#111 Cells Transportation- Passive Transport in Cells (English)

Passive transport is the cellular process of moving molecules an other substances across membranes. It differs from active transport In that it, does not involve any chemical energy. Passive transport maintains dynamic equilibrium of water, gases, nutrients and wastes between cells. The only things that can diffuse through passive transport need to be anything that is soluble in lipids, small monosaccharides, oxygen, sex hormones and carbon dioxide.

Simple Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration down a concentration gradient

Diffusion is the spreading out of particles in liquids and gases that happens because the particles are in continuous random motion. More particles move from area of high concentration to an area of lower concentration than move in the opposite direction. There is a net movement from the higher to lower concentration- a movement down the concentration gradient. Simple diffusion across membranes involves particles passing between the phospholipids in the membrane. It can only happen if the phospholipid bilayer is permeable to the particles. The centres of membranes is hydrophobic, so ions with positive or negative charges cannot easily pass through. Polar molecules, which have partial positive and negative charges over their surface, can diffuse at low rates between the phospholipids of the membrane. Small polar particles such as urea or ethanol pass through more easily than large particles.

Facilitated Diffusion

Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

Ions and other particles that cannot diffuse between the phospholipids can pass in or out of cells if there are channels for them through the plasma membrane. These channels are holes with a very narrow diameters. The walls of the channels consist of proteins. The diameter and chemical properties of the channel ensure that only one type of particle passes through.

Because these channels help particles to pass through the membrane, from a high concentration to a lower concentration.

HIB