Eukaryotic cells possess several critical differences from prokaryotic cells, including a nucleus and other membrane-bound organelles. Here is a summery of some of the defining characteristics of the cell of eukaryotes (plants, animals, fungi and protists).
* Two Basic Types of Cells *
All cells fall into one of the two major categories: prokaryotes or eukaryotes.
* Prokaryotic Cells: Evolutionarily ancient and of simple construction, prokaryotes lack a nucleus and membrane-bound organelles. They are almost always single-celled (except for prokaryote colonies), and reproduce by means of binary fission (forming another copy by dividing). Modern day bacteria and bacteria-like Archaea are prokaryotic organisms.
* Eukaryotic Cells: Eukaryotic cells are more complex, having evolved from a prokaryote-like predecessor. Most of the living things that we are familiar with, including the cells of our own body, are composed of eukaryotic cells. In addition to multi-cellular eukaryotic organisms, such as plants and animals, there are also a multitude of tiny, single-celled eukaryotes, including paramecium, amoeba and yeasts, to name a few.
* Eukaryotic Cell Structure *
Eukaryotic cells include the following structural elements:
* Nucleus: The most noticeable feature that differentiates eukaryotes from prokaryotes is the presence of a nucleus, a double membrane-bound control center separating the genetic material, DNA (deoxyribonucleic acid), from the rest of the cell.
* Plasma Membrane: All cells have a plasma membrane, the structure separating the inside from the outside of the cell. It is a phospholipid bilayer, a double membrane composed of a unique type of lipid that spontaneously organizes into two layers. The plasma membrane controls traffic of materials into and out of the cell.
* Cytoplasm: The cytoplasm of eukaryotic cells, like that of prokaryotes, is a mixture of organic and inorganic materials; the fluid portion called cytosol. Eukaryotic cytosol is less viscous (not as thick) as that of prokaryotes, and is constantly in motion. The flowing movement, known as cytoplasmic streaming, allows organelles to interact with one another.
* Cytoskeleton: This network of microtubules, intermediate filaments and microfilaments is the cellular "scaffolding" or "skeleton" within the cell.
~ maintains the cell's shape
~ protects the cell
~ enables some cell motion (using flagella and cilia)
~ plays important role in intra-cellular transport (the movement of vesicles and organelles)
~ is involved in cellular division
* Cilia and Flagella: These external appendages of the plasma membrane aid in locomotion of the cell, adhesion and movement of materials on the outside of the cell. Their internal structure consists of microtubules, and motility achieved by coordinated sliding movements of these microtubules.
* Ribosomes: These are the tiny cellular structures involved in making proteins under the instruction of DNA. Ribosomes are found attached to the rough endoplasmic reticulum or floating free in the cytoplasm.
* Mitochondria & Chloroplasts: Mitochondria are the powerhouses of the cell, where the final and most energy-productive steps of metabolism take place to generate cellular energy (ATP). Chloroplasts, possessed only by plants, are organelles containing the pigment chlorophyll. Chloroplasts harness sunlight and turn it into ATP energy. These energy-related organelles are believed to have evolved from prokaryotes that began living symbiotically within eukaryotic cells.
* The Endomembrane System: Membrane-bound Organelles: Eukaryotic cells also contain a network of internal membrane-bound structures, the organelles of the endomembrane system. These cellular organelles transport materials into, out of, and within the cell, carrying out many of the functions required for the cell to survive, thrive, grow and reproduce.
* Sources *
Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.