Eukaryotes: A Breakdown of Organelles
Organelles are unique to eukaryotes and act like organs in the cell, each performing their own specific functions to keep the whole cell running. Since they’re all in their own little sealed-off membrane-bound areas, they can provide the ideal environment for whatever function they perform—for example, they can adjust the pH or temperature—and thus this allows for much more complexity in the cell.
Most organelles fit into a single functional unit called the Endomembrane system, because they evolved in the same way. The only organelles that don’t fit into this category are the mitochondria and chloroplast—for reasons that will become clear in the next article.
So let’s take a look at the functions of the organelles in the Endomembrane system:
Nuclear envelope: This compartment contains the cell’s genetic material—the DNA. Its main function is to protect and package the DNA, but it also synthesises RNA, another kind of genetic material.
Endoplasmic reticulum: Interconnected with the outer membrane of the nuclear envelope, the ER consists of flattened tubes and sacs called cisternae. The endoplasmic reticulum is subdivided into two: the rough ER, where protein synthesis and packaging takes place, and the smooth ER, where lipid and carbohydrate synthesis takes place. Basically, the endoplasmic reticulum uses the information in DNA to create the building blocks of the cell.
Golgi body: This is composed of a group of flat, membranous sacs that deal with the goods produced in the endoplasmic reticulum. Proteins are transported by vesicles from the cis face (the part facing the nucleus) and a trans face (the part facing away from the nucleus), being packaged, modified and matured along the way. When they emerge the proteins are sent out into the wider environment.
Lysosome: This is the recycling plant of the cell, containing digestive enzymes that sort, degrade and recycle waste products. It’s actually a perfect example of how organelles can create niche environments: the lysosome maintains an acidic pH of 5, which is ideal for the breakdown of products.
Vesicle: Membrane-bound sacs used to store and transport material.
Vacuole: A large, fluid-filled “bubble” in plant cells, where food and waste products are
Cell membrane: Made mostly of lipids, this encloses the cell and separates it from the outside world. It’s selectively permeable, meaning that only some substances are allowed passage in and out.
And now the functions of those loners not in the Endomembrane system:
Mitrochondria: This is where cellular respiration takes place—where the cell’s energy (ATP) is extracted from glucose (a sugar). It’s present in all plant and animal cells.
Chloroplast: Present in plants, the chloroplast takes in light energy and converts it into chemical energy through the process of photosynthesis, ready for use in the mitochondria.
I know I’ve just dumped a whole host of terms onto you, and if they’re totally unfamiliar then they might be difficult to juggle. But trust me, we’ll come back to almost all of these in more detail later, so you’ll have time to digest and memorise.
Further resources: An interactive look at animal, plant, and bacterial cells
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