Chapter 5 – Membrane Dynamics BGYB30
Mass Balance & Homeostasis:
- The law of mass balance: if the amount of a substance in the body is to remain constant,
any gain must be offset by an equal loss.
- Total amount (or load) of substance x in the body = intake + production – excretion –
- Mass Balance = Existing body load + Intake or metabolic production – Excretion or
- The rate at which a molecule disappears from the body by excretion, metabolism, or both is
called the molecule’s clearance.
- Mass Flow (amount x/min) = concentration (amount x/vol) × volume flow (vol/min)
- Because of the free movement of water, the extracellular and intracellular compartments
can reach a state f 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
-> 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
* The intracellular fluid can be distinguished from the extracelullar fluid by the ICF’s high
concentration 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
- Passive transport doesn’t require the input of energy
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.