Chapter 6: A Tour of the Cell

Chapter 6: A Tour of the Cell • Cell theory • Cell organization and homeostasis • Studying cells – microscopy and fractionation • Eukaryotic vs. prokaryotic cells • Compartments in eukaryotic cells (cell regions, organelles) • Cytoskeleton • Outside the cell

What are the main tenets of cell theory? • What are the major lines of evidence that all presently living cells have a common origin?

Cell theory • All living organisms are composed of cells • smallest “building blocks” of all multicellular organisms • all cells are enclosed by a surface membrane that separates them from other cells and from their environment • specialized structures with the cell are called organelles; many are membrane-bound

Cell theory • Today, all new cells arise from existing cells • All presently living cells have a common origin • all cells have basic structural and molecular similarities • all cells share similar energy conversion reactions • all cells maintain and transfer genetic information in DNA • the genetic code is essentially universal

What are the main tenets of cell theory? • What are the major lines of evidence that all presently living cells have a common origin?

What is surface area to volume ratio, and why is it an important consideration for cells? • What (usually) happens to surface area to volume ratio as cells grow larger?

Cell organization and homeostasis • Plasma membranesurrounds cells and separates their contents from the external environment • Cells are heterogeneous mixtures, with specialized regions and structures (such as organelles)

Cell organization and homeostasis • Cell size is limited • surface area to volume ratioputs a limit on cell size • food and/or other materials must get into the cell • waste products must be removed from the cell • cells need a high surface area to volume ratio • BUT volume increases faster than surface area as cells grow larger…so cells usually must divide

Fig. 5.4

Cell organization and homeostasis • cell shape varies depending both on function and surface area requirements

What is surface area to volume ratio, and why is it an important consideration for cells? • What (usually) happens to surface area to volume ratio as cells grow larger?

Compare and contrast: • LM and EM • SEM and TEM • Include the terms resolution and magnification in your discussions.

Studying cells – microscopy • Most cells are large enough to be resolved from each other with light microscopes (LM) • cells were discovered by Robert Hooke in 1665 • he saw the remains of cell walls in cork with a LM • his microscope had about 30x magnification • modern LMs can reach up to 1000x

Fig. 5.2a

Fig. 5.2b

Fig. 5.2c

Studying cells – microscopy • LM resolution is limited • LM resolution (clarity) is limited • about 1 mm • due to the wavelength of visible light • only about 500 times better than the human eye, even at maximum magnification • small cells (such as most bacteria) are ~1 mm across, just on the edge of resolution • modifications of LMs and some treatments of cells allow observation of subcellular structure in some cases

Studying cells – microscopy • Resolution of most subcellular structure requires electron microscopy (EM) • electrons have a much smaller wavelength than light (resolve down to under 1 nm) • magnification up to 250,000x or more • resolution over 500,000 times better than the human eye

Transmission electron microscope Scanning electron microscope Light microscope Electron gun Electron beam Light beam Second condenser lens First condenser lens (magnet) Ocular lens Scanning coil Specimen Final (objective) lens Objective lens Projector lens (magnet) Cathode ray tube synchronized with scanning coil Specimen Condenser lens Secondary electrons Light source Specimen Film or screen Electron detector (a) (b) (c)

Studying cells – microscopy • transmission electron microscopy (TEM) • electron passes through sample • need very thin samples (100 nm or less thick) • samples embedded in plastic and sliced with a diamond knife

Studying cells – microscopy • scanning electron microscopy (SEM) • samples are gold-plated • electrons interact with the surface • images have a 3-D appearance

Compare and contrast: • LM and EM • SEM and TEM • Include the terms resolution and magnification in your discussions.

Which is SEM, and which TEM? How can you tell?

Describe cell fractionation. Why is it done, and how is it done? Include the terms lyse, centrifugation, pellet, and supernatant in your discussion.

Studying cells – fractionation • Cells can be broken and fractionated to separate cellular components • cells are broken (lysed) by disrupting the cell membrane, often using some sort of detergent • grinding and other physical force may be required, especially if cell walls are present • centrifugation is used to separate cellular components

Studying cells – fractionation • centrifugation is used to separate cellular components • samples are spun at high speeds • results in a centrifugal force thousands to hundreds of thousands times “normal” gravity • after spinning: • pellet – what gets packed down to the bottom (densest material) • supernatant– solution above the pellet

Studying cells – fractionation • cell components will end up in either the pellet or the supernatant depending on their individual properties and the details of the centrifugation • intact membrane-bound organelles often wind up in pellets (denser once first) • special treatments can determine whether a component ends up in the pellet or supernatant

Studying cells – fractionation • density gradients can be used to subdivide pellet components • based on their density • can be used to better separate similar organelles from each other, for example Golgi complex from ER

Describe cell fractionation. Why is it done, and how is it done? Include the terms lyse, centrifugation, pellet, and supernatant in your discussion.

How do prokaryotic cells and eukaryotic cells differ from each other in typical size and general organization? • Describe cytoplasm, cytosol, nucleoplasm, and the general role of membranes in cells.

Eukaryotic vs. prokaryotic cells • prokaryotic cells do not have internal membranes (thus no nuclear membrane) • main DNA molecule (chromosome) is typically circular; its location is called the nuclear area • other small DNA molecules (plasmids) are often present, found throughout the cell

Eukaryotic vs. prokaryotic cells • prokaryotic cells • plasma membrane is typically enclosed in a cell wall • often the cell wall is covered with a sticky layer of proteins and/or sugars called a capsule • do not completely lack organelles; have: • plasma membrane • ribosomes • generally just called bacteria • prokaryotic cells are typically 1-10 mm in diameter

Eukaryotic vs. prokaryotic cells • eukaryotic cells have internal membranes and a distinct, membrane-enclosed nucleus • typically 10-100 mm in diameter

How do prokaryotic cells and eukaryotic cells differ from each other in typical size and general organization? • Describe cytoplasm, cytosol, nucleoplasm, and the general role of membranes in cells.

List as many organelles as you can think of. Describe their structures and key functions. • Draw and label a typical animal cell and a typical plant cell, including organelles.

How do proteins get outside of a cell? • How do proteins get into a cell membrane? • How does a cell digest its food? • How does a cell commit suicide? • Why would a cell commit suicide?

Describe the nuclear envelope, nuclear pores, chromatin, chromosomes, and nucleoli in terms of structures and key functions. • Name something that you KNOW must get out of the nucleus for cells to function.

Compartments in eukaryotic cells • two general regions inside the cell: cytoplasm and nucleoplasm • cytoplasm – everything outside the nucleus and within the plasma membrane • contains fluid cytosol and organelles • nucleoplasm – everything within the nuclear membrane

Compartments in eukaryotic cells • membranes separate cell regions • have nonpolar regions that help form a barrier between aqueous region • allow for some selection in what can cross a membrane (more details later)

nucleus – the control center of the cell • typically large (~5 mm) and singular • nuclear envelope • double membrane surrounding the nucleus • nuclear pores – protein complexes that cross both membranes and regulate passage

Chapter 6: A Tour of the Cell