phgy 170 final- module 6

Question 1

what are some requirements for cellular function?

Answer 1

• Bring nutrients and expel wastes
• Creates cellular energy
• Synthesize new proteins
• Respond to external signals
• Transport materials throughout the cell body

Question 2

what are cell junctions?

Answer 2

• types of membrane proteins that facilitate cell-to-cell attachments
• Cells need to be able to attach to each other for communication, as well as to create barriers
• Endothelial and epithelial cells are two types of cells that form tight networks to keep unwanted material from moving from one side of these cell layers to the other
• These cells require specialized adhesion proteins called junction complexes

—–>Endothelial cells
- The body needs to create barriers to protect itself from harm
The cells that line the inside of blood vessels are called ENDOTHELIAL CELLS

—–>Epithelial cells
- The cells that line the cavities and surfaces of organs (like the inside of the digestive tract and respiratory system) are called epithelial cells

• Epithelial cells rest on the basement membrane, which is a specialized extracellular matrix (ECM) that separates the epithelial cells from underlying connective tissue

Question 3

what are proteins in the junctional complex?

Answer 3

• Epithelial cells and endothelial cells form cell-to-cell junctions known as the junctional complex
• This involves tight junctions, adherens junctions, and desmosomes

—–>Tight junctions
- Tight junctions structurally and functionally divide the plasma membrane into 3 domains

• The APICAL (top) surface of the cell
• The BASAL (bottom) surface of the cell
• Phospholipids cannot move across tight junctions, and therefore cannot move from apical to basal surfaces
• The proteins in a tight junction connect to the cell cytoskeleton
• Tight junctions also regulate paracellular transport
• Paracellular transport: the transport of material between cells

—–> adherens junctions
- Adherens junctions use transmembrane receptor proteins called cadherins that bind to other cadherins on neighboring cells

• The amount of overlap between neighboring cadherins is proportional to the bond strength between them

**Think of it like overlapping velcro

• Adherens junctions are found in neural synapses and cardiac muscle cells
• Actin bundles are adjacent to the junction at the cytoplasmic face of the plasma membrane
• Cadherins form the rods between cells and are linked to the actin cytoskeleton by anchoring proteins such as catenins

—–> Desmosomes
- Desmosomes provide structural integrity to the cell and function like “snaps” in series with each other

• They also link to the cytoskeleton of the cell, and are found in cells exposed to physical stress like skin and cardiac muscle
• Hemidesmosomes consist of one half of a desmosome complex
• They tether to the basement cell membrane of epithelial cells, strongly attaching them to the extracellular matrix ECM

Question 4

what are gap junctions?

Answer 4

• They are another type of cell communication protein, that creates open pores between two neighboring cells
• They facilitate the direct transfer of ions and small molecules between cells
• Note that the gap junctions are not part of a junctional complex

Question 5

what is the structure and function of gap junctions?

Answer 5

• Gap junctions are built of two halves, and the halves connect to cross the intercellular gap
• The cells connected by gap junctions can use channel gating to move ions, sugars, nucleotides, and other molecules directly from one cell to another
• Gap junctions are very important in cardiac muscle contractions and without them, your heart muscle cells could not all contract at the same time

Question 6

what is the extracellular matrix?

Answer 6

• The ECM is a network of molecules that fills the spaces between cells
• The proteins located in the matrix are formed by the cells to provide the external structure and support of the cells and tissues of the body
• Without the ECM, tissues are unable to hold together properly
• The basement membrane is a special matrix found below epithelial cells and on the outside of tissues
• It consists of a special sheet of collagen and other proteins that is a structural foundation and barrier for epithelial cells and a network to stabilize tissues

Question 7

what are the main types of proteins in the extracellular matrix?

Answer 7

collagen, fibronectin, elastin, laminins and proteoglycans

Question 8

what are the properties of collagen?

Answer 8

-Collagen is the most prevalent protein in the body and is the main structural protein in the ECM

• It exists as a triple helix, which then crosslinks to form fibrils
• These fibrils form collagen fibres
• The body makes multiple types of collagen that are specific to different tissue types

Question 9

what are the properties of fibronectin?

Answer 9

• Fibronectins are glycoproteins that connect cells to collagen matrices, functioning in cell adhesion
• They are expressed as dimers, and bind to integrins, which are a type of cell surface protein
• Interactions with the cytoskeleton causes the fibronectin dimers to straighten and associate with other fibronectins, resulting in fibrils at the cell surface

Question 10

what are the properties of elastin?

Answer 10

• Elastin is responsible for giving elasticity to tissues, allowing tissues to return back to their original shape after being distorted by an external force
• Elastin has hydrophobic and hydrophilic regions, which facilitate the ability of elastin proteins to return to their original shapes after being stretched

Question 11

what are the properties of laminins?

