Chapter 3

• A semipermeable membrane is a membrane which allows certain not all molecules to pass through it.
• Biological membranes are semipermeable, selectively permeable membranes for some not all substances.
• Depending on the cell need, the membrane may be a barrier for a substance at one time, and actively transport it at another time.
• The cell regulates its volume, internal ionic and molecular composition by exchanges with its environment.
• Exchanges by active or passive transport are distinguished.
• Simple diffusion (solute) occurs down a concentration gradient.
• All atoms and molecules possess a kinetic energy or energy of motion at T ° C above 0.
• Matter is solid, liquid or gas depending on the freedom of movement of its constituent particles.
• The movement of particles is unpredictable and undirected, then at least 2 regions exist: one with a higher concentration and the other with lower concentration.
• This difference is a concentration gradient.
• Lipid bilayers are impermeable to macromolecules and ions.
• Down their concentration gradient, macromolecules and ions move even there is a membrane in between.
• The stored energy (potential energy) of the concentration gradient is released when molecules move from the high to low concentration.
• The rate of diffusion is determined by the movement of particles: size, shape, electric charges, concentration gradient, liposolubility, thickness of membrane, surface of membrane and T ° C.
• Diffusion moves solutes toward the dynamic equilibrium.
• Molecules continue to move in both directions.
• Osmosis is the diffusion of the solvent (water) across a selectively permeable membrane.
• Water passes from the low concentration medium to the the high concentration medium through a semi-permeable membrane.
• Water molecules pass freely in both directions through a selectively permeable membrane but most solute molecules (sugar, salt) cannot diffuse freely through it.
• A semi-permeable membrane is a membrane which allows certain not all molecules to pass through it.
• In mammals, there are 13 known types of aquaporins, and 7 of these are located in the kidney.
• Active transport operates against their concentration gradient and requires input of energy like ATP.
• Like blood plasma and other body fluids, one may be hypertonic and the other hypotonic.
• Osmosis is important for the distribution of nutrients and release of metabolic wastes from the organism and for the maintenance of the concentration gradient inside and outside the cell.
• Water passes from the hypotonic to the hypertonic.
• If a cell is put in fresh water, the water will flow into the cell, causing it to swell.
• Osmotic pressure is the minimal pressure of a solution against a semipermeable membrane to prevent osmosis or water from flowing inward across the membrane.
• The functional AQP unit is a homotetramer, with each single monomer having six highly hydrophobic transmembrane (TM) domains of alpha-helix structure, which both carboxylic and amine termini are located in the cytosol.
• Aquaporins are integral membrane proteins from a larger family of major intrinsic proteins (MIP) that form pores in the membrane of biological cells.
• Molecules that cannot enter by simple diffusion across the membrane, their transport depends on membrane transport proteins which cross the membrane and give selected substances particular passages through the lipid bilayer.
• Passive transport dissipates the gradient and does not require input of energy.
• A cell exchanges material with the environment by taking up nutrients it needs, exchanging ions, and discarding waters from its own activity.
• The membrane domains of an AQP protein are connected by five intracellular (ICL) and extracellular (ECL) loops: A, B, C, D and E.
• The set of transport proteins present in a plasma membrane, or in the membrane of an intracellular organelle, determines which solutes will enter or leave this cell or this organelle.
• Particles may be too large to cross a semipermeable membrane separating regions of solution, therefore only water moves across the membrane.
• If a sugar solution is on one side of a semipermeable membrane and pure water is on the other side of the membrane, there will always be pressure on the water side of the membrane to try to dilute the sugar solution.
• Active transport is defined as the movement of solute against an electrochemical gradient, making it an endergonic process that requires the coupled input of energy.
• Many of these carrier proteins are referred to as pumps because analogous to a water pump which requires energy to crank the handle up and down to make water flow.
• A charged solute will tend to move in the direction of its concentration gradient.
• The K+ ion is positively charged and its intracellular concentration is much higher than the extracellular concentration, has a small electrochemical gradient across the membrane.