Introductory Biology

4.1 Passive Transport

Stars and galaxies tends to spread out. Substances tend to move down their concentration gradient (from an high concentration area to a low concentration are). Things tend to spread out until they have an equal concentration by diffusion.  Diffusion is performed by the random movement of particles.  Equilibrium is when the concentration between the substances are equal. Things that are non-polar and small can go through the cell membranes easily. For example, water and amonia can go through the  membrane  but sodium and glucose cannot.

Osmosis is the action when the diffusion of water across a selectively permeable membrane. There are hypotonic and hypertonic solutions. Hypertonic solutions have more solutes and less water. Hypotonic solutions have less solutes and more water. When an animal cell is isonic, it is just right. It is very important to keep your water balanced so if your blood is a little hypertonic, you must drink some water.

Transport proteins help move substances across the cell membrane. Ion channels are gated and are very selective. They create polar pores where ions can cross the cell membrane, without getting affected by the nonpolar lipid bilayer. Ion channels can open/close by itself. Diffusion across an ion channels is passive (from high -> low concentration) without using energy. Ions use channels to move across the membranes because of the influence of electrical charge and concentration gradient. facilitated diffusion needs a carrier protein to help molecules through the membrane. Facilitated diffusion is a passive transport (from a high -> low concentration without using energy).

This is the “3-step process”: FIrst, specific molecules combine with a carrier protein (based on shape and charge) and cause the carrier protein to change its shape. The change in the carrier proteins shape helps the molecule get to the other side of the membrane. After the molecules is released, the carrier protein goes back to its original shape. This is the facilitated diffusion process.

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