Lipids (B1.1.8, B1.1.9,B1.1.10,B1.1.11,B1.1.13)

• What are lipids?

  Lipids are a group of substances that dissolve in non-polar solvents such as ethanol, toluene and propanone. They are not very soluble in water-based solvents, meaning that they are hydrophobic. Lipids are hydrophobic because they consist primarily of long hydrocarbon chains or rings that are nonpolar. Nonpolar molecules do not form hydrogen bonds with water, a polar solvent, and are therefore insoluble in water. The lack of polarity in lipids means they cannot interact with water molecules. This property allows lipids to play key roles in forming cell membranes, storing energy, and acting as water-repellent barriers.
• What are fats?


  Fats are composed of glycerol and three fatty acid chains, forming a triglyceride. The fatty acids can be saturated (no double bonds) or unsaturated (contain one or more double bonds). They have a melting point between 20 and 37 degrees Celsius, so are solid at room and body temperature. They provide long term energy storage in animals and plants, insulation and cushioning for organs, and serve as a source of metabolic water when oxidised.
• What are oils?


  Oils are also triglycerides but primarily contain unsaturated fatty acids, making them liquid at room temperature, with a melting point below 20 degrees Celsius. They serve as an energy reserve in plants, and provide essential fatty acids for cellular function, and act as lubricants in some biological systems.
• What are waxes?


  Waxes are long-chain fatty acids bonded to long-chain alcohols or carbon rings, making them extremely hydrophobic. They have a melting point above 37 degrees Celsius, so liquefy at high temperatures. They act as waterproof barriers in plants and animals, and prevent water loss in plants.
• What are fatty acids?

  Fatty acids are actually hydrocarbons. They are an unbranched chain of covalently bonded carbon atoms, with hydrogen atoms covalently bonded. Some carbons may be double covalently bonded, and others may be single covalently bonded. Those that are linked via one covalent bond can also bond to two hydrogens, and those linked via two covalent bonds can only bond to one additional hydrogen. The acid part is the hydroxyl group (which is polar and hydrophilic) -COOH, which is one end of the chain, and the methyl group -CH3 is at the other end of the chain. Most fatty acid chains consist of 14-20 carbon atoms.
• What are saturated fatty acids?

  Fatty acids are actually hydrocarbons. They are an unbranched chain of covalently bonded carbon atoms, with hydrogen atoms covalently bonded. Some carbons may be double covalently bonded, and others may be single covalently bonded. Those that are linked via one covalent bond can also bond to two hydrogens, and those linked via two covalent bonds can only bond to one additional hydrogen. Fatty acids that have single covalent bonds between all of their carbon atoms contain as much hydrogen as physically possible- and they are saturated. The chain is straight and can tightly pack together- meaning that saturated fatty acids have higher melting and boiling points.
• What are unsaturated fatty acids?

  Fatty acids are actually hydrocarbons. They are an unbranched chain of covalently bonded carbon atoms, with hydrogen atoms covalently bonded. Some carbons may be double covalently bonded, and others may be single covalently bonded. Those that are linked via one covalent bond can also bond to two hydrogens, and those linked via two covalent bonds can only bond to one additional hydrogen. Fatty acids that have double covalent bonds between all of their carbon atoms contain less than as much hydrogen as physically possible- and they are unsaturated
• What are monounsaturated fatty acids?

  Fatty acids are actually hydrocarbons. They are an unbranched chain of covalently bonded carbon atoms, with hydrogen atoms covalently bonded. Some carbons may be double covalently bonded, and others may be single covalently bonded. Those that are linked via one covalent bond can also bond to two hydrogens, and those linked via two covalent bonds can only bond to one additional hydrogen. Fatty acids that have single bonds with one double covalent bond between all of their carbon atoms are monounsaturated.
• What are polyunsaturated fatty acids?

  Fatty acids are actually hydrocarbons. They are an unbranched chain of covalently bonded carbon atoms, with hydrogen atoms covalently bonded. Some carbons may be double covalently bonded, and others may be single covalently bonded. Those that are linked via one covalent bond can also bond to two hydrogens, and those linked via two covalent bonds can only bond to one additional hydrogen. Fatty acids that have single bonds with more than one double covalent bond between all of their carbon atoms are polyunsaturated.
• What are cis-unsaturated fatty acids?

  Fatty acids are actually hydrocarbons. They are an unbranched chain of covalently bonded carbon atoms, with hydrogen atoms covalently bonded. Some carbons may be double covalently bonded, and others may be single covalently bonded. Those that are linked via one covalent bond can also bond to two hydrogens, and those linked via two covalent bonds can only bond to one additional hydrogen. Fatty acids that have double covalent bonds between all of their carbon atoms contain less than as much hydrogen as physically possible- and they are unsaturated. If the hydrogen atoms are all on the same side, the fatty acid is known as a cis-fatty acid. There are bends in the chain at the double bond, making cis-unsaturated fatty acids (such as triglycerides) less good at packing together, and therefore have a low melting point. This is why triglycerides with cis-unsaturated fatty acids are liquid at room temperature (oils). They are found naturally in foods like olive oil and avocados, and are considered healthier for cardiovascular health.
• What are trans-unsaturated fatty acids?

