Chapter 7: Inside the Cell

7.1 The First Bacteria

The earliest evidence of life on Earth comes from fossilized bacteria, which date back approximately 3.5 billion years. These ancient microorganisms provide insight into the origins and evolution of cellular life.

• Fossil Evidence: Fossilized bacteria have been found in locations such as Australia.

• Age: The oldest bacterial fossils are about 3.5 billion years old.

• Significance: These fossils help scientists understand the timeline and development of life on Earth.

• Example: Stromatolites are layered structures formed by bacterial communities and are among the oldest known fossils.

7.1 Bacterial and Archaeal Cell Structures and Their Functions

Recent advances reveal that prokaryotic cells, including bacteria and archaea, are more complex than previously thought. Their cellular structures are specialized for survival in diverse environments.

• Prokaryotic Cells: Cells lacking a membrane-bound nucleus and other organelles.

• Complexity: Prokaryotes possess a variety of internal and external structures.

• Comparison: Both bacteria and archaea are prokaryotes, but they differ in certain structural and biochemical features.

Prokaryotic Cell Structures: A Parts List

Prokaryotic cells have an array of distinctive structures and functions that enable them to thrive in various environments.

• Chromosome: Contains genetic material; usually a single, circular DNA molecule.

• Ribosomes: Sites of protein synthesis; composed of RNA and proteins.

• Plasma Membrane: Encloses the cell, controlling the movement of substances in and out.

• Cell Wall: Provides structural support and protection; composition varies between bacteria and archaea.

• Capsule: An outer protective layer found in some bacteria.

• Flagellum: Tail-like structure used for movement.

• Pili: Hair-like appendages for attachment and genetic exchange.

Chromosome Structure in Prokaryotes

Prokaryotic cells contain at least one chromosome, which is typically circular and located in a region called the nucleoid.

• Example: Bacterial chromosome is circular, while eukaryotic chromosomes are linear.

• Nucleoid: The area within the cell where the chromosome is found, not surrounded by a membrane.

The Chromosome Is Organized into a Nucleoid

The nucleoid is the region in a prokaryotic cell where the chromosome is located. Prokaryotic chromosomes are often supercoiled for efficient packaging.

• Supercoiling: The chromosome twists upon itself to become more compact.

• Plasmids: Small, circular DNA molecules that can be exchanged between cells, often carrying beneficial genes.

• Gene Exchange: Plasmids facilitate horizontal gene transfer, allowing bacteria to share traits such as antibiotic resistance.

Supercoiled Chromosome or Plasmid

Supercoiling is a process by which DNA is twisted to fit inside the cell more efficiently.

• Function: Compacts DNA, making it easier to store within the small cell.

• Visual: Supercoiled DNA appears as tightly wound loops.

Ribosomes in Prokaryotes

Ribosomes are the molecular machines responsible for protein synthesis. Their structure is similar among all prokaryotes.

• Structure: Composed of a large and small subunit; contains sites for tRNA binding (E, P, A sites).

• Function: Translate mRNA into proteins.

• Equation:

Prokaryotes Have Organelles

Some prokaryotes possess internal compartments, or organelles, that are surrounded by membranes and perform specialized functions.

• Storage Organelles: Store ions and molecules.

• Magnetosomes: Contain magnetic crystals to help orient the cell.

• Enzyme Compartments: Concentrate enzymes for specific biochemical reactions.

• Comparison: Eukaryotes have more complex organelles, but prokaryotes also have specialized internal structures.

Cell Membranes Differ Amongst Prokaryotes

Bacteria and archaea have distinct cell membrane structures, which affect their physiology and adaptation to environments.

Additional info: Archaeal membranes are often more resistant to heat and chemical stress due to their unique lipid composition.