Cell Structure and Endosymbiotic Theory: ER, Golgi, Ribosomes, and Cell Walls

Rough ER Structure

A network of flattened sacs (cisternae) with ribosomes on its surface, giving it a rough appearance.

Rough ER Function

It folds proteins and begins modifications such as adding sugars.

Golgi Apparatus Structure

Stacks of flattened sacs without ribosomes.

Golgi Apparatus Function

It further modifies proteins, sorts them, and packages them into vesicles for transport.

Why do the rough ER and Golgi need membranes to separate their enzymes from the cytoplasm

To prevent damage and mix-ups with other cellular processes.

Membrane Function of Rough ER and Golgi

To prevent damage and mix-ups with other cellular processes.

Ribosome Structure

Made of rRNA and proteins, with a large and small subunit.

Prokaryotic vs Eukaryotic Ribosomes

Prokaryotes have smaller 70S ribosomes that float freely; eukaryotes have larger 80S ribosomes that can attach to the rough ER.

Ribosome Function

Protein synthesis.

Cell Wall Function

Provides structure, shape, and protection, and prevents cells from bursting in hypotonic environments.

Prokaryotic vs Eukaryotic Cell Walls

Prokaryotic cell walls are made of peptidoglycan and simple; eukaryotic plant cell walls are cellulose, fungal walls are chitin, and both are thicker and more complex.

Endosymbiotic Theory

That mitochondria and chloroplasts originated from prokaryotes engulfed by ancestral eukaryotic cells.

DNA Evidence for Endosymbiotic Theory

They have small circular DNA, a single chromosome, and DNA sequences that match prokaryotes.

Structural Similarity Evidence for Endosymbiotic Theory

Mitochondria and chloroplasts are similar in size and shape to prokaryotes.

Ribosome Evidence for Endosymbiotic Theory

They have 70S ribosomes, which are small and antibiotic-sensitive, like those in prokaryotes.

Reproductive Evidence for Endosymbiotic Theory

They divide semi-autonomously by binary fission.

Membrane Evidence for Endosymbiotic Theory

They have two membranes, with the inner membrane containing prokaryote-like lipids.