3.4 Permeability

Cell membranes are like bouncers at a club, letting some things in and keeping others out. This selective permeability is crucial for cells to maintain balance and function properly. It's all about controlling what goes in and out.

The membrane's structure, with its fatty core and protein gatekeepers, determines what can pass through. Small, non-polar molecules slip in easily, while big, polar ones need special help. It's a delicate dance of chemistry and biology.

Selective Permeability of Membranes

The Importance of Selective Permeability in Cellular Processes

Top images from around the web for The Importance of Selective Permeability in Cellular Processes

• 

  The Action Potential | Anatomy and Physiology I View original

  Is this image relevant?

• 

  The Plasma Membrane | Anatomy and Physiology View original

  Is this image relevant?

• 

  The Cell Membrane | Anatomy and Physiology I View original

  Is this image relevant?

• 

  The Action Potential | Anatomy and Physiology I View original

  Is this image relevant?

• 

  The Plasma Membrane | Anatomy and Physiology View original

  Is this image relevant?

1 of 3

Top images from around the web for The Importance of Selective Permeability in Cellular Processes

• 

  The Action Potential | Anatomy and Physiology I View original

  Is this image relevant?

• 

  The Plasma Membrane | Anatomy and Physiology View original

  Is this image relevant?

• 

  The Cell Membrane | Anatomy and Physiology I View original

  Is this image relevant?

• 

  The Action Potential | Anatomy and Physiology I View original

  Is this image relevant?

• 

  The Plasma Membrane | Anatomy and Physiology View original

  Is this image relevant?

1 of 3

• Selective permeability is the ability of a membrane to allow certain substances to pass through while restricting others based on their chemical properties
• The plasma membrane is selectively permeable, allowing the cell to control the entry and exit of molecules, maintaining homeostasis and facilitating essential cellular processes
• Selective permeability enables the cell to regulate its internal environment, maintain concentration gradients, and prevent unwanted substances from entering the cell
• The phospholipid bilayer of the plasma membrane is the primary determinant of selective permeability due to the hydrophobic nature of the lipid tails and the hydrophilic nature of the phosphate heads
• Membrane proteins, such as channels and carriers, also contribute to selective permeability by facilitating the transport of specific molecules across the membrane

Factors Influencing Selective Permeability

• The relative permeability of a substance depends on its polarity, size, and charge
• Small, non-polar molecules generally have higher permeability than large, polar molecules
• The hydrophobic core of the phospholipid bilayer allows small, non-polar molecules (oxygen, carbon dioxide) to pass through the membrane more easily than large, polar molecules
• Hydrophilic substances cannot easily pass through the hydrophobic core of the membrane and require specialized membrane proteins or other transport mechanisms to cross the plasma membrane

Hydrophobic vs Hydrophilic Substances

Hydrophobic Substances

• Hydrophobic substances are non-polar molecules that do not readily mix with water
• Examples of hydrophobic substances include:
  1. Lipids
  2. Steroids
  3. Certain gases (oxygen, carbon dioxide)
• The hydrophobic core of the phospholipid bilayer allows small, non-polar molecules to pass through the membrane more easily than large, polar molecules

Hydrophilic Substances

• Hydrophilic substances are polar molecules that readily mix with water
• Examples of hydrophilic substances include:
  1. Ions
  2. Amino acids
  3. Sugars
  4. Nucleic acids
• Hydrophilic substances cannot easily pass through the hydrophobic core of the membrane
• They require specialized membrane proteins or other transport mechanisms to cross the plasma membrane

Membrane Proteins for Transport

Channel Proteins

• Channel proteins form hydrophilic pores that allow the passage of specific ions or water molecules down their concentration gradients
• Aquaporins are channel proteins that selectively allow water molecules to pass through the membrane
• Ion channels are selective for specific ions (sodium, potassium, calcium) and can be gated by various stimuli, such as voltage, ligands, or mechanical stress

Carrier Proteins

• Carrier proteins, also known as transporters, bind to specific molecules and undergo conformational changes to facilitate their movement across the membrane
• Uniporters transport a single type of molecule down its concentration gradient
• Symporters cotransport two types of molecules in the same direction, using the concentration gradient of one molecule to drive the transport of the other
• Antiporters countertransport two types of molecules in opposite directions, using the concentration gradient of one molecule to drive the transport of the other against its gradient

ATP-Powered Pumps

• ATP-powered pumps, such as the sodium-potassium pump, use the energy from ATP hydrolysis to actively transport specific ions against their concentration gradients
• These pumps maintain the cell's electrochemical gradient, which is essential for various cellular processes (nerve impulse transmission, muscle contraction)