The Intercellular Environment

Question 1

What is the ECM composed of, and how does it serve the tissues of an organism?

Answer 1

The ECM is composed of proteins and polysaccharides, and it provides structural support, tensile strength, and elasticity to tissues.

Question 2

What is the ECM equivalent for plant cells?

Answer 2

The extracellular matrix of a plant cell is the cell wall.

Question 3

What is the cell wall composed of, and how does it serve a plant cell?

Answer 3

The cell wall is composed of cellulose microfibrils and other polysaccharides and glycoproteins. The cell wall provides much of the support and structure for a plant cell. It also opposes the cell’s internal turgor pressure (up to 500 atm).

Question 4

What is pressure potential, and how does a plant cell maintain its turgor pressure?

Answer 4

Pressure potential is the tendency of water to move in response to pressure. Water is free to flow into a plant cell (this is different from the controlled osmotic barrier of animal cells). The stiff cell wall pushes back with equal and opposite force on the water inside the cell, preventing it from blowing up like it would in an animal cell.

Question 5

What price must a plant pay for having a cell wall and using it to oppose gravity?

Answer 5

There are constraints on movement (little cell migration), division (difficult cytokinesis), and signaling (small molecules only).

Question 6

How does the sugar content of the ECM compare to the cell wall of a plant cell?

Answer 6

Both the ECMs of animal and plant cells have enormous amounts of sugar in them. The plant wall is different in that it is primarily composed of polysaccharides.

Question 7

What is the basic unit that forms to make plant microfibrils, and how many of them do you need?

Answer 7

Cellulose is assembled together to form microfibrils. You need 20-50 cellulose chains to do so.

Question 8

What are the four main classes of glycoproteins/proteoglycans in the animal ECM?

Answer 8

Collagens, fibronectins, laminins, and glycosaminoglycans (GAGs).

Question 9

Compare N-linked glycans with glycosaminoglycans (GAGs).

Answer 9

N-linked glycans are the most common form of glycosylation. They generate more complex, branched forms of oligosaccharides attached to core proteins. GAGs have more sugar and fewer proteins.

Question 10

Describe the composition of collagen.

Answer 10

Collagen is composed of 1,050 amino acid repeats in the pattern of glycine-X-Y (repeated 350 times). X is usually a proline and Y a lysine. These aa repeats assemble into a triple helix called a tropocollagen.

Question 11

What are collagen fibrils formed from? Describe their structure.

Answer 11

They are formed from tropocollagen (the triple helix of amino acids). Staggered ends result in a banded appearance of collagen fibrils. Tropocollagen molecules in fibril are covalently X-linked. These collagen fibrils then assemble into collagen fibers.

Question 12

Why does a lack of vitamin C result in scurvy?

Answer 12

Many prolines and lysines (two of three of the amino acids in tropocollagen) are hydroxylated; this modification requires vitamin C. VC is required for the hydroxylation and assembly of this protein into a triple helix (tropocollagen). With scurvy, tropocollagen cannot be assembled and collagen fibrils cannot be made. Hair and teeth fall out, wounds won’t heal, and the disease is quickly fatal.

Question 13

Why doesn’t scurvy occur in most animals?

Answer 13

Most animals (with the exceptions of higher primates, bats, and fish) can synthesize their own vitamin C (L-gulonolactone oxidase).

Question 14

Describe the characteristics of fibronectin (an ECM protein).

Answer 14

Fibronectin has critical roles in development/tissue integrity, forms a dimer linked by disulfide bonds, has a binding site for collagen and the cell, and is an important cross-linker within the ECM.

Question 15

What role does fibronectin play in the formation of salivary glands?

Answer 15

Salivary glands normally form lobes. If you remove fibronectin from the gland, the proper development of the salivary gland fails to happen.

Question 16

Describe the composition of glycosaminoglycans (GAGs) and proteoglycans (important ECM proteins).

Answer 16

Proteoglycan complexes are large complexes of proteins and polysaccharides (glycosaminoglycans), though they are mostly made of sugars.

Question 17

How do proteoglycans maintain water balance?

Answer 17

They bind ions and water molecules and take up lots of space, acting like a sponge to soak up water.

Question 18

List the three key components of proteoglycans and compare their monomer counts.

Answer 18

1. Keratan sulfate (monosaccharide branches of <300)
2. Hyaluronic acid (monosaccharide branches of about 25,000)
3. Chondroitin sulfate (branches of <300)

Question 19

How is a cell linked to the basal lamina?

Answer 19

Integrins and other membrane glycoproteins do this. These receptors are the mechanism by which the cell is linked to its surroundings. Integrins link F-actin to the ECM in focal contacts/adhesion plaques and also function as transmembrane receptors for collagen, laminin, fibronectins, and other ECM components.

Question 20

Describe the structure and function of integrins.

Answer 20

Integrins are dimers that work as receptors. They are composed of a and B components that bind components from the ECM. Their purpose is to form a linkage between the cytoskeleton and the ECM, acting as a bridge.

Question 21

What happens when integrins accumulate to work together?

