Diffusion and Concentration Gradients

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.

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.

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 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.

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.

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.