Lipids Generally, what are they?
Lipids include fats, oils, steroids and the main component of membranes called phospholipids. Lipids are a diverse group of molecules that include an even wider variety molecues than listed here. Generally it includes any fatty acid derived, non-polar molecule. To keep it simple, we will focus on the three main types of lipids.
Molecular Structure:
H H H3C
C
H
N+ CH3
H
C
H
H
C
H
O OH
H
H
H
C
C
C
O
O
H
O O
C
O
H
C
H
H
C
H
H
H
C
H
H
C
H
H
H
C
H
H
C
H
H
C
C
H
H
C
H
H
C
H
C
H
H
C
H
H
C
H
H
C
H
C
C
H
H
C
H
H
C
H
C
H
C
H
H
C
H
C
H
H
H H
H
C
H
H
C
H
H
C
H
H
H
C
H
H
C
H
H
C
H
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
C
H
H
C
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H
H
C
H
H
C
H
H
O
H
C
H
H
C
H
C
H H H
H
H
O
C
C
C
O O
C
H
H
C
H
H
C
H
H
H
C
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H
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C
H
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C
H
C
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H
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H
C
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C
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C
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C
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H
C
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C
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H
C
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C
H
H
C
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H
H
C
H
H
C
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C
H
H
C
H
H
C
H
C
H
H
C
H
H
Triglyceride (fats and oils)
C
O
C
H
C
H
H
O H
H
C H
H
P
H O
C
C
H
Main Function in Cells:
H
H3C
CH2 CH
CH3
CH2
CH2
HC
CH3 CH3
CH3
HO
H
Phospholipid (membrane)
Steroid (polysaccharide)
Phospholipids are perhaps the most important lipids because they form membranes. Nearly the entire structure of a cell is made from lipid membranes. A membrane is essentially a bubble (in fact, soap is a lipid). Organelles can be thought of as bubbles within bubbles.
Triglycerides (fats) are used for long-term storage of energy. Steroids are mostly used as hormones. These are chemical signals that help give cells instructions for what to do and what products to make. Cholesterol, which is also a steroid, is part of cell membranes. It is used to make them less rigid and more fluid.
Nutritional Information:
When we think about eating lipids we think about fatty, creamy, oily, or greasy food. Meat, nuts and milk products are rich in fats and most recipes call for at least some added fat in the form of cooking oil, or butter. What might surprise you about lipids is that eating fatty food does not necessarily make you fat. In fact, there is a growing body of evidence that there is little connection between your dietary fat intake and your body fat accumulation. Even though fat molecules contain more than double the amount of available energy when compared to the same amount of sugar, our bodies do not use all of the fat you have eaten for energy. They can be used to make membranes, they are important in brain and nerve functioning, they are a major component of bone marrow and are essential raw materials to make a host of other molecules our cells need. More importantly, our bodies do not respond the same way hormonally to fat molecules as they do to sugar. This means that excess fat in the blood does not directly lead to body fat deposition like sugars do. Fats and oils are needed for energy and good health. Since fats and oils are nonpolar, non-polar vitamins (like vitamins A, D, E and K) dissolve in them. Part of the reason that lipids are required in your diet is that they help you absorb these vitamins. The average North American diet likely contains far more lipids than are necessary for our health. Along with eating large amounts of meat and dairy, deep fried foods like French fries, potato chips, and chicken strips are cooked by soaking them in hot oil. Greasy foods are often thought of as comfort foods and have become a regular part of our diets. This is problematic because high levels of fat and cholesterol intake have been linked particularly strongly to heart disease. The processed food industry has found a way of turning vegetable fats (unsaturated fats) into molecules that taste and behave more like animal fats (saturated fats). These new kinds of fats are called trans-fats and new research is beginning to show that these are even more strongly associated with cardiovascular diseases than naturally occurring fats.
Dietary Sources of Lipids
Meat, seeds, nuts, dairy and small amounts of cooking oil are all good sources of dietary fat. When eating fats remember to keep the portions small.
meat
seeds and nuts
dairy
Restaurants, particularly fast food restaurants, often cook with a lot more fat and oil than you would use at home. There are many so-called comfort foods today that are very high in fat. Even a salad or sandwich can turn into a high fat meal if a lot of sauce or dressing is used. Aim to eat smaller portions and more home cooked food made from a variety of fresh ingredients more often than eating out.
