Lecture 4: Structure & Function of Molecules #3: Lipids and Proteins

9/10/25

What are the three different classes of lipids?

1. Triglycerides

2. Membrane-forming

3. Sterols

Examples of Triglycerides

Fats and oils

What forms triglycerides?

A dehydration synthesis

Describe the structure of a triglyceride

• Three carbon backbone with fatty acids attached

• Fatty acids are long carbon/hydroen chains

• Made from two-carbon units so fatty acids are often 16 or 18 carbons but rarely 17

Draw a picture of a triglyceride

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What are the two classes of membrane-forming lipids?

1. Phospholipids

2. Galactolipids 

Diagram a structural formula, space-filling model, and phospholipid symbol and a fatty acid. Include hydrophilic and hydrophobic ends.

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What are micelles and phospholipid bilayers?

A way of organization

Draw a diagram of a micelle and a phospholipid bilayer showing how hydrophobic and hydrophilic interactions stabilize these structures

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Examples of sterols in plants and animals

Plants: stigmasterol

Animals: cholesterol

Sterols main functions (3) in membranes:

1. Stiffen (fine tune how flexible membrane is)

2. Maintains fluidity (prevents fatty acids crystalizing)

3. Localized “special” regions (rafts)

Be able to identify a sterol lipid. 

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What is Olestra?

Sucrose with a-8 fatty acids

In a short version, what do proteins do?

They build tiny “machines”

What is a very important factor with proteins?

Shape

Chemical definition of a protein

A polymer of amino acids

What bond links amino acids together in a protein?

A peptide bond

How is the peptide bond in proteins formed?

By removal of water in a dehydration synthesis reaction (condensation reaction), where the carboxyl group of one amino acid reacts with the amino group of another, releasing water and creating a covalent amide bond. 

What is the opposite reaction of a dehydration synthesis reaction?

Hydrolysis reaction

What do the properties of proteins depend on?

Depend on which amino acids are used and in what order (order of amino acids is important)

How many amino acids are used in proteins (in all organisms)?

20 amino acids

Describe the structure of a protein

Carbon linked to an amino group (NH2), Carboxyl group (COOH), a hydrogen, and variable group of side chain (R) 

Be able to identify a protein

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Show the chemical structure of a peptide bond formation

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How many non-polar (hydrophobic) amino acids are there?

9 amino acids

How many polar (hydrophilic) amino acids are there?

6 amino acids

How many electrically charged amino acids are there?

5 amino acids

• 2 Acidic

• 3 Basic

Fill in the blanks: ______ is ______ into _____ that is ________ into _________

DNA is transcribed into RNA that is translated into protein

Explain why biological membranes contain other lipids, such as sterols, in addition to phospholipids

Because different lipids perform essential functions. Other lipids help in functions such as modulating membrane fluidity and permeability (cholesterol), creating specialized regions (lipid rafts) and serving as signaling molecules (phosphates)

Define amino acid

Organic compound that serves as the building block of proteins. Essential for growth, repair, and metabolism

Define polar/non-polar

Polar molecules have an uneven distribution of electrons, creating a positive and negative end (dipole)

Non-polar molecules have an even distribution of electrons, where any bond dipoles cancel out, resulting in a non-net dipole

Define side chain

A variable chemical group attached to the central alpha-carbon of an amino group (also known as the “R” group)

Define peptide bond

A covalent chemical bond that links two amino acids together to form a polypeptide chain, which is the foundation for proteins

Define protein

Complex organic compounds composed of amino acids, which are linked together in long chains

Define cholesterol

A waxy, sterol-type lipid found in all animal cells (essential component of cell membranes; maintains their structure and fluidity)

Define lipid

Fats and “fat-like” substances

Define transcription

the process of copying genetic information from a segment of DNA into a complementary molecule of messenger RNA (mRNA)

Define translation

the process where a cell reads the genetic information carried by messenger RNA (mRNA) molecule and uses it to synthesize a specific sequence of amino acids, forming a polypeptide chain that will then fold into a functional protein