Cytoskeleton

BME 615 - Tissue MechanicsRay Vanderby

Figures in this document are taken from various sources including copyrighted material and are for educational purposes only.

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.

Cell

Cell Mitosis

Nature of cells

• Each cell has common components– Plasma membrane– Region containing DNA– Cytoplasm– Organelles– Cytoskeleton

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

Cell Membranes

• Plasma membrane and internal membrane

• Lipid bilayer – structure and a molecular barrier

• Proteins – embedded or on surface; carry out most membrane functions

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.

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.

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.

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

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

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.

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

Common Organelles

• Nucleus

• Ribosomes

• Endoplasmic Reticulum

• Golgi Bodies

• Diverse vesicles

• Mitochondria

• Cytoskeleton

Nucleus

• Nucleolus

• Nuclear Membrane

• Chromosomes

Cytomembrane System

• Endoplasmic Reticulum– Smooth ER– Rough ER

• Peroxisomes

• Golgi Bodies

• Lysosomes

Mitochondria

• Powerhouse of cell

• ATP production

• Structure– Outer membrane– Outer compartment– Inner membrane– Inner compartment

Cytoskeleton

• Composition– Microtubules – Intermediate Filaments – Microfilaments

• Cell Movements– Pseudopods– Flagella– Cilia

• Microtubule Organizing Centers– Centriole

Tensegrity concept for cytoskeletonD. 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 shapeControversial

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

CytoskeletonCultured epithelial cells. Microtubules are green, actin is red, DNA blue.

(Image by Steve Rogers)

Cytoskeleton and Cytoplasm

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).

Microtubules

Microtubules

Microtubules

Microtubules

Microtubules

Microtubules – temporal changes

Golgi to RER vesicle RER to Golgi vesicle

Molecular Motors

• Dynein – movement along microtubules from + to –

• Kinesin – movement along microtubules from – to +

• Myocin – movement along actin fibers

Motor proteins that use ATP for energy to drive momements

Tubulin spindle during mitosis

Microtubule functions

Microtubule functions

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.

Intermediate Filaments - Keratin

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

Microfilament networks

Function of actin in cell motility

Actin filaments

Actin in endothelial cells

no flow (left) and flow (right)

Actin filaments

Actin filaments

Cell to Cell

• Junctions between animal cells– Tight junctions– Adhering junctions– Gap junctions