Eukaryotes are organisms whose cells are organized into complex structures by internal membranes and a cytoskeleton. The most characteristic membrane bound structure is the nucleus. This feature gives them their name, (also spelled “eucaryote,”) which comes from the Greek word referring to the nucleus. Animals, plants, fungi, and protists are eukaryotes. Microorganisms and all other living organisms are classified as prokaryotes or eukaryotes. Prokaryotes and eukaryotes are distinguished on the basis of their cellular characteristics.
For example, rokaryotic cells lack a nucleus and other memorane-bound structures known as organelles, while eukaryotic cells have both a nucleus and organelles (Figure ). The important cellular features of (a) a prokaryotic cell (a bacterium) and (b) a eukaryotic cell. Prokaryotic and eukaryotic cells are similar in several ways. Both types of cells are enclosed by cell membranes (plasma membranes), and both use DNA for their genetic information. Prokaryotes include several kinds of microorganisms, such as bacteria and cyanobacteria. Eukaryotes include such microorganisms as fungi, protozoa, and simple algae.
Viruses are considered neither prokaryotes nor eukaryotes because they lack the characteristics of living things, except the ability to replicate (which they accomplish only in living cells). Eukaryotic cells are generally larger and more complex than prokaryotic cells. They also contain a variety of cellular bodies called organelles. The organelles function in the activities of the cell and are compartments for localizing metabolic function. Microscopic protozoa, unicellular algae, and fungi have eukaryotic cells. Basic structure The basic eukaryotic cell contains the following: . lasma membrane 2. glycocalyx (components external to the plasma membrane) 3. cytoplasm (semifluid) 4. cytoskeleton – microfilaments and microtubules that suspend organelles, give shape, and allow motion 5. presence of characteristic membrane enclosed subcellular organelles Characteristic biomembranes and organelles Plasma Membrane A lipid/protein/carbohydrate complex, providing a barrier and containing transport and signaling systems. Nucleus Double membrane surrounding the chromosomes and the nucleolus. Pores allow specific communication with the cytoplasm.
The nucleolus is a site for synthesis of RNA making up the ribosome. Mitochondria Surrounded by a double membrane with a series of folds called cristae. Functions in energy production through metabolism. Contains its own DNA, and is believed to have originated as a captured bacterium. Chloroplasts (plastids) Surrounded by a double membrane, containing stacked thylakoid membranes. Responsible for photosynthesis, the trapping of light energy for the synthesis of sugars. Contains DNA, and like mitochondria is believed to have originated as a captured bacterium.
Rough endoplasmic reticulum (RER) A network of interconnected membranes forming channels within the cell. Covered with ribosomes (causing the “rough” appearance) which are in the process of synthesizing proteins for secretion or localization in membranes. Ribosomes Protein and RNA complex responsible for protein synthesis. Smooth endoplasmic reticulum (SER) A network of interconnected membranes forming channels within the cell. A site for synthesis and metabolism of lipids. Also contains enzymes for detoxifying chemicals including drugs and pesticides.
Golgi apparatus A series of stacked membranes. Vesicles (small membrane surrounded bags) carry materials from the RER to the Golgi apparatus. Vesicles move between the stacks while the proteins are “processed” to a mature form. Vesicles then carry newly formed membrane and secreted proteins to their final destinations including secretion or membrane localization. Lysosymes A membrane bound organelle that is responsible for degrading proteins and membranes in the cell, and also helps degrade materials ingested by the cell.
Vacuoles Membrane surrounded “bags” that contain water and storage materials in plants. Peroxisomes or Microbodies Produce and degrade hydrogen peroxide, a toxic compound that can be produced during metabolism. Cell wall Plants have a rigid cell wall in addition to their cell membranes. Differences between eukaryotic and prokaryotic cells The difference between the structure of prokaryotes and eukaryotes is so great that it is considered to be the most important distinction among groups of organisms.
The most fundamental difference is that eukaryotes do have “true” nuclei containing their DNA, whereas the genetic material in prokaryotes is not membrane-bound. In eukaryotes, the mitochondria and chloroplasts perform various metabolic processes and are believed to have been derived from endosymbiotic bacteria. In prokaryotes similar processes occur across the cell membrane; endosymbionts are extremely rare. The cell walls of prokaryotes are generally formed of a different molecule (peptidoglycan) to those of eukaryotes (many eukaryotes do not have a cell wall at all).
Prokaryotes are usually much smaller than eukaryotic cells. Prokaryotes also differ from eukaryotes in that they contain only a single loop of stable chromosomal DNA tored in an area named the nucleoid, while eukaryote DNA is found on tightly bound and organised chromosomes. Although some eukaryotes have satellite DNA structures called plasmids, these are generally regarded as a prokaryote feature and many important genes in prokaryotes are stored on plasmids.
Prokaryotes have a larger surface area to volume ratio giving them a higher metabolic rate, a higher growth rate and consequently a shorter generation time compared to Eukaryotes. Genes Prokaryotes also differ from eukaryotes in the structure, packing, density, and rrangement of their genes on the chromosome. Prokaryotes have incredibly compact genomes compared to eukaryotes, mostly because prokaryote genes lack introns and large non-coding regions between each gene.
Whereas nearly 95% of the human genome does not code for proteins or RNA or includes a gene promoter, nearly all of the prokaryote genome codes or controls something. Prokaryote genes are also expressed in groups, known as operons, instead of individually, as in eukaryotes. In a prokaryote cell, all genes in an operon(three in the case of the amous lac operon) are transcribed on the same piece of RNA and then made into separate proteins, whereas if these genes were native to eukaryotes, they each would have their own promoter and be transcribed on their own strand of mRNA.
This lesser degree of control over gene expression contributes to the simplicity of the prokaryotes as compared to the eukaryotes. The common features of prokaryotic and eukaryotic cells are: 1 . DNA, the genetic material contained in one or more chromosomes and located in a nonmembrane bound nucleoid region in prokaryotes and a membrane-bound ucleus in eukaryotes 2.
Plasma membrane, a phospholipid bilayer with proteins that separates the cell from the surrounding environment and functions as a selective barrier for the import and export of materials 3. Cytoplasm, the rest of the material of the cell within the plasma membrane, excluding the nucleoid region or nucleus, that consists of a fluid portion called the cytosol and the organelles and other particulates suspended in it 4. Ribosomes, the organelles on which protein synthesis takes place Comparison chart Eukaryotic Cell Prokaryotic Cell Present Absent
Number of chromosomes More than one One–but not true chromosome: Plasmids cell Type Usually multicellular Usually unicellular (some cyanobacteria may be multicellular) True Membrane bound Example Animals and Plants Bacteria and Archaea Genetic Recombination Meiosis and fusion of gametes Partial, undirectional transfers DNA Lysosomes and peroxisomes Microtubules Absent or rare Endoplasmic reticulum Cytoskeleton May be absent DNA wrapping on proteins. Eukaryotes wrap their DNA around proteins called histones. Multiple proteins act together to fold and condense prokaryotic DNA.
Folded DNA is hen organized into a variety of conformations that are supercoiled and wound around tetramers of the HU protein. Ribosomes larger smaller Vesicles Chloroplasts Present (in plants) Absent; chlorophyll scattered in the cytoplasm Flagella Microscopic in size; membrane bound; usually arranged as nine doublets surrounding two singlets Submicroscopic in size, composed of only one fiber Permeability of Nuclear Membrane Selective not present Plasma membrane with steroid Yes Usually no Only in plant cells and fungi (chemically simpler) Usually chemically complexed Cell size 10-100urn 1-1 Oum