Concentration of Ions
Understanding Resting Membrane Potential and Action Potentials
Importance of Ion Flow in Membrane Potentials
- Resting membrane potential and action potentials are closely related to the flow of ions through channels when opened.
- It is necessary to comprehend the ion concentrations across the cell membrane, specifically:
- Intracellular fluid
- Extracellular fluid
Ion Concentrations Across the Cell Membrane
- Students are typically asked to accept specified ion concentrations without question.
- Personal acknowledgment: The author found it challenging to simply memorize these concentrations without comprehension.
- Aim: To provide a narrative or framework for understanding how ion concentrations arise between the inside and outside of the cell.
The Concept of Neutrality in Cells
- Within the cell:
- The environment is said to be electrically neutral.
- Author's prior explanation:
- The universe, in its entirety, tends to be electrically neutral as well.
- Example: Atoms within a defined volume, like those in a pen, represent molar concentrations.
Molar Concentrations and Avogadro's Number
- Avogadro's number: Approximately atoms/moles.
- Discussion of how large quantities lead to a net neutrality, both in the universe and within organelles such as cells.
Concept of Average in Classes of Students (Analogy)
- Analogy used: Average age of a selection of students can reflect the average age of an entire class (
- In a class of 30 or 40 students, taking the average of just a few students still approximates the class's overall average closely.
- This concept is applied to how we think about charges and ions within a cell.
Charge Separation Across Membranes
- Neutrality within the entire system does not mean charges are uniformly distributed.
- Charge separation occurs:
- Negative charges align on the inside of the membrane.
- Positive charges align on the outside near the membrane oxygenation.
- Concept led to the development of voltage across the membrane due to this charge separation.
Ion Concentrations Across Membranes
Potassium (K⁺)
- Concentration inside the cell: approximately 140 millimolar (mM).
- Concentration outside the cell: approximately 5 mM.
- Two measurement methods:
- Milliequivalents per liter (meq/L)
- Millimoles per liter (mM)
- Inside the cell potassium is approximately 150 meq/L and outside is about 4 meq/L.
- Importance of potassium in cell function:
- Higher concentration inside may be due to early biological processes and evolution.
- Potassium channels allow K⁺ to move based on concentration gradients.
Sodium (Na⁺)
- Concentration outside the cell: approximately 140 mM or 145 meq/L.
- Concentration inside the cell: approximately 15 mM or 12 meq/L.
- Sodium concentrations inform directional flow into the cell when channels open.
Chloride (Cl⁻)
- Concentration outside the cell: approximately 110 mM.
- Concentration inside the cell: approximately 4-30 mM.
- Cl⁻ remains a negatively charged ion and its distributions are important for understanding cellular resting potentials.
Calcium (Ca²⁺)
- Concentration outside the cell: approximately 1-2 mM.
- Concentration inside the cell: approximately 0.0001 mM.
- Calcium is significant as it carries a double positive charge, influencing charge dynamics across membranes.
Summary of Ion Movement and Concentrations
- Overall presentation of charges:
- Outside the cell: roughly 145 positive charges, balanced by 110 negative charges.
- Inside the cell: approximately 140-155 positive charges balanced by negatively charged proteins.
- Potassium exits the cell, creating a surplus of positive charges outside.
- Sodium, chloride, and calcium concentrations also contribute dynamically to charge states.
Final Notes on Concentrations and Ion Movement
- Knowledge of these concentrations seems crucial for understanding ion movements:
- K⁺ moves out of the cell.
- Na⁺, Cl⁻, and Ca²⁺ move into the cell.
- Awareness of these concepts will support deeper comprehension of membrane potentials and cellular behavior when channels open.