Lipids Flashcards
Lipids, triglycerides
•Lipids contain lots of carbon and hydrogen and a small amount of oxygen. They are insoluble in water (hydrophobic), as they are non-polar, and so do not attract water molecules, but they do dissolve in alcohol. Triglycerides, phospholipids and steroids are the most important lipids, they are macromolecules, not polymers as they are made of different components bonded together.
•Triglycerides: Made up of 3 fatty acids and glycerol: an alcohol made of 3 carbon atoms, and 3 -OH groups.
Fatty acids:
•Carboxyl (-COOH) group attached to end of hydrocarbon tail. Fatty acids are acidic as the carboxyl group ionises into a free H+ ion and a -COO- group.
Saturated: There are no C=C double bonds, straight molecule.
Unsaturated: There is one or more C=C bonds – fewer hydrogen atoms bonded
•Monounsaturated: A single C=C bond (e.g. oleic acid).
•Polyunsaturated: More than one C=C bond (e.g. linoleic acid).
Having one or more C=C bonds changes the shape of the hydrocarbon chain, giving it a kink where the double bond is. As the kinks push the molecules apart slightly – making them more fluid. Animal lipids contain lots of saturated fatty acids (often solid at 20°C), melting point of unsaturated is lower.
Ester bonds:
A triglyceride consists of one glycerol molecule bonded to three fatty acids. A condensation reaction happens between the -COOH group of the fatty acid and the -OH group of the glycerol (water molecule produced). The covalent bond that forms due to this reaction is called an Ester bond. Triglyceride formation is called esterification.
Functions of triglycerides:
Energy source
Energy store
Insulation
Buoyancy
Protection
Energy source:
triglycerides can be broken down in respiration (oxidised) to release energy and generate ATP. The first step is to hydrolyse the ester bonds, then both the glycerol and the fatty acids can be broken down completely to CO2 and H2O. Note: respiration of a lipid produces more water than respiration of sugar.
Energy store :
because triglycerides are insoluble in water, they can be stored without affecting the water potential of the cell. Mammals store fat in adipose cells under the skin. Fat releases twice as much energy per gram as glucose, as lipids have a higher proportion of hydrogen atoms than carbohydrates and almost no oxygen.
Insulation:
Heat insulation (e.g. adipose tissue in whales “blubber” and animals preparing for hibernation store extra fat). Electrical insulator (Myelin sheath in nerve cells). The also form the waxy cuticle around leaves/stems of plants.
Buoyancy :
Fat is less dense than water, help keep aquatic mammals afloat.
Protection:
Humans have fat around delicate organs, such as kidneys, to act as a shock absorber.
The peptidoglycan cell wall of some bacteria is covered in a lipid-rich outer coat.
Phospholipids
(Structure)
Glycerol molecule with 2 fatty acids bonded to it by ester bonds, and instead of the third (like a triglyceride) a phosphate group is bonded by an ester bond, formed by a condensation reaction between the -OH groups on a phosphoric acid molecule (H3PO4) and one of the -OH groups of the glycerol.
- Commonly the C-chain length of the 2 fatty acids is even (e.g. 16/18) and one saturated.
•When surrounded by water the phosphate group has a negative charge (O- forms from -OH that was there), making it polar and attracted to water – hydrophilic. The fatty acid tails are non-polar and so repelled by the water – hydrophobic. This means the phospholipid molecule is amphipathic (hydrophobic polar end and hydrophilic non-polar end). In general, membrane lipids tend to be amphipathic, storage lipids are not.
•Amphipathic phospholipids may form a layer on the surface of water with tails pointing up; or may form micelle – tiny balls with tails tucked away inside, with heads pointing out into the surrounding water.
Phospholipid bilayer:
Membranes around cells and organelles. Formed of amphipathic phospholipids in 2 layers with tails pointing in and hydrophilic heads on the exterior as both sides of the membrane are aqueous solutions (cytoplasm/extracellular space). Membranes in plant and animal cells are often made of phospholipids, bacterial membranes contain more protein.
• The individual phospholipids are free to move around in their layer but will not move to a position where the hydrophobic tails are exposed to water – giving the membrane stability.
• The membrane is selectively permeable, only small/non-polar molecules can move through the bilayer (e.g. O2 and CO2), polar molecules/ions cannot travel through the non-polar hydrophobic section formed of the fatty acid tails. This gives membrane control on what enters/exits.
• Water is an exception, it is polar, but it is so small that it can travel between the phospholipids.
Cholesterol
Cholesterol is a steroid alcohol (or sterol) – not made from glycerol or fatty acids. It consists of 4 carbon- based rings or isoprene units. Only exists in eukaryotic cells, not prokaryotes.
Cholesterol is a small hydrophobic (non-polar) molecule, which means it can sit in the middle of the hydrophobic part of the phospholipid bilayer.
Mainly made in the liver in animals and transported in the blood.
• It regulates the fluidity of the membrane, preventing it from becoming too fluid/stiff, binds to hydrophobic tails of phospholipids, causing them to pack more closely together.
• The steroid-based hormones (testosterone, progesterone, oestrogen and vitamin D) are all made from cholesterol. As they are small and hydrophobic they can pass
through the hydrophobic part of the plasma membrane.
