Cell Structure, Organization, and Diversity in General Biology

Cellular Organization and Function

Internal Organization of Eukaryotic Cells

Eukaryotic cells possess a complex internal structure that enables them to efficiently perform the functions necessary for life. Internal membranes divide the cell into compartments, allowing specialized chemical reactions to occur in distinct regions.

• Energy and Matter Transformations: Internal membranes form organelles such as mitochondria and chloroplasts, which are responsible for energy conversion and biosynthesis of macromolecules.

• Genetic Information Storage and Transmission: The nucleus contains DNA, which stores genetic instructions for protein synthesis. Ribosomes, either free or bound to the endoplasmic reticulum, are the sites of protein synthesis.

• Interactions with the Environment: The plasma membrane acts as a selective barrier, regulating the movement of substances into and out of the cell.

Example: Mitochondria break down organic molecules to generate ATP, the cell's main energy currency.

Cell Size and Scale

Units of Measurement and Relative Sizes

Cells and their components vary greatly in size, and understanding these differences is essential for studying cell biology. The metric system is used to measure biological structures.

• Metric Units:

  • Meter (m): Base unit

  • Centimeter (cm): $1~\text{cm} = 10^{-2}~\text{m}$

  • Millimeter (mm): $1~\text{mm} = 10^{-3}~\text{m}$

  • Micrometer (\mu m): $1~\mu\text{m} = 10^{-6}~\text{m}$

  • Nanometer (nm): $1~\text{nm} = 10^{-9}~\text{m}$

• Relative Sizes: Most plant and animal cells are 10–100 μm in diameter. Bacteria are typically 0.5–5 μm. Viruses, ribosomes, and proteins are much smaller, measured in nanometers.

Example: Human egg cells are among the largest single cells, visible to the naked eye, while ribosomes require electron microscopy to be seen.

Cell Compartmentalization

Plasma Membrane and Selective Permeability

All cells are surrounded by a plasma membrane, which serves as a selective barrier to control the passage of substances.

• Definition: The plasma membrane is a phospholipid bilayer with embedded proteins that regulate transport and communication.

• Function: Maintains homeostasis by allowing selective entry and exit of ions, nutrients, and waste products.

Example: The plasma membrane prevents harmful substances from entering the cell while allowing essential nutrients to pass through.

Surface Area to Volume Ratio

Geometric Relationships and Cell Efficiency

The surface area-to-volume (S/V) ratio is a critical factor in determining cell efficiency. As a cell grows, its volume increases faster than its surface area, which can limit the rate of exchange with the environment.

• Formula: For a cube, $\text{Surface Area} = 6a^2$, $\text{Volume} = a^3$, where $a$ is the length of a side.

• Importance: High S/V ratios facilitate efficient transport of materials; small cells have higher S/V ratios than large cells.

Example: Microvilli in intestinal cells increase surface area for absorption without significantly increasing volume.

Prokaryotic Cell Structure

Basic Features and Diversity

Prokaryotic cells, including bacteria and archaea, lack a nucleus and membrane-bound organelles. They have unique structural features that distinguish them from eukaryotic cells.

• Key Structures:

  • Nucleoid: Region containing circular DNA.

  • Ribosomes: Sites of protein synthesis.

  • Plasma Membrane: Selective barrier.

  • Cell Wall: Provides structural support; composition varies between species.

  • Flagella: Used for motility.

  • Fimbriae: Surface appendages for attachment.

• Shapes: Spherical (coccus), rod-shaped (bacillus), spiral (spirillum).

Example: Escherichia coli is a rod-shaped bacterium commonly found in the human gut.

Gram-Positive vs. Gram-Negative Bacteria

Cell Wall Structure and Staining

Bacteria are classified based on their cell wall structure, which affects their response to Gram staining.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present
Stain Retention	Retains crystal violet (purple)	Does not retain crystal violet; stains pink/red

Example: Staphylococcus aureus is Gram-positive; Escherichia coli is Gram-negative.

Viruses: Structure and Diversity

Basic Viral Components and Types

Viruses are acellular entities composed of genetic material (DNA or RNA) enclosed in a protein coat (capsid). Some viruses have an additional lipid envelope.

• Capsid: Protein shell made of capsomeres.

• Envelope: Lipid membrane derived from host cell, present in some viruses.

• Genetic Material: DNA or RNA, single or double-stranded.

• Examples: Mosaic virus (plant), adenovirus (animal), influenza virus (animal), bacteriophage (bacteria).

Example: Influenza virus has an RNA genome and a lipid envelope with glycoprotein spikes.

Viral Life Cycle

Steps in Bacteriophage Infection

Bacteriophages are viruses that infect bacteria. Their life cycle includes several key steps:

1. Attachment: Virus binds to the bacterial surface.

2. Entry: Viral DNA is injected into the host cell.

3. Degradation of Host DNA: Host DNA is broken down.

4. Synthesis: Viral genes and proteins are produced using host machinery.

5. Assembly: New viral particles self-assemble.

6. Release: Host cell lyses, releasing new viruses.

Example: T4 bacteriophage infects E. coli and follows the lytic cycle described above.

Origin of Viruses

Competing Hypotheses

The origin of viruses is debated, with three main hypotheses proposed:

• Escaped Gene Hypothesis: Viruses originated from genetic elements that escaped from cells.

• Cell Degeneration Hypothesis: Viruses evolved from more complex cellular organisms that lost cellular functions.

• Precellular Pool Hypothesis: Viruses originated from a pool of genetic material predating cellular life.

Additional info: These hypotheses reflect the complexity and diversity of viral evolution and their relationship to cellular life.