LW

Unit 1 Lecture 7

Endomembrane system: vesicles, Golgi, and secretion vs intracellular retention

  • The rough endoplasmic reticulum (RER) and the nuclear vicinity assess proteins to decide destinies: keep them inside the cell or package them for secretion.
  • Secretory cells (e.g., sweat glands, hormone-releasing glands like thyroid or pancreas) rely on this system to export secretions outside the cell.
  • If a protein is needed inside the cell, the Golgi apparatus packages it into a vesicle for transport to its intracellular destination.
  • Vesicles are essentially bubbles of phospholipids that bud from the ER or Golgi and fuse with other membranes; their phospholipid composition matches the membranes they interact with, enabling fusion and delivery of contents.
  • Exocytosis releases vesicle contents to the outside or into extracellular space; vesicles are major players in secretion and waste disposal.

Lysosomes, endocytosis, and intracellular digestion

  • Lysosomes contain digestive enzymes that break down damaged or invading material brought into the cell by endocytosis.
  • A lysosome can fuse with a membrane-bound vesicle or phagosome to digest its contents using hydrolytic enzymes.
  • Waste materials and digested contents may be recycled or expelled via vesicles; this is part of normal cellular housekeeping.
  • Lysosomes also participate in defense: if harmful material enters the cell, lysosomal enzymes can help neutralize it.

Autophagy and programmed cell death

  • Lysosomes contain enzymes that, if unleashed within a damaged cell, can contribute to self-destruction (apoptosis).
  • Programmed cell death is normal and important during development (e.g., webbing between fingers/toes is removed by apoptosis) and in response to irreparable damage.
  • Infected cells may undergo self-destruction to prevent viral replication and spread, even if that means sacrificing the cell.

Vacuoles vs vesicles: storage vs transport

  • Vacuoles are primarily storage compartments within the cell.
  • Vesicles are primarily involved in transport of materials between organelles or to the cell membrane for secretion.

Freshwater protists: osmoregulation and intracellular energetics

  • Freshwater organisms like paramecia are 100% water with cytoplasmic saline content; they must regulate inflow of water due to osmotic pressure.
  • Contractile vacuoles act as osmoregulatory devices to expel excess water, maintaining cellular integrity.
  • These organisms contain their own DNA and can process energy, similar to mitochondria in other cells, illustrating endosymbiotic-like features across life forms.

Endosymbiotic theory: mitochondria and chloroplasts

  • Mitochondria and chloroplasts resemble independent organelles with their own DNA and machinery for replication.
  • Endosymbiotic theory proposes that these organelles originated as free-living prokaryotes that were incorporated into a host cell, becoming essential energy converters.
  • Mitochondria have an outer membrane and an inner membrane; the electron transport chain (ETC) resides in the inner membrane, and proton gradients across these membranes drive ATP synthesis.
  • Chloroplasts in plants and algae contain chlorophyll and perform photosynthesis, converting sunlight into chemical energy (glucose) and generating ATP precursors for the cell.
  • Both mitochondria and chloroplasts replicate independently of the host cell and contain their own circular DNA, supporting the endosymbiotic origin.

Energy conversion: from big food chunks to ATP

  • Food energy starts as large macromolecules (proteins, lipids, carbohydrates, nucleic acids) that are too big to enter cells directly.
  • The digestive system breaks these macromolecules into monomers (amino acids, fatty acids, monosaccharides, nucleotides).
  • Mitochondria then process these monomers and smaller units to produce ATP, the cell’s usable energy carrier.
  • Analogy: converting a large value into smaller, spendable units makes energy easier to distribute where needed.
  • Overall respiration snapshot (simplified):
    \text{C}6\text{H}{12}\text{O}6 + 6\,\text{O}2 \rightarrow 6\,\text{CO}2 + 6\,\text{H}2\text{O} + \text{ATP}_{\text{(energy)}}.

Cytoskeleton and intracellular transport

  • The cytoskeleton consists of three main filament types, varying in diameter: microfilaments, intermediate filaments, and microtubules. Diameters approximate to:
    d{\text{microfilaments}} \approx 7\ \text{nm},\quad d{\text{intermediate}} \approx 10\text{--}12\ \text{nm},\quad d_{\text{microtubules}} \approx 25\ \text{nm}.
  • Cytoskeleton provides structural support, organizes cell interior, and serves as tracks for transport of organelles and molecules.
  • Motor proteins (e.g., kinesin, dynein, myosin) move along these tracks, delivering cargo (vesicles, organelles) to specific locations within the cell.
  • During cell division, cytoskeletal components help segregate organelles and ensure each daughter cell receives a complete set of organelles and DNA copies.

Prokaryotes vs. Eukaryotes: key differences

  • Domains of life: \text{Three domains: Bacteria, Archaea, Eukarya (Eukarya = eukaryotes)}.
  • DNA location and organization:
    • Prokaryotes: DNA is typically circular and resides in the cytoplasm in a region called the nucleoid; often with plasmids.
    • Eukaryotes: DNA is linear, organized into chromosomes, and enclosed within a nuclear envelope.
  • Membrane-bound organelles:
    • Present in eukaryotes (e.g., nucleus, mitochondria, chloroplasts, lysosomes).
    • Generally absent in prokaryotes (no true nucleus or membrane-bound organelles).
  • Ribosomes:
    • Both have ribosomes, but prokaryotic ribosomes are not membrane-bound.
  • Cell size:
    • Eukaryotes are generally larger than prokaryotes.
  • Reproduction and multicellularity:
    • Prokaryotes are essentially single-celled organisms.
    • Eukaryotes include many multicellular forms, though some protozoa are unicellular eukaryotes.
  • Implications: structural organization, gene expression regulation, and cellular complexity differ markedly due to these architectural differences.

