Comprehensive Study Notes on Diffusion and Factors Affecting Diffusion Rate

Cells are surrounded by a cell membrane, which serves as the boundary controlling where molecules enter and leave the cell.
One key passive mechanism by which molecules move in and out of cells across the membrane is diffusion.

Diffusion is defined as the spreading out of particles resulting in a net movement from an area of higher concentration to an area of lower concentration.
The term "net movement" refers specifically to the overall movement of particles across a region.
Diffusion operates purely down a concentration gradient and requires no expenditure of metabolic energy.

Respiration Requirement: Cells continuously require oxygen ( $O_2$ ) for cellular respiration, a vital biochemical process carried out inside the mitochondria to generate cellular energy.
Bloodstream Transport: Oxygen is transported in the bloodstream from the lungs to tissues surrounding the cells.
Concentration Differential: Because oxygen is constantly brought by the blood and continuously consumed by mitochondria inside the cell, a higher concentration of oxygen exists outside the cell compared to a lower concentration inside the cell.
Direction of Net Movement: Oxygen molecules move into the cell by diffusion, travelling from the higher external concentration to the lower internal concentration.

Waste Gas Production: Cellular respiration inside the mitochondria generates carbon dioxide ( $CO_2$ ) as a waste gas byproduct.
Concentration Differential: Continuous respiration creates a higher concentration of carbon dioxide inside the cell relative to the lower concentration in the fluid/blood outside the cell.
Direction of Net Movement: Carbon dioxide molecules move out of the cell by diffusion, moving from the higher internal concentration to the lower external concentration.

Metabolic Waste Product: Urea is a nitrogenous waste product produced inside cells through cellular chemical processes.
Transport to Plasma: Urea diffuses out of the cells where its concentration is higher into the surrounding blood plasma where its concentration is lower.
Excretion: Once carried in the blood plasma, urea is transported to the kidneys, where it is filtered and excreted from the body.

Definition: The concentration gradient is the difference in concentration of a substance between two distinct areas (e.g., inside versus outside a cell membrane).
Effect of Gradient Size:
- A greater (steeper) concentration gradient leads to a faster rate of diffusion.
- A smaller concentration gradient leads to a slower rate of diffusion.

Relationship: The higher the temperature, the greater the rate of diffusion.
Particle Kinetic Energy: Increasing temperature supplies particles with more kinetic energy ( $E_k$ ), causing them to move faster and spread out more rapidly.

Relationship: The larger the surface area of the cell membrane surrounding a cell, the greater the rate of diffusion.
Membrane Surface Impact: A cell with a larger surface area allows a greater number of particles to cross the membrane simultaneously compared to a cell with a smaller membrane surface area.