Biomolecules: Understanding Lipids

Biomolecules

• Biomolecules are essential components of all living organisms.
• They include proteins, carbohydrates, lipids, and nucleic acids.
• Lipids are organic molecules composed mostly of carbon, hydrogen, and oxygen.
• They are used in cell membranes, energy storage, insulation, and hormones.
• Lipids are insoluble in water but soluble in non-polar organic solvents.
• They are hydrophobic or amphipathic.

Classification of Lipids

• Lipids are not polymers.
• They are classified based on molecular structures.
• Three major subclasses are recognized:
• Simple lipids: Fats and oils yield fatty acids and glycerol upon hydrolysis, and waxes yield fatty acids and long-chain alcohols upon hydrolysis.
• Compound lipids: Phospholipids yield fatty acids, glycerol, phosphoric acid, and a nitrogen-containing alcohol upon hydrolysis, glycolipids yield fatty acids, sphingosine or glycerol, and a carbohydrate upon hydrolysis, and sphingolipids yield fatty acids, sphingosine, phosphoric acid, and an alcohol component upon hydrolysis.
• Steroids: These compounds contain a phenanthrene structure and have a unique carbon skeleton with four fused rings. They differ structurally from lipids based on fatty acids.

Physical Properties of Lipids

• Lipids can be liquids or non-crystalline solids at room temperature.
• Pure fats and oils are typically colorless, odourless, and tasteless.
• Their colours, odours, and flavours come from foreign substances absorbed.
• Fats and oils are less dense than water and poor conductors of heat and electricity.

Key Principles of Fatty Acids

• Fatty acids (FAs) are water-insoluble hydrocarbons used for cellular energy storage.
• They are highly reduced, thus a rich source of stored chemical energy for cells.
• Storage as triacylglycerols is highly efficient because water is not needed to hydrate the stored fat.

Fatty Acids

• Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons long (typically 12-20).
• They contain polar carboxylic acid groups and nonpolar hydrocarbon tails, that make them overall nonpolar.

Standard Nomenclature for Fatty Acids

• In some fatty acids, the chain is fully saturated (no double bonds).
• Others contain one or more double bonds (unsaturated).
• Some contain three-carbon rings, hydroxyl groups, or methyl-group branches.
• Two naming conventions exist:
• One convention numbers the carbons from the carboxyl carbon (C-1).
• Another convention numbers the carbons from the methyl carbon (ω). The position of double bonds is designated using ω.

Physical Properties of Fatty Acids

• Fully saturated fatty acids have free rotation around carbon-carbon bonds, resulting in great flexibility of their hydrocarbon chain.
• Saturated fatty acids can pack tightly in nearly crystalline arrays.
• Unsaturated fatty acids have cis double bonds, causing kinks in their hydrocarbon chains, hindering tight packing.
• Unsaturated fatty acids have lower melting points than saturated fatty acids of the same chain length.

Common Saturated Fatty Acids

• This table provides characteristics of some naturally occurring saturated fatty acids. The table includes: Carbon skeleton, Structure, Systematic name, common name, melting point (°C), and solubility in water (mg/g solvent) and benzene.

Common Unsaturated Fatty Acids

• Includes: Carbon skeleton, Structure, Systematic name, common name, melting point (°C), and solubility in water (mg/g solvent) and benzene.
• Unsaturated fatty acids, like linoleic acid and linolenic acid, are essential fatty acids that the body cannot produce on its own.

Simple Lipids

• Fats and oils are the most abundant lipids in nature.
• They are called triacylglycerols because of their ester structure.
• They're composed of three fatty acids connected to glycerol.
• They're classified based on physical state at room temperature (solid fats vs. liquid oils).

Structure of Triacylglycerols

• The simplest lipids constructed from fatty acids are triacylglycerols.
• They're composed of three fatty acids in ester linkage with a single glycerol molecule.
• Most naturally occurring triacylglycerols are mixed, containing two or more different fatty acids.
• These molecules are nonpolar.

Esters

• Esters are distinguished by a functional group containing oxygen.
• Chemically, esters are synthesized by reacting a carboxylic acid with an alcohol.

Fatty Acid Composition of Food Fats

• Most natural fats are complex mixtures of simple or mixed triacylglycerols.
• The melting points of these fats depend on their fatty acid compositions.
• Olive oil tends to be liquid at 25°C due to its high proportion of long-chain unsaturated fatty acids.
• Butter's higher saturated fatty acid content results in a solid state at 25°C.

