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STATION ONE: THE PHOSPHOLIPID BILAYER (CELL OR PLASMA MEMBRANE)
Introduction: The cell membrane is also referred to as the phospholipid bilayer. It is called a bilayer because there are 2 layers of phospholipids. Phosphate head Fatty Acid Tail The importance of the cell/plasma membrane is to control what enters and leaves the cell. The
membrane is selective to what is permitted to come into or go out of the cell. This is why
the cell/plasma membrane is said to be selectively permeable. The cell membrane also
provides protection and support for the cell. The phospholipid bilayer also contains membrane proteins. These proteins have many important functions in the cell including transport of materials which are too large to pass through the membrane and as receptors for hormones and other molecules.
DIRECTIONS: Using the materials provided at your station construct a phospholipid bilayer,
with a transport protein. Once you are finished, raise your hand and have the teacher initial
your answer document. Once you have finished, be sure to disassemble the parts for the
next lab group.
The phosphate group is found on the outer side of the membrane and is hydrophilic (water loving). The fatty acid group is found in the middle of the bilayer and is hydrophobic (water fearing) Protein channel Carbohydrate chains Lipid bilayer
STATION TWO: PART A: PASSIVE TRANSPORT (DIFFUSION)
Introduction: The word concentration refers to how much stuff or mass is found in a given space or volume. An espresso coffee has more coffee particles than a regular cup of coffee; therefore the espresso is more concentrated. You can also say that a cup of coffee with three sugar cubes has a higher concentration of sugar then a second cup of coffee that has only one sugar cube added. When the sugar cube was first added to the coffee, the sugar was concentrated or tightly packed into a sugar cube. However, the water in the coffee gradually dissolves the sugar cube causing the sugar cube to randomly spread out in the solution. The dissolved sugar molecules moved away from the cube where it was highly concentrated into the coffee liquid where it was less concentrated. This random movement of particles from an area of high concentration to an area of lesser concentration is called diffusion. The uneven distribution of sugar particles is called a concentration gradient. Normal diffusion goes with the gradient moving particles from high to low concentrations. Diffusion stops when the particles are spread out evenly. The particles are still in constant motion, but because there is no
concentration gradient, the solution has reached equilibrium.
DIRECTIONS:
- Fill up 2 separate glass beakers - one with cold water and one with warm water.
- Drop 3 drops of food coloring one at a time into each beaker at the same time and observe what happens.
- Pour out the beakers of water into the sink, rinse beakers and return to tray.
STATION THREE: FACILITATED DIFFUSION (“HELPED”)
Introduction: The cell membrane is selectively permeable, meaning it allows only certain molecules to pass through easily. The simplest example of a cell membrane is a soap bubble. Have you ever seen the colors swirl on a bubble? Both bubbles and cells are fluid, meaning molecules move around constantly on its surface. Many smaller molecules like water, oxygen and carbon dioxide can easily pass straight through the cell/plasma membrane without harming it. The membrane has strong forces that hold the membrane together and seal it up whenever molecules pass through it. The membrane is still
fragile though. The cell must have water inside and outside the cell; not only to dissolve
materials needing to be transported through the membrane, but to hold the membrane together itself. Investigation/Procedure:
- Place the smooth side of the Styrofoam ring found on your table into the bowl of soapy solution.
- Pull the Styrofoam ring out of the bowl and observe the soap film (membrane). Notice that the soap molecules are in constant motion.
- Hold the foam ring over the piece of paper towel at your table.
- To see this self-sealing membrane in action, release a drop of food coloring above the soap film and observe what happens. Don’t contaminate the soap bowl with food coloring please!!!
- Try dripping some of the soapy water above the film and observe what happens.
- Record your observations on your notes page.
STATION THREE: FACILITATED DIFFUSION (Continued) Larger molecules such as glucose and amino acids, are unable to pass through the membrane easily. To facilitate or “help” these molecules across the membrane, bridges are used to carry
them across. These bridges are transport proteins called protein channels or pores. This
type of diffusion is called facilitated diffusion because movement of the molecules is “helped”
across the membrane by these transport proteins. Most of these proteins are specific, allowing only certain molecules to pass. This is similar to the idea of a pet door. The pet can pass through the door to get into and out of the house, but I doubt very seriously that you could crawl through the pet door. This choosy characteristic of the membrane is called selectively permeable. You will demonstrate how these transport proteins work using a loop of thread inside your soap film. Investigation/Procedure :
- Locate the tied loop of thread at your table. Dip it in the soap solution to get it wet.
- Dip the styrofoam ring into the soap solution to get a soap film. Carefully place the thread loop onto the surface of your soap film.
- Touch the inside of the loop with the end of the paper clip. It should pop the soap film inside the loop of thread but leave the surrounding soap film intact.
- Stick your finger through the opening/pore and move it around the cell.
- Now pass the straw through the thread loop to simulate facilitated diffusion.
- Record your observations.
