Cell Structure

Cell Theory – again! All organisms are composed of one or more cells. Cells are the smallest living units of all living organisms. Cells arise only by division of a previously existing cell.

Cell Characteristics Genetic material single circular molecule of DNA in prokaryotes double helix located in nucleus in eukaryotes Cytoplasm fills cell interior – sugars, amino acids, proteins - organelles Plasma membrane encloses the cell Phospholipid Membrane proteins

Cell Size Most cells are relatively small because as size increases, volume increases much more rapidly than surface area. What difficulties result?

Visualizing Cells Resolution - minimum distance two points can be apart and still be distinguished as two separate points Compound optical microscopes - magnify in stages using multiple lenses Transmission electron microscope - electrons transmitted through specimen Scanning electron microscope - electrons beamed onto surface of the specimen

Visualizing Cells

Prokaryotic Cells Simplest organisms, incl. the monera (bacteria) All have Cytoplasm, surrounded by plasma membrane and encased in a rigid cell wall (not cellulose) No membrane-bound organelles (ie no nucleus, mitochondria, etc.) Contain bacterial ribosomes Contain naked DNA (nucleoid)

Prokaryotic Cells Some use flagellum for locomotion threadlike structures protruding from cell surface Bacterial cell wall Rotary motor Flagellin Sheath

Prokaryotic Cells Some have pili – small thread-like structure which protrude through the cell wall.

Prokaryotic Cells Many have a capsule or slime layer around the cell wall, which aids water retention.

Prokaryotic Cells All reproduce a-sexually by binary fission

Endosymbiosis Endosymbiotic theory suggests engulfed prokaryotes provided hosts with advantages associated with specialized metabolic activities.

Theory of Endosymbiosis

Generalized Eukaryotic Cell