Unit 1: Introduction to Cell Biology

Introduction to Cell Biology & Cell Theory

Cell Biology is the study of cells, their structure, function, behavior, and the processes they carry out. It is fundamental to all biological sciences.

Definition: The Cell Theory

The Cell Theory is a foundational principle of biology. The modern version consists of three main tenets:

1. All known living things are composed of one or more cells.
2. The cell is the most basic structural and functional unit of life.
3. All cells arise from pre-existing cells (proposed by Rudolf Virchow).

This theory was developed from the work of Matthias Schleiden and Theodor Schwann in the 1830s, who first proposed that all plants and animals, respectively, are made of cells.

Ultrastructure of Prokaryotic and Eukaryotic Cells

Cells are broadly divided into two types: prokaryotic (lacking a nucleus) and eukaryotic (containing a nucleus).

Comparison of Prokaryotic and Eukaryotic Cells

Cytosol and Cytoplasm

These terms are often confused but have distinct meanings:

• Cytoplasm: This refers to everything inside the cell membrane *except* the nucleus. It is comprised of the cytosol and all the organelles suspended within it.
• Cytosol: This is the fluid portion of the cytoplasm. It is a complex, gel-like substance (mostly water) filled with ions, small molecules, and proteins.

Functions of the Cytosol:

• Metabolic Pathways: Many key metabolic processes occur here, including glycolysis, gluconeogenesis, and parts of the urea cycle.
• Protein Synthesis: Free ribosomes in the cytosol synthesize proteins destined for the cytosol, nucleus, mitochondria, or peroxisomes.
• Signal Transduction: It acts as a medium for transmitting signals from receptors on the cell surface to the nucleus or other targets.

Structure and Function of Motile Cells

Motility, or the ability to move, is achieved through different mechanisms. The syllabus highlights three types:

Amoeboid Movement

• Structure: This movement relies on the formation of pseudopodia ("false feet"), which are temporary extensions of the cytoplasm.
• Mechanism: It is driven by the dynamic assembly and disassembly of actin microfilaments near the cell membrane. The process involves:
  1. Extension of a pseudopod (actin polymerization).
  2. Adhesion of the pseudopod to the substrate.
  3. Contraction of the cell body (myosin motors pulling on actin) to pull the cell forward.
• Examples: *Amoeba* species, human white blood cells (like macrophages), and cancer cells during metastasis.

Ciliary Movement

• Structure: Cilia are short, hair-like projections that cover the cell surface. They are numerous.
• Mechanism: They beat in a coordinated, wave-like or oar-like motion. This is powered by motor proteins (dynein) that cause microtubules within the cilium to slide past each other.
• Core Structure (Axoneme): The core of both cilia and flagella (in eukaryotes) has a "9 + 2" arrangement of microtubules: nine doublets arranged in a circle around two central single microtubules.
• Function:
  • Locomotion: Propelling single-celled organisms (e.g., *Paramecium*).
  • Moving Substances: Moving mucus and debris out of the respiratory tract, or moving an egg cell in the fallopian tube.

Flagellar Movement

• Structure: Flagella are long, whip-like appendages. Cells usually have only one or a few.
• Mechanism: In eukaryotes, they have the same "9 + 2" axoneme as cilia. The dynein motors generate a wave-like (undulating) motion that propels the cell. (Note: Prokaryotic flagella are structurally different, made of flagellin protein, and rotate like a propeller).
• Function: Primarily locomotion (e.g., sperm cell, *Euglena*).

Cytoskeleton

The cytoskeleton is a complex network of protein filaments that extends throughout the cytoplasm. It provides structural support, facilitates cell movement, and helps transport organelles within the cell. It is not static but highly dynamic.

It is composed of three main types of filaments:

Microfilaments (Actin Filaments)

• Structure: The thinnest filaments (approx. 7 nm diameter). Composed of two intertwined strands of the protein actin.
• Functions:
  • Cell Shape: Form a network just under the plasma membrane (the cell cortex).
  • Muscle Contraction: Interact with the motor protein myosin.
  • Amoeboid Movement: As described above (pseudopodia).
  • Cytokinesis: Form the contractile ring that divides the cell during cell division.

Intermediate Filaments

• Structure: Intermediate in size (approx. 8-12 nm diameter). Composed of various proteins (e.g., keratin in epithelial cells, lamins in the nucleus). They are more stable and rope-like than other filaments.
• Functions:
  • Structural Support: Provide mechanical strength and resistance to shear stress.
  • Anchoring Organelles: Help hold the nucleus and other organelles in place.
  • Nuclear Lamina: Form a meshwork inside the nuclear envelope to support it.

Microtubules

• Structure: The thickest filaments (approx. 25 nm diameter). Hollow tubes made of the protein tubulin (a dimer of α-tubulin and β-tubulin).
• Functions:
  • Cell Shape: Resist compression, providing structural support.
  • Organelle Transport: Act as "railway tracks" for motor proteins (kinesin and dynein) to transport vesicles and organelles.
  • Cilia and Flagella: Form the core (axoneme) of these structures.
  • Cell Division: Form the mitotic spindle, which separates chromosomes.