Biological Foundations: Exam Review, Molecules, Cells, and Transport
Exam Logistics and Course Overview
Exam Schedule: An exam was theoretically due on Friday but was postponed due to two missed days. The class voted to hold it on Wednesday.
Exam Content: The exam will cover all chemistry-related topics, including introductory material, the history of biology, and core chemistry concepts.
Syllabus Reference: The original syllabus indicated readiness for an exam around this time.
Core Biological Concepts and Chemistry Foundation
OpenStax Textbook: Recommended as a good, comprehensive introductory biology textbook.
Importance of Chemical Structures: Understanding the names and structures of key molecules is crucial.
Sugars: Know the structures and names of sucrose, glucose, and fructose.
Ribose vs. Deoxyribose: The difference between ribose and deoxyribose is extremely important for understanding DNA structure.
Significance: The lack of an oxygen atom in deoxyribose is what enables DNA to form a double-stranded helical structure, which is fundamental to its function, especially replication.
Central Dogma: Students should understand the mechanisms of the central dogma, though not necessarily the exact, detailed processes at an advanced level.
Biology's Evolution: Biology began as observational (rich guys traveling, noting interesting life forms) but has become highly reductionist, breaking down complex interactions into molecules, atoms, and elements.
Modern Teaching Approach: In introductory biology (BIO 1), the approach is reversed; we start with basic building blocks (chemistry) and work our way up to cells, organs, physiological systems, and eventually full organisms (biodiversity and evolution in later courses).
Student Frustration: Acknowledged that this chemistry-first approach can be frustrating for students who signed up for organismal biology, but it is the current educational standard for building foundational understanding.
The "Weird Story" of Terpenes and Heptane
Terpenes: Compounds made by plants, found in the sap of pine trees.
Connection to Turpentine: The name "terpene" is reminiscent of turpentine, a historical product.
Historical Significance (Revolutionary War Era): Turpentine was a huge product in the early United States, particularly from Southern pine forests. Sap was collected and distilled into turpentine.
Uses: Preservative for wood, waterproofing for ships (paint soaked in turpentine), and a base for shellac (used to waterproof homes and other wooden materials).
Jeffrey Pine vs. Ponderosa Pine: Both are North American pines.
Ponderosa Pine: Found throughout the Western U.S., a traditional source of pine sap for turpentine, similar to Eastern scrub and black pines.
Jeffrey Pine: Predominantly found in California, confused with Ponderosa pine, but its sap is primarily composed of heptane () instead of terpenes.
Heptane: An alkane with seven carbons.
Historical Discovery: Early distillers in California accidentally used Jeffrey pine sap instead of Ponderosa, leading to distillery explosions because heptane is essentially gasoline (similar to octane).
Purity: Jeffrey pine sap is remarkably pure heptane, requiring little distillation. It serves as the "zero score" for octane ratings.
Biological Significance: Pine Bark Beetles and Fungus:
Beetle Predation: Pine bark beetles burrow into the bark of both Jeffrey and Ponderosa pines to eat the bark.
Tree Defense: The tree responds by producing more sap to clog the beetle's burrow and deter it from laying eggs, which would kill the tree.
Fungal Symbiosis/Pathogenesis: The beetle also carries a fungus. This fungus takes advantage of the tree's sap channels, infecting the tree and eventually killing it.
"Blue Wood" Industry: The fungus causes the wood inside the pine tree to bleach a bright blue color.
Consequence: This "blue wood" is highly prized and has led to a furniture industry in California, created by the intricate web of interactions between insect, tree, fungus, and humans.
Significance: This complex interaction highlights the interconnectedness of living things, a concept that forms the basis of biology.
The Cell: Fundamental Unit of Life
Cell Theory: The basic principle is "Omnis cellula e cellula" (Latin for "every cell from a cell"), emphasizing the cellular basis of all life and its reliance on chemistry.
Microscopy
Invention of the Microscope: A pivotal moment in biology.
Robert Hooke: A Londoner who observed thin slices of cork (dead cells) and coined the term "cell" because the regular, box-like structures reminded him of monastic cells or beehive cells. He inferred that something living once occupied these containers.
Anton van Leeuwenhoek: The first to observe living microorganisms with his simple, high-magnification microscopes, introducing the microscopic world to Europe.
Function of Microscopes: To increase the apparent size of an object by changing how light refracts and strikes the retina.
It does not change the actual size of the object.
Light Microscopes:
Magnification Limit: Approximately (1000 times better than the naked eye).
Resolution Limit: Approximately (, where ).
Understanding Size Scales (Logarithmic Scale):
A logarithmic () scale is used to represent the vast range of biological sizes on a single line, as seen in diagrams.
Examples of Sizes:
Blue whale: Tens of meters.
Chicken egg: About .
Giant squid axon: An unusually large nerve cell, crucial for early neurobiology studies (membranes large enough for electrical potential testing). Similar in concept to the mythical Kraken.
Frog egg: A couple of millimeters, still visible to the naked eye.
Typical human cell: About .
Red blood cell: Visible under a light microscope if stained.
Human oocyte/ovum: Absolutely possible to see.
Most bacteria: At the limits of light microscopy; barely visible, often viewed on pre-prepared slides.
Viruses, ribosomes, lipids, molecules: Too small for light microscopes.
Electron Microscopes:
Principle: Instead of photons, electrons are passed through or reflected off an object, creating an image on a TV-like screen.
Resolution: better than a light microscope, meaning better than the naked eye.