Lipoproteins

• Lipoproteins transport water-insoluble lipids (e.g., triglycerides, phospholipids and cholesterol) in the blood.
• They contain both lipids and proteins.
• Examples include: Chylomicrons, LDLs, and HDLs.

Triglycerides

• Triglycerides are a storage form of fatty acids in mammals.
• Blood tests often measure triglyceride levels.
• High triglyceride levels can indicate risk for atherosclerosis.

Fat Stores in Cells

• Triacylglycerols are a form of energy storage in cells as microscopic oily droplets.
• In vertebrates, specialized fat cells (adipocytes) store large amounts as fat droplets.
• Plant seeds and adipocytes contain lipases, enzymes that catalyze the hydrolysis of stored triacylglycerols to release fatty acids.
• Triacylglycerols store more energy per gram than polysaccharides.
• Fat stores insulate animals from cold temperatures.

Adipose Tissue & Fat

• Adipose tissue cushions and insulates organs.
• Lipids in fats carry flavours and vitamins to tissues.
• Fatty acids form vital compounds like prostaglandins and phospholipids.

Difference between saponifiable and non-saponifiable lipids

• Saponifiable lipids contain ester groups, undergoing hydrolysis.
• Non-saponifiable lipids do not contain ester groups and cannot be hydrolyzed.

Steroids & Their Roles

• Sterols includes compounds with a characteristic four-ring structure.
• Cholesterol is a steroid that regulates cell membrane fluidity and is a precursor for other steroids.
• Steroid hormones are signaling molecules, with functions throughout body metabolism and cell processes. Examples of sterols include Cholesterol, sex hormones, Vitamin D, and bile acids.

Waxes

• Biological waxes are esters of long-chain saturated and unsaturated fatty acids with long-chain alcohols.

Membrane Lipids

• Membrane lipids contain glycerol or sphingosine as the backbone.
• Glycerophospholipids and sphingolipids are examples of membranes.
• Phospholipids are a major component of biological membranes.
• Phospholipids are amphipathic, exhibiting both hydrophilic and hydrophobic properties.
• Glycolipids are important for cell-cell recognition and are used as blood group antigens.

Phospholipids and Glycolipids

• Phospholipids and glycolipids are amphipathic molecules that are important components of biological membranes.
• These molecules have a hydrophilic polar head group and hydrophobic tails.

Phospholipids

• Glycerophospholipids and sphingolipids are the two types of phospholipids.
• Glycerophospholipids have glycerol as a backbone; sphingolipids use sphingosine as a backbone.
• In glycerophospholipids, one of the fatty acids in the triacylglycerol is replaced by a phosphate-alcohol group.
• Lecithin is a type of phospholipid used as an emulsifier in food products like mayonnaise.

Sphingolipids

• Sphingolipids differ structurally from glycerophospholipids in that they have a sphingosine backbone instead of glycerol.
• Sphingosine replaces the glycerol component of a glycerophospholipid.

Glycolipids

• Some sphingolipids have sugars attached to the alcohol portion of the molecule.
• Glycolipids include gangliosides and globosides, significant for cell recognition and blood group antigens.
• Cerebrosides are important components for insulation in nerve fibers.

Trans Fats in Foods

• Unsaturated fatty acids, when exposed to oxygen, can react and create aldehydes & ketones.
• To improve shelf-life, some oils undergo partial hydrogenation, converting some cis double bonds to trans double bonds.
• This process changes the melting point of oils and creates trans fatty acids.
• Dietary intake of trans fats is linked to cardiovascular disease.

Commercial Hydrogenation of Fatty Acids

• Partial hydrogenation converts 'cis' unsaturated fatty acids to 'trans' unsaturated fatty acids.

An Unwanted Byproduct

• Trans fats increase low-density lipoprotein (LDL) levels.
• LDL is implicated in the deposit of cholesterol in the arteries (rather than transport to cell membranes).
• Atherosclerosis can result from this cholesterol build-up.

Explore the essential components of biomolecules with a focus on lipids. This quiz delves into the classification, structure, and functions of lipids within living organisms. Learn about simple and compound lipids and their roles in biological processes.