Diffusion Membrane Transport

Homeostasis vs. equilibrium

Homeostasis maintains dynamic steady states; the body is not necessarily at equilibrium.

Extracellular fluid (ECF)

Fluid outside cells; it acts as a buffer between cells and the external environment.

Intracellular fluid (ICF)

Fluid contained inside cells.

Osmotic equilibrium

ECF and ICF have equal total solute concentrations because water moves freely between them.

Chemical disequilibrium

Different solutes have unequal concentrations in ECF and ICF.

Electrical disequilibrium

A small separation of charge makes the inside of cells slightly negative relative to ECF.

Major ECF ions

Na+, Cl-, and HCO3- are more concentrated in extracellular fluid.

Major ICF ion

K+ is more concentrated inside cells.

Plasma

Liquid matrix of blood; contains more proteins and large anions than interstitial fluid.

Interstitial fluid (IF)

Extracellular fluid located between the circulatory system and cells.

Na+-K+-ATPase

Energy-using pump that returns leaked Na+ and K+ to their original compartments.

Total body water: standard 70-kg man

About 60% of body weight, or 42 L.

ICF fraction of body water

About 2/3 (67%) of total body water.

ECF fraction of body water

About 1/3 (33%) of total body water.

ECF distribution

About 75% interstitial fluid and 25% plasma.

Effect of sex on body water

Adult women generally have less water per kg because they have more adipose tissue.

Effect of age on body water

Infants have relatively more body water; body water decreases with aging.

Osmosis

Movement of water across a membrane in response to a solute concentration gradient.

Direction of osmosis

Water moves toward the more concentrated solution to dilute it.

Osmotic pressure

Pressure required to exactly oppose osmotic movement of water.

Molarity

Moles of dissolved solute per liter of solution (mol/L).

Osmolarity

the # of solutes present in a unit of volume of fluid.

Differences in osmolarity can drive movement of fluids or develop osmotic pressure

Osmolality

Osmoles of solute per kilogram of water.

Molarity-to-osmolarity formula

Molarity × particles per molecule = osmolarity.

Glucose dissociation

Glucose does not dissociate; 1 M glucose = 1 OsM.

NaCl dissociation in the body

At body temperature, NaCl has an approximate dissociation factor of 1.8.

Normal body osmolarity

About 280-296 mOsM; the text often rounds it to 300 mOsM.

Isosmotic

Two solutions have the same number of solute particles per unit volume.

Hyperosmotic

A solution has a higher osmolarity than the comparison solution.

Hyposmotic

A solution has a lower osmolarity than the comparison solution.

Tonicity

A physiological feature of a fluid that determines whether fluid will move into or out of cell

Hypotonic solution

Cell gains water and swells.

Hypertonic solution

Cell loses water and shrinks.

Isotonic solution

Cell volume does not change at equilibrium.

Osmolarity vs. tonicity

Osmolarity counts all particles; tonicity depends on nonpenetrating solutes and predicts cell volume.

Penetrating solute

A solute that can cross the cell membrane.

Nonpenetrating solute

A solute that cannot effectively cross the cell membrane; it contributes to tonicity.

Key nonpenetrating solute

NaCl is treated as functionally nonpenetrating in physiology.

Rule for hypotonicity

If the cell has more nonpenetrating solute than the solution, water enters and the cell swells.

Rule for hypertonicity

If the solution has more nonpenetrating solute than the cell, water leaves and the cell shrinks.

Rule for isotonicity

Equal nonpenetrating-solute concentrations cause no net water movement at equilibrium.

Hyposmotic solutions and tonicity

A hyposmotic solution is always hypotonic.

Isosmotic solutions and tonicity

May be isotonic or hypotonic, but not hypertonic.

Hyperosmotic solutions and tonicity

May be hypertonic, isotonic, or hypotonic depending on nonpenetrating solutes.

Solute-volume-concentration equation

S/V = C; rearrange as S = CV or V = S/C.

Bulk flow

Movement of fluid and dissolved solutes together because of a pressure gradient.

Diffusion

movement of molecules from an area of high concentration. To an area of low concentration.

Diffusion across the cell membrane depends on the size of molecules and membrane permeability.