Types:
Scanning Electron Microscope (SEM): Shows surface details of cells or organelles.
Transmission Electron Microscope (TEM): Allows visualization of the inside of cells and the details of internal organelles.
Sample Preparation: Cells must be prepared and stained with heavy metals (e.g., osmium).
Mechanism: Osmium attaches to lipid portions of membranes and reflects/transmits electrons.
Limitation: The process kills the cells, meaning electron micrographs always show dead cells.
Color in Electron Micrographs: Electron microscopes do not produce color images.
Reason: The structures being visualized are too small to reflect light wavelengths.
Origin of Color: Any color observed in electron micrographs is artificially added using software like Photoshop for better visualization and understanding of structures. Scientists must remember these are visualizations, not true-color representations.
The Cell Membrane (Plasma Membrane)
Definition: A lipid bilayer that separates the living cell from its non-living environment.
"Separates life from non-life."
Key Functions:
Transport: Facilitates two-way transport, allowing essential nutrients in and metabolic byproducts out.
Chemical Reactions: Serves as a site for many of the organized chemical reactions that support life.
Lipid Structure:
Diversity: Lipids vary widely; common types include steroids, fats, and phospholipids.
Generalized Structure: Typically has a glycerol backbone forming a hydrophilic (water-attracting) head and long carbon chains forming hydrophobic (water-repelling) tails.
Self-Organization: Due to their amphipathic nature:
In aqueous solutions, hydrophilic heads form hydrogen bonds with water.
Hydrophobic tails move away from water but are attracted to each other.
Result: Molecules spontaneously self-organize into micelles (small bubbles) or lipid bilayers to minimize hydrophobic interactions with water.
Phospholipids: Specialized lipids with a phosphate group attached to the glycerol head, actively promote the formation of stable lipid bilayers.
Saturated vs. Unsaturated Fatty Acid Tails:
Appearance in Diagrams: Lipid tails are often depicted as one straight and one crooked.
Straight Tails (Saturated): Every carbon bond has the maximum number of hydrogen atoms attached.
Properties: Tightly packed, less fluid, stiff (e.g., lard, butter).
Crooked Tails (Unsaturated): Have double bonds between carbons, causing kinks in the chain, and fewer hydrogen atoms.
Properties: Loosely packed, more fluid, remain liquid at lower temperatures (e.g., peanut oil, canola oil).
Impact on Membrane Fluidity and Health:
Fluidity: Unsaturated fats increase membrane fluidity; saturated fats decrease it, making membranes stiffer.
Cryopreservation: Cells with a higher proportion of unsaturated fats in their membranes survive freezing better because their membranes are more flexible and can accommodate the expansion of intracellular ice without rupturing.
Real-world Example (Milkshake Study): A single high-saturated fat milkshake could lead to the incorporation of saturated fats into cardiac cell membranes within approximately hours. While the clinical relevance is debated, prolonged high-saturated fat diets lead to stiffer cardiac muscle membranes, potentially impacting heart function.
Membrane Permeability
Permeability Types:
High Permeability: Small, non-polar molecules and some small polar molecules readily pass through.
Examples: Water, oxygen, carbon dioxide, nitrogen, glycerol.
Low Permeability: Larger molecules and ions generally cannot pass through the lipid bilayer directly.
Examples: Glucose, sucrose (require specific transport mechanisms), chlorine (), potassium (), sodium () ions (usually excluded).
Factors Affecting Permeability:
Degree of Saturation:
Unsaturated Hydrocarbons (Crooked Tails): Looser packing allows for higher permeability (analogy: a fence with wide slats).
Saturated Hydrocarbons (Straight Tails): Tighter packing creates a finer mesh, leading to lower permeability (analogy: chicken wire).
Cholesterol Content:
Adding cholesterol to the lipid bilayer decreases its permeability, acting like "corks" that block gaps between lipid tails. Experimentally shown with liposomes and glycerol movement.
Temperature:
Higher Temperature: Increases membrane fluidity, leading to higher permeability (analogy: soft butter vs. liquid oil, easier to mix ingredients).
Lower Temperature: Decreases membrane fluidity, making membranes stiffer and less permeable (analogy: hard butter, difficult to mix ingredients).
Biological Implications: Explains why chemical reactions might be slower in cold-blooded animals (like snakes) at room temperature compared to warm-blooded animals (like mice) due to membrane fluidity impacting transport and enzyme function.
Movement of Membrane Components: Phospholipids typically move laterally within the membrane but rarely "flip-flop" to the other side due to the energetic barrier of moving a hydrophilic head through the hydrophobic core.
Diffusion and Osmosis
Brownian Motion: All atoms and molecules are in constant, random motion, leading to molecular interactions.
Diffusion:
Definition: The net movement of molecules from an area of high concentration to an area of low concentration.
Direction: Occurs along a concentration gradient.
Outcome: The net effect is to equalize concentrations and achieve a steady state average, increasing entropy (tendency towards disorder).
Scope: Applies to the movement of any solute.
Osmosis:
Special Case: Exclusively refers to the movement of water.
Convention: Although technically a form of diffusion, scientists universally refer to water movement as osmosis.
Definition: The movement of water across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration.
Example: If tap water (low salt) and seawater (high salt) are separated by a membrane impermeable to salt, water will move from the tap water into the seawater until concentrations are balanced through dilution. This dilutes the higher concentration of solute.
Direction: Water moves to dilute the region with higher solute concentration, effectively moving against the water concentration gradient (from high water concentration to low water concentration).