Tissues (L24)

Question 1

What are tissues?

Answer 1

Highly organized communities of cells and the extracellular matrix

Question 2

Formation of tissues involves these three things:

Answer 2

1. Cell-cell communication
2. Differentiation (specialization) and renewal/repair
3. Cell-cell adhesion

Question 3

How are plant cells organized?

Answer 3

A supportive matrix called the cell wall

Question 4

What are the four roles of the cell wall?

Answer 4

1. Encloses
2. Protects
3. Immobilizes
4. Shapes each cell

Question 5

What allows the rigid structure of a plant?

Answer 5

Osmotic swelling of the cell limited by the resistance of the cell wall

Question 6

Newly formed plant cells have thinner primary cell walls that accommodate growth. They are made of…

Answer 6

Pectin

Question 7

After a plant cell stops growing, a secondary and more rigid cell wall is produced. It is made of … and can have …

Answer 7

Cellulose
Specialized properties

Question 8

What gives the cell wall tensile strength? What two things does it resist?

Answer 8

Cellulose microfibrils. They resist compression and tension.

Question 9

Where is cellulose synthesized?

Answer 9

On outer surface of cell by enzyme complexes in the plasma membrane.

Question 10

How is cellulose made?

Answer 10

1. UDP-glucose (activated) is transported from cytosol to the cellulose synthase complex
2. Glucose is incorporated into cellulose chains at points of membrane attachment to microtubules. These chains assemble into cellulose microfibrils.

Question 11

How is cellulose orientation determined?

Answer 11

By enzyme complex alignment with microtubules.

Microtubules serve as tracks for cellulose synthase enzymes.

Question 12

What does cellulose orientation determine?

Answer 12

Which way the plant will grow and resist tension.

Question 13

How are animal connective tissues organized?

Answer 13

By the Extra Cellular Matrix (ECM)

Question 14

What is the ECM comprised of? What is the main protein?

Answer 14

Proteins and polysaccharides. Main protein = collagen

Question 15

The ECM performs what major function?

Answer 15

Carries the mechanical load

Question 16

What provides tensile strength in animal tissues?

Answer 16

Collagen

Question 17

How is collagen delivered to the ECM?

Answer 17

1. Cells intracellularly synthesize its precursor form and excrete it (procollagen)
2. It assembles into aggregates after secretion
3. Procollagen obstructs premature assembly into collagen fibrils with peptide extensions, after they are secreted procollagen proteinases cut off the extensions

Question 18

What gives tissues their distinctive qualities?

Answer 18

The combination of macromolecules with collagen in the ECM

Question 19

What cleaves the terminal procollagen extensions that inhibit self-assembly into collagen fibrils?

Answer 19

The enzyme procollagen proteinase

Question 20

What two things allow cells to attach to collagen fibers?

Answer 20

Integrin and fibronectin

Question 21

What are integrins?

Answer 21

Transmembrane proteins. The extracellular domain binds to matrix components (collagen) and intracellular domain interacts with the cytoskeleton.

Question 22

Do integrins interact directly with collagen fibers?

Answer 22

No. Intracellular domain binds to actin filament when extracellular part binds to fibronectin.

Question 23

What is fibronectin?

Answer 23

An extracellular matrix protein that can interact directly with collagen fibrils.

Question 24

What fills the spaces between collagen? What does this do?

Answer 24

GAGs - polysaccharides fill the spaces. They are negatively charged, so they draw in clouds of cations that draw in water and result in swelling.

Question 25

How do integrins coordinate their catch/release system of movement?

Answer 25

Through conformational changes.

1. Binding on inside of PM results in activated state (stretched/extended)
2. Latching onto molecule outside of PM does same.
3. Intracellular signaling can activate integrin on cytoplasmic side
4. External attachments regulate behavior by switching on protein kinases that associate with the internal end

Question 26

How do tissues resist compression?

Answer 26

Through GAGs (glycosaminoglycans) drawing in cations and water -> swelling pressure.

Question 27

What is the difference between stratified and simple epithelium? What is an example of each?

Answer 27

Stratified: many cells thick (skin)

Simple: one-cell thick (gut)

Question 28

What regulates the movement of molecules and migratory cells through the matrix?

Answer 28

The ECM

Question 29

What are the functions of the epithelia? Where are they found?

Answer 29

Line internal cavities and cover surface of body

Functions:
1. Barrier
2. Signaling
3. Transport across

Question 30

Are epithelial sheets polarized?

Answer 30

Yes

Question 31

Where do the epithelial sheets rest?

Answer 31

On a basal lamina

Question 32

What is the basal lamina?

Answer 32

A supportive mat (ECM) that provides adhesive sites for integrins in basal plasma membranes of epithelial cells. Primarily comprised of type IV collagen and laminin

Question 33

What are the types of cell junctions found in animal epithelia?

Answer 33

1. Tight junctions
2. Adherens junction
3. Desmosome
4. Gap junctions
5. Hemidesmosome

Question 34

Tight junctions

Answer 34

Seal neighboring cells together in an epithelial sheet to prevent leakage of extracellular molecules between them. Helps polarize cells.

The junction is made of proteins called claudins and occludins that create the seal in strands along the boundary.

Prevents diffusion!

Question 35

Adherens junction

Answer 35

Joins an actin bundle in one cell to a similar bundle in a neighboring cell

The junction forms between cadherin proteins that are tethered to the cytoskeletons of their respective cells.

Anchors cells together via cytoskeletons!

Question 36

What do adherens junctions enable?

Answer 36

Tube formation and shape changing due to tension development

Question 37

Desmosomes

Answer 37

Link cells via their intermediate filaments

Cadherin connects to keratin filaments

Provides tensile strength (especially to exposed tissues like the skin)

Question 38

Gap junctions

Answer 38

Allow small molecules to pass from cell to cell directly

Proteins in each cell (connexons) connect to form hydrophilic tubes that create a cytosolic channel between cells

Direct intercystolic communication. Extracellular signals regulate the permeability of these junctions

Allow coordination (ex: muscle contraction)

Question 39

Hemidesmosome

Answer 39

Anchors the keratin (intermediate) filaments in epithelial cells to basal lamina. Integrins bind to laminin in basal lamina, inside cell the tails are bound to keratin filaments.