Biomolecules

Living organisms are made from chemicals, and many of the most important chemicals in living systems are organic molecules containing carbon.

Four major classes of biomolecules are especially important:

• Carbohydrates
• Lipids
• Proteins
• Nucleic acids

These molecules perform different jobs, but together they make cells, store and transfer energy, control chemical reactions, carry genetic information, and build the structures needed for life.

A useful overview is:

The letters represent:

C = carbon
H = hydrogen
O = oxygen
N = nitrogen
P = phosphorus
S = sulfur

Carbon: The Foundation of Biological Molecules

Carbon is especially important in living systems because each carbon atom can form four covalent bonds.

Carbon atoms can bond with:

• other carbon atoms
• hydrogen
• oxygen
• nitrogen
• phosphorus
• sulfur

Carbon atoms can form:

• straight chains
• branched chains
• rings
• double bonds
• large complex molecules

This enormous variety allows organisms to contain millions of different organic compounds.

This is why organic chemistry forms much of the chemical foundation of biology.

---

From Small Molecules to Large Biomolecules

Many biological molecules are constructed from smaller units.

A small building unit used to make a larger molecule is often called a:

monomer

A large molecule containing repeating or linked smaller units is called a:

polymer

For example:

amino acids → proteins

nucleotides → nucleic acids

many glucose molecules → some polysaccharides

However, not every major biomolecule fits a simple monomer-polymer model. Lipids, for example, are not generally considered true polymers.

---

Condensation and Hydrolysis

Biological molecules are often assembled and broken apart through two important types of reaction.

Condensation

Smaller molecules join together and water is produced.

Simplified:

small molecule + small molecule → larger molecule + water

Hydrolysis

Water is used to split a larger molecule.

Simplified:

larger molecule + water → smaller molecules

These reaction patterns appear repeatedly in biological chemistry.

---

Carbohydrates

Carbohydrates are organic compounds made mainly from:

• carbon
• hydrogen
• oxygen

Many carbohydrates contain hydrogen and oxygen in approximately a 2:1 ratio.

They include:

• sugars
• starches
• glycogen
• cellulose

Carbohydrates are particularly important for:

energy and energy storage

Some also have major structural roles.

Monosaccharides

The simplest carbohydrates are called:

monosaccharides

"Mono" means one.

"Saccharide" refers to sugar.

An important example is:

glucose

Molecular formula:

C₆H₁₂O₆

Glucose is extremely important because cells can use it during cellular respiration to release energy for biological processes.

---

Glucose and Cellular Respiration

A simplified equation for aerobic cellular respiration is:

glucose + oxygen → carbon dioxide + water + energy

Symbolically:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

The released energy is used to produce ATP, which cells use to power many processes.

Therefore, carbohydrates provide an important connection between:

food → glucose → cellular respiration → usable cellular energy

---

Disaccharides

Two monosaccharides can join together to form a:

disaccharide

Examples include:

• sucrose
• lactose
• maltose

For example, sucrose is formed from:

glucose + fructose

When two sugar molecules join, a condensation reaction can occur and water is released.

---

Polysaccharides

Many monosaccharides can join to form:

polysaccharides

"Poly" means many.

Important polysaccharides include:

starch

glycogen

cellulose

Although all can be built largely from glucose units, differences in how those units are connected give them very different biological functions.

This is an excellent example of:

structure → function

---

Starch

Starch is an important energy-storage carbohydrate in plants.

Plants produce glucose through photosynthesis.

Some glucose can then be converted into starch for storage.

Starch is commonly found in foods such as:

• potatoes
• rice
• wheat
• corn
• bread
• pasta

When starch is digested, it can eventually be broken down into glucose.

---

Glycogen

Glycogen is an important carbohydrate used for glucose storage in animals and fungi.

In humans, substantial glycogen stores are found in:

• liver
• skeletal muscles

Glycogen is highly branched.

Its branching allows glucose units to be added or removed efficiently when required.

Again:

molecular structure supports biological function.

---

Cellulose

Cellulose is a structural carbohydrate found in plant cell walls.

It provides:

• strength
• support
• resistance to stretching

Humans cannot digest cellulose because we lack enzymes capable of breaking its particular linkages efficiently.

