Biology Chapter on Plasma Membrane

Movement of Substances Across the Plasma Membrane

• Living cells require substances from the external environment to perform life processes
• Cells also produce waste products that need to be removed
• Substances must be allowed to move into and out of the cell to maintain life processes
• The movement of substances into and out of a cell is regulated by the plasma membrane

Structure of Plasma Membrane

• The fluid mosaic model describes the plasma membrane's components:
  • Proteins are embedded within a phospholipid bilayer, creating a mosaic pattern that constantly changes
  • The phospholipid bilayer, proteins, and cholesterol are dynamic, not static, and form a flexible structure
  • The fluid nature of the membrane and protein flexibility contribute to the membrane's adaptability and flexibility

Components of Plasma Membrane

• Phospholipid bilayer:
  • Hydrophilic heads (attracted to water) face the extracellular and intracellular fluid
  • Hydrophobic tails (repel water) face each other 
• Proteins:
  • Channel proteins form channels or canals, and carrier proteins transport substances across the membrane
  • Glycoproteins and glycolipids are proteins and lipids that have carbohydrate chains attached

Permeability of Plasma Membrane

• The plasma membrane is a selectively permeable membrane, allowing some substances to pass freely, while others are restricted
• The phospholipid bilayer and proteins dictate the membrane's permeability for specific substances
• Hydrophobic (nonpolar) molecules can freely pass through the phospholipid bilayer
• Hydrophilic (polar) molecules are restricted due to their interaction with the hydrophobic tails

Passive Transport

• Passive transport doesn't require energy
• Substances move from high to low concentration (down the concentration gradient)
• Types of passive transport include:
  • Simple diffusion: Small, nonpolar molecules passively move across the membrane
  • Facilitated diffusion: Large or polar molecules move across the membrane with the assistance of membrane proteins
  • Osmosis: Water passively moves from areas of high water potential to low water potential

Active Transport

• Active transport requires energy (ATP)
• Substances move from low to high concentration (against the concentration gradient)
• Carrier proteins transport the substances across the membrane
• Includes the sodium-potassium pump and proton pumps

Gaseous Exchange

• Simple diffusion is involved in the exchange of gases between alveoli and blood capillaries
• Carbon dioxide and oxygen pass from the blood capillary into the alveoli (or vice-versa) based on the concentration gradients of these substances

Reabsorption

• Water and certain solutes are reabsorbed across membranes through the action of osmosis and facilitated diffusion.

Absorption

• Water absorption by plant root hair cells occurs via osmosis
• Fructose absorption from the small intestine into the villi by facilitated diffusion.
• Absorption of glucose and amino acids in the villi is against the concentration gradient and uses active transport.
• Mineral ions are absorbed from the soil by plant root hair cells using active transport.

Isotonic, Hypotonic and Hypertonic Solutions

• Isotonic solutions have the same solute concentration as the cell cytoplasm, which results in no net water movement
• Hypotonic solutions have a lower solute concentration than the cell cytoplasm, resulting in net water movement into the cell, causing swelling
• Hypertonic solutions have a higher solute concentration than the cell cytoplasm, resulting in net water movement out of the cell, causing shrinkage

Liposomes

• Liposomes are vesicles containing aqueous solutions enclosed by a phospholipid bilayer
• Used to deliver drugs and other substances
• Used in cosmetics due to their ability to encapsulate active ingredients