biological molecules 2

What are lipids?

Lipids are biological molecules made mainly of carbon, hydrogen and oxygen.

Lipids are generally hydrophobic, meaning they are insoluble in water.

Lipids include triglycerides and phospholipids.

Q: What is the structure of a triglyceride?

• A triglyceride is made from one glycerol molecule and three fatty acid molecules.

• Glycerol contains three hydroxyl (-OH) groups.

• Each fatty acid contains a carboxyl (-COOH) group and a hydrocarbon chain.

• The three fatty acids can have different hydrocarbon chains.

Q: What is the structure of a fatty acid?

Fatty acids contain a carboxyl group (-COOH) attached to a hydrocarbon chain.

Fatty acids can be saturated or unsaturated.

what is the structure of saturated fatty acid

Saturated fatty acid

• Contains no carbon-carbon double bonds (C=C) in its hydrocarbon chain.

• Contains the maximum possible number of hydrogen atoms.

what is the structure of Unsaturated fatty acid

Unsaturated fatty acid

• Contains one or more C=C double bonds in its hydrocarbon chain.

• Therefore contains fewer hydrogen atoms than a saturated fatty acid.

• C=C double bonds can cause the hydrocarbon chain to bend/kink.

describe glycerol and draw the structure

• Glycerol is an alcohol containing three hydroxyl (-OH) groups.

• Each -OH group is bonded to a carbon atom.

Q: What is the structure of a phospholipid?

A phospholipid is made from:

• one glycerol molecule

• two fatty acid molecules

• one phosphate-containing group

Therefore, a phospholipid has two fatty acid tails instead of the three found in a triglyceride.

Q: What are the hydrophilic and hydrophobic regions of a phospholipid?

• The phosphate-containing head is hydrophilic (attracted to water).

• The fatty acid tails are hydrophobic (repel water).

• Therefore, phospholipids have both hydrophilic and hydrophobic regions.

define Amphipathic

Amphipathic = a molecule containing both hydrophilic and hydrophobic regions.

Q: How are phospholipids arranged in a bilayer?

Phospholipid bilayer

• In an aqueous environment, phospholipids arrange themselves into a bilayer.

• The hydrophilic heads face the water.

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

• This forms the basic structure of cell surface membranes.

describe the Synthesis of a triglyceride

• A triglyceride is formed when one glycerol reacts with three fatty acids.

• This is a condensation reaction.

• An ester bond forms between the glycerol and each fatty acid.

• One molecule of water is released for each ester bond formed.

• Therefore, forming one triglyceride produces three ester bonds and three water molecules.

Overall:

glycerol + 3 fatty acids - triglyceride + 3H₂O

describe the Breakdown of a triglyceride

• Triglycerides are broken down by hydrolysis.

• Water is used to break the ester bonds.

This produces:

• glycerol

• three fatty acids

Overall:

triglyceride + 3H₂O → glycerol + 3 fatty acids

Q: Why are triglycerides good energy stores?

Triglycerides → energy storage

• Triglycerides contain many C-H bonds in their fatty acid chains.

• These bonds contain a large amount of chemical energy.

• Therefore, triglycerides have a high energy content.

• Triglycerides are therefore suitable for long-term energy storage.

Why are triglycerides good energy stores?

• They contain more energy per gram than carbohydrates.

• They are insoluble in water, so they do not affect the water potential of cells.

• They can therefore be stored without causing water to enter the cell by osmosis.

Q: How do phospholipids form cell membranes?

Phospholipids → cell membranes

• Phospholipids have a hydrophilic head and hydrophobic tails.

Because they have both regions, they are amphipathic.

• In water, they arrange into a bilayer with:

• hydrophilic heads facing the water

• hydrophobic tails facing inwards

• Phospholipid bilayers form the basic structure of cell surface membranes and membranes surrounding organelles.

Q: Why is the phospholipid bilayer useful for selective permeability?

Why is the bilayer useful?

• The hydrophobic interior prevents many water-soluble and charged substances from passing directly

through the membrane.

• This contributes to the membrane's selective permeability.

Q: What is the role of cholesterol in cell membranes?

• Cholesterol has both hydrophilic and hydrophobic regions.

• It fits between the phospholipids in cell membranes.

• Cholesterol helps regulate membrane fluidity and permeability.

• It helps prevent membranes from becoming too fluid at high temperatures and helps maintain membrane stability.

• At lower temperatures, cholesterol prevents phospholipids from packing too closely together, helping

prevent the membrane becoming too rigid.