Topic 1.2.3: Lipids

What are lipids?

Lipids are a diverse group of organic molecules that play important roles in living organisms.

What do lipids consist of?

They contain carbon, hydrogen and oxygen, but generally have a lower proportion of oxygen than carbohydrates.

what is the solubility of lipids?

They are generally insoluble in water but soluble in organic solvents.

What are the major groups of lipids?

The three major groups relevant to this section are triglycerides (fats and oils), phospholipids and steroids.

What are lipids important for?

Lipids are important for energy storage and form an integral part of cell membranes.

What are fats and oils?

Fats and oils are types of lipid that are chemically very similar.

What do fats and oils both consist of?

Both consist of glycerol joined to three fatty acids, forming a triglyceride.

What state of temperature are fats like butter usually in at room temperature?

Fats, such as butter, are usually solid at room temperature.

What state of temperature are oils like olive oil usually in at room temperature?

• Oils, such as olive oil, are usually liquid at room temperature.

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What is the proportion of saturated and unsaturated fatty acids in fats and oils?

Fats are more likely to contain a higher proportion of saturated fatty acids, whereas oils are more likely to contain a higher proportion of unsaturated fatty acids.

What is a fatty acid?

A fatty acid is an organic molecule consisting of:

• A long hydrocarbon chain, which is a chain of carbon atoms with hydrogen atoms attached.

• A carboxyl group at one end.

How do fatty acids differ?

• Fatty acids vary in the length of their hydrocarbon chains.

• Many fatty acids in living organisms have chains of approximately 15–17 carbon atoms, although the length varies.

• Fatty acids also differ in the number of carbon–carbon double bonds in their hydrocarbon chains.

What are saturated fatty acids?

• A saturated fatty acid has no carbon–carbon double bonds in its hydrocarbon chain.

• Every carbon atom in the hydrocarbon chain is joined to the next carbon atom by a single covalent bond.

What is the significance of a saturated fatty acid chain being straight?

• The chain is generally straight, allowing the molecules to pack closely together.

• Close packing leads to stronger intermolecular attractions between molecules.

• This generally results in a higher melting point.

What are unsaturated fatty acids?

An unsaturated fatty acid has at least one carbon–carbon double bond in its hydrocarbon chain.

What affects the shape of the chain in an unsaturated fatty acid?

The presence of a double bond affects the shape of the chain.

How does the presence of the double carbon bond link to the melting point?

• In naturally occurring cis-unsaturated fatty acids, the double bond creates a bend or kink in the hydrocarbon chain.

• The bends prevent the molecules from packing as closely together.

• This reduces intermolecular attractions and generally lowers the melting point.

What are the two types of unsaturated fatty acids?

Monounsaturated fatty acid

- Contains exactly one carbon–carbon double bond.

Polyunsaturated fatty acid

- Contains two or more carbon–carbon double bonds.

What is the structure of glycerol?

• Glycerol is a three-carbon alcohol with the molecular formula C3​H8​O3​.

• It contains three hydroxyl groups , one attached to each carbon atom.

• Each hydroxyl group can react with the carboxyl group of a fatty acid to form an ester bond.

How is a triglyceride formed?

A triglyceride is formed when one glycerol molecule reacts with three fatty acid molecules through three condensation reactions.

What are the three stages of the formation of a triglyceride?

Stage 1: The first fatty acid joins glycerol

Stage 2: The second fatty acid joins

Stage 3: The third fatty acid joins

What is the first stage of the formation of a triglyceride?

• The carboxyl group (−COOH) of a fatty acid reacts with one hydroxyl group (−OH) of glycerol.

• A molecule of water is released.

• An ester bond forms, producing a monoglyceride.

What is the second stage of the formation of a triglyceride?

• A second fatty acid reacts with another hydroxyl group on the glycerol.

• Another water molecule is released.

• A second ester bond forms, producing a diglyceride.

What is the third stage of the formation of a triglyceride?

• The third fatty acid reacts with the remaining hydroxyl group.

• A third water molecule is released.

• A third ester bond forms, producing a triglyceride.

What is a condensation reaction?

• A reaction in which two molecules join together and a small molecule, such as water, is released.

• In triglyceride synthesis, each fatty acid reacts with a hydroxyl group on glycerol, releasing one molecule of water.

What is an ester bond?

• A covalent bond formed between the carboxyl group of a fatty acid and a hydroxyl group of glycerol during condensation.

