4.08 Diffusion

Simple Diffusion:

Definition:

• Diffusion is the passive movement of molecules or ions from a region of higher concentration to a region of lower concentration, down their concentration gradient, due to random molecular motion.
• It continues until equilibrium is reached, where concentration is balanced across the membrane.

Characteristics:

• Passive Process: Diffusion does not require energy (ATP).
• Equilibrium: Diffusion continues until molecules are evenly distributed across the membrane.

Types of Molecules and Diffusion

1. Nonpolar Molecules:
   • Small, nonpolar molecules like oxygen (O₂) and carbon dioxide (CO₂) diffuse freely through the lipid bilayer, as their hydrophobic nature allows them to pass directly through the membrane’s interior.
   • Examples: Oxygen and carbon dioxide, both of which are vital for cellular respiration and are exchanged across the cell membrane
2. Hydrophobic Molecules:
   • Small, nonpolar molecules, such as glycerol, alcohols, and steroid hormones, can diffuse through the lipid bilayer due to their compatibility with the hydrophobic membrane interior.
   • Examples: Steroid hormones like estrogen and testosterone, which diffuse through the membrane to reach intracellular receptors.

Factors Affecting the Rate of Diffusion

1. Concentration Gradient Steepness:
   • Description: The greater the concentration difference across the membrane, the faster the rate of diffusion, as molecules have a stronger directional movement.
2. Temperature:
   • Description: Higher temperatures increase the kinetic energy of molecules, causing them to move faster, which accelerates diffusion.
3. Nature of Molecules:
   • Size: Smaller molecules diffuse more quickly as they require less energy to move across the membrane.
   • Polarity: Nonpolar molecules diffuse more easily across the lipid bilayer, while polar molecules often require facilitated diffusion via channel proteins.
4. Surface Area:
   • Description: A larger surface area allows more molecules or ions to cross simultaneously, increasing the diffusion rate.
   • Adaptations: Cells may increase surface area through structures like microvilli (intestinal cells) or cristae (mitochondria) to enhance diffusion.
5. Distance:
   • Description: Shorter diffusion distances, such as thin membranes, enable faster diffusion rates.

Osmosis: A Special Case of Diffusion

• Definition: Osmosis is the passive movement of water molecules from a region of higher water concentration (low solute concentration) to a region of lower water concentration (high solute concentration) through a selectively permeable membrane.
• Why It’s Diffusion: Osmosis is a type of diffusion because it involves the movement of molecules (in this case, water) down their concentration gradient without energy.
• Aquaporins: Although water can slowly diffuse across the lipid bilayer on its own, it primarily uses aquaporins for rapid and efficient transport, especially in cells that require high water movement.
• Example: Osmosis is essential in maintaining cellular hydration and is crucial for kidney function, where aquaporins facilitate water reabsorption.

Importance of Surface Area-to-Volume Ratio

• Concept:
  • Cells maintain a high surface area-to-volume ratio to optimize diffusion.
  • As a cell’s size increases, its volume grows faster than its surface area, reducing the ratio.
  • This limitation impacts a cell’s ability to efficiently exchange materials across the membrane.

Implications for Cell Size:

• Cell Size Limitations: Cells are generally small to ensure efficient diffusion. Most eukaryotic cells are smaller than 50 micrometers, while prokaryotic cells are smaller, allowing for faster exchange across their membrane.
• Example Calculation:
  • For a cube with a side length of 1 unit:
    • Surface Area = 6 units²
    • Volume = 1 unit³
    • Surface Area-to-Volume Ratio = 6:1
  • As cell size increases, this ratio decreases, demonstrating the need for smaller cell sizes to maintain effective diffusion.

Diffusion Pathways in Cell Membranes

1. Direct Diffusion:
   • Nonpolar molecules, such as oxygen and carbon dioxide, diffuse directly through the lipid bilayer without assistance, moving in both directions to reach equilibrium.
2. Facilitated Diffusion through Protein Channels:
   • Description: Polar or charged molecules require protein channels to cross the membrane, as they cannot pass through the hydrophobic bilayer.
   • Examples:
     • Aquaporins: Facilitate water movement through osmosis.
     • Ion Channels: Permit ions (e.g., Na⁺, K⁺, Cl⁻) to pass, balancing membrane potential and enabling nerve and muscle functions.

Summary Table

Factors Affecting the Rate of Diffusion

Factors Affecting the Rate of Diffusion:

• Polarity: Nonpolar molecules diffuse readily through the lipid bilayer; polar molecules often require facilitated diffusion.
• Concentration Gradient Steepness: Higher differences in concentration increase diffusion rate.
• Temperature: Higher temperatures increase kinetic energy, speeding diffusion.
• Surface Area: Larger areas allow more molecules to pass simultaneously (e.g., microvilli in intestines, cristae in mitochondria).
• Molecule/Ion Properties:
• Size: Smaller molecules diffuse faster due to lower energy requirements.

