Passive transport

Lore & Background

Passive transport is a fundamental process in cell biology, describing how substances cross cell membranes without the use of cellular energy. It relies on the second law of thermodynamics and Fick's first law, moving substances from areas of high concentration to low concentration, increasing the overall entropy of the system. The rate of passive transport depends on the permeability of the cell membrane, which is influenced by the organization and characteristics of membrane lipids and proteins. The four main kinds are simple diffusion, facilitated diffusion, filtration, and osmosis. In many cases, such as passive drug transport, the driving force can be the difference in degree of saturation rather than simply the concentration gradient.

Reader's Guide

Passive transport is significant because it is a primary mechanism by which cells exchange materials with their environment without expending energy. It underpins essential physiological processes such as gas exchange in the lungs, where oxygen and carbon dioxide diffuse across alveolar and capillary membranes following concentration gradients. Facilitated diffusion, exemplified by GLUT2 transporting glucose into intestinal cells, allows larger or charged molecules to cross membranes via specific transport proteins. Filtration, driven by hydrostatic pressure, is critical in kidney function, where the size of membrane pores determines which solutes pass. Osmosis, the net movement of water across a selectively permeable membrane, is vital for maintaining cell volume and turgor pressure, with isotonic, hypotonic, and hypertonic solutions affecting cell behavior. The speed of diffusion is governed by the law of diffusion, where mean squared displacement is proportional to time, making diffusion fast over short distances but slow over long ones, which influences cell size and metabolic strategies.

Did You Know?

• Passive transport does not require cellular energy; it relies on the second law of thermodynamics and Fick's first law.
• The four main kinds of passive transport are simple diffusion, facilitated diffusion, filtration, and osmosis.
• In passive drug transport, the driving force can be the difference in degree of saturation, not just the concentration gradient.
• The speed of diffusion is such that to diffuse a distance x takes time ~ x²/2dD, making diffusion fast over short distances but slow over long distances.