The Endomembrane System: Functions & Components

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  • 1:00 Smooth Endoplasmic Reticulum
  • 2:18 Rough Endoplasmic Reticulum
  • 3:20 Golgi Apparatus
  • 4:36 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

In this lesson, we'll learn about the endomembrane system, which consists of the endoplasmic reticulum and the Golgi apparatus. This system is important in making, packaging, and shipping all sorts of goodies for the cell to use!

The Endomembrane System

Diagram of the endomembrane system
Endomembrane System

No matter where you live in the world or what you celebrate, you've likely heard of Santa Claus and his tiny elves. These little elves make toys and games, wrap them up, label them with the names of good little boys and girls, and send them off with Santa to be delivered for Christmas.

Christmas seems to come every day for your cells, where tiny ribosomes make proteins all throughout the year. Once they're made, these proteins need to find a way to a good home - the right good home. In your cell, this is where the endomembrane system comes in. The endomembrane system is a series of compartments that work together to package, label, and ship proteins and molecules.

In your cells, the endomembrane system is made up of both the endoplasmic reticulum and the Golgi apparatus. These compartments are folds of membranes that form tubes and sacs in your cells.

Smooth Endoplasmic Reticulum

Location of the lumen in the cell

There are two parts to the structure known as the endoplasmic reticulum. Both have a membranous tubular structure and are located near the nucleus of the cell. Notice that the membrane folds, providing plenty of surface space for work to be done, like multiple benches for Santa's elves. This also creates compartments without any open ends. The lumen is the inside compartment of the endoplasmic reticulum. The lumen is completely separated from the cytoplasm that surrounds it.

You can tell apart the two types of endoplasmic reticulum in a cell image because one is studded with small ribosomes and one is not. The one without ribosomes is known as the smooth endoplasmic reticulum, because without ribosomes, it really looks like a membranous compartment of smooth tubes. This can also be referred to as the smooth ER, or simply the SER.

The smooth ER looks like a membranous compartment of smooth tubes.
smooth endoplasmic reticulum

The smooth ER is the site of lipid and steroid synthesis. In the lumen of the smooth endoplasmic reticulum, proteins and small molecules can be chemically modified. The smooth endoplasmic reticulum is also the detoxifying space of the cell. The liver is the detox organ of your body. Therefore, your liver cells can have more smooth endoplasmic reticulum than your heart cells, for example, as it uses this cellular component to detoxify drugs and alcohol.

Rough Endoplasmic Reticulum

The rough ER has an appearance similar to bubble wrap.

The part of the endoplasmic reticulum that contains ribosomes is named the rough endoplasmic reticulum. In the rough ER, or simply RER, there are membranous compartments and sacs studded with ribosomes. Ribosomes attached to this membrane make it look like folds of bubble wrap, perfect for wrapping presents.

As we have learned, ribosomes are the site of protein synthesis, otherwise known as translation. When a ribosome begins to translate an mRNA message into a protein, it might hit a specific sequence that tells the ribosome that this is the protein that needs to be synthesized on the rough ER. This is like telling the ribosome to build a protein in an area of the workshop that has the right tools - like putting a dollhouse together near the hammer, the nails, and the pink paint. As they are being synthesized, these proteins enter the lumen of the rough ER. Here, proteins are chemically modified or folded inside the RER lumen. Proteins synthesized on the rough ER can be sent to specific cellular compartments or exported through exocytosis.

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