Cell And Tissue Form

Question 1

Hierarchy of cells

Answer 1

• Recall that a cell is the smallest basic unit of life
• In a multicellular organism, cells can come together to form → tissues
• A collection of cells that work together to perform a specific function
• Two or more tissues combine and function together to make up an → Organ

Question 2

Stem cells, somatic cells, & germ cells

Answer 2

• There are the following classes of cells in an adult multicellular organism
• Stem cells → an undifferentiated cell that can undergo an unlimited number of mitotic divisions and differentiate into any of the large number of specialized cells
• Somatic cells → a nonreproductive cell and the most common type of cell in the body of a multicellular organism
• Germ cells → a reproductive cell that produces gametes (sperm or eggs)

Question 3

Cell communities

Answer 3

• The shape of cells and organs reflects their function
• Determined and maintained by structural protein networks in the cytoplasm → the cytoskeleton
• The structural integrity of a tissue or organ depends on the ability of cells:
• To adhere to one another → via cellular junctions
• To adhere to a meshwork of proteins and polysaccharides outside the cell called the → extracellular matrix (ECM)

Question 4

Example of how cells interact - skin

Answer 4

• Skin has two main layers:
  i. The outer layer that serves as a water-resistant and protective barrier → epidermis
  ii. The layer beneath the epidermis supports the epidermis and supplies it with nutrients → dermis
• Epithelial tissue → covers the outside of the body and lines many internal structures of the body

Question 5

The epidermis

Answer 5

• Primarily composed of epithelial cells called keratinocytes → specialized to protect underlying tissues and organs
• Contains melanocytes that produce the pigments of skin
• The basal lamina supports the epithelial cells
• A specialized ECM below the bottom layer of the epidermis
• The bottom layer of keratinocytes is attached to the basal lamina → cell junctions

Question 6

The dermis (connective tissue)

Answer 6

• Connective tissue → provides structure and support
• The dermis supports the epidermis
• The dermis is mostly made up of connective tissue → a lot of ECM
• The main cell type found in the dermis is fibroblast → produces the extracellular matrix
• Also contains nerves and blood vessels

Question 7

The cytoskeleton

Answer 7

• The protein fibers of the cytoskeleton provide internal support for cells → just like bones provide internal support for the whole body
• The cytoskeleton is formed from long chains of protein subunits joined together
• They provide structural support and enable the movement of substances within cells
• All eukaryotic cells have at least two cytoskeletal elements → microtubes and microtilament.
• Animal cells also have → intermediate filament.

Question 8

Microtubules

Answer 8

• Microtubules are tubelike structures made of polymers of protein dimers
• Each dimer → made of 2 tubulin proteins
• a (alpha) tubulin
• B (beta) tubulin
• One alpha tubulin and one beta tubulin combine to form the tubulin dimer
• These dimers assemble to form microtubules

Question 9

Microtubule function

Answer 9

• Microtubules found in animal cells radiate outward to the cell periphery
• They grow out from the centrosomes
• This arrangement helps maintain the cell’s shape → allows it to withstand compression
• Many organelles are secured to microtubules → guides the arrangement of organelles in the cell

Question 10

Microfilaments

Answer 10

• Polymers of actin monomers that are arranged into a helix
• Thinnest of the cytoskeletal fibers
• Relatively short and extensively branched just beneath the cell membrane of a cell
• Reinforce the cell membrane and organize the proteins associated with it
• Present in various locations in the cytoplasm

Question 11

Example of Microfilament in Cells

Answer 11

• Epithelial cells of the small intestine contains microvilli on their surface → within the microvillus are bundles of microfilaments
• Microfilaments form a band of longer filaments that extends around the circumference of epithelial cells
• The band provides structural support to the individual epithelial cells as well as the entire layer of epithelial cells
• The band is attached to a cell junction that connects neighbouring cells

