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

• Phospholipid bilayer:
  Heads love water, tails hate water → forms barrier.
  Only small/non-polar molecules pass easily.

• Proteins:
  Act as channels or carriers → control movement in/out.

• Cholesterol:
  Adds stability and flexibility.

• Glycolipids & Glycoproteins:
  Have sugars on the outside → for recognition, protection, 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.

Prokaryotes

• No true nucleus — DNA in nucleoid

• No membrane-bound organelles — only ribosomes

• Single circular DNA + plasmids

• Cell wall:

  • Bacteria → peptidoglycan

  • Archaea → proteins or polysaccharides (no peptidoglycan)

Eukaryotes

• True nucleus with nuclear envelope

• Membrane-bound organelles (mitochondria, ER, Golgi, etc.)

• Linear chromosomes in nucleus

• Cell wall:
  

  • Plants → cellulose

  • Fungi → chitin

  • Animals → no wall

Describe peptidoglycan and contrast bacterial walls with archaeal walls

Peptidoglycan = main material in bacterial cell walls.

• Made of glycan chains (sugars) + peptide crosslinks (amino acids).

• Forms a strong, flexible lattice → gives shape, protection, prevents bursting.

Bacteria: have peptidoglycan.

Archaea: no peptidoglycan — walls of proteins or polysaccharides (adapted to extremes).

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

• Pili: help bacteria stick to surfaces or transfer DNA.

• Plasmids: small circular DNA with extra survival genes.

• Prokaryotes: simple cells — no nucleus, no organelles.

• Flagella: tail-like structures powered by protons for movement.

• Monotrichous: one single flagellum.

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.