L7 - Phospholipids, Steroids and Health

objectives

Phospholipids

• Phospholipids are similar to triglycerides, except that the glycerol molecule is bonded to two fatty acids and one phosphate group (PO43-), instead of three fatty acids that make up a triglyceride.
• The substitution of a phosphate group for one of the fatty acids results in a molecule that contains both ionic and nonpolar functional groups.

• The presence of the phosphate group results in water solubility, due to the ion-dipole attraction between the negatively charged phosphate group and the polar water molecules.
• As a result, the phospholipid consists of two portions - one that is water soluble (also known as hydrophilic, or water-loving) and one that is not (also known as hydrophobic, or water-fearing).
• With two distinct portions of the molecule, phospholipids tend to align themselves with the phosphate groups together and the triglyceride tails together.
• In aqueous environments, the phosphate 'heads' will form ion-dipole attractions with water molecules, while the fatty acid 'tails' will avoid water.
• When many phospholipids are together, they result in the formation of distinct structures known as a micelle or a phospholipidbilayer.

• The formation of micelles and phospholipid bilayers allow phospholipids to remain soluble in water.
• Micelles can form around other hydrophobic (nonpolar) molecules such as vitamins A, D, E and K, allowing them to be dissolved and transported in the bloodstream.
• The phospholipid bilayer specifically allows for the formation of structural components for the cell, the most important being the cell membrane.
• The cell membrane is a protective layer that surrounds and protects the cell, consisting of a phospholipid bilayer and other small molecules, such as proteins and cholesterol.
• Since the medium on both the inside and outside of the cell is aqueous, the hydrophilic phosphate group 'heads' form the external side of both the inside and outside of the membrane, while the hydrophobic fatty acid 'tails' are on the inner portion of the membrane, away from the aqueous medium.
• The cell membrane is selectively permeable, which means that some substances can cross the phospholipid bilayer to enter or exit the cell, while other substances cannot.

Steroids

• Steroids are chemical substances that are naturally produced in the body.
• They act as structural components in cell membranes and also act as chemical signals in an organism in the form of specialised molecules known as hormones.
• For example, testosterone is a hormone that is responsible for the growth and development of male characteristics during embryo development and puberty.
• Females naturally have low levels of testosterone and higher levels of the 'female' hormones oestrogen and progesterone, which are responsible for female traits.

• The steroidal backbone consists of 17 carbons atoms in four fused rings - three cyclohexane rings and one cyclopentane ring.
• The general structure of a steroid is shown in Figure 5a.
• The substitution of various functional groups gives rise to specific steroid molecules, such as testosterone, shown in Figure 5b.

• The steroidal backbone itself is nonpolar.
• The presence of some polar functional groups can help to increase polarity and water solubility, but many steroid molecules are not very water soluble.
• As a result, to travel through the bloodstream, which is an aqueous environment, and to cross through the cell membrane to perform functions often requires the help of carrier proteins that bind to the steroid to better increase its solubility in water.
• Other steroid molecules form structural components of the cell membrane.
• Cholesterol is a steroid that is a key component of cell membranes, adding rigidity to the otherwise flexible phospholipid bilayer.
• The four fused rings result in a structure that does not have very much flexibility and is very nonpolar.
• The hydroxyl functional group on the cholesterol molecule is attracted to the charged phosphate 'head', while the rest of the structure, being nonpolar, will be attracted to the nonpolar fatty acid tails.

• The composition of the cell membrane is integral to its function.
• The ratio of phospholipids to cholesterol can vary from one type of cell to another, depending on the function of the cell and its need to maintain rigidity or to be flexible.
• The presence of other molecules, such as proteins, are necessary to allow the passage of nutrients and waste in and out of the cell.

Uses and Negatives of Steroids

• Certain medical conditions can result in lower levels of certain steroids, requiring the use of prescription steroids for the body to have more normal levels and minimise the medical issue.
• Some people, however, choose to take steroids for non-medical reasons, such as enhanced growth or athletic performance.
• Excess intake of steroids can be very dangerous and can often have undesirable side-effects.
• The presence of higher levels of steroids in the body results in higher levels of cholesterol in the bloodstream, increasing the risk of heart disease and stroke.
• A common steroid used to enhance athletic performance is testosterone, along with other drugs that mimic the action of testosterone or drugs that increase the body's own production of testosterone.
• Testosterone is the 'male' hormone responsible for the growth and development of male characteristics during embryo development and puberty.
• Females naturally have low levels of testosterone and higher levels of the 'female' hormones oestrogen and progesterone, which are responsible for female traits.
• Increased levels of testosterone can improve strength and endurance for athletic performance, making it a suitable drug for this medicinal benefit, but steroid use may also have negative side effects on the body, such as loss of hair and decreased fertility.
• Often, athletes are taking these drugs in secret without medical supervision or sufficient counselling, making the process dangerous to the health and wellbeing of the athlete.

Health & Energy

• There is often a negative connotation to the idea of 'fats' and 'oils', but they are a necessary part of the diet.
• We must consume fats and oils for our bodies to obtain energy, and also recycle these nutrients into lipids, which perform functions and form structural components within the body.
• Without the consumption of fats and oils from our diet, our bodies would not be able to produce steroids for growth and development or build new cell membranes when older cells need to be replaced.
• Lipids also provide more energy per unit mass than other sources, such as carbohydrates and proteins, making them a more efficient fuel source.
• Over-consumption of fats and oils, however, can lead to immediate and long-term health effects, so care must be taken to ensure that healthy levels of fats and oils are consumed.

