Biology Lecture Notes Week 4 Monday

Cryo-ET Imaging Context

• Cryo-electron tomography (cryo-ET) combines multiple microscopy methods to visualize cells in detail. It uses transmission electron microscopy (TEM) to generate several two-dimensional images and then renders them into a three-dimensional reconstruction, plus cryogenic fluorescence light microscopy to enhance information about cellular components.

• Image showcases advances in capturing subcellular parts with higher resolution and 3D context.

Cell Theory and Founders

• Cell theory explains the process and structure of life; developed in the 19th century by several European scientists.

• Key contributors:

  • Matthias Schleiden

  • Theodor Schwann

  • Rudolf Virchow

  • Robert Remak (often mentioned as contributing foundational observations)

• Core statements of cell theory:

  • All living things are composed of at least one cell.

  • The cell is the basic functional unit of life.

  • New cells arise from existing cells.

• The theory underpins modern biology and the study of cellular organization across life.

Cell Size and the Surface Area–to–Volume (SA:V) Ratio

• Cells are typically very small; commonly cited size range for many cells is around:

  • 0.1\,\mu\text{m} \leq \text{size} \leq 100\,\mu\text{m}

  • Note: some larger single-celled organisms can be visible to the naked eye, but most require a microscope.

• Why small size? SA:V considerations.

  • A higher surface-area-to-volume ratio enables efficient transport of nutrients and waste across the cell membrane relative to the cell’s interior volume.

  • If a cell were too large, the membrane would have insufficient surface area to supply the larger volume with nutrients and remove wastes quickly enough.

• Conceptual takeaway: smaller cells optimize exchange with the environment and support metabolic needs more effectively.

Prokaryotes vs Eukaryotes: The Tree of Life

• Broad classifications:

  • Prokaryotes (technically: "before nucleus"; prokarya)

  • Eukaryotes ("true nucleus"; eukarya)

• Major splits:

  • Prokaryotes: Archaea and Bacteria

  • Eukaryotes: Protists, Animals, Plants, Fungi

• Key differences between the two cell types:

  • Membrane-bound organelles: present in most eukaryotes; absent in prokaryotes

  • Chromosomes: prokaryotes typically have a single circular chromosome; eukaryotes have multiple linear chromosomes

  • Size: prokaryotes are generally smaller

• Mnemonics and recall:

  • Prokarya = before nucleus; Eukarya = true nucleus

• Practical implication: compartmentalization in eukaryotes allows specialized organelles and more complex regulation of cellular processes.

Core Cellular Constituents Across All Cells

• Regardless of cell type, certain components are universal or nearly universal:

  • DNA: the genetic material; basis of life

  • Ribosomes: sites of protein synthesis

  • Cell membrane (plasma membrane): boundary that controls exchanges with the environment

  • Cytoplasm: interior content outside the membrane

  • Cytosol: fluid component of the cytoplasm

• Other components may be present in some but not all cells (not always universal):

  • Flagella/cilia can be present in both prokaryotes and eukaryotes, but they differ histologically and evolutionarily

• Notable structural terminology:

  • Nucleoid: in prokaryotes, a dense region of cytosol where DNA is located (not membrane-bound)

Prokaryotic Structures (Typical for Bacteria/Archaea)

• Cell wall: provides structure and protection; in bacteria, the cell wall is made of peptidoglycan

• Capsule: outer protective layer aiding adherence and virulence (often helps contain the cell)

• Pili (including fimbriae): hair-like projections involved in attachment and genetic exchange (conjugation via specialized pili)

• Nucleoid: region where the DNA is concentrated; not enclosed by a membrane

Eukaryotic Cells: Plant vs Animal Distinctions

• Plant cells:

  • Chloroplasts and plastids (photosynthesis-related organelles)

  • Cell wall present (made of cellulose in plants)

  • Large central vacuole (cell sap) that maintains turgor pressure and helps resist bursting when cells lose water

• Animal cells:

  • Centrosomes (microtubule-organizing centers) important for organizing microtubules during cell division

  • Small vacuoles (often several, but generally smaller than plant vacuoles)

  • Lysosomes (contain digestive enzymes)

• Comparative summary:

