Comparison of Prokaryotic and Eukaryotic Cells

4.2 Prokaryotic Cells

• Cells fall into two broad categories: prokaryotic and eukaryotic.
  • Prokaryotes = predominantly single-celled organisms in the domains Bacteria and Archaea.
  • Eukaryotes = organisms whose cells have a true nucleus and membrane-bound organelles (domains: Eukarya; includes animals, plants, fungi, protists).
• Basic, shared cell components (found in all cells):
  • Plasma membrane: outer covering that separates the cell’s interior from the environment.
  • Cytoplasm: jelly-like interior containing cytosol and organelles.
  • DNA: genetic material.
  • Ribosomes: synthesize proteins.
• Key distinction: Prokaryotes lack a nucleus and membrane-bound organelles, whereas eukaryotes have both.
• Prokaryotic DNA localization:
  • DNA is found in the nucleoid, a central region of the cell, not in a true nucleus.
• General prokaryotic cell structure (from Figure 4.5):
  • Common components: nucleoid (DNA), ribosomes, plasma membrane, cell wall.
  • Additional structures present in some bacteria: capsule (polysaccharide), sometimes a glossy outer layer.
  • Most prokaryotes have a peptidoglycan cell wall.
  • Capsule: helps the cell attach to surfaces in its environment.
  • Surface appendages (not always present):
  • Flagella: locomotion.
  • Pili: exchange genetic material during conjugation.
  • Fimbriae: attachment to surfaces, including host cells.
• Human health and hygiene context:
  • Microbes are ubiquitous on doorknobs, money, hands, etc.; handwashing is a key preventive measure against contagious illnesses.
  • Not all microbes cause disease; many are beneficial (e.g., gut microbes synthesize vitamin K; fermentation processes).
• Careers in microbiology (overview from the transcript):
  • Food industry, veterinary, medical fields.
  • Pharmaceutical sector: identifying new antibiotic sources to treat bacterial infections.
  • Environmental microbiology: bioremediation to remove pollutants from soil, groundwater, or contaminated sites.
  • Bioinformatics: modeling bacterial epidemics and other data-driven analyses.
  • Microbiologists can contribute to computer models of epidemics and other complex systems.
• HPV Pap smear example (Figure 4.4) and clinical relevance:
  • Pap smear: sample of cells from the uterine cervix examined for abnormalities that could indicate cervical cancer or infection.
  • HPV infection can alter cell appearance: infected cells may be larger and may exhibit multinucleation (two nuclei in some cells).
• Size context (cell size):
  • Prokaryotic cell diameter ≈
    0.1 \text{ to } 5.0\ \mu\text{m}
  • Eukaryotic cell diameter ≈
    10 \text{ to } 100\ \mu\text{m}
  • Small size allows rapid diffusion of ions and molecules; wastes diffuse out more quickly.
  • Larger cells require adaptations for intracellular transport.
• Why small size is advantageous (and why large size poses challenges):
  • Surface area-to-volume ratio decreases as radius increases.
  • For a spherical cell:
  • Surface area: A = 4\pi r^2
  • Volume: V = \frac{4}{3}\pi r^3
  • SA:V ratio: \text{SA:V} = \frac{A}{V} = \frac{3}{r}
  • As cells grow, diffusion efficiency drops; to remain efficient, cells may divide or develop organelles to compartmentalize functions.
• Recap of prokaryotic focus:
  • Simplicity and small size distinguish prokaryotes from eukaryotes.
  • Nucleoid as the DNA-containing region; lack of a true nucleus and membrane-bound organelles.

