4.3 Prokaryotic and Eukaryotic Cells

4.3 Prokaryotic and Eukaryotic Cells

Every cell, whether prokaryotic or eukaryotic, has four things: a plasma membrane, cytosol, ribosomes, and DNA. The plasma membrane bounds the cell. Cytosol is the aqueous solution inside that membrane. Ribosomes synthesize protein. DNA stores genetic information. Those shared parts are the baseline. Further differences concern how the DNA is housed and whether membrane-bounded organelles are present.

What every cell shares

Cytoplasm and cytosol

The word cytoplasm means the contents inside the plasma membrane. In a eukaryote that includes cytosol plus organelles. In a prokaryote, with no membrane-bounded organelles, cytoplasm and cytosol refer to nearly the same interior, with the chromosome and ribosomes in that space. A prokaryote has cytoplasm, ribosomes, and DNA. What it lacks is a nucleus.

Prokaryotic cells

No nucleus does not mean no DNA

Prokaryotes are organisms in the domains Bacteria and Archaea. A prokaryotic cell has no nucleus and no membrane-bounded organelles. Its DNA is typically one circular chromosome located in the nucleoid, a region of the cytosol that is not enclosed by a membrane. Many prokaryotes also carry plasmids, small extra DNA molecules that are usually circular and are not required for the most basic life processes. Plasmids often hold accessory genes, including some that confer antibiotic resistance. The chromosome in the nucleoid is the genome. Lacking a nucleus leaves that DNA in place.

Bacteria, Archaea, and the hot-spring case

Bacteria commonly have a cell wall containing peptidoglycan, a mesh of sugars and short peptides outside the plasma membrane. That wall helps the cell resist swelling in a watery habitat. Archaea do not have peptidoglycan. Their walls, when present, use other molecules. Archaea include many extremophiles: thermophiles that live at high temperature, halophiles that live in high salt, and methanogens that produce methane. Not every archaeon lives in an extreme habitat, but a prokaryotic cell from an extreme place is a common introductory clue. A thermophilic unicellular prokaryote is not a plant, a fungus, or an amphibian. Those three groups are eukaryotes. If the cell also lacks peptidoglycan, introductory classification places it in domain Archaea.

Where prokaryotes put metabolism

Because prokaryotes lack mitochondria and chloroplasts, they do not house respiration or photosynthesis in those organelles. Many aerobic bacteria run the respiratory electron-transport chain in the plasma membrane. Photosynthetic bacteria capture light with pigments in the plasma membrane or in infoldings of that membrane. The chemistry can resemble the eukaryotic pathways. The compartment is the cell membrane itself, or a fold of it, rather than a mitochondrion or a chloroplast. Absence of membrane-bounded organelles still leaves metabolism intact.

Binary fission and plasmids

Prokaryotes typically divide by binary fission. The circular chromosome is copied, and the cell splits into two. There is no mitotic spindle and no nuclear envelope to disassemble, because there is no nucleus. Eukaryotic mitosis and meiosis are a later topic. The organizational point is that one main chromosome in a nucleoid can be copied and segregated without a nucleus. Plasmids are copied on their own and are not the chromosome. Losing a plasmid can remove a resistance gene. It leaves the genome in the nucleoid in place.

Eukaryotic cells and the three domains

Nucleus, organelles, size, and ribosomes

Eukaryotes have a nucleus and membrane-bounded organelles such as the endoplasmic reticulum, Golgi apparatus, lysosomes, and mitochondria. Plants and algae add chloroplasts. Eukaryotic cells are generally larger than prokaryotic cells. A typical prokaryote is on the order of 1 to 10 micrometers across, and a typical eukaryote is on the order of 10 to 100 micrometers. Ribosome size differs as well. Prokaryotic ribosomes are 70S. Eukaryotic cytosolic ribosomes are 80S. The Svedberg number is a sedimentation rate, not a simple sum of masses, which is why a 30S subunit and a 50S subunit make a 70S ribosome, and a 40S subunit and a 60S subunit make an 80S ribosome. Mitochondria and chloroplasts contain 70S-like ribosomes, a reminder of their prokaryotic ancestry, but the ribosomes free in the eukaryotic cytosol are 80S.

Three domains, and why a virus is not a cell

Biologists place cellular life in three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are both prokaryotic in cell organization, yet they are separate domains. Archaea lack peptidoglycan, and some of their genetic machinery is closer to the eukaryotic version than to the bacterial version. Eukarya includes plants, animals, fungi, and protists, all with nuclei. A virus is not a member of any of these domains as a cell. A virus is genetic material inside a protein coat. It has no ribosomes, no cytosol, and no metabolism of its own, and it reproduces only inside a host cell. Some bacteria fold the plasma membrane inward for specialized chemistry, but those folds are not a nucleus, a mitochondrion, or a chloroplast.

A two-cell comparison

Picture two cells in a mixed water sample. Cell A is about 2 micrometers long, has a nucleoid and 70S ribosomes, and shows no mitochondria. Cell B is about 40 micrometers across, has a nucleus, mitochondria, and 80S ribosomes in the cytosol. Cell A is prokaryotic. Cell B is eukaryotic. If cell A also has peptidoglycan, it fits Bacteria. If it is a thermophile with no peptidoglycan, introductory classification uses Archaea. Neither cell is a virus, because both have a membrane, cytosol, and ribosomes.