Lipids for Energy

• Most of an organism's energy needs are provided by carbohydrates, also known as sugars, that are produced by plants during photosynthesis.
• The cell performs cellular respiration to break down the carbohydrates, releasing energy that is stored in the bonds of the carbohydrate molecules.
• Carbohydrates are more water soluble than lipids, ensuring that they can easily reach all cells by easily being transported through the aqueous environment of the bloodstream.

• Lipids can also be used as an energy source for organisms, reacting in a combustion reaction that converts the three free fatty acids from triglycerides to carbon dioxide and water, releasing energy.
• The digestion and metabolism of fats is a more complex process than for carbohydrates, which provide a quick and immediate source of energy.
• Since lipids are more reduced than carbohydrates, meaning they have more hydrogen and less oxygen, they produce more energy when oxidised during metabolism as more bonds must be made, which is an exothermic process.
• Lipids are not very water soluble, meaning they cannot easily be transported to be broken down by all cells.
• For a fatty acid containing the same number of carbons as glucose, the combustion equation is shown below.
• Note that more oxygen is required to produce the same products as in the equation above, yielding nearly double the energy per mole.

• The amount of energy obtained from lipids is nearly double the amount of energy obtained from carbohydrates per unit mass.
• This is because lipids require much more oxygen to oxidise than carbohydrates require.
• For carbohydrates, the ratio of carbon:oxygen is 1:1, while lipids have a carbon:oxygen ratio of far less, depending on the length of the fatty acid because the only oxygen atoms in a lipid molecule are in the carboxyl functional group.
• As a result, to oxidise a fatty acid requires a lot more oxygen to make the final product, carbon dioxide, which has a carbon:oxygen ratio of 1:2.
• This means that more energy is stored in a lipid molecule than in a carbohydrate molecule per unit mass.

Lipids & Health

• The consumption of lipids is essential for life, but the source and type of lipids consumed plays a large role in the overall health of an organism.
• An excess intake of lipids can result in a higher risk for heart disease and stroke, when lipids form blockages in the bloodstream leading to the heart or brain.
• The result of a blockage in the bloodstream leading to the heart is a heart attack, and a blockage to the brain results in a stroke.

Triglycerides

• The consumption of certain types of fatty acids can result in long-term health effects.
• When fats are not needed for energy or structural components, they are stored for later use.
• The storage of fats can result in dangerous accumulations in cells or tissues that can interfere with metabolic pathways.
• Recall that saturated fatty acids are more linear than unsaturated fatty acids and thus have greater surface area for London dispersion forces between molecules.
• As a result, saturated fats tend to be solid at room temperature (fats), while unsaturated fats tend to be liquid (oils).
• In the body, excess intake of saturated fats causes changes to the levels of cholesterol in the blood, increasing the risk of heart disease.

• For unsaturated fatty acids, most of the carbon-carbon double bonds naturally occur in the cisformation, while carbon-carbon double bonds that are formed in the synthetic production of unsaturated fats are produced in the trans formation.
• Trans fats are more linear than cis fats; therefore, trans fats resemble saturated fats in structure and function.
• Consuming high levels of trans fats also results in higher blood cholesterol levels, increasing the risk of heart disease.

Cholesterol

• Cholesterol is a key component of cell membranes and is also used as a precursor for synthesising other steroid molecules.
• The consumption of cholesterol is unavoidable, as it is present in the cell walls of all plants and animals; however, there are certain types of cholesterol that are more healthy than others.
• Since cholesterol is nonpolar, it cannot travel through the aqueous bloodstream, but must be carried by proteins, known as lipoproteins, to make it more soluble in water.
• There are two types of lipoproteins – high-density and low-density – which perform different roles in the transport of cholesterol in the bloodstream.
• High-density lipoproteins (HDL) travel through the bloodstream, absorbing excess cholesterol and transporting it to the liver for removal from the body. HDL is often referred to as 'good' cholesterol and is composed of more protein and less triglycerides.
• Low-density lipoproteins (LDL) travel through the bloodstream, depositing cholesterol. LDL is often referred to as 'bad' cholesterol and is composed of less protein and more triglycerides.
• When excess triglycerides are consumed in the diet, more LDL is produced, which results in a greater deposit of cholesterol in the walls of the blood vessels.

• Cholesterol deposited by LDL causes a buildup against the walls of the vessels in the bloodstream, known as plaque.
• Over time, the plaque can form a blockage in the blood flow, resulting in a heart attack or stroke.

Rancidity

• The storage of fats and oils for later consumption must be done carefully to avoid spoilage.
• If not consumed right away, fats and oils can react with oxygen in the air, becoming rancid.
• Rancid fats have distinct, unpleasant odours and flavours that indicate that the food substance is 'off' and should not be eaten.

Hydrolytic Rancidity

• Hydrolytic rancidity is the process of hydrolysis, turning a triglyceride back into glycerol and three fatty acid molecules.
• One of the fatty acids often produced is butanoic acid, also known as butyric acid, CH3CH2CH2COOH, which has an unpleasant odour, making a rancid fat have a characteristic smell.
• Certain enzymes, increased temperature and the presence of water can increase the rate of hydrolytic rancidity, so the use of refrigeration greatly reduces the rate of oxidation.
• Dairy products, such as milk and butter, are especially prone to hydrolytic rancidity because they contain high levels of water.

Oxidative Rancidity

• Oxidative rancidity occurs when oxygen reacts with the carbon-carbon double bonds of unsaturated fats in a free-radical mechanism, catalysed by light.
• In this process, a variety of compounds are produced, including ketones and aldehydes, which have an unpleasant aroma.
• Oxidative rancidity occurs more frequently when the substance is exposed to light and greater exposure to oxygen, so the use of food storage measures that minimise contact with light and the air greatly reduces the rate of oxidation.
• Fish, with high levels of unsaturated fats, are especially prone to oxidative rancidity.