Question 12

Other Microfilament functions

Answer 12

• Microfilaments also take part in:
• Transport of materials inside cells
• Shortening of muscle cells during contraction
• Separation of daughter cells at the end of animal cell division

Question 13

Microtubule and Microfilament are Changing

Answer 13

• Microtubules and microfilaments are always changing
• Become longer → add subunits
• Shrink → loss of subunits
• These polymers grow faster at one end than the other
• Faster-growing end → plus end
• Slower-growing end → minus end

Question 14

microtubules changing in animal cells

Answer 14

• In animal cells:
• Minus ends of microtubules are positioned at the organizing center of the centrosome
• Plus ends project outward toward the cell membrane
• Microtubules undergo random cycles of
• Rapid shrinkage → depolymerization
• Slower growth → polymerization

Question 15

Dynamic Instability In Microtubules

Answer 15

• There are cycles of polymerization (growth) and depolymerization (shrinkage) in microtubules
• This is known as → dynamic stablity
• The dramatic shrinkage results in → microtuble catastrophe
• Allows the cell to rapidly re-organize the cytoskeleton when needed
• Microtubules explore the space of the cell by growing into new areas and then shrinking back
• For example, spindle microtubules use this process to quickly find and attach to chromosomes during cell division

Question 16

Movement within the cell

Answer 16

• Microtubules and microfilaments can be joined by small accessory proteins → motor proteins
• For example, microtubules function as tracks for transport within the cell
• Two motor proteins, kinesin and dynein, associate with the tracks
  i. Kinesin (funny walking protein) moves the cargo toward the microtubule → plus end
  ii. Dynein moves the cargo toward the
  microtubule → minus end
• The energy for this movement is driven by conformational changes in the motor proteins and is powered by ATP

Question 17

Cilia and flagella

Answer 17

• Microtubules are found in structures that propel the movement of
• Cells → flagella
• Substances surrounding cells → cilia
• In cilia and flagella → microtubules associate with the motor protein dynein that causes movement

Question 18

Intermediate filaments

Answer 18

• Intermediate filaments are found in animal cells → diameter is intermediate to microfilaments and microtubules
• Provide mechanical strength to the cell
• Form strong cable-like polymers of proteins

Question 19

Types of intermediate filaments

Answer 19

• More than 100 different types of intermediate filaments
• Made up of different proteins depending on the cell type
• For example:
• Epithelial cells → keratins
• Fibroblasts → umentins
• Neurons → neurofilaments
• Nucleus → lamins

Question 20

Define cell junction and cell polarity

Answer 20

• Multicellular organisms use cell junctions to physically connect one cell to another cell or to the ECM
• Cells exhibit polarity → spatial differences in shape structure and function within a cell
• E.g. for epithelial cells:
• Facing outside body or lumen → apical
• Attached to basal lamina → basal

Question 21

Cadherins

Answer 21

• Cadherins are integral transmembrane glycoproteins →
  for cell-to-cell atachment.
• The extracellular domain of a cadherin molecule binds to the extracellular domain of a cadherin of the same type on an adjacent cell
• The cytoplasmic part of the cadherin is linked to the internal cytoskeleton
• Provides structural continuity from the cytoskeleton of one cell to the cytoskeleton of another
• Increases the strength of tissues and organs

Question 22

Integrins

Answer 22

• Integrins are integral transmembrane glycoproteins → for cells to attach to ECM
• Cytoplasmic domain (tail end of integrin inside the cell) interacts with the cytoskeleton → important for structural integrity of tissues under physical stress

Question 23

Cell Junctions function and 5 types

Answer 23

• Cell junctions connect cells to other cells or to the basal lamina and are reinforced by the cytoskeleton
• There are five types of cell junctions:
  a. Adherens junctions
  b. Desmosomes
  c. Hemidesmosomes
  d. Tight junctions
  e. Gap junctions
• Cadherins are found in both adherens junctions and desmosomes
• The complexes of cell junctions anchor cells to one another and are reinforced by the cytoskeleton

