Membrane Dynamics

BIOL 3800.001 with Lisa Welch Spring 2026

What is the movement of water across a membrane?

Osmosis

Define Electrical disequilibrium

the differences in electrical charges across cell membranes inside of a cell

Define osmotic equilibrium

water moves freely across cell membranes to equalize solute concentrations

Define chemical disequilibrium

controlled distribution of certain substances between the ICF and ECF

Define diffusion

process of ions or molecules moving from areas of high concentration to lower concentration

Diffusion enables cells to acquire ___ and exchange___.

Food; waste products

What factors can change the rate of diffusion?

presence of other molecules or the concentration gradient within a solution

Define ions

electrically charged particles that conduct electricity

What tool is used to monitor the amount of ions in solution?

Conductivity probe

Define Isotonic solution

contains equal amounts of dissolved solutes as the surrounding environment.

How does the presence of salt affect the conductivity of water?

As salt diffuses into water, conductivity of the water increases.

What compartment contains about two-thirds of body fluids?

Intracellular fluid (ICF)

What compartment contains one-third of body fluids?

Extracellular fluid (ECF)

What two compartments can we further divide ECF into?

Interstitial fluid and blood plasma

Where does interstitial fluid lie?

Between the circulatory system and the cell

___plays a critical role in transporting oxygen, hormones, and nutrients.

Blood plasma

What molecule is more concentrated in the ECF, especially in plasma?

Bicarbonate

Which molecule is a key buffer to maintain pH stability in the blood?

Bicarbonate

The concentration is greater in the ECF plasma than it is in the ICF, thus acting as a key to maintain osmotic pressure differences. What molecule is this true statement referring to?

Proteins

Define osmosis

the movement of water across a semipermeable membrane in response to solute concentration gradients.

Define aquaporins

specialized protein channels

Define osmotic pressure

the force required to prevent osmosis.

Define osmolarity

Number of osmotically active particles in a solution

How does the number of particles in a solution effect osmolarity?

More particles will result in higher osmolarity and strengthen its ability to draw water through osmosis

How does osmotic pressure help maintain fluid balance?

It regulates water movement between compartments

What are the units for molarity?

Moles per Liter

Define Isosmotic

Both solutions have the same osmolarity with an equal number of osmotically active particles

Define Hyperosmotic

Higher osmolarity with more solute particles when in comparison to another solution

Define Hypoosmotic

Lower osmolarity with fewer solute particles than the other solution.

Define Tonicity

solution's ability to change a cell's volume by affecting water movement through osmosis, determined by the concentration of solutes outside the cell compared to inside

Define Isotonic

solute concentration inside the cell is equal to the concentration outside

Define Hypertonic

solute concentration is higher outside the cell, water moves out of the cell causing it to shrink

Define Hypotonic

solute concentration is lower outside the cell, water enters the cell leading to swelling and possible lysis (bursting)

Why is it important to understand osmolarity?

Can help preparing fluids that match the osmolarity of blood plasma to avoid complications like cell shrinkage or swelling

Define solubility

The degree to which a molecule is able to dissolve in a liquid

Define concentration

A measurement of the amount of substance per unit volume of a mixture

Define solution

the combination of substances dissolved in a liquid.

Define solute

any substance that dissolves in liquid

Define solvent

liquid into which substances dissolve, for biological systems it’s water.

How do you calculate concentration?

Solute amount / Volume of solution

How is molecular mass calculated?

using the chemical formula of a substance

Define moles

an expression of the number of molecules, without regard to their weight

Define mass

the weight of the solute before its dissolves

What are the units used for ions?

Equivalents

What pieces of information do you need to know in order to calculate the molecular weight of a substance?

atomic mass of each element & the number of atoms of each element

If you were to measure out 1/1,000,000 of a liter, how would you express and abbreviate this volume measurement?

Microliter (μL)

What would you need to do to make a 13% solution of sodium chloride (NaCl)?

Add 26 g of sodium chloride to a beaker and add water to make a final volume of 200 mL.

T/F: suppose a hypothetical salt, AB, completely dissociates into A⁺ and B^- in water. One mole of AB in water would be one molar (1 M), but it would be 2 osmolar (2 OsM) solution, because there would be 1 mole of each ion dissolved in that solution.

True

Presuming that the salts listed below completely dissociate in water, which of the following solutions below has the highest osmolarity?

7 mM sodium chloride (NaCl)

Which body fluid compartment has the smallest volume?

Plasma

Water molecules can cross a cell membrane to enter a cell by several different mechanisms. Which answer choice is NOT a main way that water is transported into a cell?

Active transport

The permeability of a membrane to a particular solute is related to which two variables, and what is the relationship?

It is directly proportional to lipid solubility and inversely proportional to the size of the solute.