THE CELL MEMBRANE A LIPID BILAYER

THE CELL MEMBRANE ALSO KNOWN AS THE PLASMA MEMBRANE BOUNDARY BETWEEN THE CELL AND ITS EXTERNAL ENVIRONMENT CONTROLS THE PASSAGE OF MATERIALS INTO AND OUT OF THE CELL-MAINTAINS HOMEOSTASIS

THE CELL MEMBRANE IS COMPOSED OF TWO PHOSPHOLIPID LAYERS The individual components are called phospholipids. Polar (hydrophilic) head Nonpolar (hydrophobic) fatty acid tails Remember, water is polar; the polar heads orient themselves so that they face the watery environments inside and outside the cell. The nonpolar tails are turned inside the membrane. It is difficult for polar substances to get through this nonpolar area Lipid bilayer

FLUID MOSAIC MODEL “Fluid” The phospholipids are very mobile “Mosaic” Proteins and other biomolecules are embedded within the lipid bilayer Cholesterol Stabilizes the membrane Transport proteins Carbohydrates serve as identification tags

SELECTIVE PERMEABILITY Some, but not all, materials may cross the membrane (aka ‘semipermeable’) Allows the cell to maintain homeostasis Membrane proteins help facilitate the transport process

DIFFUSION AND OSMOSIS Diffusion-movement of molecules in a fluid or gas from a region of higher concentration to a region of lower concentration Results from the natural motion of the particles which causes the particles to collide and scatter Concentration gradient-difference in concentration from one place to another Osmosis-diffusion of water across a semipermeable membrane

HYPERTONIC, HYPOTONIC, ISOTONIC These terms usually refer to the type of environment the cell is in. They are comparisons of SOLUTE concentrations So, a hypertonic solution has MORE (hyper) solute than the cell. Conversely, a hypertonic solution has LESS water than the cell. The thing to remember is that the terms refer to the solute, but it is the movement of WATER that we discuss in these situations.

ISOTONIC SOLUTIONS In an isotonic solution the concentration of solute and water inside the cell and outside in the solution are the same. There is a constant movement of water across the cell membrane, in and out. In general, we don’t think of the solute as moving across the selectively permeable cell membrane, although some can. We are only concerned with the movement of water.

HYPERTONIC SOLUTION In this situation the solution has more solute and less water than the cell. Water is like every other fluid molecule…it will move from a region of higher concentration to a region of lower concentration. Here…water moves OUT of the cell from higher concentration inside the cell to a lower concentration of water outside the cell and the cell shrinks. The solute concentration inside the cell is lower, but the water concentration is higher than outside…so water moves out.

HYPOTONIC SOLUTION In this situation the solution outside the cell has less solute (and MORE) water. Once again, water moves from higher concentration (outside) the cell to a lower concentration INTO the cell, and the cell swells The solute concentration inside the cell is higher, but the water concentration is LOWER.

TO SUMMARIZE Animal cells will swell and burst in a hypotonic solution- they prefer an isotonic solution Plant cells prefer a hypotonic solution- they appear wilted in an isotonic or hypertonic solution. Plasmolysis

ENDOCYTOSIS AND EXOCYTOSIS Require energy, therefore these are a form of active transport Pinocytosis-movement of water into the cell Phagocytosis-white blood cells engulf large particles like bacteria