Lecture 2: Carbohydrates — Cell Structure and Imaging Notes

Prokaryotes vs Eukaryotes

• Know prokaryotes and eukaryotes: nucleus & uni-/multicellularity
• Key distinctions:
  • Nucleus presence:
  • Eukaryotes have a membrane‑bound nucleus.
  • Prokaryotes lack a true nucleus; genetic material is in a nucleoid region.
  • Cellular organization:
  • Eukaryotes contain numerous membrane‑bound organelles (e.g., mitochondria, ER, Golgi, lysosomes).
  • Prokaryotes lack most membrane‑bound organelles.
  • Multicellularity:
  • Many eukaryotes are multicellular; many prokaryotes are unicellular (with occasional simple colonies or biofilms).
  • Size and complexity:
  • Eukaryotic cells are generally larger and more complex than prokaryotic cells.
  • Genetic material packaging:
  • Eukaryotes use histones to package DNA; prokaryotes do not (DNA is in a simple nucleoid form).
  • Cell division:
  • Eukaryotes divide by mitosis/meiosis.
  • Prokaryotes divide by binary fission.
• Relevance:
  • Foundational to understanding cell function, evolution, and differences in biology across domains of life.

Structure and function of key cell components

• Cell membrane (plasma membrane)
  • Structure: phospholipid bilayer with embedded proteins; fluid mosaic model.
  • Functions: selective permeability; boundary; transport of nutrients and wastes; signal reception via membrane receptors.
  • Significance: maintains homeostasis and mediates interactions with the environment.
• Nucleus
  • Structure: nuclear envelope with pores; contains chromatin and nucleolus.
  • Functions: houses and protects genetic material; coordinates gene expression; transcription occurs here; ribosomal RNA genes located in the nucleolus.
  • Significance: central control center of the cell.
• Ribosomes
  • Structure: ribosomal RNA + proteins; free‑floating in cytosol or attached to rough ER; 70S in prokaryotes; 80S in eukaryotes; chloroplasts and mitochondria contain 70S ribosomes.
  • Functions: protein synthesis by translating mRNA.
  • Significance: essential for producing cellular proteins.
• Endoplasmic reticulum (ER)
  • Rough ER
  • Structure: ER studded with ribosomes.
  • Functions: synthesis and initial folding of membrane‑bound and secreted proteins; some glycosylation occurs here.
  • Smooth ER
  • Structure: lacks ribosomes.
  • Functions: lipid synthesis; detoxification processes; calcium storage.
  • Significance: regional specialized facilities for protein and lipid production.
• Golgi apparatus
  • Structure: stacked, flattened membrane sacs with cis (receiving) and trans (shipping) faces.
  • Functions: modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles; participates in glycosylation and vesicle formation.
  • Significance: cellular logistics and production of secreted products.
• Lysosomes
  • Structure: membrane‑bound organelles containing hydrolytic enzymes.
  • Functions: digestion of macromolecules; autophagy; waste processing and recycling.
  • Significance: cellular housekeeping and quality control.
• Vacuoles
  • Structure: membrane‑bound sacs; plant cells often have a large central vacuole; animals have smaller vacuoles.
  • Functions: storage of water, ions, nutrients, and waste; sequestration of compounds; in plants, central vacuole maintains turgor pressure.
  • Significance: contributes to cell size regulation and homeostasis.
• Mitochondria
  • Structure: double membrane; own circular DNA; inner membrane folds (cristae).
  • Functions: cellular respiration; ATP production; energy currency of the cell.
  • Significance: powerhouses of the cell; essential for energy‑dependent processes.
• Chloroplasts (in plants and algae)
  • Structure: chloroplasts with thylakoid membranes and stroma; chlorophyll pigment.
  • Functions: photosynthesis—converts light energy into chemical energy (glucose).
  • Significance: source of organic carbon and energy for photosynthetic organisms; contains own DNA and ribosomes (endosymbiotic origin).

Endomembrane system

• Components and connections:
  • Nuclear envelope (outer membrane continuous with ER)
  • Endoplasmic reticulum (rough and smooth)
  • Golgi apparatus
  • Lysosomes (in some cells via vesicle fusion)
  • Vesicles and vacuoles
  • Plasma membrane
• Functional flow:
  • Nucleus → rough ER: ribosomes synthesize secretory and membrane proteins
  • Rough ER → Golgi: proteins/lipids are modified and packaged
  • Golgi → vesicles: sent to plasma membrane, lysosomes, or outside cell
• Key roles:
  • Synthesis, folding, modification, and trafficking of proteins and lipids
  • Production of membrane components and secretory products
  • Quality control and sorting of cellular cargo

