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Chapter 5

BGYB30 Chapter 5 textbook notes

Biological Sciences
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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
metabolic removal.
- The rate at which a molecule disappears from the body by excretion, metabolism, or both is
called the molecules 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
electrical disequilibrium.
* The intracellular fluid can be distinguished from the extracelullar fluid by the ICFs 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 doesnt 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.
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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)
7-Diffusion can take place in an open system or across a partition that separates 2
*Rules for Diffusion of Uncharged Molecules Table 5-1 on page 134!
Therefore, 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
1-The rate of diffusion depends on the ability of the diffusing molecule to dissolve in
the lipid layer of the membrane
(only lipids, steroids, & small lipophilic molecules can move across the membrane by
simple diffusion)
-> water may diffuse slowly across some phospholipids membranes.
2-The rate of diffusion across a membrane is directly proportional to the surface area
of the membrane
(the larger the membranes surface area, the more molecules can diffuse)
3-The rate of diffusion across a membrane is inversely proportional to the thickness of
the membrane
(the thicker the membrane, the slower the rate at which diffusion takes place)
* The energy for diffusion comes from molecular motion.
Protein-mediated Transport:
- Simple diffusion across membranes is limited to lipophilic molecules.
- The vast majority of solutes across membranes with the help of membrane proteins, a
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