Online Activity 6.2: Plasma Membrane

Arlen Kong
October 11, 2010
Chapter 6.1, 6.2, 6.3 – Online Activities

Online Activity 6.2-Membranes Organize a Cell’s Activity
Dissect a Plasma Membrane

Objective: to understand the structure of a membrane

1. Why is the membrane described as a phospholipid bilayer?
It is described as a phospholipid bilayer because there are two layers of phospholipid molecules. 2. How does the arrangement of the phospholipids form a bilayer?
The phospholipids arranged in two layers so that the charged phosphate heads interact with the water on either side of the membrane, and the lipid-like (hydrophobic) tails point away from the water and toward each other. 3. In Chapter 4 you learned that water is polar molecule. How does this polarity influence the way in which the phospholipids are arranged in the membrane?
The phospholipid molecules have a
negatively charged head region that is attracted to the positively charged region of the water molecule. As a result, the hydrophobic nonpolar tail regions are oriented away from the water, forming the interior of the membrane.

Online Activity 6.3-Membranes regulate the traffic of molecules
Investigate movement across the membrane.
Page 1
Objective: To investigate the properties of phospholipids in a cellular membrane

1. What properties of carbon dioxide and oxygen allow them to easily cross a membrane?
Carbon dioxide and oxygen both easily cross a membrane because they are both nonpolar, small molecules. 2. What property of the sodium ion prevents it from crossing a membrane?
Sodium ion is a charged particile, which prevents it from crossing the hydrophobic region 3. What properties of glucose prevent it from easily crossing a membrane?
A glucose molecule is both large and polar, and rarely crosses a membrane by simple diffusion. 4. A benzene molecule is completely nonpolar and a little smaller than glucose molecule. Would it cross a membrane faster, or slower, than glucose? Why?
Faster; a benzene molecule is nonpolar, so it is not held back by attraction to water the way that glucose is. Because it is smaller, a benzene molecule fits between phospholipids more easily than a glucose molecule.

Page
2
Objective: To explore how ions and large molecules cross membranes

1. What 3 kinds of transport did you observe in this activity?
Diffusion, facilitated diffusion, and active transport 2. How did the large glucose molecule cross the membrane?
The transport protein allowed the glucose molecule to cross the membrane from greater to lesser concentration in a process called facilitated diffusion. Glucose could not cross the membrane without the transport protein. 3. Why do you think water, a small molecule, needs a transport protein to move rapidly through a membrane?
Water molecules cling to one another because they are polar. This makes it hard for them to escape from the water mass to enter the nonpolar region of the phospholipid bilayer. The transport protein provides a polar channel that attracts the water molecules as strongly as the surrounding water does, so they don’t have to break their polar attractions.

Page 3
Objective: To observe the processes of endocytosis and exocytosis

1. An amoeba is a single-celled organism that undergoes a similar process as the macrophage when it encounters a food particle. Describe how the amoeba feeds itself. What is this process called?
The amoeba surrounds food particles with pseudopods and engulfs the food. This process is called endocytosis. 2. How would an amoeba get rid of undigested food? What is this process called?
In the amoeba, a vesicle containing the undigested food would fuse with the plasma membrane and spill the undigested food outside the cell. The process is called exocytosis. 3. Contrast the processes of endocytosis and exocytosis.
The process of endocytosis brings materials into the cell, while exocytosis expels materials from the cell.

Assessment 1. Generally biologists rely on the _ to study the surface structures of cells and the _ to study the internal structure of cells.
B- Scanning electron microscope; transmission electron microscope. 2. Both plant and animal cells have the following organelles in common:
A- Plasma membrane, nucleus, cytoplasm 3. The chloroplast is the organelle in which
D- Photosynthesis occurs. 4. Prokaryotic cells lack _ and are much _ than eukaryotic cells.
B- A membrane-enclosed nucleus; larger 5. The plasma membrane consists of two layers of phospholipids. This bilayer is composed of
E- Hydrophilic heads and hydrophobic tails. 6. Many of the specific functions of plasma membranes are carried out by _ that are embedded in the phospholipid bilayer.
C- Proteins 7. In the plasma membrane, “transport proteins” help cells
A- Move certain substances across the membrane. 8. Diffusion is most accurately defined as _.
C- The net movement of particles from where they are more concentrated to where they are less concentrated. 9. Passive transport means that
D- No energy is expended by the cell as particles move in and out of the cell. 10. Small polar molecules diffuse into and out of the cell through a process called A- Facilitated transport 11. The passive transport of water across a selectively permeable membrane is called
E- Hydrophobic 12. A plant cell is usually healthiest when surrounded by an environment that is B- Hypotonic