Lecture 3: Structure & Function of Molecules #2: Carbohydrates and Lipids

9/8/25

What are the four main types of organic molecules that make up living things?

1. Carbohydrates

2. Lipids

3. Proteins

4. Nucleic Acids

What do all four types of organic molecules have in common structurally?

They are all carbon-based (organic), often containing hydrogen, oxygen, and sometimes nitrogen or phosphorus

What elements are carbohydrates made of?

Carbon (C), Hydrogen (H), and Oxygen (O)

What is the general formula for a simple sugar?

(CH₂O)ₙ

What is the most important sugar in our blood?

Glucose

What are the functions of carbohydrates?

• Store energy

• Transport energy

• Provide structural support

• Act as chemical signals

What two functional groups make sugars hydrophilic?

Hydroxyl (-OH) and carbonyl (=O)

What is a monosaccharide?

A single sugar molecule

How is a disaccharide formed?

By joining two monosaccharides in a condensation (dehydration synthesis) reaction, which removes water

What is the reverse of a condensation reaction?

Hydrolysis, which adds water to break bonds

What two monosaccharides make sucrose?

Glucose + Fructose

What is sucrose’s biological role?

It is the primary sugar transported in plants

What is maltose made from?

Two glucose molecules

What process produces maltose?

The breakdown (hydrolysis) of starch, often during germination of grains

What is lactose made of?

Glucose + Galactose

What is lactose intolerance?

The inability to digest lactose due to the loss of the enzyme lactase

What do you get when you polymerize many sugars?

A polysaccharide

What is a glycogen?

A branched glucose polymer used by animals for energy storage

What is starch?

A glucose polymer with some branching used by plants for energy storage

How do starch and glycogen differ structurally?

Glycogen has more irregular branching than starch

What is the difference between starch and cellulose?

• Starch: Alpha (α) 1-4 glucose linkages

• Cellulose: Beta (β) 1-4 glucose linkages

Why can’t humans digest cellulose?

We lack the enzyme to break β(1→4) bonds.

What is the most abundant biopolymer on Earth

Cellulose. Found in plant cell walls

What is chitin and where is it found?

A polymer of acetyl-glucosamine, found in insect and fungal cell walls/exoskeletons

What is the correct order of biological importance in DNA, Cellulose, and Chitin based on abundance and structure?

Cellulose > Chitin > DNA

What defines a lipid?

It is defined by being hydrophobic (water-insoluble), not by specific composition

What are the two main types of lipids?

1. Fats and oils

2. Sterols (and other isoprenoids)

What elements are common in lipids?

Mostly carbon and hydrogen; some may contain oxygen, nitrogen, or phosphorus

What is the structure of a triglyceride?

Glycerol backbone + 3 fatty acid chains

How are triglycerides formed?

By dehydration synthesis between glycerol and fatty acids

What’s the typical length of a fatty acid chain?

16 or 18 carbons (even numbers because they’re built from 2-carbon units)

What is the physical state difference between fats and oils?

• Fats: solid at room temperature

• Oils: liquids at room temperature

What two types of lipids form membranes?

Phospholipids and galactolipids

What are sterols and give examples

Lipid molecules that alter membrane properties

• Animals: Cholesterol

• Plants: Stigmasterol

What are the three main functions of sterols in membranes?

• Stiffen membranes

• Maintain fluidity (prevent fatty acid crystallization)

• Create lipid rafts (specialized regions)

What else can sterols function as?

Chemical messengers (hormones, signals)

What does the quote “High cholesterol = dead tomorrow; zero cholesterol = already dead” mean

Cholesterol is essential for life but too much can be harmful (e.g. cardiovascular disease)

Which organic molecules can act as chemical messengers?

All four: Carbohydrates, lipids, proteins, and nucleic acids

Describe what makes glucose, sucrose, glycogen, and starch both chemically similar and different

Similar: All contain the same basic elements (carbon, hydrogen, and oxygen) and are related to sugar

Difference: differ in size and structure

• Glucose: single sugar unit (monosaccharide)

• Sucrose: two linked sugar (disaccharide)

• Glycogen: long chain of glucose units (polysaccharide)

  • Highly branched and found in animals

• Starch: long chain of glucose units (polysaccharide)

  • Less branched and found in plants

Explain the structural/chemical difference between a triglyceride and a phospholipid

A triglyceride is a glycerol backbone with three fatty acid chains. It is non-polar and hydrophobic.

A phospholipid is a glycerol backbone with two fatty acids and a phosphate group instead of the third fatty acid. It is amphipathic (hydrophilic “head” and a hydrophobic “tail”)

Define bilary

Fundamental structure formed by two layers of amphipathic molecules, most notably phospholipids, that arrange themselves with their hydrophilic heads facing outward and their hydrophobic tails facing inward

Define carbohydrate

Organic compound that serves as the primary energy source for living organisms

Define cellulose

A complex carbohydrate made of long chains of glucose units that forms the structural component of plant cell walls

Define chitin

Strong, structural, natural polymer of N-acetyl-glycosmamine often found in insect exoskeleton

Define disaccharide

Type of carbohydrate formed when two monosaccharides (simple sugars) are joined together by a glycosidic bond, losing a water molecule in the process

• Made of two simple sugars: glucose and galactose

• Ex. sucrose (table sugar), lactose (milk sugar), maltose (malt sugar)

Define fructose

A simple sugar (monosaccharide) that is naturally found in fruits, vegetables, and honey

• also a component of sucrose (table sugar)

Define glucose

A simple sugar molecules that serves as the primary source of energy for the body’s cells

Define glycogen

A branched, complex carbohydrate molecules that serves as a readily available source for animals and humans

Define lactose

Natural sugar found primarily in milk and dairy products (disaccharide)

Define maltose

A natural sugar found in various foods (disaccharide)

Define monosaccharide

Any of the class of sugars that cannot be hydrolyzed to give a simpler sugar

Define polysaccharide

A carbohydrate whose molecules consist of a number of sugar molecules bonded together 

Define starch

A complex carbohydrate and the main form of energy storage in plants, made of long chains of glucose molecules linked together

Define sucrose

A natural sugar primarily found in sugarcane, composed of equal parts of glucose and fructose (disaccharide)

• Table sugar

Define hydrophilic

Any substance that has an affinity for water

• Affinity: strong attraction or liking of water

• Ex.) sugars

Define hydrophobic

Not attracted to water or repels water

Define fat

A triglyceride, a type of lipid composed of a glycerol molecule with three fatty acid chains attached

• Major source of energy

• Solid at room temperature

Define micelle

A microscopic, spherical structure formed by amphipathic molecules in an aqueous solution

Define oil

A triglyceride or mixture of triglycerides that is liquid at room temperature. Type of lipid that is non-polar (don’t mix with water)

Define phospholipid

A type of lipid molecule that forms the fundamental structure of cell membranes, known as the phospholipid bilayer

Define sterol

A lipid molecules characterized by a rigid core of four hydrocarbon rings that is essential for cell membrane structure, fluidity, and function in eukaryotes 

Define triglyceride

A glycerol backbone with three fatty acid chains. It is non-polar and hydrophobic