Question 24

Adherens Junctions

Answer 24

A belt like junctional complex of cadherins that is found around the circumference of the cell

• Intracellularly:
• The belt of cadherins attaches to a band of actin microfilaments in the cytoplasm
• near the apical side (top of cell)
• Extracellularly:
• The cadherins in adjacent cells attach to each other

Question 25

Desmosomes

Answer 25

Button-like points of adhesion that hold the cell membrane of adjacent cells together - Cadherins strengthen the connection between cells - The cadherins in the desmosome of one cell bind to the cadherins in the desmosomes of adjacent cells - The cytoplasmic domains of cadherins are linked to intermediate filaments of the cytoskeleton

Question 26

Hemidesmosomes

Answer 26

- Epithelial cells are firmly anchored to the basal lamina. It is attached via a structure like the desmosome called a hemisdesmosomes - Integrins are the most prominent cell adhesion molecules in hemidesmosomes - The extracellular domains of integrins bind to the ECM proteins in the basal lamina - The cytoplasmic domains of integrins are linked to intermediate filaments of the cytoskeleton

Question 27

Tight Junctions

Answer 27

- Since adherens junctions & desmosomes do not prevent the passage of materials between the cells → need another mechanism to prevent movement of materials - Tight junctions are able to seal the extracellular space - Only way a substance can travel from one side of a sheet of epithelial cells to the other is by moving through the cells by means of a cellular transport mechanism

Question 28

Gap junctions

Answer 28

- Gap Junctions → cell-to-cell communication - Extremely small channels (connexons) span a small gap (2-4 nm) between cells → made of connexin proteins. - Connexons allow for cytoplasmic continuity between adjacent cells → only low molecular weight material can pass between cells - E.g. ions - Integrate activities of individual cells of a tissue

Question 29

Plasmaodesmata

Answer 29

- Plasmodesmata allow plant cells to transfer RNA molecules and proteins - Much larger than gap junctions - The cell membranes between the two cells are continuous → allow plants to send signals to one another despite being enclosed within rigid cell walls

Question 30

What is the ECM?

Answer 30

- The ECM is the layer of material secreted beyond the region of the cell membrane in the extracellular area - Formed from insoluble meshwork of proteins and polysaccharides - Two general functions: - Act as a supportive & protective material - Allow for expression of different cell functions

Question 31

Plant ECM Composition

Answer 31

- The cell wall of plant cells is a type of extracellular matrix - Composed of three layers: i. Middle lamella - Made of carbohydrates - Main mechanism by which plant cells adhere to one another ii. Primary cell wall → thin and flexible - Made of cellulose fibers, pectin, and several other proteins iii. Secondary cell wall → rigid - Made of cellulose and lignin hardens the cell wall and makes it water resistant

Question 32

Animal ECM Composition

Answer 32

- The connective tissue of animals is extensive in ECM - A mixture of proteins and polysaccharides secreted by cells - It is composed of large fibrous proteins → including collagen, elastin, and laminin - These proteins are found in the gel-like polysaccharide matrix

Question 33

Collagen

Answer 33

- The most abundant protein in the animal ECM → collagen - Most abundant animal protein → 1/4th of the protein in the body - More than 20 different forms of collagen exist - Type I collagen is the most abundant - Found in the dermis of human skin → provides support - Composed of intertwined fibers that make it stronger than if it were a single fiber of the same diameter - It consists of three polypeptides wound around one another - Forms a triple helix - A bundle of these molecules forms a fibril, with fibrils then being assembled into fibers

Question 34

Basal Lamina

Answer 34

- The basal lamina is a specialized layer of the extracellular matrix that is present beneath all epithelial tissues - It provides a structural foundation for epithelial tissues - Consists of several proteins → including a type of collagen - Provides flexible support - Acts as a scaffold → for other proteins to assemble on