Cell Anatomy: Shared Characteristics

Prokaryotes

Prokaryotic cellsThe basic structural and functional units of life. are the simplest form of cells. The word prokaryotic comes from Greek, meaning “before kernel,” referring to the factA statement based on direct observation that is repeatedly confirmed. that these cells don’t have a nucleusThe control center of the cell that contains DNA and directs cellular activities. — just the “bare essentials” for life.

Bacteria, which are prokaryotic, typically come in three shapes:

• Rod-shaped (bacillus)
• Spherical (coccus)
• Corkscrew-shaped (spirillum)

Let’s take this rod-shaped bacterium as an example. It’s basically custom-built to infect you. You are the perfect temperature, and your warm, moist mucus membranes make a great vacation spot. But to set up camp, it first has to latch onto you when you cross paths.

Those pili (tiny hair-like structures) covering its surface act like Velcro strips, helping the bacterium cling to its prey. The flagellum (a whip-like tail) propels it deeper into your respiratory systemThe organ system responsible for gas exchange (oxygen and carbon dioxide).. And if your immune system comes looking for trouble? No problem. This bacterium has three layers of defense — a capsule, a cell wall, and a plasma membraneThe outer boundary of a cell that controls what enters and exits.. Think of it as wearing three winter coats in a snowstorm.

Inside, things are stripped down but effective. It has just:

• One large chromosome (a single loop of DNA)
• Ribosomes (protein factories)
• Sometimes plasmids — small, circular bits of DNA that can be traded between bacteria like passing secret notes.

The problem? If one bacterium learns how to resist antibiotics, it can share that trick with others. Suddenly, your harmless hitchhiker is running an antibiotic-resistance training seminar in your body. Bummer.

Cell Wall

Many antibiotics target the bacterial cell wall, and here’s how some of them work:

Tetracyclines stop amino acids from joining the protein assembly line at the ribosome. Without protein production, bacteria can’t grow or repair themselves. (Doxycycline is one you might hear about after dental work.)

Beta-lactams (like penicillin) attach to special proteinsLarge molecules made of amino acids with various functions in the body. called PBPs, which are responsible for cross-linking the peptidoglycan in the cell wall. Without this reinforcement, the wall collapses under osmotic pressureThe force exerted by water moving across a membrane due to osmosis. and the bacterium bursts like an overfilled waterThe universal solvent essential for life. balloon.

Fluoroquinolones mess with enzymesProteins that speed up chemical reactions in the body. needed for DNA replication, transcription, and repair. Without these enzymes, the bacterial DNA gets hopelessly tangled and can’t replicate. (Cipro and Levaquin are examples.)

Polymyxins punch holes in the bacterial membrane, causing leaks. Imagine a waterbed with a slow puncture — not good for a bacterium. Neosporin contains a polymyxin along with two other antibiotics.

Sulfonamides block the pathway bacteria use to make folic acidA substance that releases hydrogen ions (H⁺) in solution., which is essential for DNA production. No folic acid = no DNA = no survival. (Bactrim and Septra fall into this category.)

Eukaryotic Cells

Eukaryotic cells are the complex cousins of prokaryotes. They share some basicA solution with a pH above 7, having a lower concentration of H⁺ ions. features (like ribosomesSmall structures responsible for protein synthesis, either free-floating or attached to the rough ER and sometimes flagellaLong, whip-like structures used by some cells (e.g., sperm) for movement.), but they also have membrane-bound organellestructures within a cell that perform specialized functions. such as the nucleus and mitochondria — things prokaryotes simply don’t have.

The domain Eukarya includes four kingdoms:

Fungi – Yeast, mushrooms, and other fungi. Yeast can reproduce by budding, which is like cloning itself. Some fungi can even cause massive problems, like the one driving amphibians toward extinction.

Protists – For example, algae. Some have flagella so they can “swim” toward sunny areas in a pond for photosynthesis. But protists aren’t all friendly — some cause disease.

Plants – Plant cells have rigid cell walls and chloroplasts filled with chlorophyll or other light-capturing pigments.

Animals – This is our kingdom, along with everything from jellyfish to sponges to insects.

The Plasma Membrane

All cells — prokaryotic and eukaryotic — have a plasmaThe liquid component of blood. membrane. Think of it as the cell’s bouncer: it decides what gets in and what stays out. It separates the cell’s inner world from the outside environment and maintains a controlled internal space.

In microscopy images, membranes often stain pinkish-purple, making them stand out clearly. In adipose tissue, for instance, you can see the delicate outline of each fat cell thanks to that stain.

Ribosomes

Ribosomes are tiny but mighty — the protein factories of the cell. They’re made of proteins and rRNA(ribosomal RNA) – RNA that forms part of the ribosome and helps in protein synthesis. and are measured in Svedberg units (S), which describe their size and shape.

Both prokaryotic and eukaryotic ribosomes are made of two subunits:

• Prokaryotic ribosomes have a 30S small subunit and a 50S large subunit. The 16S rRNA in the small subunit acts like a bacterial fingerprint, helping scientists identify the species.
• Eukaryotic ribosomes are bigger — 40S small subunit and 60S large subunit — with around 80 proteins in total.

Where do you find them?

• In prokaryotes, they float freely in the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol..
• In eukaryotes, they’re in three places: floating in the cytoplasm, inside mitochondria and chloroplasts (where they resemble prokaryotic ribosomes), and attached to the endoplasmic reticulum, forming “rough ER.”

When assembled and working, ribosomes read mRNA instructions and link amino acids together with peptide bondsBonds that link amino acids in proteins., creating polypeptides — the building blocks of proteins. Without them, the cell’s construction site shuts down.