Concentration gradient

Difference in concentration between two regions; provides potential energy for diffusion.

Diffusion equilibrium

Molecules still move randomly, but there is no net movement between regions.

Simple diffusion

Passive movement directly through the membrane or through open channels; no carrier is required.

Fick's law: concentration gradient

A larger concentration difference increases diffusion rate.

Fick's law: surface area

Greater membrane surface area increases diffusion rate.

Fick's law: diffusion distance

Greater diffusion distance decreases diffusion rate.

Membrane permeability

How easily a substance crosses a membrane; depends on membrane and molecule properties.

Lipophilic molecules

Cross the phospholipid bilayer readily by simple diffusion.

Lipophobic molecules

Usually require membrane proteins to cross the lipid bilayer.

Membrane transporter

Membrane-spanning protein that helps lipophobic substances cross membranes.

Channel protein

Forms a water-filled pore connecting ECF and ICF; mainly transports water and ions.

Carrier protein

Binds a substrate and changes conformation; never forms a continuously open pore.

Channels vs. carriers

Channels are faster and move small ions/water; carriers are slower but can move larger molecules.

Aquaporin

Membrane protein that forms water channels.

Open channel

Usually open; allows unregulated ion movement and may be called a leak channel or pore.

Gated channel

Usually closed and opens in response to a specific signal.

Chemically gated channel

Opens or closes when a chemical ligand binds.

Voltage-gated channel

Opens or closes in response to a change in membrane potential.

Mechanically gated channel

Opens or closes in response to physical forces such as stretch.

CFTR

ATP-gated chloride channel; defective or absent CFTR causes cystic fibrosis.

Uniport

Carrier that transports one kind of molecule.

Cotransport

Carrier moves more than one kind of molecule.

Symport

Cotransported substances move in the same direction.

Antiport

Cotransported substances move in opposite directions.

Facilitated diffusion

Passive carrier-mediated transport down a concentration gradient; does not directly require ATP.

GLUT transporters

Reversible facilitated-diffusion carriers for glucose and related hexoses.

Active transport

Moves substances against their concentration gradient and requires energy.

Primary active transport

Directly uses ATP as an energy source.

Secondary active transport

Uses potential energy stored in an ion concentration gradient.

Na+-K+-ATPase stoichiometry

Uses ATP to move 3 Na+ out of the cell and 2 K+ into the cell.

SGLT

Na+-glucose symporter that uses the Na+ gradient to move glucose against its gradient.

Transporter specificity

A transporter moves only one molecule or a group of closely related molecules.

Transporter competition

Related substrates compete for the same transporter binding site.

Transporter saturation

Transport reaches a maximum rate when all available transporters are occupied.

Phagocytosis

Actin-dependent process in which a cell engulfs large particles into vesicles.

Endocytosis

Vesicular process that brings material into a cell.

Exocytosis

Vesicles fuse with the cell membrane to release contents outside the cell.

Cell-to-cell communication

Cells communicate through contact-dependent, local, or long-distance signals.

Contact-dependent signaling

Signal molecule remains attached to one cell and binds a receptor on an adjacent cell.

Gap junction communication

Direct cytoplasmic connection that permits small molecules and electrical signals between cells.

Paracrine signal

Local chemical signal that acts on nearby cells.

Autocrine signal

Chemical signal that acts on the same cell that secreted it.

Long-distance signaling

Uses endocrine chemical signals or electrical/chemical neural signals to communicate across the body.

Hormone

Chemical signal secreted into blood for transport to distant target cells.

Neurotransmitter

Chemical signal released by a neuron onto a nearby target cell.

Neurohormone

Chemical signal released by a neuron into the blood.

Cytokine

Regulatory peptide associated with cell communication, especially immune and developmental signaling.

Signal pathway

Sequence from signal reception through transduction to the target-cell response.

Receptor

Protein that binds a specific signal molecule (ligand).

Target cell

Cell that possesses the appropriate receptor for a signal.

Intracellular receptor

Receptor in cytosol or nucleus used by lipophilic signal molecules that cross the membrane.

Cell-membrane receptor

Receptor for signals that cannot cross the lipid bilayer; activates intracellular pathways.

Signal transduction

Conversion of an extracellular signal into intracellular events.