Cytoskeleton Means “cell skeleton” Internal framework of cell Has many functions Anchoring cell organelles Provide cell shape Aids in cell motility Response to environmental signals Comprises Microtubules Microfilaments Intermediate filaments

Microtubules Hollow tubes made of the protein tubulin Alternating dimers of a and b tubulin Largest of cytoskeleton filaments Is used for: Maintenance of cell shape Motility Flagella or cilia Movement of organelles through cell Often involves motor molecule Often originate from centrosome

10 µm Column of tubulin dimers Tubulin dimer   25 nm Table 6-1a

Centrioles Located in centrosome of animal cells Occur in perpedicular pair Have 9 triplets of microtubules Facilitate microtubule assembly and chromosome separation in some cells

Longitudinal section of one centriole Microtubules Cross section Fig. 6-22 Centrosome Microtubule Centrioles 0.25 µm Figure 6.22 Centrosome containing a pair of centrioles Longitudinal section of one centriole Microtubules Cross section of the other centriole

Flagellum structure Basal body links flagellum or cilia to cell surface Basal body looks just like a centriole 9 +2 arrangement of microtubules Radial spokes prevent dramatic sliding and only bending

Cross section of cilium Fig. 6-24 Outer microtubule doublet Plasma membrane 0.1 µm Dynein proteins Central microtubule Radial spoke Protein cross-linking outer doublets Microtubules (b) Cross section of cilium Plasma membrane Basal body 0.5 µm (a) Longitudinal section of cilium 0.1 µm Figure 6.24 Ultrastructure of a eukaryotic flagellum or motile cilium Triplet (c) Cross section of basal body

Motor molecules Interact with tubulin or actin Are fixed at one end and allowed to move freely at the other end Movement is directional Undulation-used for flagella and cilia movement Two microtubules moving relative to one another Organelle movement is like a ski lift tram or a monorail

Receptor for motor protein Fig. 6-21 Vesicle ATP Receptor for motor protein Motor protein (ATP powered) Microtubule of cytoskeleton (a) Microtubule Vesicles 0.25 µm Figure 6.21 Motor proteins and the cytoskeleton (b)

Cell motility Cell movement facilitated by flagella or cilia Unlike in prokaryotes, eukaryotic flagella undulate Cilia are small appendages and they move like a swimmers arm-active stroke and return stroke

How cell movement works Dynein is motor molecule that interacts with tubulin Dynein walks along one microtubule, while bound to another This results in bending If no radial spokes or organelle coat, then microtubules would walk out of cell

Figure 6.25 How dynein “walking” moves flagella and cilia Microtubule doublets ATP Dynein protein (a) Effect of unrestrained dynein movement ATP Cross-linking proteins inside outer doublets Anchorage in cell Figure 6.25 How dynein “walking” moves flagella and cilia (b) Effect of cross-linking proteins 1 3 2 (c) Wavelike motion

Microfilaments Made of two intertwined strands of actin Helps maintain cell shape Actin rearrangements allow engulfment events Psuedopod formation in ameoba Promote cytoplasmic streaming in plants Essential for muscle contraction Used by invading bacteria to move around cell Frequently being assembled and disassembled within cell

Table 6-1b 10 µm Table 6-1b Actin subunit 7 nm

Microfilaments 2 Myosin interacts with actin to cause contraction Cytoplasmic streaming and ameoboid motion are similar Cortical cytoplasm around the perimiter of cell contains perpendicular actin (wind fence) Streaming portion has parallel actin which facilitates cytoplasm movement Plant cell wall prevents amoeboid movement of plant cell

Intermediate Filaments Resemble cable in structure Are made of protein subunits Help maintain cell shape Are durable and not assembled and disassembled as other cytoskeleton components May help maintain organelle position

Fibrous subunit (keratins coiled together) Table 6-1c 5 µm Table 6-1c Keratin proteins Fibrous subunit (keratins coiled together) 8–12 nm