Structure and Function of Lipids in Plants and Animals
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Structure and Function of Lipids in Plants and Animals
Lipids are the name given to a mixed group of organic compounds. The
elements Hydrogen, Oxygen and Carbon are always found in Lipids. The
difference between them and Carbohydrates is that they have a lower
proportion of Oxygen in the molecule. Lipids are insoluble in water,
but they are soluble in organic solvents, such as ether, acetone,
chloroform or benzene.
There are several parts in the group of Lipids:
o fats and oils,
o steroids (like cholesterol, oestrogen and testosterone), and some
other related compounds.
At room temperature, fats are solids and oils are liquids. Fats and
oils are typically found in animals and plants. Fats are of animal
origin, while oils tend to be found in plants. Fats and oils are made
up of a glycerol (a type of alcohol with a hydroxyl group on each of
its three carbons) and three fatty acids (an organic acid). Since
there are three fatty acids attached, these are known as triglycerides.
A condensation reaction produces a triglyceride and water molecule
when the glycerol and three fatty acids react together (see diagram
below). The fatty acid molecules are insoluble in water because of
their long hydrocarbon tails (the tails are referred to as hydrophobic
or “water-hating”). The head of the molecule is a carboxyl group which
is hydrophilic or “water-loving”.
Fatty acids are the main component of soap, where their tails are
soluble in oily dirt and their heads are soluble in water to emulsify
and wash away the oily dirt. However, when the head end is attached to
glycerol to form a fat, that whole molecule is hydrophobic.
The digestion of triglycerides is catalysed by lipase enzymes which
are found, for example, in the digestive system of animals and in
germinating seeds, in the lysosomes inside cells and in the secretions
of saprophytic bacteria and fungi. Triglycerides are created in fat
storage depots, in such places as, in liver cells, in the adipose
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Lipids Plants Animals Fatty Acids Function Organic Compounds Fats Molecule Digestive System
cells of mammals, which make up adipose connective tissue (found under
the skin, between the skeletal muscles, around the kidneys and in the
yellow bone marrow), in the fat body of insects and also in the food
reserves and endosperm of seeds. The energy contained in triglycerides
is higher than the energy in carbohydrates or proteins, nearly twice
as much per gram is released, but the process requires less oxygen. In
addition, more water is released on oxidation than from the oxidation
of carbohydrates. This is known as metabolic water and this is
important to animals living in dry climates, such as the kangaroo rat
and camel. These store fat specifically for its metabolic water
content. Triglycerides also form compacted food reserves, which do not
upset the osmotic balance of cells.
Triglycerides are good insulators as they are poor conductors of heat.
This means that any fat which is under the skin can help prevent heat
loss from animals. Animals in Cold Arctic or Antarctic seas, such as
whales, seals and walruses, need this fact to protect them from heat
loss by building up huge deposits of blubber. Another useful thing
about the blubber is that it is less dense than water; this gives the
animals buoyancy which allows them to stay near the surface. To
protect the mammalian kidneys from damaging knocks, a thick layer of
fat also surrounds the kidneys.
Fatty acids in oils are generally:
On the other hand, fatty acids in fats are generally:
The term “saturated” means that there are no double bonds present
between the carbon atoms in the molecule. If it does have 1 or more
double bonds between the carbon atoms then it is known as an
Saturated fatty acids form saturated fats, when they are combined with
glycerol. These fats are solid when found at normal atmospheric
temperatures, and they are more common in the fats of animals. In
combination with glycerol, unsaturated fatty acids form oils, which
are liquid at normal atmospheric temperatures. They tend to be more
characteristic of plants, particularly in oily seeds and fruits, such
as castor oil seeds, sunflower seeds and coconuts. Oils can be found
in some animals, though, for example the fish liver oils of cod and
halibut, and in red fish such as sardines, pilchards, mackerel,
salmon, tuna, eels and trout which are referred to as “oily fish”.
Olive oil is formed when glycerol combines with oleic acid (a type of
unsaturated acid). Linolenic and linoleic acids (more unsaturated
acids) combined with glycerol are found in linseed oil and in corn
oils. They are both examples of “essential fatty acids” which are
acids necessary for normal health, but in small quantities and these
cannot be made by the body.
Waxes are the esters of fatty acids with long chain alcohols such as
cetyl alcohol (CH16H33OH). They are rigid and form protective
water-resistant coverings on biological surfaces, for example, the
cuticle of arthropod exoskeletons, leaf surfaces, fruits, seeds,
mammalian skin, fur, feathers. Beeswax is used to construct honeycombs
of bees. Lanolin is a waxy substance that coats mammalian hair.