4.3 Eukaryotic Cells

• Guiding principle: "form follows function" in biology; eukaryotic cells exhibit greater complexity to support specialized functions.
• Key features of eukaryotic cells that set them apart from prokaryotes:
  • True nucleus enclosed by a nuclear envelope.
  • Numerous membrane-bound organelles (e.g., endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria).
  • Several linear chromosomes.
  • Organelles enable compartmentalization of functions so different processes can occur in parallel.
• The nucleus (as an organelle):
  • Often the most prominent organelle.
  • Houses DNA and directs synthesis of ribosomes and proteins.
  • Nucleolus: condensed region of chromatin where ribosomal RNA (rRNA) synthesis occurs.
  • Chromatin: DNA-protein complex; chromosomes are linear in eukaryotes and visible during division.
  • Nucleoplasm: semi-solid fluid inside the nucleus.
  • Nuclear envelope: double phospholipid bilayer surrounding the nucleus; outer membrane is continuous with the endoplasmic reticulum.
  • Nuclear pores: control passage of ions, molecules, and RNA between nucleoplasm and cytoplasm.
• The cytoplasm:
  • The region between the plasma membrane and the nuclear envelope.
  • Contains organelles suspended in the gel-like cytosol, the cytoskeleton, and other chemicals.
  • Cytoplasm is 70–80% water but has a semi-solid consistency due to proteins and other molecules.
  • Contains sugars, polysaccharides, amino acids, nucleic acids, fatty acids, glycerol derivatives, and ions (e.g., Na⁺, K⁺, Ca²⁺).
  • Many metabolic reactions, including protein synthesis, occur in the cytoplasm.
• The plasma membrane (in eukaryotes):
  • Phospholipid bilayer with embedded proteins and cholesterol.
  • Controls passage of organic molecules, ions, water, and oxygen into and out of the cell; wastes exit via the membrane.
  • Absorptive cells have microvilli to increase surface area (e.g., intestinal lining).
  • Celiac disease example: gluten triggers an immune response that damages microvilli, reducing nutrient absorption; treatment is a gluten-free diet.
• The cytoskeleton and the cytoplasmic organization support cellular architecture and transport.
• Ribosomes (revisited):
  • Large and small subunits form ribosomes; ribosomes can be free in the cytoplasm or attached to the endoplasmic reticulum or nuclear envelope.
  • Ribosomes translate mRNA into proteins; many ribosomes cluster as polyribosomes.
• The mitochondria:
  • Double-membrane organelles with their own ribosomes and DNA.
  • Inner membrane folds (cristae) increase surface area for ATP production.
  • Mitochondrial matrix lies inside the inner membrane.
  • ATP synthesis occurs on the inner membrane (cellular respiration uses oxygen to produce ATP and CO₂ as a waste product).
  • Muscle cells have high mitochondrial density to support energy needs; in low-oxygen conditions, glycolysis yields limited ATP and lactic acid increases.
• Peroxisomes:
  • Small, round organelles with single membranes.
  • Carry out oxidation reactions that break down fatty acids and amino acids and detoxify poisons.
  • H₂O₂ produced during oxidation is decomposed to water and oxygen by catalase within peroxisomes.
  • Glyoxysomes (specialized peroxisomes in plants) convert stored fats into sugars.
• Vesicles and vacuoles:
  • Membrane-bound sacs used for storage and transport.
  • Vesicles can fuse with the plasma membrane or other membrane systems; vacuoles in plant cells typically do not fuse with other membranes.
  • Plant vacuoles may contain enzymes that break down macromolecules.
• Animal cells vs. plant cells:
  • Both possess microtubule-organizing centers (MTOCs).
  • Animal cells have centrosomes with a pair of centrioles (two perpendicular cylinders) and lysosomes; plant cells generally lack centrosomes and lysosomes.
  • Plant cells have a cell wall, chloroplasts, other plastids, and a large central vacuole; animal cells lack these features.
• The centrosome and centrioles:
  • Centrosome = MTOC near the nucleus in animal cells; contains a pair of centrioles (nine triplets of microtubules per centriole).
  • The centrosome replicates before cell division; centrioles are thought to help pull chromosomes apart, although cells can divide without centrosomes, and plant cells can divide without them.
• Lysosomes:
  • Digestive organelles containing enzymes active at low pH; break down proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles.
• The cell wall:
  • External to the plasma membrane in plant cells and many fungi and protists.
  • Primary component in prokaryotic walls is peptidoglycan; plant cell walls are rich in cellulose (β-glucose linked by 1-4 bonds).
  • Cell wall provides protection, shape, and rigidity; explains why celery is crunchy when bitten.
• Chloroplasts:
  • Plant cell organelles responsible for photosynthesis;
  • Have their own DNA and ribosomes.
  • Contain chlorophyll and internal membrane systems: outer membrane, inner membrane, thylakoids arranged in stacks called grana, and surrounding fluid called the stroma.
  • Thylakoid membranes host light-harvesting reactions; the stroma hosts sugar synthesis.
• Endosymbiosis:
  • Mitochondria and chloroplasts likely originated from endosymbiotic bacteria that were engulfed by a host cell.
  • Evidence includes their DNA and ribosomes, size similarity to bacteria, and the dependence on host cells.
  • Ancestral aerobic bacteria became mitochondria; autotrophic bacteria (cyanobacteria) became chloroplasts.
• The central vacuole:
  • Large central vacuole in plant cells regulates water balance.
  • Water movement in response to soil water potential affects turgor pressure; wilting occurs when the vacuole loses water and loses turgor.
  • When filled with water, the central vacuole supports cell expansion and growth with less cytoplasm synthesis.

4.4 The Endomembrane System and Proteins

• The endomembrane system ("endo" = within) is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.
• Major components of the endomembrane system:
  • Nuclear envelope
  • Lysosomes
  • Vesicles
  • Endoplasmic reticulum (ER)
  • Golgi apparatus
• The plasma membrane is included in the endomembrane system because it interacts with other endomembranous organelles and participates in trafficking between compartments.
• Note on scope:
  • The endomembrane system does not include mitochondria or chloroplasts.
• Functional theme:
  • The system coordinates synthesis, modification, packaging, and transport of cellular products (lipids and proteins) across compartments.

Additional context and connections

• Prokaryotic vs. eukaryotic cell organization illustrates how compartmentalization enables complex functions and specialization.
• The concept of endosymbiosis explains why mitochondria and chloroplasts have their own DNA and ribosomes, reflecting a historic symbiotic origin.
• The size and surface area-to-volume considerations underpin why cells stay small or develop internal transport systems and organelles to maintain efficient exchange with the environment.
• Real-world relevance:
  • Understanding cell structure informs medical science (e.g., Pap smear interpretation and HPV effects on cervical cells).
  • Knowledge of microbes underpins public health practices (hand hygiene) and environmental biotechnology (bioremediation and antibiotic discovery).