  • Plants: chloroplasts/plastids, cell wall, large central vacuole

  • Animals: centrosomes, smaller vacuoles, lysosomes

Cytoskeleton: Shape, Structure, and Movement

• Purpose: maintains cell shape, provides mechanical support, anchors organelles, enables movement and division

• Components:

  • Microfilaments (actin filaments): provide shape and enable cell movement; connect cell interior to exterior structures

  • Intermediate filaments: contribute to cell shape and structural integrity; help anchor organelles

  • Microtubules: organize and move organelles and chromosomes; framework for intracellular transport; essential for mitosis and meiosis

  • Centrosome: organizes microtubules in animal cells (spindle formation during cell division)

• Additional motility structures:

  • Cilia and flagella are built from microtubules; involved in cell movement and moving substances along surfaces

Cilia and Flagella: Structure and Function

• Flagella: one or a few long projections that enable locomotion

• Cilia: many short projections that can cover the cell surface and help move substances across surfaces

• Structural note: Both are composed of microtubules; evolution of these structures occurred independently in prokaryotes and eukaryotes (convergent or separate evolutionary origins)

• Functional role: propulsion in single-celled organisms and/or movement of surrounding fluids across tissue surfaces

Visual Demonstration and Practical Notes

• A short video demonstrates the movement of flagella and cilia and highlights the arrangement of microtubules on the cell exterior

• This helps visualize how cytoskeletal elements drive movement

Synthesis: Key Takeaways and Connections

• The big contrasts to memorize:

  • Prokaryotes vs Eukaryotes: membrane-bound organelles, chromosome structure, and cellular complexity

  • Plant vs Animal differences: chloroplasts, cell wall, vacuole size and function; centrosomes and lysosomes

  • Cytoskeleton roles: shape, transport, division, and movement

• Foundational links:

  • Cell theory underpins understanding of how all life is organized at the cellular level

  • SA:V ratio concept explains why cells are small and how size impacts nutrient transport and waste removal

  • Evolutionary perspective: eukaryotes and prokaryotes diverged and developed distinct cellular features (e.g., endomembrane system, cytoskeleton organization)

• Real-world relevance:

  • Imaging techniques (cryo-ET and cryo-fluorescence microscopy) enhance our ability to study cell structure in detail

  • Knowledge of cell structure informs fields like microbiology, pathology, and biotechnology

• Practical implications discussed: understanding cell structure informs lab work, experimental design, and interpretation of cellular processes

Ethical, Philosophical, and Practical Implications

• While not deeply discussed in the lecture, practice implications include responsible use of imaging technologies and the importance of accurate interpretation of cellular images for scientific conclusions.

• Philosophical note: the distinction between prokaryotic and eukaryotic organization highlights how compartmentalization enables complexity in life.

Formulas and Quantitative References

• Surface area of a sphere (model for cells):

  • A = 4\pi r^2

• Volume of a sphere (model for cells):

  • V = \frac{4}{3}\pi r^3

• Surface area-to-volume ratio (SA:V) for a sphere:

  • \frac{A}{V} = \frac{4\pi r^2}{(4/3)\pi r^3} = \frac{3}{r}

• Conceptual takeaway: as radius r decreases, SA:V increases, enhancing exchange with the environment.

Administrative Reminders from the Lecture

• Next teaching focus: Wednesday will emphasize cell function and processes (the lecture today centered on structure).

• Lab quiz reminder: There is a lab quiz this week on D2L (lab page). Deadline is Tuesday night to Wednesday night for completion.

• Note about delivery: The lecture was recorded with the instructor possibly being imperfect in voice due to a cold; content nonetheless covers the essential topics listed above.

Quick Reference: Terms to Know

• Cryo-ET, TEM, cryogenic fluorescence microscopy

• Cell theory, Schleiden, Schwann, Virchow, Remak

• Prokaryote, Eukaryote, Prokarya, Eukarya

• Nucleoid, DNA, Ribosome, Cell membrane, Cytoplasm, Cytosol

• Peptidoglycan, Capsule, Pili, Fimbriae

• Chloroplast, Plastid, Vacuole, Centrosome, Lysosome

• Cytoskeleton, Microfilaments, Intermediate filaments, Microtubules

• Cilia, Flagella

• A/V, A = surface area, V = volume (concept), $A/V = 3/r$ for spheres