Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
4/11/11 Review: Two Types of Cells. . . Coming Full Circle: From Cells to Ecosystems by Dr. Evil • PROKARYOTES: – cell membrane – cytoplasm – usually a cell wall – DNA, RNA, proteins – circular DNA – ribosomes – NO nucleus – NO complex organelles • EUKARYOTES: – cell membrane – cytoplasm – sometimes a cell wall – DNA, RNA, proteins – linear DNA (chromosomes) – ribosomes – nucleus – complex organelles Bacterial DNA is organized into one large loop, plus a number of smaller loops called plasmids. Bacteria can exchange plasmids or pick them up from the environment (remember Griffith’s “transforming principle”?) There’s a central region (the nucleoid) where the DNA is located, but this isn’t bounded by a membrane sac, and so it’s not a nucleus. Ribosomes just float free in the cytoplasm. 1 4/11/11 Some prokaryotes do contain some gasfilled sacs and other simple structures, however—but nothing very complex. Sometimes the nucleus can have a rather odd shape, as in this singlecelled organism . . . Eukaryotes (duuh!) have a defined mucleus bound by a double-layered nuclear membrane. Cross-section through a plant cell nucleus, showing the nucleolus inside. The nucleolus is where ribosomes themselves are made. . . The nucleus here, stained dark purple, is the thing that looks like a short string of beads. 2 4/11/11 The nuclear pores allow passage of RNA, ribosomes, etc. out of the nucleus. Not just simple holes, they're kept open by complex proteins, the porins. Endomembrane System • Endoplasmic reticulum (ER) – Transport network within the cell – Proteins and other products that are exported are transferred via the ER • Golgi body – "Packaging and shipping center" • Vesicles (small) and vacuoles (large) – Membrane sacs for storage and/or transport Endoplasmic reticulum (ER) Smooth endoplasmic reticulum (ER) 3 4/11/11 Close-up of rough endoplasmic reticulum showing bound and free ribosomes Golgi body Structure: Basically a stack of flat sacs. (Imagine ten deflated beach balls stacked one on top of the other.) This picture shows a cross-section through the “stack.” Anything secreted from the cell passes from the Golgi into small vesicles, which move to the outer membrane and fuse with it, releasing the contents (a process called exocytosis). Vacuoles and vesicles These plant cells contain large central vacuoles— fluid-filled sacs that show up as empty spaces in this cross-section. 4 4/11/11 Lysosomes: specialized vesicles that break down and Vacuoles and vesicles digest food particles and/or worn out bits of the cell This single-celled organism has a number of smaller vacuoles, seen as round empty spaces. A mitochondrion in cross-section. We don't have time to discuss what they do, but you can think of them as the cell's "powerplant", converting food into usable chemical energy for everything the cell does. Chloroplasts (as you saw in Lab 2) are the organelles where photosynthesis takes place—the harnessing of solar energy to build the larger molecules the cell needs. 5 4/11/11 Cytoskeleton Chloroplasts in cross-section The interior of a chloroplast is filled with stacks of flat membrane sacs. (I wish we had time to go into why!) Nucleus: stained yellow. Microtubules: stained red. This cross-section through a sperm flagellum reveals that it’s basically a bundle of microscopic tubes (seen as circles in this end-on crosssectional view. . .) 6