Interactions between Cells and the Extracellular Enviorment

Extracellular environment (Extracellular Fluid)

▫ Cells must exchange substances with the
extracellular environment in order to survive
▫ Cells must be able to communicate to help
maintain homeostasis

Plasma membrane

• Serves as physical barrier between cell and the interstitial fluid
• Regulates movement into and out of a cell
• Establishes and maintains electrochemical gradient
• Functions in cell communication

passive processes in membrane transport

do not require expenditure of cellular energy; substances move down their concentration gradient

examples of passive processes in membrane transport

diffusion and osmosis

diffusion

movement of solutes

osmosis

movement of water across semipermeable membrane; aquaporins

simple diffusion

no transport protein required

facilitated diffusion

transport protein required

channel mediated facilitated diffusion

ion moves though ion channel

carrier mediated facilitated diffusion

small polar molecule moved by carrier protein

active processes

require expenditure of cellular energy involved either the movement of a substance up its concentration gradient or the formation or loss of a vesicle

active transport

ion or small molecule moved against its concentration gradient

vesicular transport

involves a vesicle

primary active transport

energy source from ATP

secondary active transport

energy source from movement of another substance

symport

two substances moved in the same direction

antiport

two substances moved in opposite directions

exocytosis

vesicle releasing its contents from a cell

endocytosis

vesicle is formed as material is brought into a cell

phagocytosis

cellular eating

pinocytosis

cellular drinking

receptor-mediated endocytosis

targeted cellular process used to absorb specific extracellular molecules, such as hormones, nutrients, and proteins, by binding them to specialized membrane receptors

Rate of diffusion depends on

steepness and temperature

“Steepness” of concentration gradient in diffusion

• Measure of the difference in concentration between two areas
• Steeper gradient causes faster rate of diffusion

Temperature in diffusion

• Reflects kinetic energy or random movement
• Higher movement with higher temperature
• Faster rate of diffusion

Carrier-Mediated Transport

Large or polar molecules cannot diffuse across the membrane so carriers bring them across

Characteristics of the carriers

a. They are specific to a given molecule, although some can transport more than one substance
b. There may be competition for similar carriers or molecules.
c. Saturation – number of carriers is limited:
• Rate of transport increases with higher concentration
gradients until saturation is reached

normal blood glucose levels = ___ reabsorption

100

How do Osmosis and Tonicity go together?

Cell gains or loses water with osmosis along with a change in cell volume and osmotic pressure

Tonicity

ability of a solution to change the volume or pressure of a cell by osmosis

Relative concentration of solutions

• Isotonic
• Hypotonic
• Hypertonic

What kind of transport is the Na+/K+ Pump?

Active

What kind of pump is the Ca2+ pump?

An Active Transport Pump

tight junctions

impermeable junctions prevent molecules from passing through the intercellular space

demosomes

anchoring junctions bind adjacent cells together like a molecular “velcro” and help form an internal tension-reducing network of fibers

gap junctions

communicating junctions that allow ions and small molecules to pass for intercellular communication

Claudins and occludins

key transmembrane proteins that form tight junctions,

Cadherins

calcium-dependent linker proteins in the desmosomes

Connexons

protein assemblies that form the structural hemichannels of gap junctions, allowing direct chemical and electrical communication between adjacent cells

Electrical signals

Changes in the membrane potential of a cell

Methods of cell-to cell communication; Local communication

contact, electrical (gap junctions) and chemical Iparacrine or autocrine)

Chemical signals

● Secreted by cells into extracellular fluid (ECF)
● Responsible for most communication within the body
● Occur via ligands

Contact-dependent signals

require interaction between membrane molecules on two cells.

Gap junctions form

direct cytoplasmic connections between adjacent cells

Autocrine signals act on the ____

same cell that secreted them

Long distance cell to cell communication

chemical and electrical

Paracrine signals are secreted by

one cell and diffuse to adjacent cells.

what communication systems does the nervous system use?

chemical and electrical

Neurocrines are chemical signals secreted by which neurons?

▫ Neurotransmitters
▫ Neuromodulators
▫ Neurohormones

Neurotransmitters

chemicals secreted by neurons that diffuse across a small gap to the target cell.

Neurohormones

chemicals released by neurons into the blood for action at distant targets.

Cytokines

Peptides
▫ Synthesized and secreted by all nucleated cells in
response to stimuli
▫ Examples: Interleukins and TNFα

What signaling are cytokines involved in?