Answer 11

• Provide an adhesive substrate for cells, and strengthen the ECM
• Similar to collagen, laminin forms triple helical coils
  It forms a cross like structure and has multiple binding sites for ECM proteins
• Each end of the “T” or “cross” forms a connection with a neighboring molecule
  In this way, laminin can handle tension in multiple directions and it forms web-like networks that have a great deal of strength in multiple directions

Question 12

what are the properties of proteoglycans?

Answer 12

• Hydrated gel that are resistant to compressive forces and are critical for structures like cartilage in our joints
• A proteoglycan consists of a protein polypeptide core and attached sugar residues

Question 13

what are the properties of epithelial tissue?

Answer 13

• Epithelial cells play critical roles in the body and anything that needs to enter the body must cross an epithelial cell barrier which protects the inside of the body from the environment
• This exchange is not the same for all epithelial tissues and there are specialization among epithelial cells and tissue

—–> Epithelial tissues in the skin
- The skin is made of several different types of epithelial tissues

• Here are several examples: Squamous, cuboidal, and columnar epithelial cells that may be simple or stratified

—–> Epithelial tissues in the glands
- Some epithelial cells are specialized to form glands
The cells organize to form pocket-like structures into which they release secretions via ducts

**Exocrine glands
- Release their secretions to the outside of the body (such as sweat glands)

**Endocrine glands
- Release their secretions (hormones) internally into the bloodstream
Endocrine glands do not have ducts, as their cells release their products directly into the blood

—–> Epithelial tissues in the digestive tract
- Within the digestive system, epithelial tissues are specialized for absorption, secretion and protection

• Epithelial cells line the entire digestive system, from mouth to anus, and many different types of epithelial cells can be found throughout
• Using the stomach as an example, you will find that there are 3 different types of epithelial tissues that come together: those that produce and excrete proteins, those that form digestive glands, and those that are specialized for transportation

**Produce and excrete proteins
- Some epithelial tissue cells are producing and excreting, through the process of exocytosis, glycosylated proteins; these proteins form a layer of music that protects the epithelial cells from themselves and the acidic environment

**Form digestive glands
- Other epithelial tissues form glands that produce and release both digestive enzyme proteins and hydrochloric acid, which helps to break down food

• These digestive glands are another type of exocrine gland, as the lumen of the stomach is considered external to the body

—–>Facilitate transport
- Certain epithelial tissues are specialized for transportation

• The best examples of such tissues can be found in the digestive system, where nutrients must be transported across epithelial cell layers to enter the body
• In the kidney, ions and water are transported across epithelial cell layers to remove wastes from the body and to regulate urine volume

Question 14

what are the properties of nervous tissue?

Answer 14

• Nervous tissues are composed of highly specialized cells that use electrical communication to carry information over long distances within the body
• The tissues are composed of nerve cells themselves, as well as glial cells including Schwann cells that serve a supportive role

—–> How nervous tissues work
- Cells tightly control their internal environment resulting in different concentrations of ions inside and outside of a cell

• Since ions are charged, these differences in ion concentration across the membrane creates an electric potential
• Nerve cells use changes in the electric potential as a signal which is far faster than other forms of cell signaling and allows for very rapid communications throughout the body
• Think about what happens when your hand touches something hot and even before you may be aware of it, your hand pulls back from the heat
• This fast response, or reflex, serves as a protective mechanism and would not be possible if the body relied on chemical transmission alone

Question 15

what are the properties of muscle tissue?

Answer 15

• Muscle tissues can convert chemical and electrical signals into mechanical movement and are comprised of cells that specialize in contraction
• Recall from module 5 that actin is a prominent cytoskeletal protein that, along with its associated protein myosin, generates movement within cells
• It is of no surprise then that muscle cells are very rich in actin-myosin networks, since these cells are responsible for contracting and generating force
• Skeletal muscle: responsible for moving the skeleton
• Smooth muscle: smooth muscle lines most of the digestive system, the larger blood vessels, and anywhere else in the body that requires contractile activity
• Cardiac muscle: found only in the heart and is organized to pump blood throughout the body

Question 16

what are the properties of connective tissue?

Answer 16

• In module 5, you learned about the intracellular networks of proteins that make up the cytoskeleton
• One of the primary purposes of the cytoskeleton is to provide shape and structure to a cell
• Connective tissues play similar roles, but they do so outside of the cells, filling in the spaces between all of the other cells to provide mechanical strength and cushioning for protection
• They make up a larger component of the ECM, and are the cells that makeup the ECM
• There are several different types of connective tissues, each characterized by their ECM

Ex. in structures like the lungs that have a lot of movement, the ECM is rich in elastin, which is a protein with rubber-band-like properties

• Recall from section 1 that elastin allows tissues to reversibly change their shape
• In contrast, bone ECM is very rigid, which is necessary to provide the strength of the skeletal system
• One common cell type is a fibroblast

Question 17

what are the different types of tissues in the stomach and what are their functions?

