MEMBRANES MEMB1 Both prokaryotic and eukaryotic cells have ...

MEMBRANESProteins are scattered within and without the bi-layer of phospholipids. Some proteinsactually pass completely through the membrane and form channels from the outside ofthe cell to the inside of the cell (or organelle). In addition, there are often chains ofsugars attached to the surface.The one best word to describe a cell membrane is dynamic. It is constantly changing.We can radioactively tag a phosphate group and we know the phospholipids are capableof migrating within the membrane. So are the proteins.GlycocalyxA very important part of the membrane is the chains of sugars attached to the surface.These are collectively called the glycocalyx. It serves as the cell recognition factor. Itsays this is me and not you.Transport Across MembranesSo far, we have looked at what determines permeability of membranes, the structure ofmembranes, and the specifics of the glycocalyx, but we haven’t yet discussed themethods of passing materials through membranes. This process is called membranetransport.The are two ways materials are passed through the membrane: passive transport andactive transport.Passive TransportPassive transport occurs without the expenditure of energy in the form of ATP. Giventhe correct physical parameters, passive transport always occurs. There are threemethods of passive transport: (1) diffusion, (2) osmosis, and (3) facilitated diffusion.MEMB4

MEMBRANESDiffusionDiffusion is defined as the passage of particles from a region of higher concentration to aregion of lower concentration. Unless physically blocked, diffusion always occurs. Theimportant question is how many particles will move from one location to another. Theanswer is until there is no longer a difference in concentrations – in other words, untilequilibrium is reached.OsmosisOsmosis is often defined as the diffusion of water through a membrane. In this course,you will extend that definition to include any material that passes through a membrane.Selectively permeablemembraneMEMB5

MEMBRANESEndocytosisEndocytosis is simply a means of bringing material into the cell other than by activetransport. Endocytosis also requires the expenditure of energy and can work against aconcentration gradient. There are two types of endocytosis: (1) phagocytosis and (2)pinocytosis.PhagocytosisPhagocytosis occurs when the cell “engulfs” food or materials. Phagocytosis takesadvantage of the cell’s internal composition. Remember, most of the cell is verycolloidal in nature. This particular colloid is reversible. It can go from gel to sol, back togel. The gel part of the protoplasm is called ectoplasm while the sol phase of the colloidis called endoplasm. Think of how you perhaps spilled milk at the dining room table andthe milk flowed around any object on the table. When the cell comes in contact with anobject, the ectoplasm (gel) converts to endoplasm (sol) and the cell’s protoplasm flowsaround the object. Once engulfed, the endoplasm coverts to ectoplasm and a foodvacuole is formed.Endoplasm (sol)ectoplasm (gel)nucleuspseudopodfood vacuolefood particleMEMB7

MEMBRANESThree Types of Cell EnvironmentsRemember, inside cells are solutions, suspensions, and colloids and outside the cells aresolutions, suspensions, and colloids. Each cell may therefore be exposed to three types ofenvironments based on the concentrations of materials inside and outside the cells.IsotonicAn isotonic environment is one where the concentration of materials inside the cell is thesame as outside. There is no concentration gradient, and therefore no diffusion orosmosis takes place. Cells prefer isotonic environments since they don’t have to work tocounteract the effects of diffusion and osmosis. You provide cut flowers an isotonicenvironment when you add the packet of chemicals to the water in the vase to increasethe life of the cut flowers.90% H 2 O10% dissolvedmaterials90% H 2 O10% dissolvedmaterialsHypertonicCells are mostly water by weight – approximately 90%. There are two ways to definehypertonicity. You may define it based on the movement of water, or you may base it onthe movement of dissolved materials. In this course, the definition of hypertonicenvironments is based on the movement of water. Place a cell in an environment that is50% water and 50% dissolved materials. That means the concentration of water insidethe cell (90%) is greater than outside. Water will diffuse through the membrane and thecell will shrink in size. (You could also say the concentration of dissolved materialsoutside is greater than inside and the dissolved materials move into the cell.)90% H 2 O10% dissolvedmaterials50% H 2 O50% dissolvedmaterialsMEMB9

MEMBRANESAs the cell shrinks, the membrane collapses around the internal skeleton of the cells – themicrotubules. If this occurs in red blood cells or erythrocytes, the result is a crinklingeffect of the cells. This is called crenolysis in red blood cells.Plants, of course don’t undergo crenolysis. They have a rigid cell wall that will notcollapse. However, the cell membrane can collapse away from the wall and as a result,the plant cell’s contents form a tight ball of protoplasm in the center. This condition inplants is called plasmolysis. Both crenolysis in red blood cells and plasmolysis in plantcells is due to a hypertonic environment.HypotonicA hypotonic environment is where the concentration of water outside the cell is greaterthan inside. Place a cell in distilled water (100%) and you have a concentration gradient(the cell has 90% water by weight). Water diffuses into the cell. In red blood cells, thisoften results in the rupture of a cell (think of overfilling a balloon with water). Whatmakes a red blood cell red is the pigment called hemoglobin. When the red blood cellruptures, the hemoglobin leaks out and this results in empty cell casings called “ghosts”.This process of rupture in red blood cells due to a hypotonic environment is calledhemolysis.MEMB10

MEMBRANESAgain, in plant cells, you are not going to rupture the cell. The reason, again, is the cellwall. However, pressure pushes the cell membrane tightly up against the cell wall whenwater rushes in. As a consequence, prolonged pressure can destroy the cell. Thiscondition is called turgor in plant cells.MEMB11