Diffusion and Concentration Gradients

Diffusion and Statistical Probability

• Diffusion is fundamentally random motion.

  • This randomness is due to the constant thermal motion of molecules.

  • Each molecule moves independently, changing direction frequently due to collisions with other molecules.

• Despite its random nature, diffusion leads to a predictable outcome:

  • Molecules move from areas of high concentration to areas of low concentration.

  • This occurs because there are more molecules moving out of the high concentration area than moving into it, simply due to the higher number of molecules present.

• Eventually, molecules distribute themselves evenly throughout a solution.

  • At equilibrium, the rate of movement is equal in all directions, resulting in no net change in concentration.

• Diffusion rate is affected by:

  • Temperature: Higher temperatures increase the rate of diffusion.

  • Increased temperature means molecules have more kinetic energy and move faster.

  • Molecular weight: Smaller molecules diffuse faster than larger ones.

  • Smaller molecules experience less resistance and can move more quickly between other molecules.

  • Medium density: Diffusion is slower in denser media.

  • Denser media provide more obstacles, increasing the likelihood of collisions and slowing movement.

Membrane Barriers and Gradients

• A membrane barrier can establish a concentration gradient.

  • Membranes are selectively permeable, allowing some molecules to pass through while blocking others.

  • This selective permeability is due to the membrane's structure, including hydrophobic and hydrophilic regions.

• High concentration on one side.

• Low concentration on the other side.

• This gradient represents:

  • A decrease in entropy (increase in order).

  • Maintaining a concentration gradient requires energy to counteract the natural tendency for entropy to increase.

  • Storage of free energy.

  • The potential energy stored in the gradient can be harnessed to drive other processes, such as ATP synthesis.

• Types of membrane transport:

  • Passive transport: Movement across the membrane without energy input.

  • Examples include simple diffusion, facilitated diffusion, and osmosis.

  • Driven by the concentration gradient.

  • Active transport: Movement across the membrane requiring energy input.

  • Often involves transport proteins that use ATP to move molecules against their concentration gradient.

Free Energy and Molecular Movement

• Establishing a concentration gradient requires an input of free energy.

  • This energy is used to move molecules against their concentration gradient, which is thermodynamically unfavorable.

• Free energy is released when molecules move from the high concentration side to the low concentration side of the gradient.

  • This release of energy can be coupled to other cellular processes.

• This movement is still based on random motion.

  • Even though the overall movement is directional (down the gradient), individual molecules still move randomly.

• The amount of free energy released is related to the magnitude of the

Diffusion and Statistical Probability

• Diffusion is fundamentally random motion.
  • This randomness arises from the perpetual thermal motion of molecules.
  • Molecules move independently, frequently altering direction due to collisions.
  • Example: Imagine dye spreading in water; individual dye particles move randomly, yet collectively disperse evenly.
• Despite randomness, diffusion leads to predictable outcomes:
  • Molecules move from high to low concentration areas.
  • More molecules exit high concentration areas than enter, due to sheer numbers.
  • Example: Perfume sprayed in a room diffuses from the point of spraying to fill the entire room.
  • Eventually, molecules distribute evenly throughout a solution.
  • At equilibrium, movement rate is equal in all directions, causing no net concentration change.
  • Example: At equilibrium, dye molecules in water are uniformly distributed, with no discernible concentration differences.
• Diffusion rate is affected by:
  • Temperature:
  • Higher temperatures accelerate diffusion.
  • Increased kinetic energy means faster molecular movement.
  • Example: Hot tea diffuses sugar faster than cold tea.
  • Molecular weight:
  • Smaller molecules diffuse faster.
  • Less resistance allows quicker movement between other molecules.
  • Example: Helium diffuses faster than oxygen in the air.
  • Medium density:
  • Denser media slow diffusion.
  • More obstacles increase collision likelihood, slowing movement.
  • Example: Diffusion is slower in a gel than in water.

Membrane Barriers and Gradients

• A membrane barrier establishes a concentration gradient.
  • Membranes are selectively permeable, allowing passage to some molecules while blocking others.
  • Selective permeability stems from the membrane's structure, including hydrophobic and hydrophilic regions.
  • High concentration on one side; low concentration on the other.
  • Example: Cell membranes allow oxygen and nutrients to enter while preventing large molecules from escaping.
• This gradient represents:
  • Decreased entropy (increased order).
  • Maintaining a concentration gradient requires energy to counteract entropy.
  • Example: The sodium-potassium pump in nerve cells maintains ion gradients, crucial for nerve impulse transmission.
  • Stored free energy.
  • Potential energy in the gradient can drive other processes, like ATP synthesis.
  • Example: The proton gradient across mitochondrial membranes powers ATP production.
• Types of membrane transport:
  • Passive transport:
  • Movement across the membrane without energy input.
  • Includes simple diffusion, facilitated diffusion, and osmosis.
  • Driven by the concentration gradient.
  • Example: Oxygen moving from the lungs into the blood.
  • Active transport:
  • Movement across the membrane requiring energy input.
  • Often involves transport proteins using ATP to move molecules against their concentration gradient.
  • Example: The sodium-potassium pump maintains ion gradients in nerve cells using ATP.

Free Energy and Molecular Movement

• Establishing a concentration gradient requires free energy input.
  • This energy moves molecules against their concentration gradient, which is thermodynamically unfavorable.
  • Example: Energy from sunlight is used by plants to create a glucose concentration gradient during photosynthesis.
• Free energy is released when molecules move from high to low concentration.
  • This energy release can couple to other cellular processes.
  • Example: The energy from protons flowing down their concentration gradient is used to synthesize ATP in mitochondria.
• This movement remains based on random motion.
  • Despite directional movement down the gradient, individual molecules still move randomly.
  • Example: Although glucose diffuses from high to low concentration, individual glucose molecules still move randomly within the solution.
• The amount of free energy released relates to the gradient's magnitude.