HUBS 191 Lecture 7: Diffusion, Osmosis, and Tonicity

• This pre-lecture material prepares for the lecture and assists with note-taking, but is not a substitute for the lecture.
• Every effort is made to ensure this pre-lecture material corresponds to the live-lecture, but there may be differences or additions.
• University of Otago, HUBS 191, Lecture 7, Active Cell Physiology
• Study guide related reading is Martini et al. Modules 3.14 (p. 162), 3.15 (p. 164), and 11.7 (p. 456)

Diffusion and Osmosis

• Diffusion involves the movement of molecules from high to low concentration.
• Osmosis involves the movement of water across a membrane to equalize solute concentration

Cell Membrane and Water Concentration

• The cell membrane divides body fluid volumes into intracellular and extracellular spaces.
• Water concentration is dynamic on both sides of the membrane, as is the concentration of solutes and ions

Tonicity

• Osmosis works to balance tonicity between the intracellular (ICF) and extracellular (ECF) spaces.
• The ECF and ICF are in balance, with the two solutions isotonic.
• Water loss from the ECF decreases volume and makes this solution hypertonic with respect to the ICF.
• An osmotic water shift from the ICF into the ECF restores osmotic equilibrium but decreases the ICF volume

Clinical Application

• A saline or Ringer's solution is relatively isotonic compared to the intracellular fluids of blood cells, making it ideal for preserving osmotic balance when administered intravenously.
• Imbalanced tonicity can affect the integrity of cells.
• Over-consumption of water can lead to water intoxication and death.

Chemical and Electrical Gradients

• Uneven distribution of molecules across the membrane creates a "chemical gradient".
• High Na+ outside the cell and low inside, and low K+ outside the cell and high inside
• Uneven distribution of charges across the membrane creates an "electrical gradient".
• Sodium, Potassium, and Calcium are cations
• Chloride and Proteins are anions
• Chemical and electrical gradients allow rapid signalling in excitable cells.
• Ions are highly driven to move down their concentration and electrical gradients to equilibrate the inside and the outside of the cell
• The cell membrane is semi-permeable and won't allow the ions through
• The moment a pathway opens, the ions will rush in/out along their gradient.
• The bigger the gradient, the faster and stronger the signal.

Passive and Active Channels

• Passive ion channels allow the movement of ions down their chemical/electrical gradients
• The sodium-potassium exchange pump maintains the chemical and electrical gradients across the cell membrane.
• The sodium-potassium exchange pump is an ACTIVE pump, meaning it uses energy (ATP) to move molecules
• It has to be active, because it's working against chemical and electrical gradients
• It moves three sodium ions out of the cell and brings two potassium ions into the cell for every ATP used.
• This creates chemical gradients for both sodium and potassium, as well as maintaining the electrical gradient

Resting Membrane Potential

• At rest, the intracellular space has more negative charge than the extracellular space, creating an "electrical gradient": Resting Membrane Potential.
• Excitable cells (nerves, muscles) use the movement of ions as a signal via depolarization
• Positive ions enter the cell when chemical stimulus opens sodium ion channels during a depolarization, acting as a signalling event for excitable cells.
• The positive ions are then removed during repolarization to return to the resting membrane potential.