Lipids (mix)

Lipids

• Non polar organic molecules with diverse functions
• Distinct from other organic molecules as they aren't composed of recurring monomers, but may have discrete subunits
• Commonly consist of hydrocarbons arranged in chains (fatty acids) or fused rings (steriods)
• Possess hydrophobic properties causing low solubility in aqueous solvents, but can dissolve in non-polar solvents
• This makes lipids effective water-repellents

Lipid Types

• Lipids are categorized into three groups by chemical composition and physical properties
• Simple lipids are esters of fatty acids and alcohol, including waxes and triglycerides
• Compound lipids are simple lipids linked to another group, like phospholipids
• Derived lipids consist of hydrocarbon rings from simple lipid hydrolysis, such as steroids

Ester Linkages

• Simple and compound lipids contain fatty acids covalently attached to an alcohol (glycerol) via an ester bond
• This linkage involves a condensation reaction, releasing water

Fatty Acids

• Lipids have properties based on their fatty acid composition
• Triglycerides vary in physical state (solid fats or liquid oils)
• Phospholipids have different levels of fluidity
• Classified by presence/absence of double bonds between carbon atoms

Saturated Fatty Acids

• Possess no double bonds, saturated with H

Unsaturated Fatty Acids

• Have one or more double bonds

Chain Formation

• Saturated fatty acids form straight hydrocarbon chains
• Unsaturated fatty acids form kinked chains

Saturation Levels

• Unsaturated fatty acids can be monounsaturated (one double bond) or polyunsaturated (multiple double bonds)
• Polyunsaturated fatty acids will have even more kinks in their hydrocarbon chains

Fats and Oils

• Triglycerides stored as solid fats or liquid oils, based on fatty acid type
• Saturated fatty acids' straight chains allow tight packing and strong intermolecular forces, raising the melting temperature, existing as solid fat at room temperature
• Unsaturated fatty acids' kinked chains pack loosely with weaker intermolecular forces, lowering the melting point which makes them liquid oil at room temperature
• Triglyceride storage form is determined by an organism's physiology and environment
• Fatty acids need to be liquid in tissues for cell utilization, but packed tightly for storage
• Warm-blooded mammals (endotherms) produce more saturated fatty acids to stay liquid at high core temperatures
• Cold-blooded animals/those in cold climates have more unsaturated fatty acids, as saturated fats would solidify

Plant Fatty Acids

• Plants cannot control internal temperatures, generally producing unsaturated fatty acids
• Plants in warm climates produce more saturated than temperate plants

Fats vs Oils

• Fats are found in animals (endotherms), contain saturated fatty acids, have straight/tightly packed chains, are solid at room temperature, and have a higher melting point
• Oils are found in plants and fish oils, are made of unsaturated fatty acids, have kinked/loosely packed chains, are liquid at room temperature, and have a lower melting point

Trans Fats

• Unsaturated fatty acids exist in two isomeric forms: cis and trans
• Cis-isomers have hydrogen atoms on the same side of the carbon double bond, causing a kink
• Trans-isomers have hydrogen atoms on opposite sides of the carbon double bond, eliminating the kink
• Trans-fatty acids are not typically produced by living organisms (except ruminants) and are made instead through industrial cooking practices to improve shelf life

Lipid Functions

• Lipids serve different functions within cells due to property differences
• Triglycerides function as long-term energy storage and effective thermal insulators
• Phospholipids play structural roles by forming cell membranes
• Steroids may function as signalling molecules by binding to intracellular receptors

Triglycerides

• In animals, triglycerides are stored in adipose tissue
• This tissue is used for thermal insulation and a long-term energy source
• Fats have low thermal conductivity, so they are effective insulators.
• Marine mammals have thick blubber for insulation and buoyancy
• Function as an alternative energy storage option to carbohydrates

Long Term Energy Storage Properties

• Solubility: Non-polar and insoluble in water
• Osmosis: Create less osmotic pressure in cells
• Digestion: Harder to digest than carbohydrates
• ATP Production: Produce roughly twice as much energy per gram

Phospholipids

• Primary component of cell membranes
• They are amphipathic (have hydrophilic and hydrophobic regions)
• Consist of a phosphate group linked to two fatty acid chains by a glycerol molecule
• Phosphate group is polar, forming a head region, and orients away from the fatty acid tails
• In cells, phospholipids for bilayers, with the phosphate heads facing the aqueous solutions, and fatty acid tails facing inward to hold the bilayer

Steroids

• Derived lipids composed of four fused carbon rings
• Non-polar, they pass through the phospholipid bilayer and act as signalling molecules (hormones)
• Examples include the sex hormones – oestradiol (ovaries) and testosterone (testes)