However, cellulose is still important in the human diet as part of:

dietary fibre

Carbohydrate Structure and Function

Consider three molecules built largely from glucose:

starch → plant energy storage

glycogen → animal and fungal glucose storage

cellulose → plant structure

They contain related building units but differ in their bonding and arrangement.

Therefore:

same general building material ≠ same function

The arrangement of atoms matters.

---

Lipids

Lipids are a diverse group of biological molecules that include:

• fats
• oils
• phospholipids
• steroids
• waxes

They contain large amounts of:

carbon and hydrogen

and often contain oxygen.

Unlike carbohydrates, proteins, and nucleic acids, lipids are not generally true polymers composed of repeating monomers.

Triglycerides

Many fats and oils are primarily composed of molecules called:

triglycerides

A triglyceride is formed from:

one glycerol + three fatty acids

These components are connected through:

ester linkages

This connects biological chemistry directly to the organic chemistry of esters.

---

Fatty Acids

A fatty acid contains:

• a long hydrocarbon chain
• a carboxyl group, –COOH

A simplified representation is:

long carbon chain–COOH

The long hydrocarbon region is largely non-polar.

This helps explain why many lipids do not mix readily with water.

---

Saturated and Unsaturated Fatty Acids

Fatty acids can be:

saturated

or:

unsaturated

A saturated fatty acid contains no carbon-carbon double bonds in its hydrocarbon chain.

An unsaturated fatty acid contains one or more:

C=C

double bonds.

Therefore, this topic also connects with our study of:

alkenes and unsaturation

---

Fats and Oils

Fats and oils contain many of the same types of molecules but differ in their physical properties.

In general:

fats tend to be solid or semi-solid at room temperature.

oils tend to be liquid at room temperature.

The degree of unsaturation of their fatty acids can influence this difference.

C=C double bonds, particularly common cis double bonds in natural fatty acids, can introduce bends in the chains that make close packing more difficult.

---

Functions of Lipids

Lipids have several important biological roles.

They can provide:

Long-Term Energy Storage

Lipids store large amounts of chemical energy.

Thermal Insulation

Fat beneath the skin can reduce heat loss.

Protection

Fat deposits can help cushion organs.

Cell Membranes

Phospholipids are major components of cell membranes.

Chemical Signalling

Some hormones are lipid-derived molecules.

---

Phospholipids

A phospholipid has two very different regions:

• a water-attracting, or hydrophilic, head
• water-avoiding, or hydrophobic, tails

When placed in water, phospholipids naturally organize into structures such as a:

phospholipid bilayer

This forms the basic structure of cell membranes.

Structure Determines Membrane Function

The unusual structure of phospholipids makes cell membranes possible.

The hydrophilic heads interact with water.

The hydrophobic tails tend to avoid water and point inward.

This creates a stable bilayer.

The result is a flexible boundary separating:

the inside of a cell from its environment

The chemical structure of a phospholipid therefore directly determines its biological function.

---

Proteins

Proteins are large biological molecules built from:

amino acids

Proteins commonly contain:

• carbon
• hydrogen
• oxygen
• nitrogen

Some amino acids also contain:

sulfur

Proteins perform an enormous variety of jobs in living organisms.

Amino Acids

A typical amino acid contains:

• an amino group, –NH₂
• a carboxyl group, –COOH
• a hydrogen
• an R group

A simplified general structure is:

H₂N–CH(R)–COOH

Different R groups produce different amino acids.

Proteins commonly use 20 standard amino acids encoded by the genetic code.

---

Forming Proteins

Amino acids can join together by forming:

peptide bonds

A simplified reaction is:

amino acid + amino acid → dipeptide + water

Many amino acids can form:

polypeptide chains

These chains then fold into specific three-dimensional structures.

Therefore:

amino acids → polypeptide → folded protein

---

Protein Structure Determines Function

The order of amino acids in a protein affects how the protein folds.

The folded shape affects what the protein can do.

This gives another important relationship:

amino-acid sequence → protein shape → protein function

Changing even a small part of a protein's structure can sometimes significantly alter its function.

---

Functions of Proteins

Proteins perform many different roles.

Enzymes

Enzymes catalyse chemical reactions.

Structural Support

Examples include:

collagen and keratin

Transport

Hemoglobin transports oxygen.

Movement

Actin and myosin are involved in muscle contraction.

Defence

Antibodies help the immune system recognize and respond to foreign substances.

