IB Biology Topic A2.2.10: Cell Types and Cell Structures Viewed in Light and Electron Micrographs Notes

Micrographs Reveal Cell Identity

1. You identify a cell by matching its visible features and organelles to the structures each cell type is known to show.
2. This article is about recognising structures by how they look in light and electron micrographs, not about what each organelle does.

Three Cell Types Show Distinct Features

1. Cells fall into two broad categories: prokaryotic and eukaryotic.
2. Among eukaryotes, plant, animal, and fungal cells differ in a few diagnostic features.

Prokaryotic Cells

1. Prokaryotic cells, such as bacteria, look structurally simpler than eukaryotic cells.
2. To identify a prokaryotic cell, look for these signs:
   1. Small size, typically less than 5 micrometres across.
   2. Cell wall of peptidoglycan forming the outer boundary.
   3. Nucleoid region, a lighter area of cytoplasm holding circular DNA with no surrounding membrane.
   4. No membrane-bound organelles, so no nucleus, mitochondria, or chloroplasts are visible.
   5. 70S ribosomes, smaller than the 80S ribosomes of eukaryotes.

Plant Cells

1. Spot a plant cell by these features:
   1. Cell wall of cellulose giving a rigid outline.
   2. Chloroplasts, oval bodies with stacked thylakoids (grana) inside.
   3. Large central vacuole that pushes the other contents to the edge.
   4. Regular shape, often rectangular or box-like because of the rigid wall.

Animal Cells

1. Spot an animal cell by what is missing:
   1. No cell wall, so only a plasma membrane surrounds it and the shape looks irregular.
   2. No chloroplasts.
   3. Small vacuoles, if any, much smaller than a plant cell central vacuole.
   4. Centrioles, present in animal cells but absent from most plant cells.