Mrs. Engelbrecht

1. Membrane Structure Activities

What do you have to understand?

Membrane Structure:
  • The fluid mosaic model of the cell membrane and explain how the four macromolecules come together to form cell membranes.
  • Explain why cells have membranes.
  • Identify the specific parts of plasma membranes in a diagrams: carrier protein, channel protein, enzyme, transport proteins, carbohydrate, cholesterol, phospholipid, hydrophobic tails, hydrophilic heads, lipid bilayer, receptor protein
  • The functions of each part of the plasma membrane
  • Make connections between the parts to show how the molecules work together to make a cell membrane function.

Below is a copy of the activities in Juno.

What do you need to know about membrane structure in order to give a proper presentation to the parents? Well, you've done some research already on the topics below about membrane structure.

Membrane Structure:

  • The fluid mosaic model of the cell membrane and explain how the four macromolecules come together to form cell membranes.
  • Explain why cells have membranes.
  • Identify the specific parts of plasma membranes in a diagrams: carrier protein, channel protein, enzyme, transport proteins, carbohydrate, cholesterol, phospholipid, hydrophobic tails, hydrophilic heads, lipid bilayer, receptor protein
  • The functions of each part of the plasma membrane
  • Make connections between the parts to show how the molecules work together to make a cell membrane function.


So, how are we going to get more familiar with this? You've already done some independent research, but ehre are some activities to test your understanding and see what you still need to know.

While you're going through the activities, you need to fill out the note sheet below (click on the link), which is full of awesome, insightful, and inspiring questions that will help you with exactly what you need to know in order to make your presentation to the parents. ONLY ANSWER THE QUESTIONS ABOUT MEMBRANE STRUCTURE (#1-4)

Make sure you turn in this sheet to Edmodo when your are told to do so.

https://docs.google.com/document/d/1lX_-v-LPoeAItnJVnL65XZ-9_bQmywBs6-AthC66fi8/edit

The current understanding about how cell membranes are structured is called the fluid mosaic model of membrane structure. Sounds fancy, I know, but don't let all those words fool you--it's really a simple concept.

a) Go look up what a "fluid" is.
b) Go look up what a "mosaic" is.
c) Check out the picture below, which is of the fluid mosaic model of membrane structure, and then answer the question that follows this slide.


1.Now that you know what a fluid is, what a mosaic is, and what the general structure of the membrane is, describe why it is named the "fluid mosaic" model. (Put the pieces together...)

2.Still looking at that picture of the plasma membrane, look at what three macromolecules make up the membrane.

In your own way, describe how these macromolecules come together in order to form a membrane (in other words, how do the pieces of the mosaic fit together to form the larger membrane)?

3.What macromolecule is NOT a part of plasma membranes?

Now that you know the four main parts, let's look at this question:

Why do cells have membranes, anyway?

Read the statement below about the purpose of cell membranes:

"The cell's plasma membrane does not simply form a "sack" in which to keep all the cytoplasm and other cellular organelles. The plasma membrane is a very important structure which functions to allow certain substances to enter or leave the cell. It can "pump" other substance into the cell against the concentration gradient or pump other "wastes" etc. out of the cell."

4.Rephrase the quote above into your own words, explaining the main functions of cell membranes.

Check out the diagram below of a typical plasma membrane, a membrane that could be found surrounding an entire cell chock-full of organelles, or it could be found around an organelle, such as the endoplasmic reticulum, Golgi, lysosomes, mitochondria, or chloroplasts.

5.Write an analogy for what every structure in the diagram (except the gated channel protein) LOOKS like. Number your analogies.

6.Phospholipids have hydrophobic tails and hydrophilic heads--meaning that one molecule can have two ends with very different properties (neat!). The picture below shows three different ways to represent a phospholipid (some more complicated than others).





After refreshing your memory on what hydrophilic and hydrophobic are, explain how the phospholipid having hydrophilic heads and hydrophobic tails helps form the lipid bilayer. (HINT: Look at how the tails and heads of all of the phosopholipids are arranged on the diagram you saw earlier!)



Match the membrane structure on the right to the correct letter on the left.
 
7.A
 
8.D
 
9.E
 
10.F
 
11.H
 
12.I
A.hydrophobic tails of phospholipids
B.transport protein
C.hydrophilic heads
D.carbohydrate (oligosaccharide)
E.lipid bilayer
F.cholesterol

You read earlier that it is one of the jobs of the plasma or cell membrane to allow only certain substances inside and out of the cell. In other words, cell membranes are very picky as to what they let inside of cells and what they release out of cells--not just anything can pass through a membrane. We call this characteristic of membranes selective permeability, or we say that the membrane is selectively permeable.