Phospholipids are made from glycerol, two fatty acids, and (in place
of the third fatty acid) a phosphate group with some other molecule
attached to its other end. The phosphate group end of each molecule is
hydrophilic because of the oxygen’s and all of their pairs of unshared
electrons; they ionise and become soluble in water. The hydrocarbon
tails, however, are hydrophobic, non-polar and therefore remain
insoluble in water (see diagram below). This means that phospholipids
are soluble in both water and oil. Our cell membranes are made mostly
of phospholipids arranged in a double layer (bilayer) with the tails
from both layers “inside” (facing toward each other) and the heads
facing “out” (toward the watery environment) on both surfaces. (See
The fact that the layer is dynamic (fluid), i.e. it can move sideways
and exchange places in their own row, means that the membrane can shut
itself if it is punctured. This also allows substances to be passed in
and out of the cell through either phagocytosis (also called cell
eating) or by the process of pinocytosis (known as cell drinking).
This allows the protein parts of the membrane to move about easily.
The fat-soluble substances associate with the fat-soluble portions of
phospholipids and they can cross the membrane, while the water-soluble
substances are restricted of transport by the hydrophobic tails. They
therefore have to be left to the protein carriers to transport them.
Cholesterol (a steroid) is also found in some cell membranes, between
the phospholipid molecules. When it is present, it reduces the
fluidity of the membrane. Another substance that is found in the
membrane is glycolipids, these are complexes of lipid and
polysaccharides. They are normally found at the outer surface of cell
membranes as the glycocalyx. Its involvement in membranes is in the
communications with other cells, particularly during growth and
development. Viruses and bacteria also use these sites for infection.
Lipids are very important to a good diet because of their many roles
within our body. Excess intake of lipids, however, can lead to
problems with our bodies. Any triglycerides which are additional are
stored in our bodies as fat in the adipose tissue and this can cause
people to become overweight and obese. This can lead to heart problems
as the strain of having to carry around extra weight. This is also not
helped by the extra pumping needed to make blood flow through all the
capillaries in the surplus adipose tissue. Excess triglycerides,
phospholipids, some lipoproteins and cholesterol may be deposited in
and the cells on the inside of blood vessels, forming plaques or
atheromas. This can inhibit blood flow, making the heart work much
harder and therefore causing more strain on it.
To get cholesterol into our bodies, we can take it in through our
diets or it can be made from saturated fats, (which are found in high
abundance in beef, pork, butter, whole milk, eggs, cheese, palm oil
and coconut oil). This is why the intake of saturated fats should be
prevented, especially in the bodies of people with high cholesterol
levels. Unsaturated fats are better to have in our diets, because they
help to lower our cholesterol levels in the blood.
Lipoproteins are the combination of proteins and water soluble lipids.
These act as a means of carrying lipids, including cholesterol, around
in our blood. There are two main categories of lipoproteins
distinguished by how compact/dense they are:
o The first type is Low Density Lipoproteins or LDLs. They are bad
because they transport cholesterol into the blood system and lay it
inside the cells and on the walls of arteries. This increases the
cholesterol levels in the blood and therefore increases the risk of
coronary heart disease.
o The second type is High Density Lipoproteins or HDLs. They are
better because they transport cholesterol out of the system into the
liver, where it can be excreted. Therefore, they decrease the
cholesterol levels and help the body stay healthier. A good source of
HDLs is Red (oily) fish.
By taking in unsaturated fatty acids known as omega-3 fatty acids, the
risk of getting heart disease is lowered, because the cholesterol
levels will decrease. Omega-3 fatty acids are found in oily (red) fish
and in shellfish, these acids should be included in a good healthy
diet, as will keep the body healthier.
Lipids have gotten a bad reputation over the past few years due to all the hype about fats being bad, but in reality, lipids are much more than just “fat.” They are, in fact, one of the building blocks of life. In this crash course review, we will go over everything you need to know about lipids to not only be prepared for the AP Biology exam but also to better understand what an important role lipids play in biology as a whole. We’ll start with going over what lipids are in general; then we will look at how the three main types of lipids differ in structure and function; and finally we’ll have some review questions and a quick recap. By the end of this crash course review, you should feel confident enough in your knowledge of what lipids are and why they are important to be able to answer whatever the AP Bio exam may throw at you.
What are Lipids?
Lipids, like carbohydrates and proteins, are a class of organic compounds. They are hydrocarbon-based macromolecules that are grouped together because of their hydrophobic qualities. This means that all lipids are insoluble in water, which you may have already noticed if you have ever tried to wash butter or oil off of your hands. There are three main families of lipids: fats, phospholipids, and steroids. Let’s look at each of these in a bit more detail.
Fats are energy storing macromolecules that are made up of two main components: a molecule of glycerol and three fatty acids. Because of this structure, they are sometimes referred to as triglycerides, with the ‘tri-‘ prefix meaning “three.” These fatty acids are long chains made up a hydrocarbon tail with a carboxyl group head.
The fatty acids are linked to the glycerol backbone through the process of dehydration synthesis, which you may remember reading about if you have already reviewed other macromolecules like carbohydrates. If not, here is a brief explanation:
Dehydration synthesis, sometimes known simply as condensation, is a process where monomers are bound together through the loss of a water molecule. A covalent bond is formed. The reverse process of dehydration synthesis is hydrolysis.