  Fatty acids are actually hydrocarbons. They are an unbranched chain of covalently bonded carbon atoms, with hydrogen atoms covalently bonded. Some carbons may be double covalently bonded, and others may be single covalently bonded. Those that are linked via one covalent bond can also bond to two hydrogens, and those linked via two covalent bonds can only bond to one additional hydrogen. Fatty acids that have double covalent bonds between all of their carbon atoms contain less than as much hydrogen as physically possible- and they are unsaturated. If the hydrogen atoms are on opposite sides, the fatty acid is known as a trans-fatty acid. They do not have a bend at the double bonds- therefore they have straight chains, can pack closer together, an have a high melting point. This means that they are solid at room temperature- and may be used within margarine and other processed foods. They may be produced artificially via partial hydrogenation of vegetable of fish oils, and due to health concerns regarding processed food and trans-fats, the FDA in America have banned the use of industrial trans-fats.
• What are triglycerides?


  Triglycerides are three fatty acids combined with one glycerol. Every fatty acid is linked to the glycerol via a condensation reaction- therefore, three water molecules are also formed. This link (between the glycerol and each fatty acid) is called an ester bond. Ester bonds are caused when an acid reacts with the hydroxyl group (-OH) in an alcohol- and in triglycerides, this is between the carboxyl (-COOH) group on a fatty acid and a hydroxyl (-OH) on the glycerol. As these groups are the only hydrophilic parts of the fatty acids and glycerol molecules, and are used up in the condensation reaction, triglycerides are entirely hydrophobic. Triglycerides are either oils or fats.
• What are phospholipids?


  Similar to triglyceride formation, two fatty acid molecules react with two hydroxyl groups (-OH) of glycerol, forming ester bonds and releasing two water molecules. The third hydroxyl group of glycerol reacts with a phosphate group, often linked to another polar molecule (e.g., choline), via a condensation reaction. Ester bonds form between the hydroxyl groups of glycerol and the carboxyl groups of the fatty acids. A phosphoester bond forms between the hydroxyl group of glycerol and the phosphate group. The resulting phospholipid has a hydrophilic "head" (phosphate group and polar molecule) and two hydrophobic "tails" (fatty acid chains). This amphipathic nature is critical for forming the bilayer structure of cell membranes.
• What is the function of a triglyceride?

  Triglycerides are used for energy storage in both plants and in animals. In animals, the triglycerides are stored as fat, in adipose tissue. Adipose tissue is located directly beneath the skin and around organs. As triglycerides are chemically stable, energy is not lost over time.

  They are hydrophobic (immiscible with water), so form droplets in the cytoplasm, not affecting the osmolarity of the cell.

  Triglycerides can release twice as much energy per gram ,due to their long hydrocarbon chains, as a carbohydrate can- so enough energy can be stored in half the mass. This is especially important for organisms that move a lot and expend more energy.

  They are good thermal insulators, important for animals needing to conserve body heat (such as animals who maintain a body temperature higher than their environment), as they are poor heat conductors.

  They act as shock absorbers, as they are liquid at body temperature (especially important around organs like the kidneys).

  They can be metabolised slower than carbohydrates, providing sustained sources of energy for long periods of time.
• What is adipose tissue and its function?

  Adipose tissue is a good thermal insulator, important for animals needing to conserve body heat (such as animals who maintain a body temperature higher than their environment), as it is a poor conductor of heat. Adipose tissue is often located just beneath the skin (subcutaneous fat) and around vital organs, ensuring efficient insulation where heat retention is most needed. For animals in cold habitats, a thick adipose layer (blubber) allows survival in extreme conditions by maintaining core body temperature. In humans, this adaptation is critical for new-borns and individuals exposed to prolonged cold, as it minimizes energy loss needed for maintaining body heat.
• What is a steroid?

  Steroids are lipids. They may be identified via four fused rings of carbon atoms, three cyclohexane rings and one cyclopentane ring, with 17 total carbon atoms between the rings.

  Steroids may differ in the position of carbon double bonds and in the functional groups added to the four-ring structure. As they are mostly composed of hydrocarbons, steroids are hydrophobic, allowing them to pass through phospholipid bilayers easily and enter or leave cells.

  When identifying steroids in molecular diagrams, look for the four-ring backbone (a steroid nucleus) and observe any attached side chains or functional groups that modify the steroid's properties.