Signalling

Some hormones and receptors are proteins.

---

Enzymes: Structure and Function

An enzyme has a specific three-dimensional structure.

Part of the enzyme forms an:

active site

A substrate interacts with the active site.

The shape and chemical properties of the active site help determine which substrates can bind.

Therefore:

enzyme structure → active-site properties → enzyme function

Changes in temperature or pH can disrupt protein structure and reduce enzyme activity.

---

Nucleic Acids

The fourth major class is:

nucleic acids

The two most important examples are:

DNA — deoxyribonucleic acid

RNA — ribonucleic acid

Nucleic acids store, transmit, and help express genetic information.

They contain:

• carbon
• hydrogen
• oxygen
• nitrogen
• phosphorus

This is often summarized as:

CHONP

Nucleotides

Nucleic acids are built from smaller units called:

nucleotides

A nucleotide contains three main components:

• a phosphate group
• a five-carbon sugar
• a nitrogen-containing base

Therefore:

phosphate + sugar + base = nucleotide

Many nucleotides can join to form a nucleic acid.

---

DNA

DNA stores hereditary information.

DNA molecules contain two nucleotide strands arranged in a:

double helix

DNA contains four bases:

• adenine (A)
• thymine (T)
• cytosine (C)
• guanine (G)

Base pairing follows:

A pairs with T

C pairs with G

The sequence of bases stores biological information.

---

DNA Structure and Function

DNA is an excellent example of the relationship between structure and function.

Its structure allows it to:

• store information
• be copied
• pass information between cells and generations
• provide instructions used in protein production

The sequence of DNA bases acts as biological information.

Different base sequences can ultimately result in different amino-acid sequences in proteins.

---

RNA

RNA is another nucleic acid.

It differs from DNA in several ways.

RNA usually:

• contains ribose sugar
• uses uracil (U) instead of thymine
• exists primarily as a single strand, although it can fold into complex structures

RNA has several roles in cells, particularly in:

protein synthesis and gene regulation

---

From DNA to Protein

One of the most important relationships in biology is:

DNA → RNA → protein

DNA contains genetic information.

RNA helps transfer and use that information.

Amino acids are assembled into proteins according to genetic instructions.

This creates a direct connection between:

nucleic acids and proteins

Comparing the Four Major Biomolecules

---

Biomolecules and Energy

Different biomolecules play different roles in energy use.

Carbohydrates

Often provide readily accessible energy.

Glucose can enter cellular respiration.

Lipids

Are important for long-term energy storage and contain a large amount of stored chemical energy per unit mass.

Proteins

Can be used as an energy source, but their primary roles are usually structural and functional rather than energy storage.

Nucleic Acids

Their primary function is information storage and transfer rather than energy storage.

---

Biomolecules in Food

The foods we eat contain mixtures of biomolecules.

For example:

Bread

Rich in:

carbohydrates

Cooking Oil

Rich in:

lipids

Eggs

Contain significant:

proteins and lipids

Beans

Contain:

proteins and carbohydrates

Fruits

Contain:

carbohydrates, including sugars and fibre

Foods are rarely made of only one class of biomolecule.

Biomolecules in a Cell

A single cell contains all four major classes.

Carbohydrates

Can provide energy and participate in cell recognition and structure.

Lipids

Form much of the cell membrane and store energy.

Proteins

Act as enzymes, receptors, transporters, structural components, and signalling molecules.

Nucleic Acids

Store and use genetic information.

Life depends not on one biomolecule, but on the interaction of all four classes.

---

Organic Chemistry Connects the Biomolecules

The organic chemistry studied earlier appears throughout biological molecules.

Hydroxyl Groups

–OH

occur in sugars and many other biological molecules.

Carboxyl Groups

–COOH

occur in amino acids and fatty acids.

Amino Groups

–NH₂

occur in amino acids.

Ester Linkages

–COO–

occur in many lipids.

Carbon-Carbon Double Bonds

C=C

occur in unsaturated fatty acids.

Functional groups therefore help explain the chemical behaviour of biomolecules.

---

Functional Groups in Glucose

Glucose contains several:

–OH

groups.

These hydroxyl groups make glucose highly polar and allow it to interact strongly with water.

This helps explain why glucose is:

water-soluble

That property is biologically important because glucose must move through aqueous environments such as:

• blood
• cytoplasm
• tissue fluids

Again:

structure → property → biological function

---

Functional Groups in Fatty Acids

Fatty acids contain:

–COOH

at one end.

