A factor favouring the transport across a cell membrane
• Simple diffusion refers to the diffusion of the molecule across the membrane following a concentration gradient or chemical gradient, but without the help of any carrier protein.
• The rate of diffusion depends on the following factors : -
A) The rate of diffusion is directly proportionate
i) The concentration (chemical) gradient
ii) The cross-sectional area of the membrane through which diffusion takes place
iii) Lipid solubility of the substance
B) The rate of diffusion is inversely proportionate 
i) The thickness of the diffusion membrane
ii) Size of the particle
iii) Charge or polarity of the substance
• So, simple diffusion is favoured by small size, lipid solubility and absence of polarity (non-polar substance) and charge (neutral molecule) through a thin, large membrane where the concentration gradient is more.
Polar substance
High concentration gradient
Thick membrane
Big particle size