Biomacromolecules Part 1: Lipids

Biomacromolecules Biomacromolecules are BIG molecules. They play an essential role in both the structure and functions of cells. Understanding the chemical basis of life is about understanding macromolecules – how they are made and how they function.

Cells make biomacromolecules Cells import water, mineral ions (+ charge) and a host of small organic molecules, such as simple sugars, fatty acids and amino acids. Other small organic molecules are made and altered in different chemical reactions within the cell. In contrast Cells can only acquire macromolecules by making them. They are simply too big to be imported into a cell.

CLASS OF MACROMOLECULE BUILDING BLOCKS CELLULAR FUNCTIONS OF MACROMOLECULES Lipids Fatty acids and glycerol Energy store, component of cell membranes, signalling molecules Polysaccharides (complex carbohydrates) Simple sugars Energy store, structural components Nucleic acids Nucleotides Informational molecules that make up genetic material Proteins Amino acids The workhorse of the cell: controls and regulates reactions, transport, movement, receptors, defence and structure.

Are macromolecules polymers? Polysaccharides, nucleic acids and proteins are polymers. Lipids are not polymers. A polymer is made of small molecules (called sub-units or monomers) which are repetitively linked together to form long strands called polymers. The huge variety of polymers arises from the infinite number of possibilities in the sequencing and arrangement of monomers. Although lipids are large molecules, they are not made up of repeating monomers – they are made up of two distinct chemical groups of atoms (fatty acids and glycerol).

Lipids Lipids are largely composed of carbon, hydrogen and oxygen atoms. CHO Lipid is general term for fats, oils & waxes. Lipids are contained of very little water, therefore carry much more energy per molecule than other compounds.

Lipids are broadly defined as any fat-soluble (lipophilic), naturally-occurring molecule, such as fats, oils, waxes, cholesterol, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and triglycerides.

Lipids Are largely insoluble in water. This is their defining characteristic rather than any particular chemical property. Contain large non-polar hydrocarbon regions. This explains their hydrophobic character. Like any non-polar substance, lipids readily dissolve in non-polar substances. *non-polar= completely – or + charge or neutral *polar= one part – and one +

Functions of Lipids Lipids have three main functions in cells: Energy storage: lipids have twice as much useable energy, gram for gram, as sugars or starch Various lipids are essential components of cell membranes Lipids can have specific biological roles as signalling molecules (hormones), receptor sites, vitamins and coenzymes.

How are lipids formed Lipid molecules are formed when fatty acids combine with an alcohol called glycerol. A water molecule is eliminated when the acid groups reacts with the alcohol group and an ester bond is formed that links the two molecules together.

A little about fatty acids Fatty acids are hydrocarbon chains, of various length, that end in an acid functional group.

Glycerol A molecule of glycerol is made up of three carbon atoms. Each of these has a hydroxyl group attached to it. Hydrogen atoms occupy the remaining positions.

Fatty Acids A single fatty acid molecule contains an acid (COOH) group attached to a hydrocarbon chain. CH3CH2CH2COOH  

Saturated vs Unsaturated Saturated hydrocarbons All carbon-carbon bonds are single bonds. Results in long straight chains that can be packed tightly together Unsaturated hydrocarbons One or more carbon bonds are double bonds Double bonds produce kinks or bends which stop the hydrocarbon chains from being packed closely together

Examples of lipids…

Triglycerides Formed by combining three fatty acids with glycerol. The three fatty acid hydrocarbon chains can come from different fatty acids and so can vary in length and their degree of saturation. Main function is as a fuel storage molecule for the cells – stores fatty acids when they are not needed as energy.

The molecules join together through the process of condensation losing a molecule of water each time a link is made. The link between the glycerol molecule & each fatty acid is an ‘Ester Link’.

Condensation reactions A condensation reaction is a chemical reaction in which two molecules or functional groups combine to form a larger molecule, together with the loss of a small molecule usually water. Fatty acids join glycerol via this reaction, producing water as a by-product.

Phospholipids Consist of two fatty acid chains linked to a glycerol molecule. Importantly, the glycerol molecule is also linked to a negatively charged phosphate functional group with a small alcohol group attached. This attached group makes the glycerol end of the molecule a polar region and hence hydrophilic. The fatty acid hydrocarbon chains are non-polar and hydrophobic.

Phospholipids Phospholipids are an essential structural component of cell membranes. The hydrocarbon tails of each layer interact with one another forming a hydrophobic interior that acts as a permeability barrier. It is hydrophobic interactions that drive the formation of the lipid bilayer.

Lipids & water Triglycerides and phospholipids have hydrophobic and hydrophilic groups. What does this mean?

Steroids Cholesterol is a steroid molecule built up from four hydrocarbon rings linked together with a hydrocarbon tail at one end and an OH group at the other. Cholesterol is in all eukaryotic animal cell membranes. Cholesterol is the precursor for the production of other important steroid derived molecules including: bile salts, cortisol and sex hormones.