PASSIVE TRANSPORT

Introduction to transport mechanisms:

• On basis of gradiant (along concentration gradient/against concentration gradient), cell membrane transport:
• Divided into passive or active.

Passive transport:

• Physical process depending entirely on physical factors like concentration gradient, electrical gradient, pressure gradient etc.,
• In/along direction of concentration gradient i.e.,Downhill movement.
• i.e., from high concentration to low concentration.
• Not a biological process (involving enzyme actvitiy) —> Does not require energy.
• Hence, No requirement of energy expenditure.

Eg:

• Without carriers: Simple diffusion, osmosis
• With carrier:  Facilitated diffusion.

TYPES OF PASSIVE TRANSPORT:

1. Diffusion:

• Substances diffuse from higher concentration to lower concentration end. 
• i.e., in/along direction of concentration (chemical) gradient.
• Due to random molecular movement.

Two subtypes:-  

• Simple diffusion
• Facilitated diffusion.

Simple diffusion:

• Diffusion of molecule across membrane following concentration (chemical) gradient, but without help of any carrier protein.

Rate of diffusion:

• Depends on following factors:

Rate of diffusion directly proporlionate to:

• The concentration (chernical)  gradient.
• Cross-sectional area of membrane through which diffusion takes place.
• Lipid solubility of substance.

Rate of diffusion is inversely proportionate to:

• Thickness of diffusion membrane.
• Size of particle
• Charge or polarity of substance
• Simple diffusion is favoured by small size, lipid solubility & absence of  polarity (non-polar substance) & charge  (neutral molecule through a thin, large membrane where concentration gradient is more.

Fick’s law of diffusion:

• Describes rate of diffusion down (along) a concentration gradient

Rate of diffusion (dv/dt):

• Where,
• D = Diffusion coefficient.
• A = Area of diffusion membrane.
• dc/dx = Concentration gradient.
• “Minus sign” indicates direction of dlffiusion.

2. Facilitated diffusion:

• Is diffusion of large water soluble molecule by a carrier protein.
• Eg: Transport of glucose, amino acids & urea.