9/8 Signal Transmission and Osmolarity

Hydrophilic signaling molecule

- Binds to extracellular receptors

- Initiate second messenger cascades

- Alter ion channel conformations (open/close)

- Typically have a FAST effect and are metabolized/excreted quickly

Example of hydrophilic signaling molecule

Norepinephrine

Hydrophobic signaling molecule

- Binds to intracellular receptors

- Alters gene transcription and protein synthesis

- Typically have a SLOW effect and are metabolized/excreted slowly

Example of hydrophobic signaling molecule

Steroid hormones

Autocrine signaling

The target cell is also the secreting cell

Paracrine signaling

Secreting cell releases signals that act on nearby cells

Endocrine signaling

Secreting cell releases signals (hormones) that travel through the bloodstream to act on distant cells

Neurotransmission

Communication between neurons involving release of signals (neurotransmitters) across synapses that bind to nearby cells

Neurohormonal signaling

Hormones released into bloodstream by neurosecretory cells

Signal transduction

A series of molecular changes that converts a signal on a target cell's surface to a specific response inside the cell.

Why are second messengers important?

They can significantly amplify the first signal

Osmosis

Diffusion of water through a selectively permeable membrane

Aquaporins

Protein channels that allow water to diffuse across cell membranes

Osmolarity

Total solute concentration per liter of solution (includes both penetrating and non-penetrating solutes)

Milliosmole

mOsm, unit of osmolarity

Normal osmolarity of ECF and ICF

300 mOsm

How to calculate osmolarity?

Molarity * n (# of particles in solution)

Penetrating solute

A solute that freely crosses the cell membrane (e.g. urea, glucose, EtOH)

Non-penetrating solutes

Solutes that cannot cross the cell membrane (e.g. Na+, K+, Cl-)

Penetrating solute effects on osmolarity vs tonicity

Osmolarity: affected

Tonicity: unaffected (they can move freely to reach equilibrium)

Non-penetrating solute effects on osmolarity vs tonicity

Osmolarity: affected

Tonicity: affected (water must move to achieve equilibrium)

Hypoosmotic

< 300 mOsm

Isosmotic

= 300 mOsm

Hyperosmotic

> 300 mOsm

Tonicity

A solution's effect on cell volume by influencing water movement through a cell membrane. It is directly influenced by non-penetrating solutes.

Hypotonic solution

Solute concentration outside the cell is less than that inside the cell; water rushes into the cell and lyses the cell

Isotonic solution

Solute concentration outside the cell is equal to that inside the cell; no net water movement

Hypertonic solution

Solute concentration outside the cell is greater than that inside the cell; water rushes out of the cell and causes shrinkage

Can osmolarity be less than tonicity?

No, because tonicity is the part of a solution's total osmolarity that can cause water movement across a membrane.

Hemolysis

Rupture of red blood cells