Cells Overview and Classification

Cells Overview

• Fundamental unit of life; all living organisms consist of cells.
• Organisms classified as unicellular (single-celled) or multicellular (multiple cells).
• Unicellular examples: Bacteria, Archaea, Protozoa.
• Multicellular examples: Plants, Animals, Algae, and Fungi.

Classification of Cells

• Two primary cell types:
  • Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; examples include Bacteria and Archaea.
  • Eukaryotic Cells: Contain a nucleus and several membrane-bound organelles.

Prokaryotes

• First life forms on Earth, characterized by simplicity and small size (0.5 to 2.0 µm in diameter).
• DNA is a single chromosome in direct contact with cytoplasm, found in nucleoid region.
• No membrane-bound organelles present.

Eukaryotes

• More complex cells with organized internal structures, such as membranes and a cytoskeleton.
• Characterized by the presence of a nucleus; typically larger (up to fifteen times wider than prokaryotes).
• Eukaryotic DNA organized into linear chromosomes associated with histone proteins.
• Most chromosomal DNA is stored in the nucleus; some organelles (e.g., mitochondria) also contain DNA.

Types of Cell Tissues

• Four main tissue types:
  • Epithelial
  • Connective
  • Muscle
  • Nervous

Epithelial Tissue

• Forms coverings or linings of organs and cavities; characterized by tightly joined cells with tight junctions.
• Tight junctions provide barriers against pathogens and fluid loss.
• Distinct arrangements of epithelial cells:
  • Cuboidal: secretion
  • Simple columnar: secretion and active absorption
  • Simple squamous: material exchange through diffusion
  • Stratified squamous: multilayered, regenerates quickly for protection.

Connective Tissue

• Comprised of loose and fibrous connective tissues; provides support and insulation.
• Loose connective tissue example: Adipose tissue, stores fat for energy and insulation.
• Types of connective tissue:
  • Blood: Composed of plasma, erythrocytes (RBC), leukocytes (WBC), and platelets.
  • Cartilage: Contains collagenous fibers embedded in chondroitin sulphate, provides cushioning between bones.
  • Bone: Formed by osteoblasts; collagen matrix combined with minerals for hardness, includes blood vessels and nerves in central canals surrounded by osteons.

Muscle Tissue

• Composed of muscle fibers enabling voluntary or involuntary movement.
• Muscle movement is triggered by nerve signals.
• Types of muscle in vertebrates:
  • Skeletal: Striated and voluntary.