Answer 21

Though integrins can work on their own, they can also work together to form points of strong adhesion. These points are called hemidesmosomes, and they are substrate-adhesion junctions. They act as a bridge between internal keratin intermediate filaments and external ECM filaments.

Question 22

How do integrins play a role in the development and maintenance of multicellular organisms?

Answer 22

Integrins maintain muscle attachments. Pulling against the strong integrin junctions linked to the tendons allows for movement of the muscle. Integrins also guide nerve cell growth and control cell adhesion–a cell can migrate by integrin adhesions to its environment.

Question 23

What are the five types of epithelia in the body, and what structure organizes all of them?

Answer 23

The types of epithelia are simple, stratified, columnar, cuboidal, and squamous. All of them are organized by the basal lamina.

Question 24

What is the adhesion belt of adhering junctions?

Answer 24

The adhesion belt is a belt of actin filaments running through several cells and linked by the adhesion junction. It provides a band of strength to the cells.

Question 25

What is the role of a desmosome, and how does it accomplish this?

Answer 25

The primary role of a desmosome is adhesion. It does not provide a band of strength but a point of contact. This contact is accomplished using intermediate filaments (keratin).

Question 26

What is the role of transmembrane cadherin-family adhesion proteins?

Answer 26

These proteins link the actin cytoskeleton of adjacent cells. Their intracellular domains bind linker proteins (a and B catenins) that bind the cytoskeleton. Thus, cadherin proteins bridge the cytoskeletons of cells, providing a strong linkage either during development/movement (transient) or in mature tissues (permanent).

Question 27

What does it mean to say that cadherins are self-recognizing molecules?

Answer 27

It means they can bind to cadherins from other cells because they have a high degree of specificity. Their extracellular domains bind each other with high affinity (homophilic adhesion: self-loving adhesion).

Question 28

Describe an example of transient cadherin-mediated adhesion.

Answer 28

In nervous system development, transient cadherin-mediated adhesion separates cells of different types. The epidermis expresses E-cadherin, neural folds express N-cadherin, and somites (made from fibronectin) will go on to form the ribs and the cytoskeletal components of the body.

Question 29

Describe an example of cadherin junctions at maturity.

Answer 29

Epithelial cells lining the gut have cadherins permanently at their junctions. These cadherins link a band of actin filament from one cell to its neighboring cell. This generates a constant bond of actin linkage between cells called an adhesion belt, maintaining the structural integrity of the gut.

Question 30

How do the cadherin receptors at a desmosome junction differ from those at an adherens junction?

Answer 30

They are the same; the same class of cadherin transmembrane proteins link both F-actin (adherens junctions; connective adhesion belts) and keratin intermediate filaments (desmosomes; “spot weld” function).

Question 31

What type of force binds together cadherins?

Answer 31

Cadherins are bound together by homophilic interactions. They also link together with linker proteins such as y-catenin. These linker proteins link the cadherins to the intermediate cytoskeleton (keratin).

Question 32

How are hemidesmosomes different from desmosomes?

Answer 32

Hemidesmosomes are a part of a cell-ECM interaction. Desmosomes are a part of cell-cell interactions. The desmosome is mediated by the homophilic cadherin proteins that span the membrane whereas the hemidesmosome is mediated by integrin receptors.

Question 33

How do hemidesmosomes cause adhesion, and what do they cause adhesion between?

Answer 33

Hemidesmosomes link keratin filaments to the ECM via integrin-family adhesion proteins. This causes adhesion between the cell and the substrate.

Question 34

What are tight junctions composed of?

Answer 34

They are composed of long strands of proteins–claudins and occludins. It typically requires five strands of protein to ensure the tight seal characteristic of a tight junction.

Question 35

What is the function of tight junctions?

Answer 35

Tight junctions act as diffusion barriers. They prevent diffusion of extracellular molecules across epithelial cells, block movement between cells, prevent diffusion of proteins, and block movement within membranes.

Question 36

How do you experimentally show the presence of a tight junction?

Answer 36

Inject tracer dye into the extracellular space. That dye can then be traced around the cell and barriers can be discovered when the dye stops moving. The tight junctions act as gaskets to form a water-tight barrier between cells, preventing the flow of materials across epithelial barriers.

Question 37

What is the function of gap junctions?

Answer 37

Gap junctions provide a direct passage between two cells and provide a continuous channel/pore between the cytosols of two adjacent cells. This is the means by which cells can couple their energy metabolism, synchronize development/function, and for fast intercellular communication (in the nervous system). ATP produced in one cell can travel through a gap junction into another cell.

Question 38

How can you experimentally prove the existence of a gap junction?

Answer 38

A technique known as dye-coupling. Inject a dye into one cell and watch it pass into its neighbors through the gap junctions.

Question 39

What are the limitations of cytoplasmic coupling?

Answer 39

Cytoplasmic coupling is limited to small molecules (<1000 Da). It can pass monomers but not polymers. It can pass signals or small metabolites such as ATP cAMP, Ca2+, but cannot pass RNA or DNA.

Question 40

Describe the point of contact at gap junctions and name the technique used to observe it.

