Diffusion and Facilitated Diffusion

Diffusion

Diffusion is a fundamental process in cellular transport, involving the movement of molecules from areas of high concentration to areas of low concentration.

Definition: Diffusion is the movement of molecules down a concentration gradient until there is a uniform distribution.

Factors affecting the rate of diffusion include:

• Thickness of the exchange surface's membrane
• Surface area of the membrane
• Size and nature of the diffusing molecule
• Temperature

Facilitated Diffusion

Facilitated diffusion is a specialized form of diffusion that allows molecules unable to pass through the membrane naturally to be transported across.

Highlight: Facilitated diffusion is a passive process that occurs along the concentration gradient, requiring no energy input.

Two types of proteins facilitate this process:

1. Channel proteins: Molecules with pores lined with polar groups, allowing small polar molecules to pass through. These channels can open and close based on cellular needs.

2. Carrier proteins: Enable diffusion of large polar molecules like sugars and amino acids. They work by binding to a molecule, changing shape, and releasing the molecule on the other side of the membrane.

Example: Glucose transport into cells often occurs through facilitated diffusion using specific carrier proteins.

Active Transport

Active transport is an energy-requiring process that moves ions and molecules across the membrane against a concentration gradient.

Key features of active transport include:

• Movement from lower to higher concentration
• Utilization of intrinsic carrier proteins
• Energy requirement (usually in the form of ATP)
• Rate limitation by the number and availability of carrier proteins

Highlight: The difference between active transport and diffusion is that active transport requires energy and moves substances against the concentration gradient, while diffusion is passive and follows the concentration gradient.

The process of active transport involves:

1. Molecule or ion binding to a specific carrier protein
2. ATP transferring a phosphate group to the carrier protein
3. Carrier protein changing shape and transporting the molecule across the membrane
4. Release of the molecule or ion into the cytoplasm
5. Regeneration of ATP and return of the carrier protein to its original state

Vocabulary: Co-transport is a type of facilitated diffusion that brings molecules and ions into cells together on the same transport protein.

Osmosis and Water Potential

Osmosis

Osmosis is a special case of diffusion that specifically involves the movement of water molecules.

Definition: Osmosis is the diffusion of water molecules across a selectively permeable membrane from an area of higher water potential to an area of lower water potential.

Water Potential

Water potential is a crucial concept in understanding osmosis and cellular water balance.

Definition: Water potential is a measure of the free energy of water molecules and their tendency to move.

Key points about water potential:

• Pure water has the highest water potential (0)
• Adding solutes to water lowers its water potential, giving it a negative value
• Water always moves from an area of higher water potential to an area of lower water potential

Solute Potential

Solute potential is closely related to water potential and affects the movement of water in and out of cells.

Definition: Solute potential measures how easily water molecules move out of a solution.

Important aspects of solute potential:

• The more solute present, the more tightly water molecules are held
• Higher solute concentration results in a lower (more negative) solute potential
• Solute potential affects the overall water potential of a solution

Highlight: Understanding osmosis, water potential, and solute potential is crucial for comprehending cellular water balance, plant physiology, and many biological processes.