Inside the Cell

Introduction to Cells

Cells are the fundamental unit of life, and all living organisms are composed of one or more cells. Viruses, while complex biological entities, exhibit some characteristics of life but do not meet all criteria to be classified as living organisms.

• Cells are the smallest units capable of independent life and reproduction.

• Viruses require host cells for replication and lack many features of living cells.

Macromolecules in Cells

Cells contain four major classes of macromolecules, each with distinct functions essential for cellular life.

• Proteins: Perform most cellular functions, including catalysis (enzymes), structural support, transport, and signaling.

• 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 sources, 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.

• Cytoplasm: The internal fluid of the cell, containing organelles and cytosol.

• Ribosomes: Molecular machines responsible for protein synthesis.

Cell Types and Classification

Cell Morphology: Prokaryotes vs. Eukaryotes

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

• Eukaryotes: Have a membrane-bounded nucleus and various membrane-bound organelles.

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

Example: Bacterial cells are prokaryotic, while animal and plant cells are eukaryotic.

Cell Phylogeny: Domains of Life

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

• Eubacteria (true bacteria)

• Archaea (ancient bacteria)

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

The Tree of Life illustrates the evolutionary relationships among these domains, with prokaryotes comprising Eubacteria and Archaea, and eukaryotes forming a separate branch.

Comparison of Prokaryotic and Eukaryotic Cells

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

Additional info: Eukaryotic cells are generally larger and more complex than prokaryotic cells, with specialized organelles for compartmentalized functions.

Prokaryotes

Overview of Prokaryotic Cells

Prokaryotic cells are simpler in structure and lack internal membrane-bound organelles. Their genetic material is located in the nucleoid region, and they reproduce primarily by binary fission.

• Examples include Escherichia coli (E. coli) and Bacillus subtilis.

• Prokaryotes play essential roles in ecosystems, such as nitrogen fixation and decomposition.

Additional info: For more detailed information, refer to section 7.1 in your textbook.