PHGY 215 mod 1

Define homeostasis

Ability of a cell/organism to regulate + maintain its internal environment regardless of the influences of the external environment

Define set point

the range or point at which a physiological state tends to stabilize

• optimal conditoin

3 components of homeostatic control + purpose of homeostatic control

components

• sensor

• integrator

• effector

purpose

• maintenance of a set point

describe the 3 components of homeostatic control

sensor: responsible for detecting environmental variable

integrator: compares the variable being detected to its set point

effector: the effector is responsible for initiating the changes to restore the variable back to the set point

2 types of homeostatic regulation

• Intrinsic regulation (local/autoregulation)

• extrinsic regulation

describe intrinsically controlled homeostasis

• sensor, integrator, effector located within tissue

• regulates own internal environment

describe extrinsically controlled homeostasis

• regulatory mechanisms outside of tissue/organ

• regulates / used more than intrinsic

describe negative feedback

• used my majority of homeostatic control

• change in environment → effector initiates response in opposite direction → restoring set point

how is blood glucose is regulated

Negative feedback

1. Homeostasis (concentration of glucose in blood is regulated to maximize its energy making potential)

2. Imbalance (after meal, blood glucose rise)

3. Response (insulin is released by the pancrease in response to high blood glucose levels)

4. Effect on tissue (insulin lowers blood glucose by increasing body cells ability to uptake glucose from blood)

5. Effect on liver (insulin also upregulates livers ability to convert glucose and store it as glycogen)

blood glucose

• regulated variable

• sensor

• integrator

• effector

example of one negative and one positive feedback

negative

• blood glucose

positive

• temperature

describe positive feedback

• occurs when the effector causes changes that amplify the initial signal

• not homeostatic

Describe one full example of positive feedback

body temperature

• 1. Brain stimulates the pituitary gland to secrete oxytocin

• 2. oxytocin is carried in the bloodstream to the uterus

• 3. Oxytocin stimulates uterine contractions which push the baby towards the cervix

• 4. The head of the baby pushes against the cervix

• 5. the nerve impulses from the cervix are transmitted to the brain

match

roles of the plasma membrane

• regulating internal fluid composition (it controls what goes in and out of a cell)

• allows nutrients to enter cells and allows waste products to leave

• cell to cell communication

• joins cells together to form tissues and organs

3 primary functions of the plasma membrane

• Ensures the cells survival

• Maintainb homeostasis

• Function cooperatively + in coordination w/ surrounding cells

Different components of the plasma membrane

• Phospholipids

• Cholesterol

• Membrane proteins

• Ion channels

• Carbohydrate chains

identify the components of a phospholipid

• polar head

• negatively charged phosphate group

• 2 non-polar fatty acid tails

Describe cholesterol role

• tucked in between phospholipids

• prevents fatty acid chains from packing too tightly together + forming rigid structures

• keeps membrane fluid

Describe membrane proteins location

• inserted into phospholipid membrane

• associated w the inner side + outer side

• can pass all the way through plasma membrane

describe membrane protein function

• maintain cell structure

• regulate cell function

• allow transport across the cell membrane

• facilitate signalling

Describe ion channels

• span the entire lipid membrane

• permits the entry/exit of ions

describe what carbohydrate chains attach to + what they can turn into

short chains of carbohydrate can attach to proteins or the bilayer forming glycoproteins / glycolipids

Function of carbohydrate chains

• stabilizes membrane structure

• acts as cell surface receptors

• participate in transportation acorss the cell membrane

2 clinical disorders of the plasma membrane

• Cystic fibrosis

• Alzheimers disease

Describe cystic fibrosis

• a defect in chloride ion channel

  • involved in controlling the amount of fluid and mucous within the lungs

• patients experience a build up of fluid in the lungs making breathing difficult

Alzeimer’s disease

• Results in oxidative stress

• condition that leads to the alteration and degredation of phospholipids within neurons

• reduction in phospholipids compromises membrane integrity → impacts neural function

  • results in membrane loss and reduced cognitive function

describe Fluid Mosaic Model

Fluid: Phospholipid bilayer is viscous; phospholipids can move laterally → membrane is fluid + flexible.
Mosaic: Proteins and other molecules are embedded throughout the lipid bilayer, creating a mosaic and carrying out various functions.