They also contain a long hydrocarbon chain.

The carboxyl end is relatively polar.

The hydrocarbon chain is largely non-polar.

This combination helps determine the behaviour of fatty acids and the larger lipid molecules constructed from them.

---

Functional Groups in Amino Acids

Amino acids contain:

–NH₂

and:

–COOH

This allows amino acids to participate in acid-base chemistry and join together through peptide bonds.

Different amino acids also contain different:

R groups

These side chains give proteins enormous chemical diversity.

---

Structure and Function: Carbohydrates

The structure of a carbohydrate influences its function.

For example:

glucose

is small and soluble, making it suitable for transport and cellular respiration.

glycogen

is highly branched, allowing rapid release of glucose units.

cellulose

forms strong fibres suitable for supporting plant cell walls.

Same broad biomolecule class:

different structures → different functions

---

Structure and Function: Lipids

Lipids contain large non-polar regions.

This makes many lipids:

hydrophobic

That property is important for:

• energy storage
• waterproofing
• biological membranes

Phospholipids contain both hydrophilic and hydrophobic regions.

That special structure causes them to organize into membranes.

---

Structure and Function: Proteins

Proteins provide perhaps the clearest example of structure determining function.

Their amino-acid sequence affects:

folding

Folding produces:

three-dimensional shape

Shape determines:

interactions with other molecules

Those interactions determine:

function

Therefore:

sequence → shape → function

---

Structure and Function: DNA

DNA's structure allows it to store information in the sequence of its bases.

Complementary base pairing allows DNA to be copied with high fidelity.

The double-stranded structure also helps protect and organize genetic information.

Therefore:

DNA structure → reliable information storage and transmission

---

The Chemistry of Life Is Interconnected

Consider what happens when a person eats food containing carbohydrates, lipids, and proteins.

Digestion breaks large molecules into smaller components.

Carbohydrates can yield:

monosaccharides

Proteins can yield:

amino acids

Lipids can yield:

fatty acids and other components

Cells then use these molecules to:

• release energy
• build new molecules
• repair tissues
• make membranes
• produce enzymes
• produce hormones and signalling molecules

DNA contains information that helps determine which proteins cells make.

All four biomolecule classes therefore interact.

---

Worked Example: Identify the Biomolecule

A molecule is built from amino acids.

What class does it belong to?

Protein

Why?

Proteins consist of amino-acid residues joined by peptide bonds.

---

Worked Example: Identify the Biomolecule

A molecule contains a phosphate group, a sugar, and a nitrogenous base.

What is the smaller unit?

Nucleotide

Which major biomolecule is constructed from these units?

Nucleic acid

---

Worked Example: Energy Storage

A plant stores excess glucose as a large polysaccharide.

Which molecule?

Starch

An animal stores glucose in a highly branched polysaccharide.

Which molecule?

Glycogen

---

Worked Example: Cell Membrane

Which biomolecule class forms the basic bilayer of a cell membrane?

Lipids

More specifically:

phospholipids

Why?

Their hydrophilic heads interact with water while their hydrophobic tails avoid it.

---

Worked Example: Enzyme

An enzyme catalyses a reaction inside a cell.

Which biomolecule class does it most commonly belong to?

Protein

Why can enzymes perform specific reactions?

Their three-dimensional structures create active sites with specific chemical properties.

---

Worked Example: Genetic Information

A molecule stores genetic instructions using a sequence of A, T, C, and G.

Which molecule?

DNA

Which biomolecule class?

Nucleic acid

---

Common Mistakes

Thinking All Biomolecules Are Polymers

Lipids are major biomolecules, but they are not generally considered true polymers.

Thinking Carbohydrates Only Provide Energy

Some carbohydrates have structural functions.

Cellulose is an important example.

Thinking Lipids Only Store Energy

Lipids also form membranes, provide insulation, protect organs, and participate in signalling.

Thinking Proteins Are Only for Muscles

Proteins function as enzymes, antibodies, receptors, transporters, hormones, structural materials, and much more.

Thinking DNA Is a Protein

DNA is a:

nucleic acid

Confusing Amino Acids with Nucleotides

amino acids → proteins

nucleotides → nucleic acids

Confusing Fatty Acids with Amino Acids

Fatty acids are important components of many lipids.

