Transport of substances across membranes

The cell membrane is selectively permeable, allowing certain substances to pass while restricting others. Transport mechanisms are classified as passive (no energy required) or active (requires energy), depending on whether they move substances along or against a concentration gradient.

---

I. Passive Transport

Definition: Movement of substances down their concentration gradient (from high to low concentration) without the use of energy.

1. Simple Diffusion

• Involves small, nonpolar molecules (e.g., O₂, CO₂).
• Movement directly through the lipid bilayer.
• No protein channels or carriers involved.

2. Facilitated Diffusion

• Involves larger or polar molecules (e.g., glucose, ions).
• Requires membrane proteins:
  • Channel proteins: form pores for ions or water.
  • Carrier proteins: bind and transport specific molecules by changing shape.
• Still passive—no energy required.

3. Osmosis

• Special case of diffusion involving water molecules.
• Water moves across a semipermeable membrane from an area of low solute concentration to high solute concentration.
• Often uses specialized channels called aquaporins.

---

II. Active Transport

Definition: Movement of substances against their concentration gradient (from low to high concentration) using energy in the form of ATP.

1. Primary Active Transport

• Direct use of ATP to move molecules.
• Example: Sodium-Potassium Pump (Na⁺/K⁺-ATPase):
  • Pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions in.
  • Maintains electrochemical gradients essential for nerve impulse transmission.

2. Secondary Active Transport (Cotransport)

• Does not use ATP directly.
• Utilizes the energy from the movement of one molecule (often Na⁺) down its gradient to move another molecule against its gradient.

Types:

• Symport: Both molecules move in the same direction.
• Antiport: Molecules move in opposite directions.

---

III. Bulk Transport (Vesicular Transport)

Used for transporting large molecules or large quantities of substances. Requires energy (ATP).

1. Endocytosis (into the cell)

• Phagocytosis: Uptake of large solid particles ("cell eating").
• Pinocytosis: Uptake of fluid and dissolved solutes ("cell drinking").
• Receptor-mediated endocytosis: Specific uptake of substances via receptor-ligand binding (e.g., cholesterol uptake via LDL receptors).

2. Exocytosis (out of the cell)

• Transport of substances out of the cell via vesicle fusion with the plasma membrane.
• Important in secretion of hormones, enzymes, and neurotransmitters.

---

IV. Comparison Table

Transport Type	Energy Required	Direction of Movement	Involves Proteins	Examples
Simple Diffusion	No	High → Low concentration	No	O₂, CO₂
Facilitated Diffusion	No	High → Low concentration	Yes	Glucose, Na⁺, Cl⁻
Osmosis	No	Toward higher solute	Sometimes (aquaporins)	Water
Primary Active Transport	Yes (ATP)	Low → High concentration	Yes	Na⁺/K⁺ pump
Secondary Active Transport	Indirect (ion gradient)	Low → High concentration	Yes	Na⁺/glucose symport, Na⁺/Ca²⁺ antiport
Endocytosis	Yes	Into the cell	No	Phagocytosis, receptor-mediated uptake
Exocytosis	Yes	Out of the cell	No	Hormone or neurotransmitter release