The Endoplasmic Reticulum (ER)

Jill, Edward, and Nicole

General StructureThe ER is surrounded by a single continuous membrane which encloses the entire organelle.

Inside the membrane is a large complex lumen. The lumen, or the cisternal space, is the internal cavity of the ER.

The membrane of the ER allows selected molecules to move from the cytoplasm into the lumen.

The Endoplasmic Reticulum is primarily concerned with the synthesis, storage, and exportation of important molecules.

Rough ER Structure

The Rough ER is composed of a network of flattened sacs.

The sacs are studded with ribosomes which make it appear “rough” in electron micrographs.

The membrane of the Rough ER and the membrane of the Nuclear Envelope is continuous.

The first major function is to make more membrane.

Certain proteins are made by the ER enzymes and sent to the membrane of the ER.

The membrane enlarges and the ER may produce membrane that ends up in other organelles.

The second major function is to produce secretory proteins.

Secretory proteins are proteins which are discharged by certain cells, for example white blood cells discharge antibodies.

The ER synthesizes polypeptides, with data from the Nucleus, and manufactures functional proteins.

Rough ER Functions

Important Processes in the Rough ER

Process:

1) a polypeptide is synthesized as it enters the ER

2) chains of sugar are linked to the polypeptide making it glycoprotein

3) the glycoprotein is packed in a transport vesicle

4) the vesicle buds from the membrane (see diagram slide 8)

5) the vesicle is sent to the golgi apparatus for processing.

Ribosomes:

The membrane in the rough ER holds receptors that make the larger ribosomal subunits attach. Through a pore in the receptor the ER allows newly synthesized proteins into the lumen.

Ribosomal Interaction with the Rough ER

This diagram shows how the ribosomes attach to

the membrane and enter the lumen.

This diagram shows the regulation by the ribosomes of proteins entering the ER.

Interactions with Other Cell Parts

Golgi Complex:

The ER sends vesicles with secretory proteins to the Golgi Body.

Resident Proteins:

Most proteins move through the ER temporarily, but a few are essential to the operation of the ER. These essential molecules are called “resident proteins,” they have a specific sequence of amino acids that allows them to remain in the ER.

ex. BiP, the ‘chaperone protein’ operates within the ER. It identifies improperly built proteins and keeps them from leaving and harming the cell.

Nuclear Envelope:

The ER is located next to the Nuclear envelope and around the nucleus for quick communication. If the ER starts to build up unfolded proteins it can quickly send a signal to the nucleus to slow ribosome production and prevent a lethal build-up.

Budding Vesicles and the exchange between the ER and other organelles.

This diagram shows how vesicles are made. Vesicles are a piece of the

membrane that breaks off and surrounds the proteins that the ER

exports.

This shows the exchange of the proteins between the ER and other

organelles.

Smooth ER Structure

The Smooth ER is completely continuous with the rough ER.

The Smooth ER consists of interconnected tubules rather than sacs.

They appear “smooth” because no ribosomes attach to their exterior.

Smooth ER FunctionsThe Smooth ER is designed to:

Synthesize lipids, fatty acids, phospholipids, steroids etc.

Regulate sugar levels released from liver

Break down drugs or harmful substances within liver

Side effect: As the ER builds immunity against a harmful drug it can become equally immune to beneficial drugs.

Store calcium ions

To move a muscle cell, the ER leaks calcium into the cytoplasmic fluid to trigger a contraction.

The Smooth ER Transitional Function

The smooth ER is responsible for the exportation of vesicles.

In #4, the finished proteins from the Rough ER accumulate in the smooth ER.

In #6, the proteins are enclosed by the vesicles and exported from the Smooth ER into the cytoplasm.

In #7, the vesicle arrives in the Golgi Body to be further processed.

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