Amino acids are the building blocks of proteins.

Thinking All Carbohydrates Are Sugars

Carbohydrates include sugars as well as large polysaccharides such as starch, glycogen, and cellulose.

Thinking All Lipids Are Fats

Lipids include fats, oils, phospholipids, steroids, and other molecules.

Thinking Structure Does Not Matter

Small structural differences can produce major differences in biological function.

Forgetting Nitrogen in Proteins

Proteins contain nitrogen because amino acids contain amino groups.

Forgetting Phosphorus in Nucleic Acids

DNA and RNA contain phosphorus in their phosphate groups.

Thinking Organic Means "Natural"

In chemistry, organic generally refers to carbon-based chemistry. It does not simply mean natural, healthy, or produced without synthetic chemicals.

---

Key Terms

Biomolecule — A molecule associated with living organisms and biological processes.

Organic compound — A carbon-based compound belonging to the field of organic chemistry.

Carbon skeleton — The framework of carbon atoms forming part of an organic molecule.

Carbohydrate — A class of biomolecules including sugars, starch, glycogen, and cellulose.

Monosaccharide — A simple sugar molecule that can serve as a carbohydrate building unit.

Glucose — A monosaccharide with formula C₆H₁₂O₆ used extensively in cellular energy metabolism.

Disaccharide — A carbohydrate formed from two monosaccharides.

Polysaccharide — A carbohydrate consisting of many linked monosaccharide units.

Starch — A major glucose-storage polysaccharide in plants.

Glycogen — A highly branched glucose-storage polysaccharide in animals and fungi.

Cellulose — A structural polysaccharide found in plant cell walls.

Lipid — A diverse group of largely hydrophobic biological molecules including fats, oils, phospholipids, and steroids.

Triglyceride — A lipid formed from glycerol and three fatty acids.

Glycerol — A three-carbon alcohol component of triglycerides and many phospholipids.

Fatty acid — A molecule containing a hydrocarbon chain and carboxyl group that forms part of many lipids.

Saturated fatty acid — A fatty acid without C=C bonds in its hydrocarbon chain.

Unsaturated fatty acid — A fatty acid containing one or more C=C bonds.

Phospholipid — A lipid containing hydrophilic and hydrophobic regions and forming a major component of cell membranes.

Hydrophilic — Having an affinity for or interacting favourably with water.

Hydrophobic — Having little affinity for water.

Protein — A biological macromolecule composed of one or more polypeptide chains.

Amino acid — A building block of proteins containing amino and carboxyl functional groups.

Peptide bond — The –CO–NH– linkage joining amino-acid residues.

Polypeptide — A chain of amino-acid residues joined by peptide bonds.

Enzyme — A biological catalyst, usually a protein.

Nucleic acid — A biomolecule constructed from nucleotides and involved in genetic information.

DNA — Deoxyribonucleic acid, the primary hereditary information-storage molecule in cells.

RNA — Ribonucleic acid, a family of nucleic acids with important roles in gene expression and other cellular processes.

Nucleotide — A building unit of nucleic acids containing a sugar, phosphate group, and nitrogenous base.

Nitrogenous base — A nitrogen-containing component of a nucleotide.

Polymer — A large molecule formed from many linked smaller units.

Monomer — A smaller molecular unit that can be linked into a larger structure.

Condensation reaction — A reaction in which molecules join and a small molecule such as water is produced.

Hydrolysis — A reaction using water to break a larger molecule into smaller components.

Cellular respiration — A set of reactions through which cells transfer energy from molecules such as glucose into usable forms such as ATP.

ATP — Adenosine triphosphate, a molecule used in cellular energy transfer.

Functional group — A specific group of atoms that influences the characteristic reactions and properties of an organic molecule.

Structure-function relationship — The principle that molecular structure influences biological properties and function.

