Macromolecules

Carbohydrates • Lipids • Proteins • Nucleic Acids

All living cells are built from four major classes of macromolecules.
These molecules form the structure, store the information, and power the reactions that sustain life.

This page introduces each class at a beginner-friendly level, with focus on:

• what they are
• what they do
• where you find them
• whether they are visible under optical microscopy
• examples relevant to yeast, plants, and general biology

1. What Are Macromolecules?

Macromolecules are large biological molecules made up of smaller repeating units.

The four major types are:

1. Carbohydrates – sugars & starches
2. Lipids – fats, oils, membranes
3. Proteins – enzymes, receptors, motors
4. Nucleic Acids – DNA & RNA

Together, these molecules form the structure of cells and carry out most cellular functions.

2. Carbohydrates

Carbohydrates are molecules made of carbon, hydrogen, and oxygen.
They serve as:

• energy sources
• structural materials
• cell signaling molecules

2.1. Monosaccharides (simple sugars)

Examples:

• glucose
• fructose
• galactose

Cells break down glucose to produce ATP through glycolysis.

2.2. Disaccharides

Two sugars linked together:

• sucrose (glucose + fructose)
• lactose
• maltose

2.3. Polysaccharides (complex carbohydrates)

Many sugar units linked together.

Examples:

• starch (plants — energy storage)
• glycogen (animals & yeast — energy storage)
• cellulose (plants — cell walls)
• chitin-like mannans (yeast cell wall components)

2.4. Visibility under optical microscopy

Carbohydrates cannot be seen directly, but:

• Cell walls of plants → visible due to cellulose
• Yeast cell walls → visible due to glucans/mannoproteins
• Starch granules → visible in some plant tissues (not onion epidermis)

3. Lipids

Lipids include fats, oils, waxes, and phospholipids.
They are mostly hydrophobic (do not mix with water).

3.1. Major types of lipids

• Triglycerides – energy storage
• Phospholipids – form cell membranes
• Sterols – cholesterol in animals, ergosterol in yeast
• Fatty acids – building blocks of lipids

3.2. Functions of lipids

• form membranes (phospholipid bilayers)
• energy storage
• cell signaling (hormones)
• insulation and protection

3.3. Lipids in yeast

• membranes contain ergosterol (instead of cholesterol)
• yeast store lipids in lipid droplets under stress conditions

3.4. Visibility under optical microscopy

Lipids are not visible directly, except:

• large lipid droplets in some cells (not typical in onion or yeast without special stains)
• membranes are too thin to resolve (5–10 nm)

4. Proteins

Proteins are polymers of amino acids.
They perform the vast majority of work in the cell.

4.1. Functions of proteins

• enzymes (accelerate reactions)
• receptors (receive signals)
• pumps and channels (membrane transport)
• structural components (cytoskeleton)
• motors (kinesin, myosin)
• regulatory factors (transcription factors)

4.2. Where proteins are found

Everywhere:

• cell membrane
• cytoplasm
• nucleus
• mitochondria
• secreted outside the cell
• embedded in cell walls (yeast, bacteria)

4.3. Visibility under optical microscopy

Proteins are too small to see individually:

• 5–10 nm (20–40x smaller than microscope resolution)

Proteins only become visible indirectly if:

• they form massive complexes (e.g., actin fibers with fluorescence staining)
• they are tagged with GFP (fluorescence microscopy)

5. Nucleic Acids (DNA & RNA)

Nucleic acids store and transmit genetic information.

5.1. DNA (Deoxyribonucleic Acid)

• double helix
• stores genetic instructions
• found in nucleus and mitochondria

5.2. RNA (Ribonucleic Acid)

Several types:

• mRNA (messenger RNA)
• tRNA (transfer RNA)
• rRNA (ribosomal RNA)
• miRNA, siRNA, lncRNA

Functions:

• carriers of genetic information
• building blocks of ribosomes
• regulators of gene expression

5.3. Visibility under optical microscopy

Nucleic acids are not visible directly.

Exceptions:

• condensed chromosomes during mitosis in onion root tips
• heavily stained nuclei reveal “dark spots” (mostly rRNA-rich nucleoli)

Yeast chromosomes and nuclei are too small to be resolved.

6. Summary Table: Major Macromolecules

7. Why Macromolecules Matter in Cell Biology

Understanding macromolecules helps explain:

• why yeast cell walls are visible
• why mitochondria are not visible
• why DNA is invisible except during mitosis
• how ATP is produced
• how membranes behave
• how proteins create nearly all cellular functions

Macromolecules form the foundation of:

• metabolism
• genetics
• cell structure
• aging
• microscopy
• biotechnology
• every experiment you will do in your home lab

8. Quick Beginner Summary

• Carbohydrates → energy & structure
• Lipids → membranes & storage
• Proteins → do almost everything
• Nucleic acids → store information

Life is built from combinations of these four macromolecules.