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What are organelles?
What are organelles?
One of the ways by which cells are able to get larger despite their increased energy demands is by utilizing specialized organelles. Organelles are basically specialized components of a cell - basically, they are parts of a cell that have a single or a small number of jobs to perform.
Just like in all of biology, the function is dependent on structure, so as you look at specific organelles, it is important to think about why they may be shaped how they are, contain the membranes they have, etc.
Organelles
Organelles
You are responsible for being familiar with all of the organelles shown in this diagram. There is also a simplified chart provided here for a less-detailed account of some of the important organelles to be familiar with.
The most important for you to recall are the ones that are part of the endomembrane system
As a quick reminder when using the table provided:
Eukaryotic cells are cells that have a nucleus
Prokaryotic cells do not contain a nucleus
ENDOMEMBRANE SYSTEM
The endomembrane system includes :
the nuclear envelope,
lysosomes, vesicles
ER
Golgi apparatus
plasma membrane.
The endomembrane system (endo = “within”) is a group of membranes and organelles within Eukaryotic cells that work together to modify, package, and transport lipids and proteins.
The Nuclear Envelope
is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 1). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.
Endoplasmic Reticulum
The endoplasmic reticulum is a membranous network that is responsible for synthesizing secretory materials
Rough ER
is embedded with ribosomes and synthesizes proteins destined for extracellular use
Smooth ER
is involved in lipid synthesis and also plays a role in carbohydrate metabolism
Materials are transported from the ER when the membrane bulges and then buds to create a vesicle surrounding the material
Golgi apparatus (also called the Golgi body) is , a series of flattened membranes
Golgi apparatus (also called the Golgi body) is , a series of flattened membranes
Sorting, tagging, packaging, and distributing lipids and proteins takes place in the Golgi apparatus.
Transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the Golgi apparatus’ lumen. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is adding short sugar molecule chains. These newly modified proteins and lipids then tag with phosphate groups or other small molecules in order to travel to their proper destinations.
Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the Golgi’s trans face.
Lysosome
Lysosome
A lysosome is a membrane-bound cell organelle that contains digestive enzymes. Lysosomes are involved with various cell processes. They break down excess or worn-out cell parts. They may be used to destroy invading viruses and bacteria.
Transport Vesicles
Transport Vesicles
Vesicles are sacs of the membrane that bud off of the endomembrane system. These small compartments function as trucks to transport cargo. The cargo includes molecules such as proteins, large molecules that cannot pass through membranes, chemicals, and wastes. Vesicles are constant-forming structures that are made by the Golgi apparatus, the endoplasmic reticulum, and parts of the plasma membrane
Plasma Membrane
Although it's not technically inside the cell, the plasma membrane is also part of the endomembrane system. As we'll see, the plasma membrane interacts with the other endomembrane organelles, and it's the site where secreted proteins (like the pancreatic enzymes in the intro) are exported.
Endomembrane Transport
Endomembrane Transport
Membrane and secretory proteins are synthesized in the rough endoplasmic reticulum (RER). The RER also sometimes modifies proteins.
In this illustration, a (green) integral membrane protein in the ER is modified by attachment of a (purple) carbohydrate.
Vesicles with the integral protein bud from the ER and fuse with the cis face of the Golgi apparatus. As the protein passes along the Golgi’s cisternae, it is further modified by the addition of more carbohydrates.
After its synthesis is complete, it exits as integral membrane protein of the vesicle that bud from the Golgi’s trans face and when the vesicle fuses with the cell membrane the protein becomes integral portion of that cell membrane
Shipping Routes of Proteins:
As a protein is made in a ribosome, it passes step by step through a shipping "decision tree." At each stage, the protein is checked for molecular tags to see if it needs to be re-routed to a different pathway or destination. The first major branch point comes shortly after translation starts. At this point, the protein will either remain in the cytosol for the rest of translation, or be fed into the endoplasmic reticulum (ER) as it is translated
Proteins that have a signal peptide are fed into the ER during translation. In general, proteins bound for organelles in the endomembrane system (such as the ER, Golgi apparatus, and lysosome) or for the exterior of the cell must enter the ER at this stage.
Proteins that do not have a signal peptide stay in the cytosol for the rest of translation. If they lack other "address labels," they'll stay in the cytosol permanently. However, if they have the right labels, they can be sent to the mitochondria, chloroplasts, peroxisomes, or nucleus after translation.
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