Cell Discovery and the Plasma Membrane

Chapter 7.1: Cell Discovery and Theory

Learning Target: Understand how cells were discovered and how the plasma membrane maintains cell homeostasis.
Preview: Progression from the first microscopes to modern cell theory and membrane structure.

History of the Cell Theory

1665: Robert Hooke utilized a simple microscope to observe cork, which is made of dead oak bark cells.
- He noticed small, box-shaped structures resembling monastery rooms and termed these structures "cellulae" (Latin for small rooms), which led to the term "cell." His observations primarily focused on the cell walls of these dead plant cells, highlighting them as rudimentary compartments.
- He hypothesized that these cellulae were the fundamental structural and functional units of all living organisms.
Late 1600s: Anton van Leeuwenhoek, a Dutch scientist, created his own microscopes, significantly improving magnification capabilities influenced by Hooke's findings.
- He made significant discoveries, observing living organisms—which he called "animalcules"—in various substances like pond water, milk, and even plaque from his teeth. This marked a crucial differentiation from Hooke's observations of dead cells.
- His observations revealed a previously unseen microscopic world and established new scientific fields.
Reflection Question: How do you think people reacted to learning about unseen living organisms?

The Development of Cell Theory

1838: Matthias Schleiden (German botanist) concluded that all plants are composed of cells, identifying the cell as the structural unit of plants.
1839: Theodor Schwann (German zoologist) reported that animal tissues also consist of individual cells, unifying the understanding that cells are the universal building blocks of all life.
1855: Rudolph Virchow (German physician) proposed the principle "Omnis cellula e cellula," meaning that all cells arise from the division of existing cells. This refuted the idea of spontaneous generation of cells.
These contributions laid the foundation for modern cell biology.

The Three Principles of Cell Theory

All living organisms are composed of one or more cells.
Cells are the basic unit of structure and organization in all living organisms.
Cells arise only from previously existing cells, passing genetic material to daughter cells.

The cell theory is fundamental to modern biology, serving as one of its unifying principles.
Learning Target Check-in: Can you explain why cells cannot appear spontaneously?

Microscope Technology Evolution

Cell discovery was made possible due to advancements in microscopy, greatly expanding human perception beyond the naked eye.
Improved magnification (enlarging an image) and resolution (clarity, the ability to distinguish two close objects) allowed scientists to observe finer cellular details.
Modern microscopes enable observations of cells in previously unimaginable ways, with advanced models costing upwards of 2,000,000 due to sophisticated optics and electronics.

Compound Light Microscopes

Modern compound light microscopes use a series of glass lenses and visible light to magnify specimens.
Each lens amplifies the image produced by the preceding lens (e.g., an ocular lens of 10x and an objective lens of 10x combine for a total magnification of 10x \times 10x = 100x).
Scientists often stain cells with various dyes to enhance visibility and differentiate structures, as most biological cells are tiny, colorless, and translucent. Common stains include methylene blue and iodine.
Maximum magnification is approximately 1000x before the scattering of light waves causes blurring, limiting the resolution of very small organelles.

Electron Microscopes

Developed in the 1940s during World War II, these microscopes utilize beams of electrons instead of light, allowing for significantly greater magnification and resolution by overcoming the limitations of light wavelengths.
Transmission Electron Microscope (TEM): Electrons pass through ultra-thin specimens (typically less than 100 nm thick) to a fluorescent screen or photographic film. Denser areas scatter more electrons, appearing darker.
- Can magnify biological specimens up to 500,000x or more, revealing internal cell structures and organelles in great detail.
- Disadvantage: Specimens must be dead, fixed, dehydrated, sectioned extremely thin, and observed in a vacuum, which can introduce artifacts.
Scanning Electron Microscope (SEM): Electrons scan the surface of a specimen, and secondary electrons emitted from the specimen surface are detected to create highly detailed, three-dimensional images.
- Provides excellent topographical information about the specimen's surface.
- Disadvantage: Like TEM, only nonliving cells and tissues can be examined, and specimens usually need to be coated with a thin layer of heavy metal (e.g., gold) for conductivity.

Modern Microscope Innovations

Scanning Tunneling Microscope (STM): Produces 3D computer images of objects as small as individual atoms by measuring the tunneling current between a sharp conducting tip and a conducting surface.
- Can operate in various environments, including air or liquid.
Atomic Force Microscope (AFM): Measures forces between a tiny probe tip (attached to a cantilever) and the cell surface, allowing for topographical imaging at atomic resolution.
- Advantage over TEM and SEM: These can be utilized with live specimens and do not require a vacuum, making them invaluable for studying biological processes in more natural conditions.
These technologies allow scientists to observe DNA, proteins, and other cellular components in remarkable detail, pushing the boundaries of biological imaging.
Reflection Question: How might future microscope innovations change our understanding of cells?

Chapter 7.2: The Plasma Membrane

Basic Cell Types and Plasma Membrane

All cells, regardless of their complexity, share a critical structure: the plasma membrane (also known as the cell membrane).
- Cells are broadly categorized into two main groups based on their internal organization: prokaryotic and eukaryotic.
The plasma membrane functions as a specialized boundary, a dynamic barrier controlling what enters and leaves the cell, comparable to a "cell's skin." It encases the cytoplasm and organelles.
This structure is essential for maintaining homeostasis—a steady internal balance—in all living cells by regulating the passage of substances.