Now, back to fats. There are two main types of fatty acids: saturated and unsaturated. Saturated fats contain only single bonds between carbon atoms. All the carbons are bonded to hydrogens, and there are no carbon double bonds. Generally, saturated fats come from animals (but also some tropical oils like coconut and palm oil), and they are solid at room temperature. Consumption of saturated fats is linked to heart disease due to plaque deposits in the blood vessels. A good example of saturated fat is butter.
Unsaturated fatty acids have at least one double bond in their chains. This is formed by removing hydrogen atoms from the carbon skeleton, meaning that unsaturated fatty acids have fewer hydrogen atoms than saturated fatty acids (i.e., they are less saturated with hydrogen). Unsaturated fatty acids usually come from plants or fish and are liquid at room temperature. When fats are in liquid form, they are known as oils. Some good examples unsaturated fatty acids include vegetable oils like canola oil and olive oil as well as fish oil.
You may have also heard of a third type of fat called trans fats. Trans fats are sometimes known as ‘partially hydrogenated oils’ because they are created through an industrial process where hydrogen is added to vegetable oils to make them more solid. A good example of this is margarine, which is vegetable oil but can be bought in a solid form similar to butter. Consuming lots of trans fats increases your risk of heart disease, stroke, and type 2 diabetes.
So if fats increase the risks for all these bad things like heart disease and stroke, what are they good for? Fats are incredibly important for energy storage: 1g of lipid will release nine calories when burned, while in comparison 1g of carbohydrates only releases four calories. Although we are lucky enough to live in a time and place where our food sources are abundant, this was not always the case, and certainly still is not the case for most life on earth. Plants and animals need to be able to store energy as fat to be able to access it in times when food is scarce, and their fat reserves are all they can live off of.
Fats also serve the important function of protecting the organs and insulating the body. Whale blubber, for example, is entirely made of fat.
Phospholipids are very similar to fats in their structure, but instead of having three fatty acids bound to glycerol, they have two fatty acids and a phosphate group (PO4) bound to glycerol.
Phospholipids serve essential functions in the structure of cell membranes. While the fatty acid tails are hydrophobic, the PO4 head of a phospholipid is hydrophilic. This allows them to arrange themselves into a phospholipid bilayer, where the hydrophilic heads face outward, and the hydrophobic tails face inward to create a nonpolar zone that is essentially a barrier in water. This is how cell membranes are formed.
Steroids are a family of lipids that have quite a different structure compared to fats and phospholipids. Steroids have four fused hydrocarbon rings with various chemical attached to them that determine which specific steroid it is. One steroid you will need to know for AP Biology is cholesterol. Cholesterol is a component of the plasma membranes in animal cells, making it a vital part of cell structure – it helps keep membranes flexible and fluid. It is also the precursor to many other important steroids, such as the sex hormones testosterone, estradiol, and progesterone.
We’ve gone through the three main types of lipids, but there are a few more less-common types that are also worth mentioning. Waxes, for example, are also considered lipids due to their hydrophobic nature. Wax can coat the outside of some plants, as well as the feathers of birds and even the fur of some animals to keep them dry from rain and other water. Omega fatty acids like Omega-3 and Omega-6 are also lipids and are essential for normal growth and brain health. They also protect against cardiovascular disease.
Question 1. Why are sex hormones considered lipids?
A) They consist of fatty acids
B) They are essential to the structure of cell membranes
C) They store energy
D) They are hydrophobic
E) They are hydrophilic
Question 2. What happens when hydrogen is added to vegetable oils?
A) The hydrogenated vegetable oil will have fewer trans fats
B) The hydrogenated vegetable oil will be solid at room temperature
C) The hydrogenated vegetable oil will be less likely to cause heart disease
D) The hydrogenated vegetable oil will become a saturated fat
Question 3. True or False: Of the three main families of lipids, phospholipids are most important for energy storage.
Question 1. The correct choice is option D – they are hydrophobic. The criteria that all lipids must meet to be considered lipids is that they must be insoluble in water.
Question 2. The correct answer is option B – the oil will be solid at room temperature. The process of hydrogenation creates trans fats that cause many health problems.
Question 3. The correct answer is B – False. Of the three main classes, the group of lipids that is most important for energy storage is fats. Phospholipids serve a vital purpose in providing structure for cell membranes.
Crash Course Review Recap
• Lipids are hydrophobic organic compounds that are divided into three main categories: fats, phospholipids, and steroids.
• Fats are composed of a glycerol and three fatty acids and are used for energy storage.
• Saturated fats have single bonds, are solid at room temperature, and generally come from animal sources.
• Unsaturated fats have double bonds, are liquid at room temperature (oils), and generally come from plant sources.
• Trans fats are created industrially by adding hydrogen to vegetable oils.
• Phospholipids have a glycerol, two fatty acids, and a phosphate group; they are essential for the structure of cell membranes.
• Steroids are made of four fused hydrocarbon rings and are important for structural and endocrine functions; main example to know is cholesterol.
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