Movement Through the Membrane 3 PDF Biology Notes

The Cell Membrane Regulates what enters and leaves the cell. Provides some protection and support. Also called a Phospholipid Bipayer – contains phospholipids with hydrophilic (polar) heads (water loving) and hydrophobic (non-polar) tails (water fearing). Also has integral (go all the wa...

The Cell Membrane Regulates what enters and leaves the cell. Provides some protection and support. Also called a Phospholipid Bipayer – contains phospholipids with hydrophilic (polar) heads (water loving) and hydrophobic (non-polar) tails (water fearing). Also has integral (go all the way through) and peripheral protiens.  Cell Membrane (cont.) Note that both the outside and inside of a cell are mostly water. Thus, cells exist in as aqueous environment. The plasma membrane is often referred to as a “fluid mosaic” because a “mosaic” is a structure made up of many different parts. Furthermore, the plasma membrane is a flexible structure in which the various components float and move laterally (side-to-side). So how do molecules that are in water get through the membrane with these qualities?  Diffusion The movement of molecules across a membrane from an area of high concentration to an area of low concentration. Does not require the use of energy The rate of diffusion is affected by temperature, the molecule’s size, the steepness of the concentration gradient, etc.  Selectively Permeable Membrane or SEMI-PERMEABLE MEMBRANE: A membrane through which only certain molecules may pass. In other words, it lets certain molecules through, but prevents other molecules from crossing. Such functionality is critical to the homeostasis of cells – the ability to maintain and regulate internal environment and to maintain the various processes of life. What determines whether or not a molecule can pass through the lipid bilayer of the cell membrane? SIZE and CHARGE!!!!!! Small molecules will pass more readily through the phospholipid bilayer than large molecules will. Non-polar molecules, since they are hydrophobic and NOT water soluble, ARE soluble in the non-polar fattyacids that form the hydrophobic interior of the cell membrane. Therefore, they can more easily pass through the membrane. Polar molecules (such as water) or ions (positively or negatively charged atoms or molecules) are both hydrophilic and water soluble. Therefore, they CANNOT pass through the non-polar fatty-acids that form the hydrophobic interior of the cell membrane.  Osmosis The movement of WATER across a semi-permeable membrane from an area of low solute concentration (i.e. high WATER concentration) to an area of high solute concentration (i.e., low WATER concentration) Hypotonic solution: the concentration of solutes OUTSIDE the cell is LOWER than the inside of the cell; water moves INSIDE the cell, in extreme cases an animal cell may burst in a process called “lysis”. Isotonic solution: the concentration of solutes OUTSIDE the cell is EQUAL to that inside the cell; net water movement = 0, as water is moving out of and into the cell equally. Hypertonic solution: the concentration of solutes OUTSIDE the cell is HIGHER than the inside of the cell; water LEAVES the cell, in extreme cases the cell shrivels up and dies. This is called plasmolysis. HOW DO THINGS GET IN AND OUT OF A CELL? PASSIVE TRANSPORT: The movement of a substance across a cell membrane WITHOUT the use of a cell’s ENERGY. – Simple diffusion: The passage of small, non-polar molecules (e.g. oxygen gas, O2, carbon dioxide gas, CO2) directly across the cell membrane – Moves molecules from HIGH concentration to LOW concentration – Does NOT require any energy input (like a ball rolling down hill) Passive Diffusion through the membrane  Facilitated diffusion: – Materials pass through the cell membrane with the help of a transport protein (such as a channel protein or carrier protein) – These transport proteins basically form a doorway through the hydrophobic interior of the phospholipid bilayer – Moves molecules from high concentration to low concentration – Does NOT require any energy input (like a ball rolling down hill…)  ACTIVE TRANSPORT: – Uses cellular ENERGY (ATP!) to move materials across the cell membrane – Needed to move material from an area of LOWER concentration to an area of HIGHER concentration – It’s like “swimming upstream” or “rolling the ball uphill” – Example… The sodium-potassium pump (Na+-K+ pump)!!! Neurons constantly perform active transport to set up an ion concentration gradient that is the basis for the electrochemical signal of an active neuron!!!  BULK MOVEMENT: Getting the really BIG stuff in and out… Exocytosis: Movement of materials OUT of the cell (exiting the cell). Vesicles (e.g. from Golgi apparatus) fuse with the cell membrane and release their contents OUTSIDE the cell. – Or by neurons to release neurotransmitters into the synaptic cleft (the space between the terminal end of one neuron’s axon and the next neuron receiving the signal…) and thereby regulate behavior, personality, and mood!!!!  Endocytosis: Movement of materials INTO the cell (entering the cell). Part of the cell membrane engulfs or surrounds the material, then pinches off to form a vesicle to bring the material INSIDE the cell. TYPES of endocytosis: – 1. Phagocytosis: “cellular eating,” for example of an invading bacterium by the white blood cells of your immune system – 2. Pinocystosis: “cellular drinking” to bring in a substantial volume of extracellular fluid