---

Key Takeaways

• The four major classes of biomolecules are carbohydrates, lipids, proteins, and nucleic acids.
• Carbon forms the chemical backbone of an enormous variety of biological molecules.
• Carbon can form four covalent bonds.
• Biomolecules commonly contain C, H, O, N, P, and S in different combinations.
• Carbohydrates contain mainly carbon, hydrogen, and oxygen.
• Carbohydrates have important roles in energy supply, energy storage, and structure.
• Glucose is an important monosaccharide.
• Starch stores glucose in plants.
• Glycogen stores glucose in animals and fungi.
• Cellulose provides structural support in plant cell walls.
• Lipids contain large amounts of carbon and hydrogen.
• Lipids include fats, oils, phospholipids, and steroids.
• Triglycerides contain glycerol and three fatty acids.
• Lipids provide long-term energy storage, insulation, protection, and membrane structure.
• Phospholipids form the basic bilayer of cell membranes.
• Proteins are constructed from amino acids.
• Proteins contain carbon, hydrogen, oxygen, and nitrogen, and some contain sulfur.
• Amino acids join through peptide bonds.
• Protein shape is closely related to protein function.
• Proteins can act as enzymes, transporters, antibodies, receptors, structural materials, and signalling molecules.
• Nucleic acids are built from nucleotides.
• DNA and RNA are nucleic acids.
• Nucleic acids contain carbon, hydrogen, oxygen, nitrogen, and phosphorus.
• DNA stores hereditary information.
• RNA has important roles in using and regulating genetic information.
• DNA information can ultimately determine amino-acid sequences in proteins.
• Functional groups studied in organic chemistry occur throughout biomolecules.
• Fatty acids contain carboxyl groups.
• Amino acids contain amino and carboxyl groups.
• Many lipids contain ester linkages.
• Unsaturated fatty acids contain C=C bonds.
• Sugars contain several polar functional groups, including hydroxyl groups.
• Structure strongly influences biological function.
• Organic chemistry provides much of the chemical foundation needed to understand living systems.

A useful summary is:

Carbohydrates → energy, storage and structure

Lipids → long-term energy, membranes and insulation

Proteins → enzymes, structure, transport and countless cellular functions

Nucleic acids → genetic information

And the central theme is:

MOLECULAR STRUCTURE → CHEMICAL PROPERTIES → BIOLOGICAL FUNCTION

---

Check Your Understanding

1. Name the four major classes of biomolecules.

2. Which element forms the backbone of organic molecules?

3. Why can carbon form such a large variety of molecules?

4. Which three elements are most commonly associated with carbohydrates?

5. What is a monosaccharide?

6. Give an important example of a monosaccharide.

7. State one major biological function of glucose.

8. What is a polysaccharide?

9. What is the main role of starch?

10. What is the main role of glycogen?

11. What is the main role of cellulose?

12. Name four types of lipid.

13. What components form a triglyceride?

14. What functional group occurs in fatty acids?

15. Explain the difference between saturated and unsaturated fatty acids.

16. Give three biological functions of lipids.

17. What type of lipid forms much of the cell membrane?

18. Explain why phospholipids naturally form bilayers in water.

19. What are the building blocks of proteins?

20. What bond joins amino acids?

21. Explain why protein shape is important.

22. Give four different functions of proteins.

23. What are the two major nucleic acids?

24. What are the building units of nucleic acids?

25. Name the three components of a nucleotide.

26. What is the main biological role of DNA?

27. Explain the relationship:

DNA → RNA → protein

28. Which major biomolecule contains phosphorus as a characteristic element?

29. Explain why lipids are not usually described as true polymers.

30. Challenge: A cell contains glucose, phospholipids, enzymes, and DNA.

a. Classify glucose by biomolecule type.
b. State one important function of glucose.
c. Identify the biomolecule class of phospholipids.
d. Explain why phospholipids are suitable for forming cell membranes.
e. Identify the biomolecule class to which most enzymes belong.
f. Identify the building blocks of proteins.
g. Name the bonds connecting amino-acid residues.
h. Explain why the shape of an enzyme affects its function.
i. Identify the biomolecule class of DNA.
j. Identify the building blocks of DNA.
k. Name the three components of a nucleotide.
l. State the main role of DNA.
m. Explain how information in DNA can influence the structure of a protein.
n. Identify a functional group found in amino acids.
o. Identify a functional group found in fatty acids.
p. Explain how ester chemistry is connected to many lipids.
q. Explain how C=C bonds are connected to unsaturated fatty acids.
r. Compare the major energy-related roles of carbohydrates and lipids.
s. Explain why proteins are generally better described as functional and structural molecules than as energy-storage molecules.
t. Use one example from each of the four biomolecule classes to explain the principle:

structure → properties → biological function.