Compare and contrast prokaryotic and eukaryotic cells using structural and functional evidence (Tennessee Academic Standards for Science, Biology I, BIO1.LS1).

What this topic is asking

The Tennessee LS1 standards ask you to compare and contrast the two basic kinds of cell using structural and functional evidence. For the Biology I EOC that means knowing the one feature that separates prokaryotic from eukaryotic cells, the features they share, and the further differences between plant and animal cells among the eukaryotes. The exam often gives you a labeled diagram or a list of parts and asks you to classify the cell, so this is a high-frequency, point-rich topic.

What every cell has in common

Because all cells share these four parts, an EOC item that asks "which is found in all cells" should point you to one of them, while an item that asks "which is found only in eukaryotes" points to the nucleus or another membrane-bound organelle.

The defining difference: the nucleus

The single feature that separates the two cell types is the membrane-bound nucleus.

• A prokaryotic cell (a bacterium or an archaeon) has no nucleus. Its DNA is usually a single circular loop that floats free in a region of the cytoplasm called the nucleoid. It has no membrane-bound organelles either, so its reactions all happen in the cytoplasm or at the membrane. Prokaryotic cells are typically small (about 1 to 10 micrometers).
• A eukaryotic cell (a plant, animal, fungus, or protist) has a true nucleus: a double membrane that encloses the DNA and separates it from the rest of the cell. Eukaryotes also have membrane-bound organelles (mitochondria, endoplasmic reticulum, Golgi apparatus, and others) and are typically larger (about 10 to 100 micrometers).

The word roots help: pro-karyote means "before the nucleus," and eu-karyote means "true nucleus."

Why the eukaryotic design matters: compartmentalization

The advantage of having internal membranes is compartmentalization. By walling off different reactions into different organelles, a eukaryotic cell can run many incompatible processes at the same time and keep the right conditions (pH, enzymes, concentrations) in each compartment. For example, the digestive enzymes of a lysosome are kept away from the rest of the cell, and the reactions of respiration are concentrated inside the mitochondrion. A prokaryote, lacking these compartments, is more limited, which is one reason eukaryotic cells can grow larger and support more complex organisms.

A second split: plant versus animal cells

Among eukaryotes, the EOC expects you to know what makes plant cells different from animal cells. Both have a nucleus, mitochondria, ribosomes, a membrane, and cytoplasm. Plant cells add three structures that animal cells do not have:

• A cell wall of cellulose, outside the membrane, that gives shape and support.
• Chloroplasts, which capture light energy for photosynthesis.
• A large central vacuole that stores water and keeps the cell firm (turgid).

So a parts list that includes a cell wall, chloroplasts, and a large vacuole points to a plant cell, while the same list without those three points to an animal cell.

Try this

Q1. State the one feature that defines a eukaryotic cell. [1]

• Cue. A true, membrane-bound nucleus enclosing the DNA.

Q2. A plant cell and an animal cell are compared. Name three structures found in the plant cell but not the animal cell, and give the function of each. [3]

• Cue. Cell wall (support and shape), chloroplasts (photosynthesis), large central vacuole (stores water and keeps the cell firm).