Organelles: distribution across organisms

• What organisms have which organelles/structures:
  • Eukaryotes (plants, animals, fungi, many protists):
  • Have nucleus, mitochondria, endomembrane system (ER, Golgi, lysosomes, vesicles), and (in plants/algae) chloroplasts.
  • Contain ribosomes (80S in cytosol; 70S in mitochondria/chloroplasts).
  • Prokaryotes (bacteria, archaea):
  • Do not have a true nucleus or most membrane‑bound organelles.
  • Contain ribosomes (70S), cell membrane, cytoplasm, and usually a cell wall; chlorophyll‑containing photosynthetic membranes may be present in some, but not chloroplasts.
  • General note:
  • Chloroplasts and mitochondria are hallmark organelles of eukaryotes (endosymbiotic origin).

Be able to draw and recognize a cell with organelles

• Core features to include in a labeled diagram:
  • Nucleus with nuclear envelope and nucleolus
  • Ribosomes (cytoplasmic and/or rough ER)
  • Rough ER and Smooth ER
  • Golgi apparatus
  • Mitochondrion
  • (If plant cell) Chloroplasts and large central vacuole
  • Lysosome (where applicable)
  • Vesicles and plasma membrane
• Recognition tips:
  • Chloroplasts indicate a plant/algal cell; mitochondria are present in all eukaryotes.
  • A cell wall suggests a plant, fungal, or certain algal cells (not typical for animal cells).
  • Dense stacks of cristae in mitochondria vs smooth ER ribbons for smooth ER.
• Practice approach:
  • Draw a generic eukaryotic cell first; add plant vs animal features as appropriate.
  • Use arrows to indicate trafficking paths: nucleus → ER → Golgi → vesicles → plasma membrane/lysosome.

Differences between SEM and TEM

• Scanning Electron Microscopy (SEM)
  • What it shows: surface topology and 3D-like exterior structures.
  • How it works: beams of electrons scan the specimen surface; detectors capture secondary electrons.
  • Sample prep: usually coating with a thin metal layer (e.g., gold); often preserved and mounted on a stub.
  • Resolution/magnification: good surface detail, lower depth‑of‑field than TEM; typical resolution in nanometer range.
• Transmission Electron Microscopy (TEM)
  • What it shows: internal ultrastructure of thin sections.
  • How it works: electrons transmitted through a very thin section; heavy metal staining enhances contrast.
  • Sample prep: ultra-thin sections, preserved, stained; no surface imaging.
  • Resolution/magnification: highest resolution for cellular ultrastructure; reveals organelles, membranes, and macromolecular complexes.
• Practical distinctions:
  • Use SEM when you need 3D surface morphology; use TEM when you need internal architecture at high resolution.

Connections to foundational principles and real-world relevance

• Foundational concepts:
  • Cell theory: all living organisms are composed of cells; cells are the basic unit of life; new cells arise from existing cells.
  • Structure–function relationship: organelle presence and arrangement reflect cellular roles and metabolism.
• Real‑world relevance:
  • Understanding organelles informs pathology (e.g., lysosome storage diseases, mitochondrial disorders).
  • Imaging techniques (SEM/TEM) are essential in research and diagnostics for examining cell structure and disease-related alterations.
  • Endomembrane system dysfunctions can impact secretion, enzyme delivery, and membrane integrity, relevant to biotechnology and medicine.

Practical implications and study tips

• Visualize with analogies:
  • Nucleus = headquarters; ER = factory floors; Golgi = shipping/receiving; lysosomes = recycling/waste disposal; mitochondria = power plants; chloroplasts = solar power stations in plants.
• Practice drawing regularly: label all major organelles and indicate direction of trafficking between compartments.
• Review imaging concepts: be able to justify when SEM or TEM would be preferred for a given research question.
• Know organism differences: recall which organelles are present or absent in prokaryotes vs eukaryotes and why.

Ethical, philosophical, or practical implications

• Not discussed in the transcript; practical implications discussed above focus on scientific and medical applications of organelle knowledge and imaging techniques.

Summary of key takeaways

• Distinguish prokaryotes and eukaryotes by nucleus presence, organelle complexity, and cellular organization.
• Master the structure and function of major organelles: membrane, nucleus, ribosomes, ER, Golgi, lysosomes, vacuoles, mitochondria, and chloroplasts.
• Understand the endomembrane system and the flow of cellular cargo from synthesis to secretion or degradation.
• Be able to identify which organisms have specific organelles and to recognize a labeled cell diagram.
• Compare SEM and TEM imaging techniques and their applications in studying cell structure.
• Connect these concepts to broader biology principles and real‑world applications.