However, membranes don't just wake up one day and decide to be picky; the properties of the macromolecules that make up the membrane give the membrane its selectively permeable properties. For example, all lipids--including phospholipids and cholesterol-- are hydrophobic (either the entire molecule or part of the molecule), which means they do not like to be near water. This is because they contain lots of bonds between the elements carbon and hydrogen, which are nonpolar covalent bonds. These are bonds in which electrons are shared equally between atoms, so there are no partial positive or negative charges that develop on either end of the molecule. This means that there are no charges here to attract the polar water molecules to form hydrogen bonds. Because there are no charges, molecules or regions of molecules that contain nonpolar covalent bonds are known as nonpolar molecules. Not only are lipids hydrophobic in some way, but some proteins can also have parts or regions of themselves that are hydrophobic, which means those regions contain nonpolar covalent bonds.


Nonpolar molecules like to be near other nonpolar molecules when they meet in solution; conversely, polar molecules like to be near other polar molecules when they are in solution because the opposite charges on the ends of these molecules attract each other to form hydrogen bonds.

Located in the lipid bilayer of the cell membrane are various proteins. The middle part of a membrane protein is mostly nonpolar; it is attracted to the interior of the lipid bilayer but is repelled by the water on either side of the lipid bilayer. In contrast, the inner and outer parts of the protein are mostly polar and are therefore attracted to water. This dual attraction to water holds the protein in the lipid bilayer. However the motion and fluidity of the phospholipids enable the cell-membrane proteins to move around within the lipid bilayer.

Proteins can have many functions in the membrane--they can transport molecules across the membrane, they can join one cell's membrane to another cell's membrane, they can receive chemical signals from outside of the cell and transmit those signals to the inside of the cell, they can be enzymes, catalyzing chemical reactions at the surface of the membrane, they can have carbohydrates attached to them that help other cells recognize what type of cell they are. However, the transport proteins and those proteins that receive signals (receptor proteins) have a special property called specificity. This means that transport proteins are very specific, or picky, about what molecules they will let across the membrane. This means that some proteins will only transport one type of substance across a membrane. For example, some transport proteins form channels that will only let K+ (potassium ions) across and nothing else. Some proteins in your neurons (nervous system cells) will only transport specific neurotransmitters (such as serotonin) from one neuron to the next when passing nerve signals down a nerve. In the case of receptor proteins, each receptor protein will only be able to receive one type of chemical signal from the extracellular fluid--the fluid that surrounds all cells which is full of molecules--to transmit to the inside of the cell.

13.Refer to what you just read, and then look again at the pictures of phospholipids as well as the picture of the entire plasma membrane.

Because the hydrophobic tails of phospholipids, cholesterol, and parts of proteins in the middle of the membrane are nonpolar, the interior of any cell membrane is called the hydrophobic region of the membrane.

Let's say a small glucose molecule--a monosaccharide-wants to cross a membrane. Glucose is a small polar molecule. Would it be able to cross the hydrophobic interior of the membrane? If YES, explain why. If NO, then explain why not and how it could get across (because cells take up glucose all the time to break down for energy).

14.In the reading, you read that certain transport proteins can only transport certain molecules across, and K+ (an ion) was given as an example.

Knowing what you know about the hydrophobic interior, why would K+ and all other ions need to go through a transport protein rather than through the hydrophobic interior of the membrane?

15.Summarize:

a) Why any cell membrane is called "selectively permeable," giving two pieces of evidence for your answer
b) Why a membrane would need to be selectively permeable.

16.Go check out the online activity below about the functions of each part of the plasma membrane, watching for the functions of each of these parts of the plasma membrane:

phosopholipids
cholesterol
transport proteins
enzymes
receptor proteins
carbohydrates


http://www.wisc-online.com/Objects/ViewObject.aspx?ID=ap1101

Match the function on the right to the correct membrane structure on the left.
 
17.carbohydrate
 
18.transport protein
 
19.enzyme
 
20.phospholipids
 
21.cholesterol
 
22.receptor protein
A.allows it to recognize other cells and have other cells recognize it
B.speeds up chemical reactions at the membrane's surface
C.helps move polar molecules or ions across the hydrophobic interior of the membrane
D.make up the bulk of the cell membrane, and make it selectively permeable by creating a hydrophobic interior.
E.receives chemical signals from outside of the cell (such as hormones) and transmits those signals to the inside of the cell
F.keeps the hydrophobic tails from crowding together at low temperatures so the membrane does not solidify

Have you filled out your note sheet as you go through these activities? If not, go back through it now! Only answer the questions regarding membrane structure.

Go back through your note sheet. Grab someone (not literally), sit down with them, and explain your answers for questions #1-4. Have them listen to your answers and see if there's anything you've left out, or anything that is unclear or could be explained better.

Check out the document below, and fix anything that needs fixing:

http://www.sc.chula.ac.th/courseware/2303101j/X-membrane-and-transport.pdf

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