Cell Membrane Transport Quiz

Membrane Physiology

• Extracellular Fluid:
  • Na+: 142 mEq/L
  • K+: 4 mEq/L
  • Ca²⁺: 2.4 mEq/L
  • Mg²⁺: 1.2 mEq/L
  • HCO₃⁻: 103 mEq/L
  • Phosphates: 28 mEq/L
  • Glucose: 90 mg/dL
  • Amino Acids: 30 mg/dL
  • Cholesterol: 0.5 g/dL
  • Phospholipids: 0.5 g/dL
  • Neutral Fat: 2 to 95 g/dL
  • PO₂: 46 mm Hg
  • PCO₂: 35 mm Hg
  • pH: 7.4
  • Proteins: 2 g/dL
• Intracellular Fluid:
  • Na+: 10 mEq/L
  • K+: 140 mEq/L
  • Ca²⁺: 0.0001mEq/L
  • Mg²⁺: 58 mEq/L
  • HCO₃⁻: 4mEq/L
  • Phosphates: 4 mEq/L
  • Glucose: 10 to 20 mg/dL
  • Amino Acids: 200 mg.dL
  • PO₂: 20 mm Hg?
  • PCO₂: 50 mm Hg?
  • pH: 7.0
  • Proteins: 16 g/dL

Diffusion

• All molecules and ions in body fluids are in constant motion.
• Motion of particles is called "heat".
• Motion never ceases except absolute zero.
• Substances move from areas of high concentration to low concentration.

Transport through Cell Membrane

• Transport occurs through diffusion and active transport.
• Diffusion through the cell membrane involves random movement of substances either through intermolecular spaces within the membrane or through carrier proteins.
• Diffusion is caused by the kinetic energy of molecules.

Simple Diffusion

• Molecules or ions move through membrane openings or intermolecular spaces without interacting with carrier proteins.
• Rate is determined by amount of substance, velocity of kinetic motion, and number and size of openings in the membrane.

Facilitated Diffusion

• A carrier protein aids the passage of molecules or ions.
• Binding chemically with molecules/ions.
• Rate approaches maximum (Vmax) at high concentrations.
• Glucose and most amino acids are carried through this process.

Diffusion Through Protein Pores and Channels

• Protein channels are selective and have gates.
• Voltage Gating: Channels open/close in response to changes in electrical potential (ex. sodium channels respond to membrane losing negativity).
• Chemical (Ligand) Gating: Channels open/close upon binding of a ligand (ex. acetylcholine gated sodium channels).

Active Transport

• Movement of ions or substances against an energy gradient (uphill).
• Requires energy besides kinetic energy.
• Carrier protein initiates movement and involves ATP, usually.
• Substances actively transported include sodium, potassium, calcium, iron, hydrogen, chloride, iodide, urate, several sugars, and most amino acids.

Primary and Secondary Active Transport

• Primary active transport: Energy comes from direct ATP breakdown
• Secondary active transport: Energy originates from ion concentration gradients created by primary active transport. Uses ion movement to drive movement of other substances.

Sodium-Potassium Pump

• Pumps 3 sodium ions outward and 2 potassium ions inward.
• Important for maintaining sodium and potassium concentration differences across the membrane.
• Establishes a negative electrical voltage inside the cells.
• Controls cell volume.

Calcium Pumps

• Two pumps for calcium to the outside and inside of cells / organelles.
• Maintains low intracellular Ca²⁺ concentrations.

Hydrogen Pumps

• Primary active transport of hydrogen ions in gastric glands of the stomach, and distal tubules and cortical collecting ducts of kidneys.

Co-Transport and Counter-Transport

• Co-transport (symport): Coupled movement of two substances in the same direction.
• Counter-transport (antiport): Coupled movement of two substances in opposite directions.

Membrane Potentials and Action Potentials

• Resting Membrane Potential: About -70 mV in large nerve fibers
• Action Potentials: Rapid changes in membrane potential that spread along nerve fibers (nerve impulse).

Stages of Action Potentials in Nerves

• Resting Stage: Membrane is polarized (-70mV).
• Depolarization: Membrane becomes permeable to sodium ions; rapid inward diffusion of Na⁺ neutralizes negative potential; potential rises rapidly.
• Repolarization: Sodium channels close, potassium channels open; rapid outward diffusion of K⁺; potential returns to negative resting level.

Initiation of Action Potential

• Any event causes initial rise in membrane potential towards zero.
• Opening of voltage-gated sodium channels allows rapid Na⁺ inflow.
• Positive feedback cycle leading to action potential.