STATION FIVE: OSMOSIS
Introduction:
Osmosis is a specialized case of diffusion that involves the passive transport of water
across the cell/plasma membrane. In osmosis, water moves through a selectively permeable membrane from a region of higher concentration (more water) to a region of lower concentration (less water). I always remember is as H 2 O smosis. The cell membrane is selectively permeable which means it allows passage of certain types of molecules while restricting the movement of others. Water is allowed to pass freely, and without using energy, through the cell/plasma membrane. Therefore, osmosis if a form of passive transport. If there was a membrane with twice as many water molecules on one side as there were on the other (and remember, water can move freely through the membrane), what do you think would happen to the water molecules? Correct, the side with twice as many water molecules would move in one direction to the other side where the concentration was lower until eventually causing the concentration of water to be equal or in equilibrium on both sides. After that the flow of water would then diffuse through the membrane in both directions to maintain equilibrium.
REMEMBER:
Water is considered the Universal Solvent. While discussing osmosis, when we use the word solves we will always be referring to water (Ex. Salt Solution, Sugar Solution...) ISOTONIC SOLUTION, HYPERTONIC SOLUTION AND HYPOTONIC SOLUTION
Isotonic : The concentration of solutes in the solution is equal to the concentration of the
solutes inside the cell. As a result, water will move equally in both directions and the cell remains equal in size (Equilibrium).
Hypertonic: The solution has a higher concentration of solutes and a lower concentration of
water than what is inside the cell. As a result, water will move from inside cell out into the solution and the cell will shrink in size.
Hypotonic: The solution outside of the cell has a lower concentration of solutes and a higher
concentration of water than what is inside the cell. As a result, water moves from the solution into the cell and the cell swells and bursts open (cytolysis). Example: Blood is isotonic to body cells. This means that the body cells will not shrink or burst when they come in contact with blood. Example: Putting salt on a slug (snail would kill it by drawing water out of the slug (dehydrating the slug). Example: Putting 100% pure water into a patient’s IV bag would cause excess water to get into their cells. To keep cells from bursting, IV’s usually contain a salt or sugar solution.
Station 2: Passive Transport (Diffusion)
- Define the term diffusion: ________________________________________
- Define the term concentration: ____________________________________
- What is a concentration gradient? _________________________________
- When does diffusion stop? ______________________________________________
- What does it mean when a solution has reached its equilibrium? ______________
Investigation:
- Describe what happened when you dropped the food coloring into the warm water and the cold water : _____________________________________________ ___________________________________________________________
- What conclusion can you make about diffusion and the water temperature? _______ ____________________________________________________________
- Why did the extracts diffuse out of the balloon but the sand did not? ____________ ____________________________________________________________
- How far away can you get and still smell the balloons? ________________________
- Why can’t you smell them after a certain distance away? ______________________
- Why is diffusion a type of passive transport? ______________________________
Station 3: Facilitated (“ Helped”) Diffusion
Observation: Characteristic #
- Describe what happened when you dropped food coloring onto the soapy membrane: _______________________________________________________
- How is the soapy membrane similar to a cell/plasma membrane? _____________
- What happened when you dribbled the soapy solution onto the soapy membrane?
Observation: Characteristic #
- Describe what happened when you placed the thread loop onto the soapy membrane: _______________________________________________________
- In this simulation, the hole made inside the thread loop represents? ___________
- Transport proteins called _________________ or ___________________________ move molecules across the cell during facilitated diffusion.
- Does facilitated diffusion require energy? _________
- Does the concentration gradient play a role in facilitated diffusion? ________ Explain your answer: ________________________________________________
Station 5: Osmosis Draw examples of what a cell would look like in an Isotonic, Hypertonic and Hypotonic Solution: Be sure to label each solution.
- What is Osmosis? _________________________________________________
- What is a solution? ________________________________________________
- What is a solute? ___________________________________________
- What is a solvent? __________________________________________
- Define the term isotonic solution: _______________________________ _________________________________________________________
- Give an example: ___________________________________________________
- Define the term hypertonic solution: _____________________________ _________________________________________________________
- Give an example: ___________________________________________________
- Define the term hypotonic solution: ______________________________ _________________________________________________________
- Give an example: __________________________________________________
Egg Lab: The purpose of this lab is to observe the process of osmosis occurring through the
membrane of an egg.
Day 1: Gently place an egg in a glass beaker and using the graduated cylinder,
measure 150 ml of vinegar and carefully pour the syrup over the egg. Record any
observations and then cover with a piece of foil and set aside.
Day 1: What happened when you poured the vinegar over the egg? ____________________
Draw what your egg looked like when you placed it in the vinegar: Day 2: Observe and record what happened to the egg and record the amount of vinegar left in the beaker once the egg was removed (use the graduated cylinder to get an accurate reading).
- What happened to the egg? ___________________________________________________
- How much vinegar was left in the beaker once the egg was removed? _________________
- What do you think happened to the missing vinegar? _______________________________
- What type of solution do you think the vinegar is? _________________________________
- Explain your answer: _________________________________________________________
- Draw what your egg and solution looked like after being in vinegar: Be sure to draw arrows showing the direction of water flow. Rinse off the egg gently, removing all of the white powdery substance on the egg and place back into the glass beaker. Using the graduated cylinder, carefully measure 150 ml of corn syrup and gently pour over the egg, cover with the foil and let sit.
