1. BME 615 - Tissue Mechanics Ray Vanderby
Cytoskeleton
BME Tissue Mechanics
Ray Vanderby
Figures in this document are taken from various sources including copyrighted material and are for educational purposes only.

2. Cell Theory
The Cell is the smallest biological entity that retains the characteristics of life, i.e. the basic unit of life.
All organisms are composed of one or more cells.
New cells arise only from cells that exist.

3. Cell

4. Cell Mitosis

5. Nature of cells Each cell has common components Plasma membrane
Region containing DNA
Cytoplasm
Organelles
Cytoskeleton

6. Cell Types Prokaryotic cells
No nucleus –nuclear region not separated by membrane from the cytoplasm
Bacteria
Eukaryotic cells (only these are considered)
Nucleus present which is separated by a membrane from the cytoplasm
All higher life forms

7. Cell Membranes Plasma membrane and internal membrane
Lipid bilayer – structure and a molecular barrier
Proteins – embedded or on surface; carry out most membrane functions

9. Cell Membranes Cell membrane has many functions
Serves as protection layer for interior of cell
Determines how cell will act and with whom it will act
Membrane connects to other cells with gap junctions
enables cells to share information
Regulates exchange between exterior environment and cell contents for homeostasis
Phospholipid layer of membrane
Repels all polar molecules
Regulates amounts of substances that cell needs.
Polar, hydrophilic head of phospholipid is attracted to water
Non-polar, hydrophobic tail of phospholipid repels water.

10. Cell Membrane Transport
Two transport systems occur across cell membrane.
Passive transport
This transport moves substances across the membrane while going down a gradient concentration and
It does not require any energy.
Simple diffusion occurs which is the diffusion of water or dissolved gasses.
Passive transport creates facilitated diffusion which is the diffusion of molecules through a carrier protein.
Allows for osmosis across the membrane. This is the diffusion of water across a permeable membrane which is more permeable to water than dissolved molecules.

11. Cell Membrane Transport
Active transport is 2nd transport mechanism across membrane
Requires energy for transport (ATP)
Moves substances (small individual molecules or ions ) across a membrane mostly against a concentration gradient
Endocytosis and Exocytosis occur.
Endocytosis is movement of large substances into cell by surrounding extracellular material. Membrane creates membrane bound sacs that go into cytoplasm.
Exocytosis is movement of large substances outside cell by encircling the material in a membrane sac. Sac moves to surface of the cell and is diffused away when the sac fuses with plasma membrane and is opened to the outside.

12. Cytoplasm Watery environment inside the cell. Composition
salts
organic molecules, including many enzymes to catalyze reactions
water
Thick soup or gel of proteins, carbohydrates, salts, sugars, lipids, nucleotides, and amino acids
Plasma membrane separates the cytoplasm from exterior cell environment and encircles compartments in interior of cell.
Bacterial cytoplasm contains ribonucleic acid (RNA), on which proteins are sythesized.
Contains other organelles which store and produce energy
Contains everything within the cell, except the nucleus
Storage place within the cell

13. Cytosol
The cytoskeleton and cytosol are structural elements that provide the cell with its structure.
Cytosol is main component of cytoplasm.
Both cytoskeleton and cytosol, are "filler" structures that do not contain essential biological molecules but perform structural functions.
Cytosol
Comprises more than 50% of a cell's volume
Provides structural support
Provides site for protein synthesis to occur
Provides a home for centrosomes and centrioles

14. Surface to Volume Ratios
Size constraints
When a cell expands in diameter, its volume increases more rapidly than its surface area.
Surface area to volume ratio must be such that nutrients can flow in and wastes flow out.
Shape constraints
Spherical cells have less surface area than irregularly shaped or skinny cells.
The smaller or more stretched out the cell, the more efficiently materials can cross its surface.

15. Eukaryotic Cells Contain internal membranes and organelles Organelles
Physically separate chemical reactions (space)
Temporally separate chemical reactions (time)
Plants and Animals
Plants: cell walls, choroplasts, central vacuoles
Animals: no cell walls, no choroplasts, no central vacuoles

16. Common Organelles Nucleus Ribosomes Endoplasmic Reticulum Golgi Bodies
Diverse vesicles
Mitochondria
Cytoskeleton

17. Nucleus
Nucleolus
Nuclear Membrane
Chromosomes

19. Cytomembrane System Endoplasmic Reticulum Peroxisomes Golgi Bodies
Smooth ER
Rough ER
Peroxisomes
Golgi Bodies
Lysosomes

23. Mitochondria Powerhouse of cell ATP production Structure
Outer membrane
Outer compartment
Inner membrane
Inner compartment

25. Cytoskeleton Composition Cell Movements Microtubule Organizing Centers
Microtubules
Intermediate Filaments
Microfilaments
Cell Movements
Pseudopods
Flagella
Cilia
Microtubule Organizing Centers
Centriole

28. Tensegrity concept for cytoskeleton D. Engber theory
Tensegrity is a concept by J. Buckminister Fuller in which a stabile structure is created from compression tubes and tensile cables.
D. Engber suggested that the cytoskeleton is made up this way – allowing it to rapidly change shape
Controversial

30. Cytoskeleton Network of protein fibers in cytoplasm that
Gives shape to a cell
Without CSK, cell would have no shape
Shape necessary for cell to function and stay in homeostasis
Holds and moves organelles
Involved in cell movement

31. Cytoskeleton Cultured epithelial cells
Cytoskeleton Cultured epithelial cells. Microtubules are green, actin is red, DNA blue. (Image by Steve Rogers)

32. Cytoskeleton and Cytoplasm

33. Microtubules
Microtubules are tubes made up of spiraling, two-part subunits. 
Each spiral has 13 dimers.
Made of  and  tubulin. 
Aids in movement of
Chromosomes
Organelles
Cilia
Flagella
MTOC - where nucleation of (-) end occurs. Then, polymerization proceeds outward to (+) end.
-tubulin combines with several other proteins to become -tubulin ring complex (-turc) at (-) end for nucleation to start / tubulin dimers in polymerization.
Polymeration grows outward on (+) end.
GTP binds to -tubulin stabilizing tubes, forming GTP-tubulin.
GTP binding to  tubulin can hydrolize (GDP-tubulin) which can lead to rapid disassociation of microtubule (depolymerization and shrinkage).

37. Microtubules

38. Microtubules

39. Microtubules

40. Microtubules

41. Microtubules

43. Microtubules – temporal changes

44. Golgi to RER vesicle
RER to Golgi vesicle

45. Molecular Motors
Motor proteins that use ATP for energy to drive momements
Dynein – movement along microtubules from + to –
Kinesin – movement along microtubules from – to +
Myocin – movement along actin fibers

46. Tubulin spindle during mitosis

47. Microtubule functions

48. Microtubule functions

49. Intermediate Filaments
Intermediate filaments are made of eight subunits in rope-strands. 
The proteins structure varies with different tissue types. 
They help maintain shape.
They support nerve cell extensions.
They attach cells together.

53. Intermediate Filaments - Keratin

55. Microfilaments Twisted double strands of proteins (actin).
From 7 nm to several cm long. 
Functions include
Contraction of muscle contraction
Maintenance of cell shape
Transport within cytoplasm

57. Microfilament networks

58. Function of actin in cell motility

59. Actin filaments

60. Actin in endothelial cells
no flow (left) and flow (right)

61. Actin filaments

62. Actin filaments

63. Cell to Cell Junctions between animal cells Tight junctions
Adhering junctions
Gap junctions