Threshold for Action Potential

• Sudden membrane potential rise of 15-30 mV required in large nerve fibers.
• -55mV often considered threshold.

Propagation of the Action Potential

• Action potential elicited at any point excites adjacent membrane resulting in propagation along the membrane.
• Depolarization process travels along the entire fiber in both directions (nerve/muscle impulse).

Re-establishing Sodium and Potassium Ionic Gradients

• After AP completes, sodium ions that diffused inside, and potassium ions that diffused outside, must be returned to original state.
• Achieved by action of the Na⁺-K⁺ pump.

Plateau in Some Action Potentials

• In some cases, the membrane does not repolarize immediately; instead, the potential remains on a plateau.
• Happens in heart muscle fibers where plateau lasts 0.2-0.3 seconds and causes sustained contraction.

Skeletal and Smooth Muscle Contraction

• Skeletal Muscle Fiber:
  • Contains myofibrils (with actin and myosin filaments).
  • Has a sarcolemmal and sarcoplasmic reticulum.
• Smooth Muscle:
  • Composed of fibers that are both shorter and smaller than skeletal muscle fibers.
  • Two types: multi-unit and single-unit.
• Multi-unit smooth muscle: Discrete, separate fibers that contract independently.
• Single-unit smooth muscle (unitary): Arranged in masses that contract as a single unit.

Sources of energy for muscle contraction

• Glycolysis: Breaks down glycogen in absence of oxygen, about 2.5x faster than oxidative source (sustains max contraction ~1 minute).
• Phosphocreatine: Combined energy of stored ATP and phosphocreatine can cause max muscle contraction for ~5-8 seconds.
• Oxidative metabolism: Combining oxygen with end products of glycolysis/carbohydrates, fats and proteins, creates ATP (sustains long term, hours)

Excitation of Skeletal Muscle (Neuromuscular Junction)

• Neuromuscular junction: Junction of myelinated nerve fiber and muscle fiber.
• **Acetylcholine (ACh) ** released when a nerve impulse reaches the junction.
• ACh Receptors: Acetylcholine binds to receptors on the muscle fiber membrane.
• Action potential generation: This initiates an action potential to the muscle fiber stimulating contraction.
• Re-excitation prevention: The enzyme acetylcholinesterase breaks down acetylcholine preventing continued muscle re-excitation.

Excitation-Contraction Coupling

• Transmission of the action potential from the outside to interior of the muscle fiber.
• T-tubules communicate with the extracellular fluid and contain extracellular fluid in their lumens.
• Potential changes spreads along the T tubules to the deep interior.
• T-tubule action potentials cause release of calcium ions into the muscle fiber initiating contraction.

Smooth Muscle Contraction

• Chemical Factors:
  • Low oxygen levels/High Carbon Dioxide/High H⁺ levels cause smooth muscle relaxation.
• Hormonal Factors:
  • Many hormones affect smooth muscle contraction. (ex. norepinephrine, epinephrine, acetylcholine, angiotensin)
• Smooth muscles can be stimulated to contract by nervous signals, hormonal, stretch of the muscle, and chemical environment changes.

Membrane Potentials and Action Potentials in Smooth Muscle

• Intracellular potential in smooth muscle is usually -50 to -60 mV.
• Action potentials of visceral smooth muscle (unitary):
  • Spike potentials: Similar to skeletal muscle.
  • Action potentials with plateaus: Repolarization is delayed for hundred milliseconds - long contractions (e.g., ureter and uterus).

Calcium Channels and Action Potential in Smooth Muscle

• More voltage-gated calcium channels.
• Sodium channels participate less in action potential (compared to skeletal muscle).
• Calcium ion flow to the interior of the fiber mainly responsible for the action potential (calcium channels open slower than sodium channels, causing plateau in some smooth muscles)

Spontaneous Generation of Action Potentials in Unitary Smooth Muscles

• Some smooth muscle cells are self-excitatory and generate their own action potentials (e.g., slow wave rhythm in digestive tract), without external stimuli.
• Slow wave rhythm originates from waxing and waning of calcium ions pumping out of the cell. Strong enough slow waves initiate action potentials, but waves alone cannot cause contraction.

Test your knowledge on the mechanisms of cell membrane transport including diffusion and facilitated diffusion. This quiz covers key concepts such as selective permeability, carrier proteins, and gated channels. Challenge yourself to understand how molecules move in and out of cells and the factors that influence these processes.