3 means in which cells adhere to each other

• extracellular matrix

• cell adhesion molecules

• cell junctions

3 major protein fibers found in the ECM (interstitial fluid)

• Collagen

• Elastin

• Fibronectin

Describe the ECM

• network of fibrous protein s

• surrounds all cells in tissues + keeps them in place

The majority of ECM is secreted by what?

fibroblasts located within the interstitial space

What are cell adhesion molecules (CAM)

Transmembrane proteins that help cells stick to other cells or the ECM

What do the intracellular and extracellular sides of cell adhesion molecules interact with?

Intracellular: cytoskeleton
Extracellular: other CAMs or ECM

What are the 4 main Cell Adhesion Molecules families?

Cadherins, selectins, NCAMs, integrins

What is the main function of desmosomes?

Strongly anchor adjacent cells together and provide mechanical strength/stretch.

What are desmosomes made of?

Plaques + cadherin-containing glycoprotein filaments that connect neighboring cells.

What do desmosomes connect to inside the cell?

Cytoskeletal proteins/filaments, forming a strong network through tissues.

What is the main function of tight junctions?

Form a tight seal between adjacent cells to prevent substances from passing between cells.

Where are tight junctions mainly found?

Epithelial tissues, where they create selective barriers between compartments.

What is a “kiss site”?

The point where junctional proteins from neighboring cells meet.

What forms a connexpn ( half a gap junction? )

six connexin protein subunits

what happens when connexons aligns with a connexon of an adjacent cell?

forms a tunnel that connects their intracellular spaces and allows them to communicat directly

what can pass through gap junctions?

small water soluble substances

How do cells communicate thorugh gap junctions?

Tunnels open and close as needed to control cell to cell comomunication

where are gap junctions common?

cardiac and smooth muscles

function of gap junctions

• spread waves of excitation (electrical activity)

• allows spread of secondary messengers between connected cells

  • “communication junctions”

define what it means for a membrane to be permeable

substance can freely corss the membrane

define what it means for a membrane to be impermeable

substance cannot corss the membrane

what is the permeability of a plasma membrane

semi permeable

• some substance may pass freely

• others cannot

factors which determine membrane permeability

size + solubility

describe how molecule size determines membrane permeasbility

• small substances (ex. ions) → enter membrane through ion channels

• larger substances (ex. glucose) → transport protein

describe how molecule solubility determines membrane permeability

lipophilic → easily cross

• uncharged or nonpolar

Lipophobic → cannot pass unaided

• charged or polar substances

passive transport examples

• diffusion

• facilitated diffusion

active transport examples

• carrier mediated

• vesicular transport

describe diffusion

• molecules spread from areas of high density to areas of low density

  • down a concentration gradient

what is dynamic equilibirum

• concentration gradient no longer exists

• movement of substances are said to be in a steady state

• no net movement

Diffusion in the presence of a permeable membrane

• net diffusion pasively down its concentration gradient across the membrane until equilibrium has been achieved

Diffusion in the presence of a impermeable membrane

• no diffusion will occur

• concentration gradient remains in place

what are aquaporins

specific membrane proteins

• channels that allow water molecules to freely pass

what is osmosis?

• net diffusion of water

  • water moving down its concentration gradient by diffusion

when does osmosis occur + finish

when a membrane seperates pure water from a solution of non-penetrating solute

• finishes when both water and solute are equally distributed acorss the membrane

what is osmotic pressure + describe when its greater

• underlying force that moves water down its concentration

• greater the gradient → greater the osmotic pressure

Describe hydrostatic pressure + describe when its greater

force created by a given volume of water

• greater the volume → greater the hydrostatic pressure

what happens when osmotic pressure is equal to net hydrostatic pressure

no net movement of water even though a concentration gradient still exists

how do large, lipophobic molecules cross the plasma membrane?

• facilitated diffusion

• carrier mediated transport

• vesicular transport

what type of transport is carrier mediated transport

can be either facilitated diffusion or active transport

describe facilitated diffusion

• does not require energy

• uses a carrier to assist in the transport of a substance down its concentration gradient

identify and describe an Example of facilitated diffusion

• transport of glucose into cells

• glucose carrier proteins bind extracellular glucose and transports it into the cell

identify and describe an example of active transport

Na K ATPase pump

• transports Na+ out of cell

• brings K+ in to cell

Go into depth about carrier mediated transport: facilitated diffusion (4 steps)

1. binding site faces high concentration

2. solute binds

3. carrier changes shape → faces low concentration

4. solute released → carrier returns to original shape

High → low, no ATP

Go into depth about carrier mediated transport: facilitated diffusion: active transport

• Inside: 3 Na⁺ bind → ATP phosphorylates pump.