Fungi and microbial diversity: decomposers, parasites, and symbioses

  • Fungi live as saprophytes, parasites, or mutualists; roles include decomposition and nutrient cycling.
  • Saprophytes: obtain nutrients from dead organic matter (substrates like leaves and detritus).
  • Parasites: obtain nutrients from living hosts (e.g., athlete's foot, yeast infections).
  • Mutualistic associations: lichens form a partnership between a fungus and an alga (or cyanobacterium), enabling nutrient exchange and survival in harsh environments.
  • Fungal reproduction:
    • Vegetative growth spreads hyphae and mycelia.
    • Sexual and asexual reproduction both occur via spores.
    • Asexual spores can be produced on specialized structures (e.g., conidia).
    • Sexual spores result from the fusion of compatible hyphae or gametes; spores are dispersed by wind and can establish new organisms.
  • Fruiting bodies (e.g., mushrooms) contain structures with gills or other spore-bearing surfaces; these release thousands of spores into the environment.
  • Spore terminology:
    • Asexual spores are often called conidia rather than sporangiospores; sexual spores require fusion of compatible partners.
  • Real-world relevance: fungi are responsible for decomposition, fermentation (bread, beer, wine), pathology, and significant ecological roles.

Fungal reproduction in everyday contexts and healthcare relevance

  • Asexual reproduction (conidia/spores) enables rapid dissemination and colonization.
  • Sexual reproduction adds genetic diversity, which can influence pathogenicity and drug resistance.
  • Healthcare relevance: fungi can cause opportunistic infections, especially in immunocompromised individuals; hospital- and community-acquired infections are important considerations.

Opportunistic infections and ecological context

  • Opportunistic infections arise when a normally managed pathogen exploits a weakened immune system.
  • They are common in settings where patients have compromised immunity due to illness, medications, or co-infections.
  • Fungi are frequently implicated as opportunistic pathogens in such scenarios.

Algae, phytoplankton, and ecological significance

  • Phytoplankton (algae) are major oxygen producers in aquatic ecosystems, supporting marine life including whales and other organisms.
  • Red tide events are caused by certain algal blooms producing toxins; these toxins can kill fish and pose health risks to humans who encounter contaminated water or seafood.
  • Climate change and nutrient loading can influence the frequency and intensity of red tide events, impacting fisheries and coastal health.

Protozoa: diversity, unicellularity, and malaria lifecycle

  • Protozoa are a diverse group of mostly unicellular eukaryotes (though some form colonies).
  • Approximate global diversity: about 65{,}000 species.
  • Malaria lifecycle: transmitted by mosquitoes (e.g., Anopheles spp.).
    • Part of the life cycle occurs in the mosquito vector; part occurs in the human or animal host.
    • If the parasite cannot transition between mosquito and host, it cannot complete its life cycle and survive.
  • The mosquito and its role as a critical host in the Plasmodium lifecycle illustrate the complex host-vector relationships that sustain protozoan diseases.

Practical and cross-cutting themes

  • Cellular organization underpins function: endomembrane system, energy production, and cytoskeletal transport coordinate to meet cellular needs.
  • Evolutionary context: endosymbiotic theory explains the origin of mitochondria and chloroplasts, linking cellular biology to evolutionary biology.
  • Ecology and health: fungi, algae, and protozoa influence ecosystems, industry (fermentation), and human health (opportunistic infections, malaria lifecycle).
  • Metabolic integration: digestion, monomer transport, mitochondrial ATP production, and membrane transport illustrate how energy flow is managed from nutrients to usable cellular energy.

Quick recap of key contrasts and connections

  • Endomembrane system roles: ER -> Golgi -> vesicles -> secretion or intracellular targeting.
  • Lysosomes as digestive hubs and players in apoptosis.
  • Vacuoles for storage; vesicles for transport and secretion.
  • Endosymbiotic theory: mitochondria and chloroplasts as former free-living organisms with own DNA.
  • Cytoskeleton as cellular highways with motor proteins enabling organelle and cargo movement.
  • Prokaryotes vs. Eukaryotes: DNA organization, membrane bound organelles, and cellular complexity.
  • Fungi: diversity of lifestyles (saprophyte, parasite, mutualist); reproduction via spores; ecological and clinical relevance.
  • Microbial ecology and health: fermentation (yeast), opportunistic infections, red tide, malaria lifecycle.

Connections to foundational principles and real-world relevance

  • Structure-function: how membrane composition enables fusion, vesicle trafficking, and selective transport.
  • Energy transformation: from large food macromolecules to ATP via digestion and mitochondrial respiration.
  • Evolutionary biology: endosymbiosis explains organelle origins and the distribution of genetic material.
  • Ecology and public health: decomposition, nutrient cycling, algal blooms, and vector-borne diseases shape ecosystems and human activities.