▫ Autocrine
▫ Paracrine
▫ Endocrine

There is an electrical potential on either side
of membranes when:

The # of ions is different across the membrane
The plasma membrane provides a resistance to ion flow

Role of Ion Channels

• To allow passage of ions in + out of cells

Passive, or leakage ion channels are ___

always open

Ligand-gated ion channels

open with binding of a specific NT (or ligand)

Voltage-gated ion channels

open and close in response to membrane potential changes
(i.e. once a specific voltage is reached)

Resting Membrane Potential

The separation of charge (–70 mV) across the membrane of a resting cell

when acetylcholine receptor sites are not occupied,

the sodium channels remain closed

when the acetylcholine binds to the receptor sites,

the sodium channels open

once acetylcholine binds to the receptor, it causes

sodium ions to diffuse through and enter the cell

what is a ligand?

a small molecule that binds to a membrane bound receptor

initially, bound to the alpha subunit of the g-protein is a(n)

GDP molecule

as a result of the ligand binding to its site

the G protein changes conformation and GTP replaces the GDP on the alpha subunit

the alpha subunit’s seperatism from the remainder of the G-protein can be repeated as long as the ligand is ____

bound to the receptor

inactivation of the alpha subunit occurs when its own phosphorylase activity removes a phosphate from the

GTP

an action potential arriving at the presynaptic terminal causes ____

voltage-gated calcium ion channels to open, and calcium ions diffuse into the cell

acetylcholine has which effect on the postsynaptic neuron

ligand-gated sodium ion channels open and sodium diffuses inward

the sodium-potassium pump uses bulk transport to move the ____

sodium and potassium ions

acetylcholine is actively transported from the presynaptic membrane to the postsynaptic membrane

false

which of the following occurs when an action potential arrives at the presynaptic terminal?

Synaptic vessels fuse with the plasma membrane and release acetylcholine

Which of the following statements about the resting membrane potential is TRUE?

The exterior of the cell has a net positive charge and the interior has a net negative charge.

During depolarization, which of the following statements about voltage-gated ion channels is true?

Na+ gates open before K+ gates

the nerve impulse is an electrical current that ___

travels along dendrites or axons

the sodium potassium ATPase pump is involved in

establishing the resting membrane potential

depolarization occurs because

more Na+ diffuses into the cell than K+ diffuses out of it

depolarization occurs because _____

sodium ions are diffusing into the cell through voltage-gated ion channels faster than potassium ions are diffusing of the cell

depolarization occurs

potassium ions continue to diffuse out of the cell after inactivation gates of the voltage-gated sodium ion channels begin to close

hyper polarization or after potential occurs because _____

the increased potassium ion permeability lasts slightly longer than the time required to bring the membrane potential back to it resting level

after the passage of the action potential, the sodium potassium pump helps to ____

reestablish the resting membrane potential

an action potential causes

the inside of the neuron cell membrane to become positively charged in reference to the outside

an action potential generates local currents that tend to ____ the membrane immediately adjacent to the action potential

depolarize

the absolute refractory period period causes action potential propagation to

occur in one direction

action potentials can travel in

both direction on the axon

threshold is the minimum current required for the

cell membrane to generate an action potential

ESPS and ISPS _____ act upon a post-synaptic neuron at the same time according to the all-or-none principle

cannot

IPSPs cause hyper polarization of a postsynaptic membrane. one way to accomplish this task would be either

open sodium channels or potassium channels

cells continually generate carbon dioxide and must get rid of it. the mechanism whereby carbon dioxide moves out of the cell is called ____

simple diffusion

what does the hydrolysis of ATP do in a Ca 2+ pump or Na+/K+ pump?

allows the ion to bind to the inside of the carrier

the extracellular matrix includes

collagen

two solutions are said to differ in ____ if they have different concentrations of non nonpenetrating solutes

tonicity

when sugar is mixed with water, equilibrium is reached when

the dissolved sugar molecules are evenly distributed throughout the solution

which of the following factors affects the rate of diffusion?

• size of molecules

• temperature and size

• steepness of the concentration gradient

• temperature

the molecules in a solid lump of sugar do not move

false

diffusion is one of the processes whereby material are exchanged

between a cell and its environment

simple diffusion is defined as the movement of ____

molecules from areas of higher concentration to areas of lower concentration

the sodium potassium pump functions to pump ____

sodium ions out of the cell and potassium ions into the cell

what is the source of energy used to power the sodium-potassium pump?

the breakdown of ATP

during one cycle, the sodium-potassium pump binds and moves ____

3 NA+ out of and 2 K+ into the cell