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

receptor required

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

Osmosis and Tonicity

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 allows ions and small molecules to pass for intercellular communication

Claudins and occludins

key transmembrane proteins that form tight junctions,

Cadherins

calcium-dependent linker proteins 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
● 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 commutation 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

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