Feeling Salty

      The title of this post really describes how I felt when I had to make this blog for my AP Biology Class. Not really, but I hope reading my blog is not a painful experience. Anyway, this week we're supposed to talk about cell membranes. Cell membranes are barriers that regulate what substances enter and leave the cell, and they consist of a double phospholipid bilayer with the hydrophilic phospholipid heads facing on the outside and the hydrophobic tails on the inside. Proteins are also embedded inside and attached to the outside of this bilayer. This is shown in the fluid mosaic model shown below:

  

The phospholipid heads are facing outwards while the tails are facing inward. It's like in a movie when two characters (in this case the phospholipids) are cornered by another group (water) in a fight scene, and they fight back to back! That's what I thought of for some reason. Anyway, cell membranes are also semi-permeable meaning they allow certain substances, such as water, to pass over the membrane more easily than others. This allows osmosis (the diffusion of water) to occur across the cell membrane, and it also gives one of the reasons for this blog post. If you survived the small biology lesson above, more interesting stuff will follow now! 

     So I was sitting my biology class, and for some reason we were talking about slugs and how they ruin gardens. This is why you could use egg shells to keep them out of your garden, but someone suggested pouring salt on the plants. Honestly, I started laughing because I'm a nerd. Also because I knew if someone did this, they wouldn't have just dead slugs but dead plants too. Civilizations actually used to pour salt on crop fields of countries they were at war with because it would ruin their crops which would hurt the nation's food supply needed to fight the war.
      Then why does salt kill plants? By pouring salt on plants, a hypertonic environment is created outside of plant cells because of this increase in solute concentration causing water to have a net flow out of plant cells and into the environment. The environments surrounding a cell can be described using three terms: hypertonic, hypotonic, and isotonic. As mentioned before, hypertonic describes an environment in which the solute concentration is greater than the solute concentration in the cell. Hypotonic describes an environment that has a lesser solute concentration than the solute concentration inside the cell. Then there's isotonic, which describes a state of equilibrium in which the solute concentrations in the cell and environment are equal. Also based on water potential, if enough salt is added, the environment will have a solute potential greater than the water potential within the plant cells once again explaining why water would move from the cell into the surrounding environment. This would cause cell membranes to shrink away from the cell wall, a condition known as plasmolysis. An example of plasmolysis in elodea cells is shown below:

Notice how the green cell membrane shrinks away from the rigid cell wall.

Plasmolysis is also the reasons why plant wilt because if the cell membranes do not push against the cell wall, the cells and plant are not turgid and thus lack the pressure to stand up. Therefore when plants are sufficiently watered, they are turgid and do not wilt. This water lost would also make it more difficult for the cell to carry out basic functions to sustain life explaining why plants die. In the end, the cell wall would probably be more shriveled than an old person left in a pool for too long.
     Even though all cells have cell membranes, the membrane proteins in this phospholipid bilayer vary greatly based on function. This set of membrane proteins include glycoproteins which have a polysaccharide chain attached and act as recognition proteins. Glycoproteins are like ID tags letting the immune system of organisms recognize which cells are foreign to the body allowing antibodies to be produced in order to eradicate them. Since membrane proteins are distinct to certain cells, scientists are using a method known as neutron reflectometry to learn and model the structures of bacteria cell walls in order to produce better antibiotics. As stated in this article, "over 60% of currently available drugs and 40% of new drugs target membrane proteins."  
     Not only are cell membranes vital to cell function with inner and outer cell activities, but membranes of organelles are also extremely important as they allow compartmentalization. Compartmentalization in very simple terms allow all necessary substances and materials to be in one place, or membrane bound organelle, for a certain cellular process. For example all of the substances needed for cellular respiration are found in the mitochondria while all substances needed for photosynthesis are found in the chloroplasts. Thus membranes keep cells from becoming like Ukraine at the moment: an ugly mess. Compartmentalization can be related to any sports team, like a soccer team. One team acts together as a cell, but the players are compartmentalized into defense, midfield, and offense so they can carry out different functions and get the job done. Compartmentalization also reminds me of a cruise ship. If the cruise ship is the cell, different floors are compartmentalized based on their purpose. So one floor will have only rooms for guests, another will have entertainment, another will have dining, and another will have pools and decks.    

     Though tattoos mainly depend on a machine that was based off of Benjamin Franklin's engraving machine (yeah, you're basically putting an engraving machine into your skin), these designs also depend on principles of biology with cell membranes. I also wanted to relate cell membrane properties to something more relevant than pouring salt on plants and elodea cells. So here is a video showing the process in really slow motion. Skip to 3:11, to see the cool part!

So tattoos work by using this needle-like machine to insert ink into the skin. Cohesion allows molecules that make up the ink to stay together in between the needles. The information explaining the biology of tattoos was found here. Tattoos do not insert ink into cells of the epidermis since this is where skin cells are shed very often. Instead, they insert ink into the layer of cells beneath the epidermis so ink can be taken up by dermal cells. Once the ink is in the dermal cells, the cell membrane's selectively permeable nature may not allow the molecules making up the ink to cross over the membrane and leave the cell. This is why tattoos stay in your skin, and even if the dermal cells die, they are taken up by other cells with the ink. Now if you cringe whenever you pick up salt thinking of this article or refuse to get a tattoo after seeing this video, then I consider this a job well done!