8 - Cell Junctions and Tissues

Question 1

3 categories of cell junctions

Answer 1

1. Adhering junctions: these mechanically hold cells together. E.g. desmosomes
2. Impermeable junctions: these prevent the passage of molecules between cells. E.g. tight junction
3. Communicating junctions: these allow the passage of small molecules from one cell to another. e.g. gap junction or chemical synapse

Question 2

Desmosomes (adhering junction)

Answer 2

commonest in tissues, which are subject to mechanical stresses
They hold cells together to give a tissue structural integrity
The junction consists of a desmosome plaque on the insider of each cell and a desmosome core joins the plaques
the core is filled with glycoproteins called cadherins.

Question 3

3 types of desmosomes

Answer 3

Belt, spot, hemi

Question 4

Belt desmosomes

Answer 4

form a continuous band around the cell, common in epithelial cells where they are associated with actin filaments (inside the cell)

Question 5

Spot desmosomes

Answer 5

spot-weld cells together, and are attached on the inside of the cell to keratin filaments, which serve to spread the stresses from the spot desmosome throughout the cell

Question 6

Hemi-desmosomes

Answer 6

attach the base of the cell to the basal lamina

Question 7

Tight junctions (impermeable junction)

Answer 7

Tight junctions prevent molecules from leaking between adjacent cells. They also serve to separate the apical plasma membrane from the basal plasma membrane, thus allowing them to have different compositions

Question 8

Why are there tight junctions present in the gut epithelium?

Answer 8

The apical surface of the gut epithelium contains H+ and Cl- ion pumps which produce HCl - if this was pumped into the blood it would be extremely dangerous so the tight junctions prevent the apical membrane components and the basal membrane components from mixing

Question 9

Gap Junctions (communicating junctions)

Answer 9

Gap junctions are tiny pores composed of proteins called connexins. Twelve connexin (six from each cell) form the gap junction.
The permeability of gap junctions is changed by decreased intercellular pH or increased intracellular calcium
Cells with gap junctions can share small molecules

Question 10

What are connexins?

Answer 10

The protein subunits that make up gap junctions

Question 11

What is the medical significance of gap junctions?

Answer 11

Gap junctions form the cell-to-cell pathways for propagation of the patterns of electrical flow which govern the regular rhythm of a healthy heart.
The reduction in number of gap junctions or change in their distribution leads to a variety of cardiac diseases

Question 12

Tissues

Answer 12

• Most animals are composed of specialized cells organized into tissues that have different functions
• Tissues make up organs, which together make up organ systems

Question 13

4 categories of tissue

Answer 13

epithelial, connective, muscle, and nervous

Question 14

Epithelial tissue

Answer 14

consists of tightly packed cells - covers outside of body and lines organs and cavities inside the body.
the epithelium protects against mechanical injury and also provides a barrier against microbes and fluid loss

Question 15

What is stratified epithelium?

Answer 15

Epithelia that is several layers thick

Question 16

T/F?: Epithelial tissue never has secretory functions

Answer 16

FALSE - epithelial tissue often has secretory functions

Question 17

Connective tissue

Answer 17

Provides mechanical strength - binds and supports other tissues. Consists of an extracellular matrix through which cells are sparsely scattered.

Question 18

What are the two types of connective tissue?

Answer 18

Dense and loose

Question 19

Dense connective tissue

Answer 19

such as cartilage bone and tendon has great mechanical strength and elasticity. e.g. Bone Tendon, ligaments, sclera, cornea, cartilage
Made up of :
1. Fibrous proteins (mainly collagen and elastin)
2. Ground substance usually proteoglycans

Question 20

Loose connective tissue

Answer 20

holds small glands and epithelia together and includes the basal lamina of cells

Question 21

T/F?: Elastin is the main component of dense connective tissue

Answer 21

FALSE - collagen is the main component of dense connective tissue

Question 22

t/f? The basic structural unit of collagen is a triple helix

Answer 22

true

Question 23

Three muscle types in body are:

Answer 23

• skeletal muscle - responsible for voluntary movement
• smooth muscle - responsible for involuntary body movement
• cardiac muscle - responsible for contraction of heart

Question 24

t/f? Muscle tissue generates physical force for movement

Answer 24

true

Question 25

What does nervous tissue contain?

Answer 25

• neurons that transmit the nerve impulses

- glial cells/glia that help nourish, insulate and replenish neurons