Answer 40

Under an electron microscope, you can see that gap junctions occur where the adjacent membranes of two cells are very rigid and the space in between is consistent (2-4 nm). Freeze fracturing reveals plaques of intra-membrane protein contact.

Question 41

In what types of tissues are gap junctions common?

Answer 41

Gap junctions are common in developing tissues and mature cardiac muscle, liver, and the retina. They are also used in electrical synapses between neurons for most rapid communication.

Question 42

Name, compare, and contrast the gap junction proteins for vertebrates and invertebrates.

Answer 42

For vertebrates, cannexin. For invertebrates, innexin. Both cannexin and innexin are 4-pass transmembrane proteins and 6 proteins are required to make 1 channel. However, cannexin and innexin show no amino acid conservation, so we know this is a case of convergent evolution since they are not related to each other.

Question 43

Describe the structure of the vertebrate gap junction.

Answer 43

This gap junction is composed of connexon channels made of connexin transmembrane protein hexamers. Connexin is the basic subunit, and six together create one ringed connexon. Two connexon create one gap junction channel about 1.5 nm in length. This cuts off any molecules greater than 1000 Da from passing through.

Question 44

What are the four types of cell-cell signaling?

Answer 44

1. Contact-mediated
2. Paracrine
3. Endocrine
4. Neuronal

Question 45

How does contact-mediated cell-cell signaling work?

Answer 45

There is a signaling cell and a target cell. The signaling cell attaches a ligand to the receptor of the target cell.

Question 46

How does paracrine cell-cell signaling work?

Answer 46

Paracrine signaling involves the secretion of some kind of diffusible signal.

Question 47

What is autocrine signaling?

Answer 47

Autocrine signaling is a special type of paracrine signaling in which the cell makes both the signal and the receptor. It “talks to itself.”

Question 48

How does endocrine cell-cell signaling work?

Answer 48

Endocrine signaling uses hormones as its signal. The only difference between paracrine signaling and endocrine signaling is the distance. The endocrine signal is typically secreted into the blood stream where it is transported to the target cell.

Question 49

How does neuronal cell-cell signaling work?

Answer 49

Neuronal signaling is really just a specialized type of paracrine signaling. The process is the one used by neurons with their cell body, axon, synapse, and receiving cell.

Question 50

List the common features of cell-cell signaling pathways.

Answer 50

1. Signaling molecule or “key”
2. Receptor or “lock”
3. Relay cascade
4. Responses
5. Inactivation

Question 51

Describe what happens when a diffusible signal is small and nonpolar.

Answer 51

The molecule crosses the plasma membrane by simple diffusion and binds within the cell to intracellular receptors. The receptors for this hydrophobic signaling molecule then translocates as a ligand-receptor complex into the nucleus to regulate gene expression. An example of this type of molecule is steroid hormones.

Question 52

Describe what happens when a diffusible signal is large and polar.

Answer 52

The molecule cannot cross the plasma membrane and binds to cell surface receptors. Transmembrane receptors for hydrophilic signaling molecules activate a wide variety of intracellular signal transduction pathways in the cytosol and nucleus.

Question 53

How does a ligand-gated ion channel act as a cell-surface receptor?

Answer 53

The binding of the ligand causes a conformational change that opens a pore, and ions will flow into the cell. Calcium ions are an important example of this.They act as a second messenger because the first messenger was outside the cell (the ligand). The binding of a single ligand in the ECM can cause the influx of tens of millions of calcium ions into the cell.

Question 54

How does an enzyme-linked receptor act as a cell-surface receptor?

Answer 54

A typical case has transmembrane receptors that exist as individual proteins in the plasma membrane. When they bind to their signaling ligand, it can cross-link the two receptors. When they are cross-linked, they typically become catalytically active. This generates some molecules such as cAMP, cGMP, kinase, and phosphatase, as examples.

Question 55

How many genes are dedicated to making G-protein coupled receptors?

Answer 55

Over 100 genes. G-protein coupled receptors are one of the most common types of receptors in the body.

Question 56

Describe the structure of a G-protein coupled receptor.

Answer 56

This receptor is a 7-pass “serpentine” receptor coupled to a class of three G-proteins: a, B, and y (gamma) proteins.

Question 57

When the G-protein coupled receptor is not bound to a ligand, what is the a-protein bound to?

Answer 57

The a-protein is bound to GDP, acting as a type of GTP switch.

Question 58

What happens to the G-protein coupled receptor when a ligand binds to it?

Answer 58

When a ligand binds to it, it acts to kick off that GDP and replace it with a GTP. This is the activating step of the GTPase switch step.

Question 59

In a G-protein coupled receptor, after the a-protein binds to GTP, what does it do?

Answer 59

It dissociates from the B and y subunits and goes into the cell to act as a second messenger to mediate all types of signaling inside the cell.

Question 60

In a G-protein coupled receptor, what happens to the B and y subunits after the a-protein dissociates?

Answer 60

Simultaneously, the B and gamma subunits also dissociate and act as a second messenger.

Question 61

In the G-protein coupled receptor, what step must occur to restore the original condition of the complex?

Answer 61

The complex is reassembled when GTP is removed from the a-protein in exchange for a GDP.