14: Cell Diversity

Explain why “oil and water don’t mix” and describe how this drives phospholipid bilayer formation.

Water molecules are polar — they have a partial negative charge (δ–) on the oxygen atom and partial positive charges (δ+) on the hydrogens.
Because of this polarity, water molecules strongly attract each other through hydrogen bonds.

Oil (lipids) are non-polar, meaning they lack charges. Water molecules can’t form hydrogen bonds with them, so instead they push the oil molecules away.
This exclusion is called the hydrophobic effect — water sticks to itself and forces non-polar molecules to cluster together.

Draw and label a simple cell membrane: phospholipids (heads/tails), integral/transmembrane proteins, cholesterol, glycolipids/glycoproteins, and the glycocalyx

The cell membrane, also called the phospholipid bilayer, is made up of several important parts that work together to control what enters and leaves the cell and how it communicates.

First, the phospholipid bilayer has hydrophilic heads that love water and hydrophobic tails that hate water. This creates a barrier — only small or non-polar molecules can pass through easily.

Then there are proteins. Some proteins sit on the surface, and others go all the way through the membrane — these are called integral or transmembrane proteins. They act like channels or carriers, letting specific molecules in or out.

Cholesterol is tucked between the phospholipids. It doesn’t transport things — instead, it helps keep the membrane stable and flexible, especially when temperatures change.

Glycolipids are lipids with a carbohydrate (sugar) attached.

Glycoproteins are proteins with carbohydrates attached.

These carbohydrates stick out on the outside of the cell and form what’s called the glycocalyx, which helps with cell recognition, protection, and communication

Define “amphipathic” and identify amphipathic molecules in the membrane (phospholipids, cholesterol).

Amphipathic means a molecule has both hydrophilic (water-loving) and hydrophobic (water-fearing) regions within the same structure.

In cell membranes:

• Phospholipids are amphipathic because they have a polar phosphate head (hydrophilic) and non-polar fatty acid tails (hydrophobic).

• Cholesterol is also amphipathic — its –OH (hydroxyl) group is hydrophilic, while the four-ring carbon structure and side chain are hydrophobic.

Describe how cholesterol packs into membranes and justify its role in fluidity buffering (too cold vs too hot).

• When it’s too hot: cholesterol’s rigid rings restrict phospholipid movement, preventing the membrane from becoming too fluid or leaky.

• When it’s too cold: cholesterol prevents phospholipids from packing too tightly, keeping the membrane from becoming stiff or solid.

Differentiate endocrine (no duct → ISF/blood) from exocrine (duct) glands; give one example of each.

• Endocrine glands have no ducts. They release hormones directly into the interstitial fluid or bloodstream, where they travel to target cells.
  → Example: Thyroid gland or pituitary gland (both secrete hormones into blood).

• Exocrine glands have ducts that carry secretions to the body surface or into body cavities.
  → Example: Sweat glands (secrete sweat through ducts to the skin surface) or salivary glands.

Classify proteins as fibrous vs globular and predict where hydrophobic vs hydrophilic R-groups reside in a globular protein

• Fibrous proteins are long, strand-like molecules that provide structure and strength.
  → Examples: Collagen (connective tissue), keratin (hair, nails).

• Globular proteins are compact, spherical molecules that perform functional roles such as enzymes, transporters, or hormones.
  → Examples: Hemoglobin, enzymes, insulin.

In globular proteins:

• Hydrophobic R-groups face inward, hidden from water.

• Hydrophilic R-groups face outward, interacting with the surrounding watery environment.

Compare and contrast prokaryotes vs eukaryotes: nucleus, organelles, chromosome location, cell wall composition.

Feature	Prokaryotes	Eukaryotes
Nucleus	❌ No true nucleus — DNA lies free in the cytoplasm (in a region called the nucleoid).	✅ True nucleus enclosed by a nuclear envelope.
Organelles	❌ No membrane-bound organelles. Have ribosomes only.	✅ Membrane-bound organelles such as mitochondria, ER, Golgi, lysosomes, chloroplasts (in plants).
Chromosome location	Single circular DNA in the nucleoid; may also have plasmids (small extra DNA loops).	Linear chromosomes inside the nucleus.
Cell wall composition	Bacteria: made of peptidoglycan (glycan chains + peptide crosslinks). Archaea: no peptidoglycan — wall of proteins or polysaccharides.	Plants: cellulose wall. Fungi: chitin wall. Animals: ❌ no cell wall (only membrane).

Describe peptidoglycan and contrast bacterial walls with archaeal walls

Peptidoglycan is the main structural material in bacterial cell walls.
It is made of:

• Glycan chains – long chains of sugar units (polysaccharides).

• Peptide crosslinks – short chains of amino acids that connect the glycan strands together.

This forms a strong, flexible lattice that gives the cell shape, protection, and prevents bursting from osmotic pressure.

In contrast:

• Bacterial walls = contain peptidoglycan.

• Archaeal walls = lack peptidoglycan; instead, they’re made of proteins or other polysaccharides, often adapted to extreme environments (e.g. high temperature, no oxygen).

Explain pili and plasmids in prokaryotes (genetic exchange) and outline the proton-driven flagellar motor concept.

1.Pili (singular: pilus)

Description:

Pili are short, hair-like protein structures found on the surface of many prokaryotic cells (especially bacteria).

Main function:

They help bacteria attach to surfaces or to other cells, and some special pili are used in DNA transfer.

Key process:

• The sex pilus is used during conjugation, a type of genetic exchange.

• It connects two bacteria and forms a bridge to transfer plasmid DNA.

2.Plasmids

Description:

Plasmids are small, circular pieces of DNA separate from the main bacterial chromosome.

Main function:

They often carry extra genes that give bacteria advantages — like antibiotic resistance or metabolic traits.

Key process:

During conjugation, a plasmid can be copied and sent to another bacterium.

3.Prokaryote Genetic Exchange

Description:

Even though bacteria reproduce asexually (by binary fission), they still exchange genes in several ways to increase diversity.

Main function:

To share useful genes like antibiotic resistance, toxin production, or metabolic capabilities.

Key processes:

1. Conjugation — DNA transfer through a pilus (plasmid transfer).

2. Transformation — Bacteria take up naked DNA from the environment.

3. Transduction — Viruses (bacteriophages) transfer DNA between bacteria.

State the relationship between the endoplasmic reticulum (ER) and the nuclear envelope.

• The endoplasmic reticulum (ER) is continuous with the outer membrane of the nuclear envelope.

• They form one connected membrane system.

• This continuity allows mRNA to move easily from the nucleus to ribosomes on the rough ER (RER).

• RER ribosomes translate these mRNAs into proteins that enter the ER for folding, modification, and transport.

• This link directly connects gene expression → protein synthesis → secretion or membrane insertion.