Inside the Cell

Introduction to Cells

Cells are the fundamental unit of life. All living organisms are composed of cells, which carry out the essential processes necessary for life. Viruses, while complex and capable of some life-like functions, are not considered fully living because they lack many characteristics of true cells.

• Macromolecules in Cells:

  • Proteins: Perform most of the cell’s functions, including catalyzing metabolic reactions, providing structural support, and facilitating communication.

  • Nucleic acids: Store, transmit, and process genetic information (e.g., DNA and RNA).

  • Carbohydrates: Provide chemical energy, carbon, support, and identity (e.g., glucose, cellulose).

  • Lipids: Serve as energy storage, building blocks for cell membranes, and signaling molecules (e.g., phospholipids, steroids).

• Plasma membrane: A selectively permeable barrier that separates the cell from its environment and regulates the movement of substances in and out.

• Cytoplasm: The internal fluid of the cell, containing organelles and the cytosol (gel-like aqueous matrix).

• Ribosomes: Molecular machines used for protein synthesis.

Types of Cells

Cell Classification by Morphology

Cells are classified into two fundamental types based on their structure (morphology):

• Eukaryotes: Have a membrane-bounded nucleus that contains the cell’s genetic material.

• Prokaryotes: Lack a membrane-bounded nucleus; their DNA is located in a region called the nucleoid.

Example: Bacteria are prokaryotes, while animals, plants, fungi, and protists are eukaryotes.

Cell Classification by Phylogeny

Based on evolutionary history (phylogeny), all organisms are divided into three domains:

• Bacteria (Eubacteria)

• Archaea

• Eukarya (includes animals, plants, fungi, and protists)

The Tree of Life illustrates these relationships, with prokaryotes (Bacteria and Archaea) forming separate branches from eukaryotes.

Comparison of Prokaryotic and Eukaryotic Cells

The following table summarizes the main differences between prokaryotic and eukaryotic cells:

Additional info: Eukaryotic cells are generally larger and more structurally complex than prokaryotic cells.

Prokaryotes

Prokaryotes include the domains Bacteria and Archaea. They are characterized by the absence of a nucleus and other membrane-bound organelles. Their genetic material is located in the nucleoid region. Prokaryotes are typically unicellular and have simpler internal structures compared to eukaryotes.

Additional info: Prokaryotes play essential roles in ecosystems, including nutrient cycling and as part of the human microbiome.