Microbiology For Dummies
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Many cellular structures are important for cellular function, and these differ between eukaryotic and prokaryotic cells. Here is a list of structures important to prokaryotic cells.

  • Nucleoid: The nucleoid is the region in the cell where the tangled mass of genetic material, called the chromosome, is found. Unlike the nucleus of eukaryotic cells, the nucleoid does not have a membrane. The chromosome of prokaryotes is often circular but can be linear.

  • Plasmids: Plasmids also contain genetic material but in smaller circular forms that are independent from the chromosome and replicate on their own.

  • Cytoplasm: The cytoplasm is the sea in which the rest of the structures swim. It’s not just water, though — it’s gelatinous and contains filaments that span from one end to the other, providing structure to the cell.

  • Ribosomes: Ribosomes are involved in making protein and are plentiful in growing cells. They’re smaller and less dense in prokaryotes than they are in eukaryotes. Ribosomes are made of subunits — 30S and 50S — that are named for their size and shape. (Well, actually they’re named for how far they move through a solution when you spin them very fast, but that’s a function of size and shape.) Many antibiotics target bacterial ribosomes as a way of slowing down their growth.

  • Glycocalyx: The glycocalyx, an extracellular polymer that surrounds the outside of the cell, is called the capsule when firmly attached; when loosely attached, it’s called a slime layer. The capsule protects bacteria from being phagocytosed (taken up) by immune cells.

    A biofilm is formed when one or several bacterial populations use a glycocalyx called an extracellular polymeric substance (EPS) to attach to a surface. Biofilm formation not only helps microbes to form a niche for themselves but also helps them survive environmental conditions such as drought or stress from antibiotics.

  • Inclusion: An inclusion is the concentration of a substance inside of the prokaryote cell. Many types of inclusions exist, such as those for nutrient storage (such as polysaccharides, lipids, or sulfur granules) or those that have a function such as carboxysomes (for carbon dioxide fixation), vacuoles (filled with gas for buoyancy), or magnetosomes (which contain magnetic compounds for direction).

  • Endospores: Endospores are formed as a survival mechanism by some bacteria. When nutrients are depleted or conditions start to become unfavorable, an endospore can begin forming inside the growing cell (called the vegetative cell).

    Endospores have a tough spore coat, contain very little water, and take with them only the bare essentials to start growing again when conditions improve. After it’s released from the vegetative cell, the endospore doesn’t actively metabolize or grow; instead, it lies waiting, sometimes for thousands (or millions!) of years until it can germinate near a suitable food source.

Eukaryotic cells differ from prokaryotic cells in several ways. Although they contain some of the same structures as prokaryotic cells, for the most part eukaryotic cells are larger and more complex. One major difference is that eukaryotic cells contain compartmented structures called organelles, membrane-bound structures that usually have a special function. Here are some important organelles:

  • Nucleus: The nucleus is the largest organelle in the cell and contains the chromosomes. DNA in eukaryotes is associated with DNA-binding proteins called histones.

  • Mitochondria: Mitochondria are the powerhouse of the cell and produce adenosine triphosphate (ATP) through respiration. They have their own DNA and bear an evolutionary relationship to bacteria.

  • Chloroplasts: Chloroplasts are organelles within some eukaryotic cells (such as algae and plants) that are responsible for the photosynthetic reaction.

Other important organelles include the endoplasmic reticulum (where protein is made), the Golgi body (where proteins are processed), and lysosomes (containing digestive enzymes).

About This Article

This article is from the book:

About the book authors:

Jennifer C. Stearns, PhD, is an assistant professor in the department of medicine at McMaster University. She studies how we get our gut microbiome in early life and how it can keep us healthy over time. Michael G. Surette, PhD, is a professor in the department of medicine at McMaster University, where he pushes the boundaries of microbial research.

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