Cells are the structural units of all livingthings.The principle of complementarity statesthat the biochemical activities of cells aredetermined and made possible by thespecific subcellular structures of cells.
The generalized or composite cell hasthree major regions - the nucleus, thecytoplasm, and plasma membrane.
Membrane transport Interstitial fluid is an extracellular fluid derived from blood, and baths the cells. The plasma membrane is a selectively permeable barrier, which allows some substances to pass while excluding others.
Passive processes cause substances topenetrate the membrane without anyenergy input from the cell, while in activeprocesses the cell provides ATP to drivethe transport mechanism.
Diffusion is the tendency of molecules orions to scatter evenly throughout theenvironment.Molecules tend to move away from areasof high concentration, and toward areasof low concentration, along theirconcentration gradient.
The diffusion of a solvent (water) thougha selectively permeable membrane iscalled osmosis.The total concentration of all soluteparticles in a solution is referred to as thesolution’s osmolarity.
Hydrostatic pressure is the back pressureexerted by water against the membrane.Tonicity is the ability of a solution tochange the tone or shape of cells byaltering their internal water volume.Solutions with concentrations equal tothose found in cells are isotonic.
Solutions that have a higherconcentration of solutes than the cell arehypertonic.Solutions that are more dilute than cellsare called hypotonic.
In facilitated diffusion, moleculescombine with protein carrier molecules inthe plasma membrane and are releasedinto the cytoplasm.
Filtration is the process by which waterand solutes are forced through amembrane or capillary wall byhydrostatic pressure. The moleculesmove along a pressure gradient instead ofa concentration gradient.
Active transport (solute pumping) issimilar to facilitated diffusion, except thatsolute pumps move solutes against theirconcentration gradient by coupling withATP.
If two transported substances are movedin the same direction, the system is asymport system.If the the substances cross the membranein opposite directions, the system is anantiport system.
In primary active transport, the energy isprovided directly by hydrolysis of ATP.In secondary active transport, thetransport is driven indirectly by passiveion gradients.An example of a primary active transportsystem is the sodium/potassium pump.
In phagocytosis, part of the plasmamembrane and cytoplasm protrude andflow around some relatively large or solidmaterial, forming a vesicle called aphagosome.Most phagocytes move by means ofamoeboid motion.
In pinocytosis, the plasma membranesurrounds a droplet of extracellular fluid.Receptor-mediated endocytosis usesproteins that bind only with certainsubstances.
Generation and maintenance of aresting membrane potential A membrane potential is an electrical voltage across the plasma membrane, resulting from a separation of oppositely charged ions. All cells exhibit a resting membrane potential of approximately -20 to -200 mV. Therefore, all cells are polarized.
The greater outward diffusion ofpotassium (than inward diffusion ofsodium) leads to a charge separation atthe membrane (inside negative). Thischarge separation is maintained by theoperation of the sodium-potassium pump.
Ions diffuse according to electrochemicalgradients, which takes into account theeffect of both electrical and chemicalforces.
Cell-environment interactions Cell adhesion molecules (CAMs) are composed of glycoproteins that anchor cells to molecules in the extracellular space, propel the cells past one another, and recruit white blood cells to traumatized tissue.
Membrane receptors are proteins andglycoproteins that serve as binding sites.Signaling chemicals that bind specificallyto membrane receptors are called ligands.
G protein-linked receptors use G proteinsas a relay to activate or inactivate amembrane-bound enzyme of ion channel. The intracellular chemical signals areknown as second messengers.Ionic calcium and cyclic AMP are twoimportant second messengers, whichusually activate protein kinase enzymes.