Prokaryotic and Eukaryotic Cells
Compare typical bacterial, plant and animal cells by their DNA location and cell structures. Distinguish optional plasmids from the main chromosome, and compare measured cell sizes in micrometres.
What a learner can do afterwards
- Names shared cell structures and explains how typical bacterial, photosynthetic plant and animal cells differ in nuclei, walls, chloroplasts and vacuoles.
- Orders given bacterial, animal-cell and plant-cell measurements in micrometres and calculates a size ratio without assuming a fixed order for all cells.
- Explains how a transferable plasmid can spread a trait such as antibiotic resistance without replacing the main chromosome.
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Plant and animal cells are eukaryotic. Bacteria and archaea have a prokaryotic cell plan: no membrane-bound nucleus and no membrane-bound organelles such as mitochondria. Both cell plans include a cell membrane, cytoplasm, DNA and ribosomes. Ribosomes make proteins, so finding ribosomes does not distinguish the two plans.
The main bacterial chromosome lies in the nucleoid, a DNA-containing region without a surrounding membrane. In most bacteria the chromosome is circular. Some bacteria also have plasmids, small DNA molecules separate from the main chromosome. Plasmids are usually circular, but not every bacterium has them. Most bacteria have a cell wall.
Animals, plants, fungi and protists are eukaryotes. In a typical eukaryotic cell, most DNA is in a membrane-bound nucleus. Mitochondria help supply usable energy from food. A photosynthetic leaf cell has a wall, chloroplasts and usually a large central vacuole. Animal cells lack walls and chloroplasts but may have small vacuoles. Some plant cells, such as root cells, lack chloroplasts.
A micrometre is one millionth of a metre. Bacteria are usually smaller than plant and animal cells, but sizes vary. Order measurements, not displayed image sizes: micrographs may use different magnifications. Divide the larger length by the smaller for a size ratio. Here, 30 divided by 3 is 10. The animal cell is ten times as long as the bacterium. A plant cell is not always larger than an animal cell.
Plasmids can carry genes that give a bacterium useful traits. Some carry antibiotic-resistance genes. When a transferable plasmid passes to another bacterium, those genes can spread too. Not every plasmid transfers between cells or carries resistance. Plasmids do not supply a nucleus or have to replace the main chromosome.
Prokaryotes lack a membrane-bound nucleus. Typical eukaryotic cells have one. Plasmids are optional, and cell-size comparisons need measurements.
Lesson, questions, explanations and study cards written for this dataset. Cell-biology sources were consulted for facts and teaching design.
Supplementary readings and study cards written for this dataset. Linked videos are credited to Khan Academy; their licensing terms have not been verified.
More to read
Finding DNA in a bacterial cell
A nucleus has membranes around it. A bacterial nucleoid does not: it is the region where the main chromosome is concentrated. Most bacteria have a circular main chromosome, but that common arrangement is not the definition of a prokaryote.
Some bacteria also carry plasmids. These are separate DNA molecules, usually circular. A plasmid can carry genes for a useful trait. If it carries antibiotic-resistance genes and can transfer to another cell, those genes can spread.
Bacteria still need a membrane, cytoplasm and ribosomes. Most also have a wall. Their lack of a nucleus does not mean they lack DNA or protein-making machinery. Bacteria have many roles, and they do not all cause disease.
Comparing named plant and animal cells
A photosynthetic leaf cell and a cheek cell share many structures. Both typically have a nucleus and mitochondria, as well as cytoplasm, ribosomes and a cell membrane. Most DNA is in the nucleus. Mitochondria have some DNA of their own; chloroplasts also contain DNA where they are present.
The leaf cell has a supporting wall and chloroplasts that use light energy to make sugars. It usually has a large central vacuole. An animal cell lacks the wall and chloroplasts, although it may contain small vacuoles.
A root cell is also a plant cell, but it generally has no chloroplasts. Name the cell being compared. One leaf-cell diagram cannot show the structures of every cell in a plant.
Comparing cells in microscope images
A microscope image is called a micrograph. Before comparing cells in two micrographs, check their scale bars or measured lengths. A highly magnified bacterium can look larger on a page than an animal cell photographed at lower magnification.
Light microscopes can show living cells, although stains and other preparation may kill them. Magnification enlarges an image. Resolution determines whether nearby details can be distinguished; enlarging an unclear image does not reveal new structures.
Electron microscopes can reveal finer details. Transmission electron microscopy can show internal structures in thin sections. Scanning electron microscopy shows surface features. Conventional electron-microscope preparation does not permit viewing living cells. These methods do not all use the same preparation steps.
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