Function of the Plasma Membrane

The plasma membrane's primary role is maintaining homeostasis, which refers to the balance of the internal environment, crucial for cell survival.
Key Property:
- Selective Permeability (or semipermeability): A fundamental characteristic meaning it allows specific substances (like water, small nonpolar molecules) to pass through freely or with assistance, while restricting or regulating the passage of others (like large polar molecules, ions).
- Analogy: Similar to a fish net, where water passes through but fish cannot, depending on the mesh size. This "mesh size" in the plasma membrane is determined by its molecular composition.
The membrane rigorously regulates how, when, and how much substances enter and exit the cell, ensuring necessary nutrients come in and waste products are removed.
Think-Pair-Share: Why is selective permeability crucial for cell survival?

Structure of the Plasma Membrane

Composed primarily of a phospholipid bilayer, along with embedded proteins, cholesterol, and carbohydrates.
Phospholipids:
- Each phospholipid molecule contains a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails.
- They are arranged in a phospholipid bilayer: two layers with the hydrophobic tails facing inward towards each other, creating a nonpolar core. The hydrophilic heads face outward towards the aqueous cellular environment (cytosol) and the extracellular fluid.
- This arrangement forms a stable barrier, preventing water-soluble substances and ions from passing easily through the nonpolar interior.
Cholesterol:
- Embedded within the hydrophobic core of the bilayer.
- Functions as a fluidity buffer, helping the membrane remain fluid at low temperatures and less fluid at high temperatures, maintaining its integrity and flexibility.
Proteins:
- Integral proteins are embedded within the lipid bilayer, often spanning the entire membrane (transmembrane proteins), acting as channels, carriers, or receptors.
- Peripheral proteins are loosely bound to the surface of the membrane (either on the cytoplasmic or extracellular side), involved in cell signaling or structural support.
- Proteins carry out most of the membrane's specific functions, including transport, enzymatic activity, signal transduction, cell-cell recognition, and attachment to the cytoskeleton or extracellular matrix.
Carbohydrates:
- Short chains of sugars that can be attached to proteins (forming glycoproteins) or lipids (forming glycolipids) on the extracellular surface of the plasma membrane.
- They form a "sugar coating" called the glycocalyx, crucial for cell-cell recognition, adhesion, and protection.

The Fluid Mosaic Model

Describes the plasma membrane as a dynamic and flexible structure.
The phospholipid bilayer behaves like a fluid, where individual phospholipid molecules and many proteins can shift from side to side (lateral movement), creating fluidity similar to molecules floating freely in a barrel of water (like apples).
The term "mosaic" refers to the diverse array of proteins and carbohydrate chains that are embedded in or attached to the bilayer, creating a dynamic, ever-changing pattern. This emphasizes that nothing is fixed in position, allowing for constant reorganization and adaptation.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells:
- Generally smaller and simpler in structure, typically between 0.1 and 5.0 \mu m in diameter.
- No true nucleus; their genetic material (a single circular DNA molecule) is located in a region called the nucleoid.
- Lacks membrane-bound organelles (like mitochondria, ER, Golgi); metabolic processes often occur in the cytoplasm or on the plasma membrane.
- Possess a cell wall (containing peptidoglycan in bacteria) and often a capsule outside the cell wall.
- Examples: Bacteria and Archaea.
Eukaryotic Cells:
- Generally larger and more complex, typically between 10 and 100 \mu m in diameter.
- Contain a true nucleus, which houses their linear genetic material (DNA organized into chromosomes).
- Possess numerous membrane-bound organelles that compartmentalize cellular functions, allowing for greater efficiency and specialization.
- Examples: Animals, plants, fungi, and protists.

Chapter 7.3: Structures and Organelles

Eukaryotic Cell Organelles

Learning Target: Identify and explain the functions of eukaryotic cell organelles.
On a scale of 1-5, assess confidence regarding cell structures.
Preview: Cells are likened to factories with specialized departments, with each organelle performing a specific job for the overall function of the cell.

Prokaryotic vs. Eukaryotic Cells Comparison

Prokaryotic Cells:
- Features include:
  - Nucleoid region containing circular DNA.
  - No membrane-bound organelles.
  - Small (typically 0.1-5.0 \mu m); unicellular.
  - Reproduces primarily via Binary Fission.
  - Older evolutionary structure, thought to be the first forms of life.
  - Cell wall present (peptidoglycan in bacteria).
  - Metabolic processes occur in the cytoplasm and on the plasma membrane.
Eukaryotic Cells:
- Features include:
  - True nucleus housing linear DNA organized into chromosomes.
  - Nucleus and various membrane-bound organelles (e.g., mitochondria, ER, Golgi).
  - Larger (typically 10-100 \mu m); can be unicellular or multicellular.
  - Reproduction occurs via Mitosis/Meiosis.
  - Younger evolutionary structure, evolved from prokaryotic ancestors.
  - Cell wall present in plants and fungi (cellulose in plants, chitin in fungi), absent in animal cells.
  - Metabolic processes are compartmentalized within specific organelles.

Cytoplasm: The Cell's Foundation

Cytoplasm: Refers to the entire contents within the plasma membrane, excluding the nucleus in eukaryotic cells. It consists of the cytosol (the semifluid, gel-like material) and the organelles suspended within it.
- In prokaryotic cells, all chemical processes occur directly in the cytosol as there are no compartments.
- In eukaryotic cells, the cytosol is the site of many metabolic pathways (e.g., glycolysis), while other processes take place in specialized organelles.

Cytoskeleton: The Cell's Framework

Cytoskeleton: A dynamic and intricate supporting network of protein fibers extending throughout the cytoplasm of eukaryotic cells, providing structural support and anchoring organelles.
- Composed of three main types of protein filaments:
  - Microtubules: Thick, hollow cylinders (approx. 25 nm diameter) made of the protein tubulin. They act as