Facilitated Diffusion

Facilitated Diffusion

• Definition:
  • Facilitated diffusion is the passive movement of molecules across the cell membrane through specific transport proteins, allowing substances that cannot directly cross the hydrophobic lipid bilayer to move into or out of the cell.
  • This process requires no energy input, as molecules move down their concentration gradient.

Why Facilitated Diffusion is Needed

• Certain molecules are unable to cross the lipid bilayer directly due to their size, charge, or polarity.
• Facilitated diffusion provides a way for these substances to pass through the membrane:
  • Large Polar Molecules: Examples include glucose and amino acids, which are too large and polar to diffuse directly through the membrane.
  • Ions: Charged molecules like Na⁺, K⁺, and Cl⁻ cannot pass through the hydrophobic interior of the bilayer without assistance.
• Transport Proteins create hydrophilic pathways, enabling these molecules and ions to diffuse across the membrane.

Types of Transport Proteins in Facilitated Diffusion

1. Channel Proteins
   • Structure: These proteins have water-filled pores that allow specific ions and small molecules to pass through.
   • Function: Channel proteins provide a direct pathway for ions to diffuse across the membrane. Channels are often selective, allowing only certain ions or molecules to pass.
   • Gated Channels: Many channel proteins are gated, meaning they can open or close in response to specific signals (e.g., voltage, ligand binding), controlling the passage of ions.
   • Examples:
     • Ion Channels: Potassium (K⁺) and sodium (Na⁺) channels in nerve cells play a crucial role in transmitting electrical signals.
     • Voltage-Gated Channels: In nerve cells, voltage-gated channels regulate Na⁺ entry during an action potential and K⁺ exit during repolarization.
   • Energy Requirement: Facilitated diffusion via channel proteins is passive; however, some gated channels may use ATP to open or close, depending on the type of gate.
2. Carrier Proteins
   • Structure: Unlike channel proteins, carrier proteins change shape to transport specific molecules across the membrane.
   • Mechanism:
     • Carrier proteins have a binding site that alternates between being open to the inside and outside of the membrane, allowing molecules to move across.
     • Carrier proteins are highly specific, only allowing certain molecules, like glucose, to bind and be transported.
   • Types:
     • Facilitated Diffusion Carriers: These carrier proteins enable passive movement down a concentration gradient without energy.
     • Pumps (Active Transport): While not part of facilitated diffusion, some carrier proteins act as pumps and require energy (ATP) to move substances against their concentration gradient.
   • Examples:
     • Glucose Transporter: Allows glucose to enter red blood cells.
     • ADP and ATP Transporter: Moves ADP into mitochondria and ATP out into the cytosol.

Mechanism of Facilitated Diffusion

1. Binding and Shape Change:
   • In carrier proteins, the molecule binds to the protein on one side of the membrane, triggering a rapid shape change (conformational change). This change moves the molecule to the other side, where it is released.
2. Triggering and Opening of Pores:
   • For channel proteins, specific molecules or signals can cause channels to open temporarily, allowing ions or molecules to diffuse from high to low concentration.

Key Characteristics of Facilitated Diffusion

• Passive Process: Does not require energy input; movement occurs down the concentration gradient, driven by the kinetic energy of molecules.
• Specificity: Each transport protein is specific to certain molecules or ions, ensuring selective permeability of the membrane.
• Rate-Dependent Factors:
  • Concentration Gradient: The steeper the gradient, the faster the diffusion rate.
  • Number of Transport Proteins: More available channel and carrier proteins increase the rate of facilitated diffusion.
  • Gate Status of Channels: For gated channels, the rate depends on whether the channels are open, allowing ion flow.

Factors Affecting the Rate of Facilitated Diffusion

1. Concentration Gradient: Facilitated diffusion occurs faster when there is a steeper concentration gradient.
2. Number of Transport Proteins: A higher number of channels and carriers increases the rate of diffusion.
3. Channel Gate Status: For gated channels, the rate depends on whether the channels are open.

Summary Table

.

Diffusion vs. Facilitated Diffusion:

1. Diffusion

• Definition: The passive movement of molecules from an area of higher concentration to lower concentration until equilibrium is reached.
• Mechanism: Molecules move directly across the phospholipid bilayer without assistance.
• Molecules Involved: Small, nonpolar molecules (e.g., oxygen, carbon dioxide) and some small polar molecules.
• Energy Requirement: No energy required (passive process).
• Rate Factors: Concentration gradient, temperature, surface area, and molecule size.
• Example: Oxygen diffusing into cells from the bloodstream.

2. Facilitated Diffusion

• Definition: The passive movement of molecules across the cell membrane with the help of transport proteins.
• Mechanism: Uses channel proteins or carrier proteins embedded in the membrane to assist movement of larger or charged molecules.
• Molecules Involved: Larger, polar molecules (e.g., glucose) and ions (e.g., Na⁺, K⁺).
• Energy Requirement: No energy required (passive process).
• Rate Factors: Concentration gradient, number of available transport proteins.
• Example: Glucose moving into cells through a glucose transporter protein.

Summary Comparison