• Shape changes: 3 Na⁺ released outside.

• Outside: 2 K⁺ bind → pump dephosphorylates.

• Shape changes back: 2 K⁺ released inside.

• Repeats: 3 Na⁺ OUT, 2 K⁺ IN, 1 ATP
  

key: na+ moves in , K+ moves out

both against their concentration gradients

3 characteristics of carrier mediated transport

• Specificity: Carrier recognizes specific substances; related substances may share a transporter.

• Saturation: Limited number of carriers → reaches a transport maximum (Tm).

• Competition: Related substances using the same carrier compete → each cannot reach its individual Tm.

types of vesicular transport

• exocytosis

• endocytosis

3 classifications of endocytosis

• pinocytosis

• receptor mediated endocytosis

• phagocytosis

Describe pinocytosis

cell membrane engulfs + internalizes a small drolet of extracellular fluid

How does the vesicle form during pinocytosis?

→ Coat proteins deform the inner plasma membrane

→ form an endocytic pouch

→ dynamin pinches it off

→ internalized vesicle forms.

what type of transport is pinocytosis

non-selective endocytosis

• the vesicle will contain whatever substances were close to the membrane at the time

3 steps of pinocytosis

1. Solute molecules and water molecules are outside the plasma membrane

2. membrane pockets inward, enclosed solute molecule and water molecules

3. pocket pinches off as endocytic vesicle containing sample of ECF

3 stps of receptor mediated endocytosis

1. Substances attach to membrane receptors

2. membrane pockets inward

3. pocket pinches off as endocytic vesicle containing target molecule

difference between receptor mediated endocytosis + pinocytosis

the trigger to create a vesicle is dependent upon the binding of a substance to a specific receptor on the cell surface

what does phagocytosis involve

The internalization of large multimolecular particles and only occurs in a few specialized cell types (ex. white blood cells)

Process of phagocytosis

1. Pseudopods begin to surround prey

2. pseudopods close around prey

3. prey is enclosed in endocytic vesicle that sinks into cytoplasm

4. lysosome fuses with vesicle, releasing enzymes that attack material inside vesicle

What uses exocytosis?

materials produced by endoplasmic reticulum + golgi complex, which are destined for the plasma membrane or beyond

what is the purpose of exocytosis

• releases large polar molecules

• enables a cell to move proteins (ex. carrier proteins, ion channels, receptors)

what happens when the vesicle membrane fuses with the plasma membrane?

any protein in the vesicle membrane becomes part of the plasma membrane

Summar steps of exocytosis

1. secretory vesicle formation

2. budding from golgi

3. uncoating

4. docking at plasma membrane

5. exocytosis

Exocytosis steps

→ Golgi sorting markers recognize proteins with the correct sorting signals

→ coatomer curves the membrane and forms a bud → bud pinches off

→ vesicle uncoats, exposing v-SNAREs

→ v-SNAREs bind matching t-SNAREs on the target plasma membrane

→ vesicle fuses and releases contents outside the cell.

what is an electrochemical gradient

a gradient that consists of an electrical and chemical gradient across a membrane

What is a membrane potential?

A difference in electrical charge across the plasma membrane caused by unequal distributions of cations (+) and anions (-)

Which cells use membrane potential?

excitable cells

• neurons + muscle cells

What is Ohm’s Law in excitable cells?

V = IR

• V = membrane potential (voltage)

• I = ionic current (ion movement across membrane)

• R = membrane resistance to ion movement
  
  

• → Higher V → higher I; higher R → lower I.

4 types of ion channels

• Voltage-gated

• chemically gated

• mechanically gated

• thermally gated

describe voltage-gated + steps

voltage gated ion channels open + close in response to changes in membrane potential

1. closed at resting potential

2. open in response to nerve impulse

3. inactivated for a brief period following activation

When do chemically gated channels open

• open when a specific chemical messenger interacts w/ it

Where are chemically gated ion channels found?

Dendrites of neurons

when do mechanically gated ion channels open?

In response to mechanical deformations such as stretch

example of where mechanically gated ion channel is found

• found in chcolea of the year

what do thermally gated ion channels respond to?

• respond to changed in temperature

where are thermally gated ion channels present in?

specialized neurons that project to the skin to act as temperature detectors