Cell Membrane Transport and Membrane Potential: Study Notes for Anatomy & Physiology

Cell Membrane Transport

Overview of Cell Membrane Transport

Cell membrane transport refers to the mechanisms by which substances move across the selectively permeable plasma membrane. These processes are essential for maintaining cellular homeostasis and function.

• Passive Transport: Movement of substances without energy input from the cell.

• Active Transport: Movement of substances requiring cellular energy, usually in the form of ATP.

• Cytosol: Fluid portion of cytoplasm inside cells.

• Extracellular Fluid (ECF): Fluid outside cells.

• Concentration Gradient: Difference in concentrations of a substance between two areas.

Diffusion

Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration, driven by kinetic energy.

• Occurs in air or water.

• Stops when equilibrium is reached, but movement continues.

• Passive: Uses molecules' own kinetic energy.

Rate of Diffusion

The rate at which diffusion occurs is influenced by several factors:

• Concentration Gradient: Rate increases with a steeper gradient.

• Molecular Size: Rate decreases with larger molecules.

• Distance: Rate decreases with greater distance.

• Temperature: Rate increases with higher temperature.

Membrane Proteins

Membrane proteins facilitate transport across the cell membrane. All are considered transporters and include:

• Channel Proteins: Form tubes for substances to pass through.

• Carrier Proteins: Bind and move substances across the membrane.

• Pumps: Use energy (ATP) to move solutes against their gradient.

• Na+/K+ ATPase Pump: An important pump that moves sodium and potassium ions across the membrane.

Types of Transport

Transport across the membrane can be classified as follows:

• Simple Diffusion (Passive): No membrane protein required; moves with concentration gradient.

• Facilitated Diffusion (Passive): Uses channel or carrier proteins; moves with concentration gradient.

• Primary Active Transport: Uses ATP to move substances against their concentration gradient (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses energy from the movement of another substance down its gradient (symport or antiport).

Transport Table

Type of Transport	Plasma Membrane Protein	Energy Source	Direction	Types of Molecules Moved
Simple Diffusion (passive)	None	Solute kinetic energy	With concentration gradient	Lipid soluble (non-polar), oxygen, carbon dioxide
Facilitated Diffusion (passive)	Channel/Carrier	Solute kinetic energy	With concentration gradient	Glucose, ions (Na+, K+, Ca2+), amino acids
Primary Active Transport	Na+/K+ Pump	ATP	Against concentration gradient	Ions: Na+ out, K+ in
Secondary Active Transport	Na+/K+ Pump, Carrier	ATP (indirect), concentration gradient of first substance	One substance sodium is transported down its gradient, second substance against its gradient	Glucose, Cl-, bicarbonate ions, H+
Carrier = symport (cotransport)	Carrier	Both molecules move same direction	Na+ in and glucose in
Carrier = antiport (countertransport)	Carrier	Molecules move in opposite directions	Na+ in and H+ out

Osmosis and Osmolarity

Osmosis

Osmosis is the movement of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This is the opposite of the diffusion rule for solutes.

• Water follows solute: water moves from low solute to high solute concentration.

• High solute outside cell (dehydration): cells shrink (crenate).

• Low solute outside cell: cells swell (lyse).

Osmolarity and Particles

Osmolarity is the concentration of solute particles per liter of solution, measured in osmoles (Osm).

• 1 mole of substance always has the same number of molecules ($6.02 \times 10^{23}$).

• Some substances split into particles in solution (e.g., NaCl splits into Na+ and Cl-).

• Osmolarity depends on the number of particles, not just molecules.

Osmolarity Table

Solution/ECF	Tonicity of solution/ECF	Water movement
300 mOsm	Isotonic	No net movement
500 mOsm	Hypertonic	Out of cell to higher solute (shrink/crenate)
200 mOsm	Hypotonic	Into cell to higher solute (swell/lyse)

Tonicity

Tonicity describes the effect a solution has on cell volume:

• Isotonic: Solution has same concentration as cell (300 mOsm); no net water movement.

• Hypertonic: Solution has higher concentration than cell; water moves out, cell shrinks.

• Hypotonic: Solution has lower concentration than cell; water moves in, cell swells.

Membrane Potential and Gradients

Membrane Potential

Membrane potential is the electrical gradient caused by separation of charges across the cell membrane. It is essential for nerve and muscle function.

• Cell membrane allows separation of electrical charge.

• Inside cell is more negative; outside is more positive.

• Most of cell and ECF is neutral.

• Ion concentrations are unequal:

  • Inside cell: High concentration K+

  • Outside cell: High concentration Na+

  • Role of Na+/K+ pump: Maintains the K+ and Na+ ion concentrations, creates gradients.

Types of Gradients

• Electrical Gradient: Charge inside cell negative; opposite charges attract; ions move into cell.

• Chemical Gradient: Concentration gradient; K+ moves out, Na+ moves in.

• Electrochemical Gradient: Sum of electrical and chemical gradients for an ion.

Summary Table: Electrochemical Gradients

Ion	Concentration in Cell	Concentration Out of Cell	Chemical Gradient	Electrical Gradient	Electrochemical Gradient	Equilibrium Potential (mV)
Na+	Low	High	Into cell	Attraction into cell	Into cell	+60
K+	High	Low	Out of cell	Attraction into cell	Out of cell	-94

Ion Channels and Gates

• Only way an ion can move across membrane is when there is an electrochemical gradient.

• Leak channel: Always open.

• Ligand-gated channel: Receptor opens/closes with binding of specific ligand (chemical).

• Voltage-gated channel: Opens/closes with change in membrane voltage.

Equilibrium Potential

Equilibrium potential is the specific membrane potential for each ion at which the electrical gradient exactly opposes the chemical gradient ($\text{electrochemical gradient} = 0$).

• Ion will cross the membrane until the membrane reaches equilibrium potential.

• Example: If cell were at +10 mV, K+ would have strong electrochemical gradient because +10 mV is far from -94 mV.

Resting Membrane Potential

The resting membrane potential is the voltage across the cell membrane when the cell is at rest. It is primarily determined by the movement of K+ through leak channels and is close to the K+ equilibrium potential.

• Neuron resting potential: Approximately -70 mV.

• Na+ (in) & K+ (out) electrochemical gradients.

• K+ diffuses through leak channels more easily (out).

Endocytosis and Exocytosis

Endocytosis

Endocytosis is the process by which cells take in large molecules or particles by engulfing them in vesicles.

• Phagocytosis: Cell eating; large molecules/particles (e.g., bacteria, debris).

• Pinocytosis: Cell drinking; small molecules/nutrients.

• Receptor-mediated endocytosis: Specific substances bind to receptors on membrane (e.g., cholesterol, iron, hormones).

Exocytosis

Exocytosis is the process by which cells release substances out of the cell via vesicles (e.g., hormones, neurotransmitters, enzymes).

Key Equations and Concepts

• Osmolarity: $\text{Osmolarity} = \text{moles/liter}$

• Number of molecules in a mole: $6.02 \times 10^{23}$

Summary of Water Movement Rules

• Water moves from lower solute concentration to higher solute concentration.

• Water moves from glucose side with 6 particles to the NaCl side with 12 particles.

Additional info:

• These notes cover foundational concepts in cell membrane transport and membrane potential, which are essential for understanding nerve impulse transmission, muscle contraction, and cellular homeostasis in Anatomy & Physiology.