French fries
potato chips
cooking oil
other deep fried foods
Hamburgers
ice cream
cheesy pizza
cookies / baked goods
The Biochemistry of Lipids By the end of this section you should be able to: Recognize the chemical structure of the three main types of lipids. Describe how dehydration reactions are used to make triglycerides and phospholipids Explain why triglycerides are used for long-term energy storage Explain how the polarity / hydrophobicity of phospholipids makes them form membranes in water Explain the role of cholesterol and steroid hormones in the body
Subunits, But Not Monomers Lipids are the only macromolecules that are not true polymers. Their subunits cannot be linked over and over again to form increasingly larger and more complex molecules; however, even though they are not polymers, they are still made subunits. The structure and chemistry of their subunits varies which gives variety to the structure and function of fully assembled lipids. The subunits used to make most lipids are glycerol and fatty acids. glycerol
fatty acids (these are hydrocarbon chains with a carboxyl group on one end)
H H
C
OH
H
C
OH
H
C
OH
Saturated fatty acid (contain only single bonds) O HO
C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Unsaturated fatty acid (contain at least one double bond) HO
O
H
C
C H
H C
H C
H
H
H H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
C
H
H
H H
H
H
H
H
H
H
H
H
Triglycerides (fats and oils) Triglycerides are large molecules that are almost entirely non-polar. Very large, saturated triglycerides form solid, or creamy substances that we call fats. Smaller, and/or unsaturated triglycerides form liquids that we call oils. The long hydrocarbon chains of their “tails” can be broken down to produce cellular energy much like carbohydrates. Since triglycerides are non-polar, they are also hydrophobic. This means that they do not mix with water, so they cannot dissolve in our cells. Fats form a separate, solid layer in our bodies that is stored without using water. Without water, this makes fat a very lightweight, high energy molecule that is ideal for storing extra energy for later use. Triglycerides are also used by our bodies as insulation for warmth, and the oil on our skin helps lock in moisture in the layers beneath.
Triglycerides are formed by combining glycerol with three fatty acids using dehydration reactions. H H
H
H
C
C
C
OH
HO
+
OH
OH
HO
HO
H
O
H
H
H
H
H
H
H
H
H
H
H
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H
H
C
C
C
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H
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H H
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H
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H H
H
+
3 H2O
H
remove water
glycerol
fatty acids (x3)
triglyceride
water
The size and the level of saturation of the fatty acids in a triglyceride determine its texture properties. solid
creamy
Degree of Saturation of Fatty Acids
Size of Fatty Acids Long
liquid
Short
saturated (single bonds)
unsaturated monounsaturated polyunsaturated (one double bond)
(2+ double bonds)
Impact on the physical state of the triglyceride (more solid, or more liquid)
Phospholipids (form membranes) Phospholipids simply replace one of their fatty acid tails with a polar head. This one simple change radically changes the properties of the molecule. Now, instead of being entirely non-polar like a triglyceride, phospholipids are part polar and part non-polar. If you place phospholipids in water, the non-polar, hydrophobic tails will associate with each other through hydrophobic interactions. The polar, hydrophilic heads will face outward toward the water Chemical structure of a phospholipid H3C
CH3 H
H
N+
C
C
CH3 H
H
O O
P
Phospholipid Bilayer (membrane)
H O
C
H
C
O
OH
H
C H
O
O
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
O
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H C
H C
H
H C
H
H C
H
H C
H
H
H C
H
H
H C
H
H
Phospholipids bilayers give cells control of what goes in and out. They are semi-permeable which means that only some molecules can pass through them. Membranes in cells are extremely effective barriers to most molecules. water (H2O) oxygen (O2) carbon dioxide (CO2)
To give a cell very specific control of which molecules come in and out of their membranes, cells can embed proteins into the phospholipid bilayer. These act like highly specific pores for desired molecules.
most other molecules
carrier protein embedded in the membrane
Steroids (cholesterol and hormones) Steriods are derived from fatty acids. They are folded into structures made of three rings of carbon. Various side functional groups and side chains can be added to this same basic overall structure to create molecules that serve as chemical signals called hormones. Cholesterol H3C
CH2 CH2
CH
CH2
HC
CH3
CH3 CH3
CH3
HO
Testosterone
Estradiol (an estrogen) OH CH3
OH CH3
O
Cortisol O
CH3
O
There are many more steroids found in the blood which serve as hormones rather than as membrane additives. You may be familiar with a few of them like estrogen, testosterone and cortisol. Each different type of cell in your body responds to these chemical signals in their own way. Some do not need to respond at all.
HO
HO
Cholesterol is the most notable steroid that is not a hormone. It is mostly non-polar, however the hydroxyl group on one end is polar. This means that it can act similarly to a phospholipid. Cholesterol is part of the cell membrane, and because of its size and shape, it helps space out the phospholipids. Spacing out the phospholipids makes a membrane less rigid and more fluid.
CH3
C
CH2 OH OH
Anabolic steroids are synthetic hormones designs to increase muscle gain. Since hormones have different effects on different tissues, taking anabolic steroids has been linked to adverse health effects including heart disease, liver damage, infertility, and psychological disorders and a host of other side effects.
PRACTICE! 1) What are the subunits that make up triglycerides? Describe how they combine. __________________________________________________________________________________ __________________________________________________________________________________ 2) Identify each type of lipid being shown in the diagrams below. Explain how you were able to distinguish between them and indicate the main function of each. a)
b)
c)
Type of molecule Main function
Distinguishing features (how did you identify the molecule)
3) Explain how phospholipids form membranes. How does this relate to bubbles? __________________________________________________________________________________ __________________________________________________________________________________ __________________________________________________________________________________ __________________________________________________________________________________ 4) Explain why triglycerides are a better long term energy molecule than a carbohydrate. __________________________________________________________________________________ __________________________________________________________________________________ __________________________________________________________________________________ __________________________________________________________________________________ 5) Steroid hormones usually have many different types of target cells. This means that having them in your blood with cause several different cell types to change their behaviour. Why is this good to know for someone considering using anabolic steroids to enhance muscle gain? __________________________________________________________________________________ __________________________________________________________________________________
