- Some prokaryotic cells are able to formendosporesthroughsporulationto survive in a dormant state
when conditions are unfavorable. Endospores cangerminate, transforming back intovegetative cellswhen
conditions improve. - In prokaryotic cells, thecell envelopeincludes aplasma membraneand usually a cell wall.
- Bacterial membranes are composed of phospholipids with integral or peripheral proteins. The fatty acid
components of these phospholipids are ester-linked and are often used to identify specific types of bacteria.
The proteins serve a variety of functions, including transport, cell-to-cell communication, and sensing
environmental conditions. Archaeal membranes are distinct in that they are composed of fatty acids that are
ether-linked to phospholipids. - Some molecules can move across the bacterial membrane by simple diffusion, but most large molecules must
be actively transported through membrane structures using cellular energy. - Prokaryotic cell walls may be composed ofpeptidoglycan(bacteria) orpseudopeptidoglycan(archaea).
- Gram-positive bacterial cells are characterized by a thickpeptidoglycanlayer, whereas gram-negative
bacterial cells are characterized by a thin peptidoglycan layer surrounded by an outer membrane. - Some prokaryotic cells produceglycocalyxcoatings, such ascapsulesandslimelayers, that aid in attachment
to surfaces and/or evasion of the host immune system. - Some prokaryotic cells havefimbriaeorpili, filamentous appendages that aid in attachment to surfaces. Pili
are also used in the transfer of genetic material between cells. - Some prokaryotic cells use one or moreflagellato move through water.Peritrichousbacteria, which have
numerous flagella, userunsandtumblesto move purposefully in the direction of a chemical attractant.
3.4 Unique Characteristics of Eukaryotic Cells
- EukaryoticcellsaredefinedbythepresenceofanucleuscontainingtheDNAgenomeandboundbyanuclear
membrane(ornuclear envelope) composed of two lipid bilayers that regulate transport of materials into and
out of the nucleus through nuclear pores. - Eukaryotic cell morphologies vary greatly and may be maintained by various structures, including the
cytoskeleton, the cell membrane, and/or the cell wall - Thenucleolus, located in the nucleus of eukaryotic cells, is the site of ribosomal synthesis and the first stages
of ribosome assembly. - Eukaryotic cells contain80S ribosomesin the rough endoplasmic reticulum (membrane bound-ribosomes)
and cytoplasm (free ribosomes). They contain 70s ribosomes in mitochondria and chloroplasts. - Eukaryotic cells have evolved anendomembranesystem, containing membrane-bound organelles involved
in transport. These include vesicles, the endoplasmic reticulum, and the Golgi apparatus. - Thesmooth endoplasmic reticulumplays a role in lipid biosynthesis, carbohydrate metabolism, and
detoxification of toxic compounds. Therough endoplasmic reticulum contains membrane-bound 80S
ribosomes that synthesize proteins destined for the cell membrane - TheGolgi apparatus processes proteins and lipids, typically through the addition of sugar molecules,
producing glycoproteins or glycolipids, components of the plasma membrane that are used in cell-to-cell
communication. - Lysosomescontain digestive enzymes that break down small particles ingested byendocytosis, large particles
or cells ingested byphagocytosis, and damaged intracellular components. - Thecytoskeleton, composed ofmicrofilaments,intermediate filaments, and microtubules, provides
structural support in eukaryotic cells and serves as a network for transport of intracellular materials. - Centrosomesare microtubule-organizing centers important in the formation of the mitotic spindle in mitosis.
- Mitochondriaare the site of cellular respiration. They have two membranes: an outer membrane and an inner
membrane with cristae. The mitochondrial matrix, within the inner membrane, contains the mitochondrial
DNA, 70S ribosomes, and metabolic enzymes.
Chapter 3 | The Cell 133