Q.What are the factors affecting the rate of diffusion?

Diffusion is the movement of molecules or ions from a region of their higher concentration (or higher chemical potential/free energy) to a region of their lower concentration, driven purely by the random kinetic motion of particles, with no expenditure of metabolic energy. Several physical factors govern how fast this movement occurs:

• Concentration gradient (density gradient): the steeper the gradient between two regions, the faster the net diffusion — diffusion is directly proportional to the gradient.
• Temperature: higher temperature gives particles more kinetic energy, so they move and collide more rapidly, speeding up diffusion.
• Pressure: an increase in pressure increases the frequency of molecular collisions and can drive faster movement, especially for gases.
• Permeability of the membrane (if one is crossed): the more permeable the membrane to the diffusing substance, the faster diffusion proceeds; thickness and composition of the membrane also matter.
• Size/molecular mass of the diffusing substance: smaller, lighter molecules diffuse faster than larger, heavier ones (diffusion rate is inversely related to the square root of molecular mass, as in Graham's law for gases).
• Medium of diffusion: diffusion is faster through air than through a liquid, and faster through a liquid than through a solid.

Diffusion is the spontaneous, passive movement of molecules, ions or particles of a substance from a region where they are present in higher concentration to a region of lower concentration, until the concentration becomes uniform throughout the available space. It occurs because of the inherent kinetic energy of particles and requires no input of metabolic (ATP) energy; it can take place in gases, liquids and solids, and — unlike osmosis — does not require a membrane.

The rate at which diffusion proceeds is governed by several factors:

• Concentration (density) gradient: Diffusion is fastest when the difference in concentration between the two regions is largest. As the gradient decreases (concentrations approach equilibrium), the net rate of diffusion slows down and eventually stops once equilibrium is reached.
• Temperature: Diffusion rate increases with rising temperature because molecules possess greater kinetic energy at higher temperatures, so they move faster and collide more frequently, spreading out more quickly.
• Pressure: Higher pressure increases the frequency of molecular collisions, which can increase the rate of diffusion, particularly of gases.
• Permeability of the membrane: When diffusion involves crossing a membrane (for example, a cell membrane), the rate depends on how permeable that membrane is to the diffusing substance — its thickness, lipid composition, and the presence of channel or carrier proteins all affect this.
• Size and molecular mass of the diffusing substance: Smaller and lighter molecules diffuse faster than larger, heavier ones. For gases this follows Graham's Law — the rate of diffusion is inversely proportional to the square root of the molecular mass.
• The medium in which diffusion occurs: Diffusion is fastest in air (gaseous medium), slower in liquids, and slowest in solids, because particle mobility decreases as the medium becomes denser.

Because diffusion does not need a carrier or energy input, it is an important mechanism for the short-distance movement of gases (like O₂ and CO₂ during photosynthesis and respiration) and other small molecules within and between plant cells.

Recall the definition of diffusion as passive, energy-independent movement down a concentration gradient, then list each physical variable that changes how fast that movement happens: the gradient itself, temperature, pressure, membrane permeability (if relevant), molecular size/mass, and the density of the medium. Relate each factor logically to particle kinetic energy or collision frequency rather than just naming it.

• Confusing diffusion with osmosis and stating that a semi-permeable membrane is always required — diffusion can occur without any membrane at all.
• Forgetting that diffusion is a passive process, and wrongly including ATP/energy expenditure as a factor.
• Omitting the medium (gas vs liquid vs solid) and molecular size/mass as factors, and listing only concentration gradient and temperature.