Chapter 5 ± Membrane Dynamics BGYB30
Because of the free movement of water, the extracellular and intracellular compartments can reach a
state of osmotic equilibrium, in which the total amount of solute per volume of fluid is equal on the two
sides of the cell membrane. At the same time, however, the body is in a state of chemical
disequilibrium, in which the major solutes are more concentrated in one of the two body compartments
than in the other.
-> Na+, Cl- and HCO3
- are more concentrated in extracelullar fluid than in intracellular fluid.
-> Whereas, K+ are more concentrated inside the cell.
-> Ca2+ is more concentrated in the extracelullar fluid than in the cytosol, although many cells store Ca2+
inside organelles such as the endoplasmic reticulum and mitochondria.
- The inside of the cell is slightly negative relative to the extracelullar fluid.
- Homeostasis is not the same as equilibrium. The intracellular and extracelullar compartments of the
body may be in osmotic equilibrium, but they are also in chemical and electrical disequilibrium.
of K+ ion and low concentration of Na+, and Cl- ions.
- Cell membranes are selectively permeable; the lipid and protein composition of a given cell membrane
determines which molecule will enter the cell and which will leave.
- The size of the molecule and its lipid solubility influence its movement across cell membranes.
- Passive transport }v[µ]Z]vµ}(vPÇ
- Active transport requires the input of energy (e.g. high-energy phosphate bond of ATP)
- Diffusion is the movement of molecules from an area of higher concentration of the molecules to an
area of lower concentration of the molecules.
1- Diffusion is a passive process
2- Molecules move from an area of higher concentration to an area of lower concentration (along
a chemical gradient)
3- Net movement of molecules occurs until the concentration is equal everywhere
4- Diffusion is rapid over short distances but much slower over long distances
5- Diffusion is directly related to temperature (higher temp, molecules move faster)
6- Diffusion rate is inversely related to molecular size (the larger the molecule, the slower its
- Diffusion is the passive movement of uncharged molecules down their concentration gradient due to
random molecular movement.
- Diffusion directly across the phospholipids bilayer of a membrane is called simple diffusion.
- Simple diffusion across membranes is limited to lipophilic molecules.
- The vast majority of solutes move across membranes with the help of membrane proteins, a process