Membrane Transport — Interactive Mind Map

Root

Permeability

Passive Transport

Active Transport

Vesicular Transport

ⓘ Click nodes to expand/collapse · Hover ⓘ for details

🧬 Membrane Transport

Movement of Substances Through Membranes

▶ 🔬 Membrane Permeability

🚫 Impermeable — complete barrier, nothing crosses in either direction

✅ Freely Permeable — all substances cross freely (not compatible with life)

⚖️ Selectively Permeable — cell membranes use THIS: size · charge · lipid solubility i

Selective permeability is the basis of ALL transport mechanisms. The three selection criteria are: (1) Molecular size — smaller crosses easier, (2) Electric charge — charged ions are repelled by the hydrophobic core, (3) Lipid solubility — nonpolar molecules dissolve in the bilayer and cross freely.

▶ ⛰️ Passive Transport

▶ 💨 Simple Diffusion

🧱 Crosses lipid bilayer directly — no membrane protein needed

⚗️ Only small nonpolar molecules: O₂ · CO₂ · fatty acids · steroid hormones · H₂O

❌ Cannot cross: Na⁺ · K⁺ · Cl⁻ · Glucose · Amino acids (polar/charged)

📐 4 Factors affecting rate: Temperature · Molecule size · Medium density · Concentration gradient i

↑ Temperature → faster kinetic energy → faster diffusion. ↑ Molecule size → slower. ↑ Medium density → slower. ↑ Concentration gradient → faster net flux toward equilibrium.

▶ 💧 Osmosis

🌊 Water diffusion: HIGH [H₂O] → LOW [H₂O] (toward more solute)

📊 Osmolarity = total dissolved particles: Glucose=1 · NaCl=2 · MgCl₂=3 osmol/L i

NaCl dissociates into Na⁺ + Cl⁻ = 2 particles per molecule. MgCl₂ → Mg²⁺ + 2Cl⁻ = 3 particles. Higher osmolarity = more dissolved particles = less free water.

💪 Osmotic Pressure — force resisting water entry (used in clinical settings)

Tonicity: Isotonic = normal shape (0.9% NaCl) · Hypotonic = cell swells/bursts · Hypertonic = cell crenates

▶ 🚪 Facilitated Diffusion

▶ 〰️ Channel Mediated

🔑 Ligand-gated — channel opens when specific molecule binds

⚡ Voltage-gated — channel opens with membrane electrical change

🧪 Ion passage: Na⁺ · K⁺ · Cl⁻ · Ca²⁺ (electrochemical gradient driven)

▶ 🔄 Carrier Mediated

🎯 Specific Binding — one substrate type per carrier protein

🔃 Shape Change — conformational transition carries substrate across

📈 Saturation Point — maximum rate when all carriers occupied

🍬 Glucose via GLUT carriers — passive entry into most cells i

GLUT (Glucose Transporter) carriers allow glucose to enter most body cells passively — down the glucose gradient. Glucose is metabolized immediately, keeping intracellular levels low and ensuring continuous inward flux. Exception: intestines and kidneys use ACTIVE transport for glucose uptake.

▶ ⚡ Active Transport

▶ 🔋 Primary Active Transport

⚡ ATP Hydrolysis — direct energy from ATP powers the pump (ATPase)

▶ 🔁 Na⁺/K⁺ ATPase Pump

3 Na⁺ pumped OUT — against gradient (per cycle)

2 K⁺ pumped IN — against gradient (per cycle)

1 ATP consumed per pump cycle

⚡ Electrogenic → creates −70 mV resting membrane potential i

3 positive charges out vs 2 in = net negative inside. This −70 mV resting membrane potential is the foundation of all nerve impulses and muscle contraction. Nerve cells spend 70% of their energy on this pump alone.

Na⁺: ICF ~15 mEq/L (LOW) ↔ ECF ~142 mEq/L (HIGH)

K⁺: ICF ~150 mEq/L (HIGH) ↔ ECF ~5 mEq/L (LOW)

Ca²⁺: ICF ~0.0001 mEq/L (VERY LOW) ↔ ECF ~2.4 mEq/L (HIGH)

▶ 📡 Ca²⁺ ATPase Pump

Pumps Ca²⁺ OUT of cytoplasm (plasma membrane · ER · mitochondria)

Cytoplasmic Ca²⁺ kept extremely low: ~0.0001 mEq/L

Ca²⁺ rise = key signal → triggers muscle contraction, hormone secretion, exocytosis

▶ ⚗️ H⁺ ATPase Pump

Stomach parietal cells → secretes HCl (gastric acid)

Kidney tubules → acidifies urine

Mitochondria → H⁺ gradient drives ATP synthesis (oxidative phosphorylation)

▶ 🔗 Secondary Active Transport

⚡ Ion Gradients Energy — uses Na⁺ gradient stored by Na⁺/K⁺ pump (indirect ATP) i

No direct ATP used. Instead, the Na⁺ gradient created by the Na⁺/K⁺ pump acts as stored energy. Na⁺ flows down its gradient into the cell and this released energy drives a 2nd molecule uphill against its own gradient.

▶ ↓↓ Symport — SAME direction

🍬 Na⁺ + Glucose → BOTH into intestinal cells (absorption after digestion)

🧬 Na⁺ + Amino acids → BOTH in (same mechanism, different carrier)

🏥 SGLT2 Inhibitors — Diabetes drugs block Na⁺/glucose symport in kidney i

SGLT2 = the Na⁺/glucose symporter in kidney tubules that reabsorbs glucose from filtrate back to blood. Drugs empagliflozin, dapagliflozin, canagliflozin block it → glucose lost in urine → blood sugar falls. This is a direct clinical application of secondary active transport.

▶ ↓↑ Antiport — OPPOSITE direction

Na⁺ enters cell while Ca²⁺ is expelled simultaneously

Double safety: Ca²⁺ ATPase pump + Na⁺/Ca²⁺ antiport both maintain low cytoplasmic Ca²⁺

▶ 📦 Vesicular Transport

▶ 📤 Exocytosis — OUT of cell

① Package — cargo proteins packaged into secretory vesicles (Golgi apparatus)

② Migrate — vesicle travels along cytoskeletal filaments to plasma membrane

③ Fuse & Release — SNARE proteins mediate membrane fusion → contents released to ECF

Example: Insulin release from pancreatic β cells · Neurotransmitter release at synapses

▶ 📥 Endocytosis — INTO cell

▶ 💧 Pinocytosis — "Cell Drinking"

Small vesicle — takes in extracellular fluid + dissolved solutes

Occurs continuously in almost ALL eukaryotic cells

Function: nutrient intake, membrane recycling, ECF sampling

▶ 🦠 Phagocytosis — "Cell Eating"

Large phagosome — engulfs bacteria, viruses, cellular debris

ONLY in neutrophils and macrophages (specialized white blood cells)

🛡️ Innate immune defense: phagosome fuses with lysosomes → digestive enzymes destroy pathogen

⛰️ Passive Transport — No ATP

Simple Diffusion · Osmosis · Facilitated Diffusion
Direction: High → Low concentration
Spontaneous — like a ball rolling downhill

⚡ Active Transport — ATP Required

Primary (direct ATP) · Secondary (Na⁺ gradient)
Direction: Low → High concentration (uphill)
Requires specific pump or carrier proteins

📦 Vesicular Transport — Bulk

Exocytosis (out of cell) · Endocytosis (into cell)
For large cargo — proteins, bacteria, whole particles
Membrane vesicles carry the cargo