Prokaryotic and Eukaryotic Cells Study Pack
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Last updated May 27, 2026
Prokaryotic and Eukaryotic Cells Study Guide
Distinguish the structural and functional differences between prokaryotic and eukaryotic cells, from the nucleoid region and 70S ribosomes of bacteria to the nuclear envelope, organelles, and endosymbiotic origins of mitochondria found in eukaryotes.
Key Takeaways
- •Prokaryotic cells lack a membrane-bound nucleus and membrane-enclosed organelles, while eukaryotic cells contain both, making compartmentalization a defining difference between the two domains.
- •All prokaryotes have a plasma membrane, cytoplasm, ribosomes, and a nucleoid region where circular DNA is concentrated, but no nuclear envelope separates the DNA from the cytoplasm.
- •Eukaryotic cells house DNA in a true nucleus bounded by a double membrane called the nuclear envelope, and carry out specialized functions in organelles such as mitochondria, the endoplasmic reticulum, and the Golgi apparatus.
- •Prokaryotic ribosomes are 70S (composed of 30S and 50S subunits), while eukaryotic cytoplasmic ribosomes are 80S (40S and 60S subunits) — a difference exploited by many antibiotics that selectively target bacterial ribosomes.
- •Many prokaryotes possess a cell wall made of peptidoglycan, a mesh of sugar chains cross-linked by peptides, which maintains cell shape and protects against osmotic lysis; eukaryotic animal cells have no cell wall, though plant and fungal cells have walls made of cellulose or chitin, respectively.
- •The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by a host cell, supported by evidence including their double membranes, 70S ribosomes, and circular DNA.
- •Prokaryotes reproduce asexually by binary fission, a rapid process that copies the chromosome and splits the cell, whereas eukaryotes reproduce through mitosis or meiosis, which require spindle apparatus and take considerably longer.
Defining the Two Cell Types
Every living organism is built from one of two fundamentally different cell architectures: the simpler prokaryotic design and the more structurally elaborate eukaryotic design. Understanding what each cell type has, lacks, and why those differences matter is the foundation for all of cell biology.
Prokaryotic Cell Identity
- •Prokaryotes include Bacteria and Archaea — two of the three domains of life — and are almost always unicellular.
- •The term 'prokaryote' reflects the absence of a true nucleus: genetic material sits in a region of the cytoplasm called the nucleoid, but no membrane encloses it.
- •Prokaryotic cells are generally 1–10 micrometers in diameter, roughly 10 times smaller than a typical eukaryotic cell.
Eukaryotic Cell Identity
- •Eukaryotes include all protists, fungi, plants, and animals — organisms in the domain Eukarya — and can be unicellular (e.g., yeast, Amoeba) or multicellular.
- •The defining feature is a membrane-bound nucleus that houses linear chromosomes packaged with histone proteins.
- •Eukaryotic cells range from about 10–100 micrometers and contain an extensive internal membrane system that divides the cell into functionally distinct compartments.
Shared Structures Found in All Cells
Despite their many differences, prokaryotic and eukaryotic cells share a set of core structures that reflect their common ancestry and the universal requirements of life.
Plasma Membrane
- •Every cell is enclosed by a plasma membrane — a phospholipid bilayer embedded with proteins that controls the movement of substances into and out of the cell.
- •The fluid mosaic model describes the membrane as a dynamic structure in which proteins float within or span the bilayer.
Cytoplasm and Ribosomes
- •The cytoplasm is the gel-like fluid filling the interior of the cell, in which biochemical reactions occur.
- •All cells contain ribosomes, the molecular machines that translate mRNA into protein; their presence in every cell reflects the universal use of the genetic code.
- •Prokaryotic ribosomes are 70S; eukaryotic cytoplasmic ribosomes are 80S — the size difference arises from distinct ribosomal RNA sequences and protein compositions.
Genetic Material as DNA
- •All cells store heritable information as double-stranded DNA, and all use the same triplet codon system to encode proteins.
- •Both cell types use DNA polymerase for replication and RNA polymerase to transcribe genes into mRNA.
Structures Unique to Prokaryotic Cells
Prokaryotes have evolved a distinct set of structures that allow them to thrive in virtually every environment on Earth, many of which have no equivalent in eukaryotic cells.
Nucleoid and Plasmids
- •The nucleoid is a region — not an organelle — where the single circular chromosome is compacted by nucleoid-associated proteins; it has no surrounding membrane.
- •Many prokaryotes also carry plasmids: small, circular, extrachromosomal DNA molecules that replicate independently and often encode advantageous traits such as antibiotic resistance.
Prokaryotic Cell Wall and Capsule
- •Most bacteria have a cell wall composed of peptidoglycan, a polymer of N-acetylglucosamine and N-acetylmuramic acid cross-linked by short peptide bridges.
- •Gram-positive bacteria have a thick peptidoglycan layer that retains crystal violet dye; Gram-negative bacteria have a thin peptidoglycan layer sandwiched between two membranes and do not retain the dye.
- •Some prokaryotes secrete a polysaccharide capsule outside the cell wall that helps them evade host immune responses and resist desiccation.
Flagella and Pili
- •Prokaryotic flagella are composed of the protein flagellin and rotate like a propeller, driven by a proton gradient across the plasma membrane; they are structurally distinct from eukaryotic flagella.
- •Pili (singular: pilus) are protein filaments on the cell surface; some types anchor the bacterium to surfaces, while sex pili mediate the transfer of plasmid DNA between cells during conjugation.
Binary Fission
- •Prokaryotes reproduce by binary fission: the chromosome replicates, the two copies are pulled to opposite ends of the cell, and the cell pinches in two — a cycle that can complete in as little as 20 minutes under ideal conditions.
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What is the key structural feature that defines a eukaryotic cell and distinguishes it from a prokaryotic cell?
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Prokaryotic vs. Eukaryotic Cell Organization
Explain the fundamental differences between prokaryotic and eukaryotic cells in your own words. What structural features define each type, and why does compartmentalization matter for how eukaryotic cells function?
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