Eukaryotes are cells categorized as having a nucleus and a variety of other membrane-enclosed organelles. Eukaryotes include plants, animals, fungi, and protists. Eukaryotic cells have greater mass than prokaryotic cells and have far more internal complexity. Within each eukaryotic cell, internal membranes create multiple compartments. These compartments, called organelles, carry out specific functions through independent chemical reactions.
In eukaryotic cells, the DNA is separated from the rest of the cell in the nucleus by the nuclear membrane. This separation is important in the regulation of protein synthesis, as the cellular structures that produce proteins are found outside the nucleus. The nuclear membrane limits the type of molecules that can reach the DNA and also limits the type of molecules that can leave the nucleus and reach the protein synthesis machinery of the cell. Eukaryotic cells have more precise regulation of the production of proteins and their RNA intermediates than do prokaryotes, in which the DNA is not sequestered from the rest of the cell.
One feature of eukaryotic cells, particularly within multicellular organisms, is the variety of shapes and sizes of these cells. The shape of the cell is related to the function it performs. Nerve cells, for example, have fingerlike projections that allow them to communicate over a distance. In addition, cells aggregate to form larger structures, such as tissues and organs, that perform specific functions.
Other Organelles and Cellular Structures
Eukaryotic cells contain other organelles that are made up of membranes, which either have a single membrane (endoplasmic reticulum, Golgi body, vacuole) or double membrane (mitochondrion, chloroplast). However, there are organelles without a membrane, called non-membrane bound organelles. These organelles include ribosomes, centrosomes, centrioles, cilia/flagella, and microtubules.The endoplasmic reticulum (ER) is a network of membranes that helps process molecules such as proteins and lipids in a cell. Specifically this organelle is a folded series of membrane spaces where cellular components, including proteins and lipids, are made. It consists of smooth endoplasmic reticulum (smooth ER), which lacks ribosomes, and rough endoplasmic reticulum (rough ER), which has attached ribosomes, hence their appearance and names. A ribosome is a small cellular structure composed of RNA and protein that constructs proteins based on the instructions provided by DNA. The Golgi apparatus is a series of membrane stacks that attaches chemical markers to molecules produced in the endoplasmic reticulum in order to transport the molecules to their places inside or outside a cell. Cells also contain many other structures. The cells of animals include a type of organelle called a lysosome (an organelle that digests bacteria that enter a cell, eliminates toxins, and recycles worn cell materials), large protein complexes, and structures related to cell division, such as centrioles. A centriole is a structure that coordinates formation of microtubules found in pairs near the nucleus. A centrosome coordinates the formation of microtubules, which allows cell division to proceed during reproduction. The cells of plants contain a large central vacuole, a large membranous sac that plays an important role in storage and plant cell growth. These structures are all suspended in the cytosol of the cytoplasm, which is the aqueous fluid that fills the cytoplasm inside the cell. The cytoplasm is surrounded by the cell membrane, which encloses the cell and controls transport of materials into and out of the cell.
Cell Structures and Their Functions in Eukaryotic Cells
|Organelle||Description||Animal Cells||Plant Cells|
|Cell membrane||Controls the passage of substances entering or leaving the cell|
|Cell wall||Provides structural support for the plant cell|
|Central vacuole||Stores water and inorganic ions inside the plant cell|
|Centrosome (contains a pair of centrioles)||Organizes microtubules; will ultimately form the poles of the mitotic spindle during the cell cycle|
|Chloroplast||Converts sunlight energy to chemical energy during photosynthesis in the plant cell|
|Cytoplasm||Serves as the cell's internal environment for all of the organelles|
|Endoplasmic reticulum||Synthesizes lipids, makes proteins by means of the ribosomes that it contains, and detoxifies drugs or poisons|
|Golgi apparatus||Prepares the lipids and proteins made in the endoplasmic reticulum for transport|
|Mitochondrion||Performs the Kreb's citric acid cycle and oxidative phosphorylation to produce ATP|
|Nucleus||Directs the cell's functions and its division; contains most of the cell's DNA|
|Ribosome||Translates messenger RNA (mRNA) into protein|