Eukaryotic Subcellular Structures (Continued)

Vacuoles

• Vacuoles: storage compartments formed by a lipid bilayer

• Plant cells have one large vacuole while animal cells have multiple small vacuoles

• Different functions of vacuoles

• The central vacuole in plants

  • stores organic molecules and water

  • is responsible for creating turgor pressure to support the plant

  • maintains fluid balance

• Phagosomes: vacuoles storing food that the cell takes in

• Contractile vacuoles: vacuoles that expand and contract to expel excess water in the cell

  • They are responsible for osmoregulation

Mitochondria

• Mitochondria are found in most eukaryotes

  • Photosynthetic organisms also have mitochondria!!

• Mitochondria are the site of cellular respiration

Chloroplasts

• Chloroplasts are the site of photosynthesis

• They are only found in photosynthetic eukaryotes

Cellular Evolution

Origin of the Endomembrane System

• The endomembrane system originated from the infolding of the plasma membrane to increase available surface area.

Endosymbiotic Theory

• Symbiosis: a close, long-term interaction between two species

• The modern mitochondria and chloroplasts used to be free-living prokaryotes that were engulfed by larger prokaryotes

  • Mitochondria are related to proteobacteria

  • Chloroplasts are related to cyanobacteria

• The mitochondria or chloroplast and the larger prokaryotic cell developed a symbiotic relationship

  • They became so dependent on each other that they became one cell

Evidence for the Endosymbiotic Theory

• Mitochondria and chloroplasts are similar in size to prokaryotes

• Mitochondria and chloroplasts replicate by binary fission

• They have a double membrane

  • The inner membrane is similar to a prokaryotic plasma membrane

  • Outer membrane maybe originated from phagocytosis when the mitochondria or chloroplast was engulfed by a large prokaryote

• Mitochondria and chloroplasts have 70S ribosomes

• Their genome is circular

• Genome sequence similarity

  • The mitochondrial genome is similar to proteobacterial genomes

  • The chloroplast genome is similar to cyanobacterial genomes

• Reduced organelle genomes

  • The mitochondria or chloroplast and host transferred DNA amongst each other by horizontal gene transfer

    • Horizontal gene transfer: non-sexual movement of genetic information between genomes of the same generation

  • Lots of genes needed by the mitochondria or chloroplast are now in the nucleus

    • These genes are similar to genomes of proteobacteria or cyanobacteria