• A triglyceride contains three ester bonds.

What is hydrolysis of triglycerides?

• Hydrolysis is a reaction in which a molecule is broken down by the addition of water.

• During triglyceride hydrolysis, water is used to break the three ester bonds.

• Complete hydrolysis produces one glycerol molecule and three fatty acid molecules.

What are the three main functions of lipids?

• Energy storage

• Waterproofing

• Insulation

Explain how lipids are adapted to conducting efficient energy storage?

They contain many carbon–hydrogen bonds.

• Fatty acids have long hydrocarbon chains containing many carbon–hydrogen bonds.

• These bonds contain a large amount of chemical energy that can be released during aerobic respiration when the fatty acids are oxidised.

• Consequently, lipids store a large amount of energy per unit mass.

2. They are insoluble in water.

• Triglycerides are hydrophobic and do not dissolve in the aqueous cytoplasm.

• They can therefore be stored in cells as droplets without significantly affecting the water potential of the cytoplasm.

• This avoids causing excessive water movement by osmosis.

• 3. They have a low density.

  - Lipids have a relatively low density, making them suitable for storing substantial energy without adding as much mass as a denser store would.

Explain how lipids are adapted to the function of waterproofing?

Lipids are useful for waterproofing because they are hydrophobic and insoluble in water.

• Their hydrocarbon regions are non-polar and do not form favourable interactions with water.

• Lipids can therefore form a water-repellent coating on surfaces.

• This reduces water loss from organisms and prevents surfaces from becoming easily wetted.

Explain how lipids are adapted to the function of insulation?

Lipids are useful for insulation because they form a layer that reduces heat transfer.

• Triglycerides can accumulate beneath the skin as adipose tissue.

• This layer acts as a thermal insulator, reducing heat loss to the surroundings.

• It helps animals maintain a relatively stable body temperature.

What is a phospholipid?

A phospholipid is a lipid molecule consisting of:

• One glycerol molecule.

• Two fatty acid molecules.

• One phosphate group.

Unlike a triglyceride, which has three fatty acids, a phospholipid has two fatty acids and a phosphate-containing head group.

What are the hydrophilic heads in a phospholipid?

• The phosphate group is polar and carries a negative charge.

• It can interact with water molecules, which are polar.

• Therefore, the phosphate-containing head is hydrophilic, meaning it is attracted to water and interacts readily with it.

• The head is soluble in water.

What are the Hydrophobic tails in a phospholipid?

• The two fatty acid tails consist mainly of non-polar hydrocarbon chains.

• They cannot form favourable interactions with water.

• Therefore, the tails are hydrophobic and do not dissolve in water.

• They tend to move away from water and associate with other hydrophobic regions.

What does amphipathic mean in terms of phospholipids?

Phospholipids are amphipathic molecules because they have both a hydrophilic region (the head) and hydrophobic regions (the tails).

How do phospholipids behave in water?

The hydrophilic heads and hydrophobic tails determine how phospholipids arrange themselves when they come into contact with water.

What is a monolayer?

A monolayer is a single layer of phospholipid molecules.

• It can form at an interface between air and water.

• The hydrophilic heads face towards the water.

• The hydrophobic tails point away from the water and towards the air.

• This arrangement allows the hydrophilic heads to interact with water while keeping the hydrophobic tails away from it.

What is a micelle?

A micelle is a roughly spherical cluster of molecules with hydrophilic and hydrophobic regions.

• It forms in an aqueous environment.

• The hydrophilic heads face outwards towards the surrounding water.

• The hydrophobic tails point inwards, away from the water.

• The hydrophobic tails form the interior of the micelle.

What is a bilayer?

A bilayer consists of two layers of phospholipids arranged with their hydrophobic tails facing each other.

• The hydrophilic heads face outwards towards the aqueous solutions on both sides.

• The hydrophobic tails point inwards, away from the water.

• The tails form a hydrophobic interior between the two layers.

• This arrangement is energetically favourable because it minimises contact between the hydrophobic tails and water.

Why is the phospholipid bilayer suitable for cell membranes?

Phospholipids are amphipathic, with hydrophilic phosphate heads and hydrophobic fatty acid tails. In an aqueous environment, they form a bilayer, with the hydrophilic heads facing the water on either side and the hydrophobic tails facing inwards. This forms a continuous, flexible membrane with a hydrophobic interior that acts as a barrier between the cell and its surroundings.