1.3 Prokaryotic and eukaryotic cell structure

Prokaryotic and Eukaryotic Cell Structure

All living organisms are built from one of two fundamental cell types: prokaryotic or eukaryotic. Understanding the structural differences between them is central to cell biology, because those differences explain how cells carry out their functions, how they evolved, and why life is so diverse.

Prokaryotic vs. Eukaryotic Cell Structures

The biggest distinction is compartmentalization. Eukaryotic cells contain membrane-bound organelles that separate different chemical processes into distinct spaces. Prokaryotic cells lack these internal compartments, so their chemical reactions all occur in the cytoplasm or at the cell membrane.

Here's how the two cell types compare across key features:

• Membrane-bound organelles: Prokaryotes have none. Eukaryotes have many (nucleus, mitochondria, lysosomes, etc.), which lets them run different processes simultaneously without interference.
• Genetic material location: Prokaryotes store their DNA in a nucleoid region, an area of the cytoplasm with no surrounding membrane. Eukaryotes house their DNA inside a true nucleus enclosed by a double-membrane nuclear envelope with nuclear pores.
• Cell wall composition: Most prokaryotes have a cell wall made of peptidoglycan. Some eukaryotes also have cell walls, but the material differs: cellulose in plants, chitin in fungi. Animal cells have no cell wall at all.
• Cell membrane: Both cell types have a plasma membrane that controls what enters and exits the cell.
• Ribosomes: Prokaryotic ribosomes are smaller (70S), while eukaryotic ribosomes are larger (80S). Both carry out protein synthesis, but this size difference is clinically important because certain antibiotics can target 70S ribosomes without harming eukaryotic cells.
• External and structural features: Prokaryotes may have flagella (for motility), pili (for attachment to surfaces or other cells), and capsules (a protective outer layer). Eukaryotes rely on a cytoskeleton made of microfilaments, intermediate filaments, and microtubules to maintain cell shape, enable movement, and organize internal structures.

Major Eukaryotic Cell Organelles

Each organelle handles a specific set of tasks. Think of the eukaryotic cell as a factory with specialized departments:

• Nucleus: Contains the cell's DNA, surrounded by the nuclear envelope. Nuclear pores regulate the transport of molecules (like mRNA) between the nucleus and cytoplasm. This is where DNA replication and transcription occur.
• Endoplasmic reticulum (ER): Comes in two forms. Rough ER is studded with ribosomes and synthesizes and modifies proteins, especially those destined for secretion or membrane insertion. Smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification of drugs and poisons, and calcium storage.
• Golgi apparatus: Receives proteins and lipids from the ER, then modifies, sorts, and packages them into vesicles for delivery to their final destinations (other organelles, the plasma membrane, or outside the cell).
• Mitochondria: The site of cellular respiration, where glucose is broken down to produce ATP (the cell's energy currency). Mitochondria have their own circular DNA and 70S ribosomes, which is key evidence for the endosymbiotic theory.
• Lysosomes: Membrane-bound sacs filled with digestive enzymes that break down worn-out organelles, food particles, and cellular debris. They maintain an acidic internal pH (~4.5–5.0) to keep those enzymes active.
• Peroxisomes: Contain enzymes that break down fatty acids and detoxify harmful substances. A major function is converting toxic hydrogen peroxide ($H_2O_2$) into water and oxygen.
• Chloroplasts (plants and algae only): The site of photosynthesis, where light energy is converted into chemical energy (glucose). Like mitochondria, chloroplasts have their own circular DNA and 70S ribosomes, further supporting the endosymbiotic theory.

Genetic Material in Cell Types

How DNA is organized differs significantly between the two cell types:

• Prokaryotic DNA is typically a single, circular chromosome located in the nucleoid region. It is not wrapped around histone proteins (though some archaea do use histone-like proteins). Many prokaryotes also carry small circular DNA molecules called plasmids, which can provide advantages like antibiotic resistance.
• Eukaryotic DNA consists of multiple, linear chromosomes housed inside the nucleus. The DNA is tightly wound around histone proteins to form chromatin, which condenses further into visible chromosomes during cell division. This packaging allows for more complex regulation of gene expression.

Evolution of Cell Structures

Prokaryotic cells appeared first in Earth's history (around 3.5 billion years ago), while eukaryotic cells emerged roughly 1.5–2 billion years ago. The leading explanation for how eukaryotes arose is the endosymbiotic theory, proposed by Lynn Margulis in 1967.

The theory works like this:

1. An ancestral prokaryotic cell engulfed a smaller aerobic (oxygen-using) prokaryote. Instead of being digested, the smaller cell survived inside the host.
2. Over time, the two organisms developed a mutually beneficial relationship. The engulfed cell provided ATP through aerobic respiration, while the host provided nutrients and protection.
3. The engulfed cell eventually became the mitochondrion.
4. A similar event occurred later when a mitochondria-containing cell engulfed a photosynthetic prokaryote, which became the chloroplast.

Several lines of evidence support this theory:

• Mitochondria and chloroplasts have their own circular DNA, similar to bacterial DNA.
• Both organelles have 70S ribosomes, the same size found in prokaryotes.
• Both replicate by binary fission, the same method bacteria use to divide.
• Both are surrounded by a double membrane, consistent with an engulfing event.

The evolution of membrane-bound organelles gave eukaryotic cells two major advantages: compartmentalization (running incompatible chemical reactions in separate spaces) and specialization (dedicating entire organelles to specific tasks). The true nucleus, in particular, allowed for more sophisticated control over when and how genes are expressed, paving the way for the complex multicellular organisms that exist today.