Gradients in Physiology: Sodium and Potassium

Gradients in Physiology

Introduction to Gradients

• Gradients are fundamental to many chemical reactions in the body.
• We will focus on the gradients of sodium (Na+) and potassium (K+).

Cell Membrane and Selective Permeability

• Cells are often drawn as circles for simplicity, but this isn't always the actual shape.
• The purple line in the diagram represents the plasma membrane (cell membrane).
• The cell membrane is selectively permeable.
  • Allows certain substances to pass in and out.
  • Relevant here: sodium and potassium ions.

Sodium Gradient

• Sodium is represented as Na^+, with brackets indicating concentration: [Na^+].
• Ion: A charged atom.
• Extracellular fluid (ECF): fluid outside the cell.
• Intracellular fluid (ICF): fluid inside the cell (cytoplasm).
• Sodium concentration:
  • Much higher outside the cell (ECF).
  • Much lower inside the cell (ICF).
• Concentration refers to the ratio of a substance (e.g., sodium) to the amount of water in a space (ECF or ICF).

Movement Down the Chemical Gradient (Diffusion)

• Entities like sodium move down their chemical gradient (from high to low concentration).
• This is a passive process, requiring no energy.
• Sodium moves into the cell down its chemical gradient.
• Requires a channel in the selectively permeable membrane (not shown in the initial diagram).

Diffusion Defined

• Diffusion: The passive movement of any entity from an area of high concentration to an area of low concentration.
• Example: Skunk odorant molecules diffusing from high concentration (near the skunk) to lower concentrations.
• Diffusion has a limited range.
  • Skunk smell detectable over a distance, but not miles away.
  • Same limitation applies to sodium diffusion within the body.

Chemical Gradient

• Chemical gradient is synonymous with concentration gradient.
• Sodium's chemical gradient: high concentration outside the cell to low concentration inside the cell.

Potassium Gradient

• Potassium is represented as K^+.
• Potassium concentration:
  • High concentration inside the cell (ICF).
  • Low concentration outside the cell (ECF).
• Potassium moves passively out of the cell down its chemical gradient (from high to low concentration).

Clarification on Arrow Direction

• The direction of the arrow in the diagram doesn't necessarily indicate directionality (up or down).
• The arrow simply shows the movement of the ion near its channel.

Introduction to Electrical Gradients

• Electrical gradients consider the charge difference across the cell membrane.
• Resting cells generally have a negative charge inside and a positive charge outside.
• The human body is electrically neutral overall, but cells have a charge differential at the membrane.
• For now, the exact voltage is not important; focus on the charge difference.

Electrochemical Gradient of Sodium

• Sodium (Na+) is positively charged.
• The negative charge inside the cell attracts sodium.
• Opposite charges attract (physics/chemistry principle).
• Sodium is incentivized to move into the cell due to:
  • Chemical gradient (high to low concentration).
  • Electrical gradient (positive attracted to negative).
• Sodium moves passively into the cell down its electrochemical gradient (both chemical and electrical gradients).

Potassium's Electrical vs. Chemical Gradient

• Potassium (K+) is also positively charged (cation).
• Chloride (Cl^-) is a negatively charged anion.
• Potassium has a chemical gradient to leave the cell.
• However, potassium's electrical gradient would favor it staying inside the cell (positive charges repel).
• Potassium leaves the cell due to its strong chemical gradient, despite the opposing electrical gradient.

Summary of Sodium and Potassium Gradients

• Sodium has an electrochemical gradient favoring its movement into the cell.
• Potassium has only a chemical gradient favoring its movement out of the cell in resting cells.

Importance of Gradients in Physiology

• Gradients are the basis of many cellular processes.
  • Muscle cells.
  • Nerve cells.
  • Pancreatic beta cells (release insulin).
• Beta cell activity and insulin release are based on these gradients (to be discussed in a later video).