macromolecule. And cellulose is an example of what
we call a carbohydrate. Our four types of biological
macromolecules are proteins, lipids, nucleic acids, and carbohydrates.
Since nucleic acids and cellulose
belong to other categories, we know that they’re not lipids. The second thing to note here is
that lipids are characterized by the possession of a hydrocarbon chain. We can see hydrocarbon chains in
the triglyceride molecule, the phospholipid molecule, and the cholesterol
molecule.
Nucleic acids are composed of a
phosphate group, a sugar, and a nitrogen-containing base. They do not possess hydrocarbon
chains. And cellulose is made of a
repeating pattern of sugars, also without hydrocarbon chains. Since triglycerides, phospholipids,
and cholesterol possess hydrocarbon chains, we know that they’re classified as types
of lipids.
Let’s wrap up our lesson by
reviewing what we’ve learned. In this video, we learned about the
properties and the functions of lipids. We learned about saturated and
unsaturated fatty acids and how their structures give them specific properties. We learned about triglycerides and
phospholipids and how their structures are directly related to their function. And we learned how lipids are
classified as well as the role that the derived lipid, cholesterol, plays in the
animal cell.
