3.2 - Concept of Movement of Substances across a Plasma Membrane

Classification of Substances Moving Across the Membrane

Substances that can pass through the phospholipid bilayer freely

• Lipid-soluble substances: Fatty acids, glycerol, steroid compounds (such as progesterone, estrogen, testosterone), and fat-soluble vitamins (Vitamins A, D, E, K).

• Small uncharged and non-polar molecules: Oxygen ($O_2$), carbon dioxide ($C{O}_2$), and water molecules ($H_{2}O$, which are polar but small enough to squeeze through the bilayer).

Substances that CANNOT pass through the phospholipid bilayer freely

• Large uncharged polar molecules: Glucose and amino acids (require carrier proteins).

• Ions / Charged particles: Sodium ions ($N{a}^{+}$), potassium ions ($K^{+}$), calcium ions ($C{a}^{2+}$), and chloride ions ($C{l}^{-}$) (require channel proteins).

The movement of substances across a plasma membrane occurs through passive and active transport.

Passive Transport

• Does not require metabolic energy (ATP).

• Occurs down the concentration gradient (from a region of higher concentration to a region of lower concentration).

Simple Diffusion

• The net movement of molecules or ions from a region of higher concentration to a region of lower concentration until dynamic equilibrium is reached.
• Examples: Gaseous exchange of oxygen and carbon dioxide at the alveolus and body cells.

Osmosis

• The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a selectively permeable membrane.

• Examples: Absorption of water by plant root hair cells.

Facilitated Diffusion

• The movement of large or charged substances down a concentration gradient with the aid of transport proteins (carrier or channel proteins). No energy (ATP) is required.

• Channel Protein Mechanism: Forms a specific channel that allows specific ions to pass through.

• Carrier Protein Mechanism:

  1. The target molecule (e.g., glucose) binds to the active site of the carrier protein.
  2. The carrier protein changes shape.
  3. The molecule is released to the opposite side of the membrane, and the protein returns to its original shape.

Active Transport

• Requires metabolic energy (ATP) generated during cellular respiration.

• Occurs against the concentration gradient (from a region of lower concentration to a region of higher concentration).

• Requires specific carrier proteins (often called pumps).

• Mechanism:

  1. The substance binds to the carrier protein.
  2. An ATP molecule binds to the carrier protein and splits into ADP and phosphate, releasing energy.
  3. The phosphate group binds to the carrier protein, causing it to change shape.
  4. The substance is pumped to the opposite side.

Examples of Active Transport in Organisms

• Sodium-Potassium Pump: Pumps $3N{a}^{+}$ ions out of the cell and $2K^{+}$ ions into the cell.
• Proton Pump: Pumps $H^{+}$ ions out of the cell (e.g., in stomach parietal cells or plant root cells).
• Absorption of mineral ions by plant root hair cells.
• Absorption of glucose and amino acids in the villi of the human ileum.