Cell ultrastructure and the secretion pathway

What you’ll learn

• Identify the main structures of eukaryotic and prokaryotic cells.
• Relate organelles to their cellular functions.
• Trace a secreted protein through the endomembrane system.

Understand the idea

All living organisms are cellular. Cells share a plasma membrane, cytoplasm, genetic material and ribosomes, but their internal organisation differs. A eukaryotic nucleus contains chromosomes and has a double-membrane envelope with pores. The nucleolus is involved in ribosome production. Mitochondria have an inner membrane folded into cristae and support aerobic ATP production. Lysosomes contain digestive enzymes; centrioles in many animal cells help organise microtubules, including those of the spindle.

Rough endoplasmic reticulum has ribosomes on its surface and is involved in producing proteins for secretion or membranes. Smooth endoplasmic reticulum lacks these attached ribosomes and has functions including lipid synthesis. A secreted enzyme enters the rough ER during synthesis, moves in transport vesicles to the Golgi apparatus, and is modified and sorted into secretory vesicles. Fusion of a vesicle with the plasma membrane releases its contents by exocytosis. A Golgi stack often appears as flattened curved sacs with nearby vesicles in an electron micrograph.

A typical bacterial cell has circular DNA in a nucleoid region rather than a membrane-bound nucleus, small ribosomes and a cell wall made of peptidoglycan. Some bacteria also have plasmids, a protective capsule, pili for attachment or DNA transfer, and flagella for movement. These optional structures must not be assigned to every bacterial species. Prokaryotes lack the membrane-bound organelles of eukaryotes; their metabolic reactions still occur, using enzymes in the cytoplasm and appropriate membrane systems.

Worked example

A cell releases large quantities of a digestive enzyme. Explain why an electron micrograph may show extensive rough ER, a prominent Golgi apparatus and many mitochondria.

1. Ribosomes associated with rough ER synthesise the secreted protein and the ER supports its processing and transport.
2. The Golgi modifies and sorts the protein into vesicles for secretion.
3. Mitochondria supply ATP for energy-requiring processes such as biosynthesis and vesicle transport.

Answer: The organelles support high rates of enzyme synthesis, processing and export.