4! Lesson Cell Structure and Function Week #2B Lesson #4: What's Due? July 26th August 1st Biology: Concepts and Connections, Chapter 5 Lab #3-The Effect of Salt Concentration on Osmosis in Potato Cells (remember the application paragraph) Quiz #2 Saturday, August 1st
Due Date:
Hello Class,
This week we are going to continue our study of the cell, focusing this week on a cellular structure that should
already be fairly familiar the cell membrane. Consequently, our reading will be focused on chapter 5 of Biology: Concepts and Connections As we saw in our Beet lab, cell membranes are extremely important to the survival of the cell. They are literally a barrier that protects the cell, while still allowing the cell (unless the membrane is in turn surrounded by a cell wall) a lot of flexibility.
Things to do
If you live outside of the Phoenix area, and if you have not already done so, I suggest you contact the Proctor Testing Office and set up your proctored midterm exam right away.
Read and study Lesson #4 from your Rio online course: Be sure to take good notes on each of the Assessing Your Learning essays that are part of this lesson. One of them may be on the midterm. Read the brief overview below. The chemical properties of a cell membrane also are key to its role. Cell Membranes are made up of a phospholipid bilayer, in which the polar "head" of the phospholipid is in contact with an aqueous environment either inside or outside of the cell. The interior of the membrane bilayer is characterizes by the nonpolar lipid "tails" of the phospholipid. This nonpolar hydrophobic region is an effective barrier against large polar hydrophilic molecules such as sugar or glycerol, as well as being a fairly effective barrier against water and ions, most of which are regulated through channels in the membrane. Since some substances can permeate the membrane, while others cannot, the membrane is characterized as semi permeable. We will also be discussing how cells import and export substances through active and passive transport. In order to understand these concepts we must first understand diffusion and concentration gradients. Diffusion is simply the movement of molecules from areas of high concentration to low concentration. A good way of visualizing the process of diffusion is to drop a few drops of food coloring into a glass of water. The food coloring does not spread through out the water evenly, rather, it is so random that it can take minutes for the water to be completely, evenly colored. Another example of diffusion is the spread of a smell of cooking food from the kitchen (high concentration) throughout the house (low concentration). Another property of diffusion is that it is a random process: while the molecules involved travel from high to low concentration, they do so randomly and in no particular direction. A concentration gradient is just the difference between areas of high concentration and low concentration, and when the substance flows from high concentration to low concentration, we refer to that as going down the gradient. Think of a gradient like a waterfall the water at the top flows to the river at the bottom. Passive transport is a way in which cells take advantage of diffusion. Typically, there is a low concentration of sugars and other nutrients inside the cell as cells use them fairly rapidly. If there is a high concentration of sugar or other nutrients outside the cell, they can diffuse into the cell. However, this process stops at equilibrium, because there is no longer a gradient. Once equilibrium is reached the only way to get more that of nutrient into the cell is via active transport (see below). Obviously sugars and other polar molecules cannot just diffuse across the cell membrane, so the cell has channels and transporters in order to help. However, Steroids and other nonpolar molecules can easily diffuse across the membrane. Active transport, as the name implies, requires the cell to spend energy in order to transport a substance. The universal cellular energy currency throughout the Biological realm is Adenosine Triphosphate, or ATP for short. ATP is formed by all organisms via the break down of sugar or other metabolic compounds, as we will see later. The key form of
active transport in the Animal cell is the SodiumPotassium Pump, which is described by your book on page 83. This pump is nothing more than a membrane protein that pumps out three ions Sodium (Na+) ions and imports in two Potassium (K+) ions. In some cases, the SodiumPotassium pump is used to import key metabolic compounds into the cell with the Potassium. Facilitated diffusion is another form of passive transport that cells use. In this case, a large bulky molecule, such as sugar or a chain of amino acids, is paired with a smaller, more common ion, usually sodium. The sodium flows down its gradient through special channels into the cell, dragging the large molecule with it. This is another reason why the sodium potassium pump is important, as it keeps pumping out sodium, establishing a gradient that can be harnessed for work. Finally, Osmosis is the diffusion of water. Water also moves from high concentration to low concentration. Thus water will flow from a cell into salty water or from fresh water into a cell. Your lab this week will refer to the process of osmosis, so pay particular attention to section 55 in your book. I have attached a power point that may help you conceptualize these topics.
Here are 3 web sites that will assist you in learning about the Cell membrane
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html This site gives a description of the cell membrane with some good diagrams. http://www.bbc.co.uk/education/asguru/biology/01cellbiology/05pathways/06passivesimple/index.shtml This site provides excellent animations of passive diffusion, facilitated diffusion, active transport, osmosis, and exocytosis/endocytosis. http://physioweb.med.uvm.edu/diffusion/ This site offers great animations that are highly interactive in changing concentration gradients.
Assignments: Lab #3 Report: The effect of salt concentration on Potato cell membranes In this lab you get to play around with potato cells and see what happens when they are exposed to liquids that are either isotonic, hypertonic, or hypotonic (there will be one of each). A few reminders as you prepare yourself for this lab:
Please review the lab scoring rubric. The questions asked in this week's lab tied extremely closely to that of the lab scoring rubric. Please make sure to answer these completely in paragraphs. Make sure to use DISTILLED WATER only, tap water has trace amounts of solutes that could throw off your results. Make sure to include a detailed data table. Please include your raw data as well as your percentages in this table. Make sure to explicitly state your hypothesis. This report must include an application paragraph, as do all lab reports and essays.
Quiz #2: Over lessons 3 and 4 (found in online lesson): You can print out the quiz if you want to for tracking your answers. You may want to keep written copy of all quiz answers until you are ready to submit. Once you submit the quiz you may not go back and redo the quiz!