Answer 17

—–>Epithelial tissue
- As discussed in section 1, the inside of the stomach is lined with epithelial tissues

• They protect the underlying layers from the digestive proteins and acids released by epithelial glands

—–>Muscle tissue
- Under the epithelial tissues, the outside of the stomach is lined with layers of smooth muscle cells, and these cells rhythmically contract to both mix the stomach contents and to help propel them into the intestine

—–>Nervous tissue
- The stomach wall contains nervous tissue, which help to control and coordinate the muscle contractions and some gland secretions

—–>Connective tissue
- Between all these different cell types you will find connective tissues to hold everything together and to provide the shape to the stomach

• All together, these tissue types create the organ

Question 18

what is homeostasis?

Answer 18

• Homeostasis is defined as the ability of a cell or organism to regulate and maintain its internal environment, regardless of the influences of the external environment
• When the external environment changes and tries to alter the internal environment, physiological mechanisms counter these changes to maintain the internal environment
• At the organism level, and even at the cellular level, the internal environment is maintained under the conditions that optimally support life
• These optimal conditions are what we consider to be the set point
• However, since everything is happening in the body is very dynamic, to achieve a set point is very difficult and it may be easier to consider environment parameters as physiological ranges

Question 19

what are the 3 components of the homeostatic control system?

Answer 19

-The sensor: detects an environmental variable

• The integrator: compares the variable’s value to its set point
• The effector: if the value is different from the set point, then the effector initiates changes to restore the set point

Question 20

how is body temperature maintained by homeostatic control systems?

Answer 20

• When the external environment gets cold enough that the internal environment starts to decrease in temperature, the body compensates and responds by generating heat to increase the temperature back towards the set point
• This can be done by shivering, which is just muscle contractions, as heat is released during the contractions of muscles

**Set point
The body’s set point is 37 degrees C but generally fluctuates in the physiological range of 36-38 degrees C

—–>Sensor
- temperature : monitoring nerve cells are always sensing the temperature and sending that information to the thermoregulation centre

—–>The integrator
- The thermoregulation centre takes the information from the sensor and compares it to the set point

• If the sensed temperature is different from the set point, the thermoregulation centre will send signals to the effector (Ex. can be the brain)

—–>The effector
- The integrator causes the blood vessels in the skin to constrict, minimizing heat loss, and skeletal muscles rapidly contract, causing shivering, which generates heat (Ex. can be blood vessels, skeletal muscles)

**NOTE: when the set point temperature is restored and homeostasis has been reached, the integrator stops communicating with the effector which causes the effectors actions

Question 21

what are intrinsically vs. extrinsically controlled systems?

Answer 21

• Homeostatic control systems are grouped as intrinsically controlled and extrinsically controlled systems

—–> Intrinsically controlled
- Intrinsic control, also called local control, is where the sensor, integrator, and effector of a system are all located within a tissue, such that the tissue can regulate its own internal environment

• An example of this is an exercising skeletal muscle that needs lots of oxygen to produce ATP
• When the local oxygen concentration drops, this is sensed by the blood vessels within the muscle which dilate (get bigger) to increase the amount of oxygen that can be delivered
• Ex. the blood clotting feedback loop is an example of positive feedback and is intrinsically controlled since the signal is localized at the site of the damaged blood vessel

—–> Extrinsically controlled
- Extrinsic control is when any of the regulatory mechanisms are outside of the tissue or organ, such as in the case of body temperature regulation

• The majority of homeostatic control systems in the body are dependent upon extrinsic control
• The blood pressure feedback loop is an example of a negative feedback loop that is extrinsically controlled

Question 22

what is an example of a negative feedback loop?

Answer 22

• The majority of homeostatic control systems operate on negative feedback
• Negative feedback refers to a change in an environmental parameter that causes the corresponding effector to initiate a response in the opposite direction, thus restoring the parameter to its set point
• Once the set point is achieved, the actions of the effector stop
• The example of body temperature regulation that was used earlier to describe homeostasis is an example of a negative feedback loop
• Recall that upon completion, the set point was restored

Question 23

what is an example of a positive feedback loop?

Answer 23

• Occurs when the effector causes changes that amplify the initial signal
• Positive feedback is not homeostatic, and as a result, there are few examples of it in the human body
• For example, a child breastfeeding stimulates more milk production
• A positive feedback loop does not occur in thermoregulation in humans and other mammals
• Hypothetically, if it did exist, a positive feedback loop effector would further decrease body temperature, which would in turn further activate the sensor and try to decrease temperature more
• Childbirth feedback loop is positive, and contractions and pressure on the uterus leads to more production of hormones released in the blood