Study Guide for Endomembrane System and Protein Transport

Structure:
- Composed of an outer nuclear membrane and an inner nuclear membrane.
- The outer membrane is continuous with the endoplasmic reticulum (ER).
- The nuclear intermembrane space is continuous with the ER lumen.
Communication:
- The nucleus communicates with the cytosol through nuclear pores.

Function: Site of new membrane synthesis.
Types:
- Rough ER:
- Associated with ribosomes on the cytosolic side.
- Responsible for the synthesis of proteins that are either sorted into the ER membrane or the ER lumen.
- Smooth ER:
- Functions include synthesizing steroid hormones.
- Serves as Ca²⁺ storage, facilitating uptake and release in response to extracellular signals (e.g., during neuronal signaling).
Free Ribosomes:
- Synthesize cytosolic proteins.

Function:
- Modifies proteins and lipids received from the ER before transporting them to other cellular compartments or to the extracellular space.
- Engages in the breakdown of lipids.
- Contributes to detoxification processes.

Function:
- Involved in the breakdown of damaged organelles and certain endocytosed macromolecules.

Additional Function:
- Engages in the degradation of macromolecules and cellular waste.

It is hypothesized that these membrane-bound organelles likely originated from the invagination of the plasma membrane.
Evidence includes the structural similarity to certain prokaryotes where DNA is often attached to the plasma membrane.

The mitochondrion is not part of the endomembrane system.
Chloroplasts and mitochondria are believed to originate in a fundamentally different way than the endomembrane system entities.

Organelles are held in place by attachments to the cytoskeleton.
Components include:
- Actin filaments
- Microtubules
- Example: Centrosome with a pair of centrioles.
- Chromatin (DNA)
- Nuclear pore
- Extracellular matrix
- Nuclear envelope
- Vesicles
- Nucleolus
- Peroxisome
- Ribosome
- Plasma membrane
- Golgi apparatus
- Intermediate filaments
- Lysosome
- Nucleus
- Endoplasmic reticulum
- Mitochondrion
Role of Cytoskeleton:
- Provides tracks for organelle movement, which is ATP-dependent.

Proteins can be:
- Delivered directly from the cytosol.
- Delivered via the ER.
The final location of a protein depends on its sequence, specifically the presence of a sorting signal.
Translocators are involved in the transportation process.
Folding States:
- Proteins remain folded during transport to most destinations.
- Proteins unfold during transport into mitochondria or chloroplasts.

If potential sorting signals are removed or added prior to protein synthesis, it can affect their destination.
However, if a protein is already in the ER lumen and the sorting signal is removed, the protein will stay in the ER lumen.