BiomacromoleculesPart 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 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 (less oxygen atoms compared to hydrogen and carbon).

• They are generally known as fats and oils.• The name applies to a diverse group of

molecules that, due to their large molecular mass, are commonly grouped as macromolecules.

Lipids are DEFINITELY NOT polymers.

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 such as chloroform.

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.• Fatty acids in biological systems usually contain an

even number of carbon atoms, typically 14 to 24.• The length of the chain and the amount of

saturation largely determine the properties of fatty acids and other lipids derived from them.

• Unsaturated fatty acids have a lower melting point compared to saturated fatty acids, given the hydrocarbon chains are of the same length.

• Animal fats contain saturated fatty acids and are usually solids at room temperature.

• Oils contain unsaturated fatty acids and are liquids at room temperature

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

Classes of LipidsCLASS EXAMPLE FUNCTION

Fatty acids Stearic acidOleic acid

Energy sourceSubunit of other lipids

Triglycerides Fats and oils Energy storage

Phospholipids Phospholipids Structural component of plasma membranes

Glycolipids Glycolipids Recognition sites on plasma membranes

*Steroids*(only found in eukaryotic cells)

Cholesterol

Sex hormones

Component of plasma membraneSignalling molecules

Terpenes Vitamin A Antioxidant

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.

• Lipase (produced in the pancreas) acts on the ester bond linking fatty acids to glycerol in order to break down triglycerides.

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.

• Any molecule that contains both hydrophilic and hydrophobic regions is called an amphipathic molecule.

Phospholipids• Phospholipids are an essential structural component of cell

membranes. • Membrane formation is a consequence of their amphipathic

character.• 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.

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 found in varying amounts in virtually all eukaryotic animal cell membranes.

• It is arranged between the fatty acid chains of phospholipids, with the OH group being attracted to the polar end.

• Cholesterol is the precursor for the production of other important steroid derived molecules including: bile salts, cortisol and sex hormones.