Cell: The Unit of Life

Cell: The Unit of Life

Structural Unit of Life

• Every organism, whether unicellular or multicellular, is composed of cells.
• Structures smaller than a complete cell do not ensure independent existence.
• A single cell can perform all essential functions of life.

Functional Unit of Life

• Cells function as the smallest unit that can carry out life processes.

Exception: Viruses

• Viruses do not have a complete cell structure.
• They cannot survive independently and are considered non-living.

Discovery of the Cell

• Robert Hooke:
  • First to observe cells by examining cork under a microscope (dead cells).
  • Described the structure as resembling a honeycomb.
• Anton Van Leeuwenhoek:
  • First to observe living cells, including:
  • White blood cells
  • Red blood cells
  • Sperm cells
  • Yeast cells
  • Bacteria

Key Cell Components

• Nucleus:
  • Discovered by Robert Brown
  • Contains:
  • Nuclear pore
  • Chromatin
  • Nucleolus
  • Nucleoplasm
  • Nuclear envelope

Contributions to Cell Theory

• Matthias Schleiden (1838):
  • Proposed that plants are composed of different tissues and cells.
• Theodor Schwann (1839):
  • Contributed to the cell theory including both plant and animal cells.
  • Identified the plasma membrane as a unique structure.

Cell Theory

• Formulated by Schleiden and Schwann:
  • All living organisms are composed of cells.
  • All cells are products of cells.
  • Drawback: Did not explain how new cells are formed.
• Modified by Rudolf Virchow (1855):
  • Proposed that all cells come from pre-existing cells (via division).
  • Stated: "Omnis cellula e cellula."

Modern Cell Theory

1. All living organisms are composed of cells and their products.
2. All cells arise from pre-existing cells.

Overview of Cells

• Plant Cells:
  • Example: Onion cells have a cell wall and plasma membrane.
• Animal Cells:
  • Example: Human cheek cells only have a plasma membrane.

Cell Structures

• Cell Wall:
  • Present in prokaryotes and eukaryotic plant cells.
  • Rigid outermost layer; dead structure.
• Plasma Membrane:
  • Present in all living organisms; living layer, also called the outer thin delimiting layer.

Nucleus

• Dense membrane-bound structure containing genetic material (DNA) in the form of chromosomes.

Cytoplasm

• Semi-fluid matrix present in all living cells.
• Main site for cellular activities and chemical reactions.

Cell Organelles

• Types of Organelles:
  • Membrane-bound:
  • Mitochondria (double membrane)
  • Plastids (only in plants)
  • Golgi body
  • Endoplasmic reticulum
  • Lysosomes
  • Vacuoles
  • Non-membrane bound:
  • Ribosomes
  • Centrioles (only in animal cells)

Size and Shape of Cells

• Size Variations:
  • Mycoplasma: Smallest living cell (~0.3 μm).
  • Bacteria: Average length 3-5 μm.
  • Red Blood Cells (RBC): Diameter ~7 μm.
  • Ostrich Egg: Largest isolated single cell.
  • Neuron (Nerve Cell): Longest cell, found in giant squid with long extensions.