prokaryotic cell
A cell lacking a membrane-bounded
nucleus or membrane-bounded
organelles. Prokaryotic cells are
thus more primitive than eukaryotic
cells, which evolved from them. A
prokaryotic organism, such as a
bacterium, consists of single
prokaryotic cell.
A general prokaryotic cell can be
considered to have three
architectural regions, though not all
the components are always
present:
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Generalized prokaryotic cell. Image: Sinauer Associates, Inc
a cell envelope consisting of
a capsule, a cell wall, and a cell membrane
a cytoplasmic region that contains the cell genome (DNA), ribosomes, and various
sorts of inclusions
appendages, sometimes present, called flagella and pili
In a prokaryotic cell, most of the functions of organelles, such as mitochondria,
chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic cell membrane.
eukaryotic cell
A cell that contains
membrane-bound
compartments in which
specific metabolic
activities take place.
Most important among
these compartments is
the nucleus, which
houses the eukaryotic
cell's DNA. It is this
nucleus that gives the
eukaryote – literally, "true
nucleus" – its name.
Eukaryotic organisms
also have other
specialized, membranebounded structures,
called organelles, which
are small structures
within cells that perform
Comparison of a typical eukaryotic cell with a typical prokaryotic cell (bacterium).
The drawing on the left highlights the internal structures of eukaryotic cells,
including the nucleus (light blue), the nucleolus (intermediate blue), mitochondria
(orange), and ribosomes (dark blue). The drawing on the right demonstrates how
bacterial DNA is housed in a structure called the nucleoid (very light blue), as well
as other structures normally found in a prokaryotic cell, including the cell
membrane (black), the cell wall (intermediate blue), the capsule (orange),
ribosomes (dark blue), and a flagellum (also black).
dedicated functions. Eukaryotic cells are typically 10 to 100 micrometers across, or about
10 times the size of prokaryotic cells.
Origin of eukaryotic cells
The set of ideas most favored by biologists to explain how eukaryotic cells first came about
is called the endosymbiotic theory. This theory is able to account well for the fact that two
of the organelles found in eukaryotic cells, mitochondria and chloroplasts, have their own
DNA that is completely distinct from the DNA housed in the nucleus. According to the
endosymbiotic theory, the eukaryotic cell evolved from a symbiotic community of
prokaryotic cells. Specifically, the mitochondria and the chloroplasts are what remains of
ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, whereas the
rest of the cell seems to be derived from an ancestral archaean prokaryote cell.
The origin of the eukaryotic cell was a milestone in the evolution of life. Although
eukaryotes use the same genetic code and metabolic processes as prokaryotes, their
higher level of organizational complexity has permitted the development of truly
multicellular organisms. Without eukaryotes, the world would lack mammals, birds, fish,
invertebrates, fungi, plants, and complex single-celled organisms.
Structure
All eukaryotic cells have a number of features
in common. These include:
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nucleus
cytoplasm
cell (plasma) membrane
mitochondria
endoplasmic reticulum
Golgi apparatus
ribosomes
cytoskeleton
centriole
However, there are also some important
differences between animal cells and plant
cells.
A typical eukaryotic cell
Comparison of eukaryotic and prokaryotic cells
feature
eukaryotic cells
prokaryotic cells
types of
organism
Found in "complex" organisms, including all plants and animals
Found in "simple" organisms,
including bacteria and
cyanobacteria
specialization
Can specialize for certain functions, such as absorbing nutrients from food or
transmitting nerve impulses; groups cells can form large, multicellular organs and
organisms
Usually exist as single, virtually
identical cells
size
Most animal cells are 10–30 micrometers across; most plant cells are 10–100
micrometers across
Most are 1–10 micrometers across
nucleus
Contain a nucleus and many other organelles, each surrounded by a membrane (the
nucleus and mitochondrion have two membranes)
Lack a nucleus and other
membrane-encased organelles
nucleolus
One (or more) present in each nucleus
Absent
DNA
DNA always in combination with histone proteins
Simple duplex not associated with
histones (i.e. basic
proteins)Absent
spindle
Present temporarily during mitosis and meiosis
Absent
sexual system
Complete nuclear fusion between gametes, with equal contributions from both
genomes
Unidirectional transfer of genes
from donor to recipient
cell wall
Present in plant cells, but never contain muramic acid
Present, but chemically different
in many respects from eukaryotes
(e.g. presence of muramic acid)
internal
membranes
Complex compartmentalization into endoplasmic reticulum, Golgi bodies,
lysosomes, etc
Usually simple and often
transient, if present at all
ribosomes
80 S with subunits (60 S + 40 S)
70 S* with subunits (30 S + 50 S)
photosynthesis Complex chloroplasts (membrane-bounded organelles)
Simple chromatophores
respiration
Virtually all forms are aerobic, though a few are faculatively anaerobic (e.g. yeasts);
Simple chromatophores
and, uniquely, the trichomonads are obligate anerobes
electron
transport
system & ATP
synthesis
Found on the inner membrane of special membrane-bound organelles: mitochondria
(oxidative phosphorylation) and chloroplasts (photophosphorylation). Virtually all
Localized on the cell membrane
forms are aerobic, though a few are faculatively anaerobic (e.g. yeasts); and,
uniquely, the trichomonads are obligate anerobes