Chp.4 Cell Transport

How is a cell membrane (plasma membrane) like a castle wall?

The walls of a castle, like the cell membrane, are designed to keep out dangerous things. Whether you’re concerned about an enemy army or disease-causing bacteria, you don’t want to allow everything to enter! However, in order to survive, there are some things that the cell (or the castle) does need to let in.

Introduction to Cell Transport

Cells are found in all different types of environments, and these environments are constantly changing. For example, one-celled organisms, like bacteria, can be found on your skin, in the ground, or in all different types of water. Therefore, cells need a way to protect themselves. This job is done by the cell membrane, which is also known as the plasma membrane.

Controlling the Cell Contents

The cell membrane is semipermeable, or selectively permeable, which means that only some molecules can pass through the membrane. If the cell membrane were completely permeable, the inside of the cell would be the same as the outside of the cell. If it never let anything in, then the cell would not get enough food or oxygen. Allowing things into the cell and keeping things out maintains the cell’s homeostasis.

Without these properties, it would be impossible for the cell to maintain homeostasis. Homeostasis means maintaining a stable internal environment. Keeping oxygen levels normal, removing waste products, and obtaining nutrients all help the cell maintain homeostasis.

Composition of the Cell Membrane

Molecules in the cell membrane allow it to be semipermeable. The membrane is made of a double layer of phospholipids (a "bilayer") and proteins (Figure below). Recall that phospholipids, being lipids, do not mix with water. It is this quality that allows them to form the outside barrier of the cell.

A single phospholipid molecule has two parts:

1. A polar head that is hydrophilic, or water-loving.

2. A fatty acid tail that is hydrophobic, or water-fearing.

FIGURE 1.1

The cell membrane is made up of a phospholipid bilayer, two layers of

phospholipid molecules. Notice the polar head group of the phospholipid is attached to the phosphate, and the tails are two fatty acid chains. The head group and tails are attached by a glycerol backbone.

There is water found on both the inside and the outside of cells. Since hydrophilic means water-loving and they want to be near water, the heads face the outside of the cell where water is found. The water-fearing, hydrophobic tails face each other in the middle of the cell membrane.

An interesting quality of the plasma membrane is that it is very "fluid" and constantly moving. This fluid nature of the membrane is important in maintaining homeostasis. It allows the proteins in the membrane to float to areas where they are needed and serve as gatekeepers.

Due to the composition of the cell membrane, small molecules such as oxygen and carbon dioxide can pass freely through the membrane, but other molecules, especially large molecules, cannot easily pass through the plasma membrane. These molecules need assistance to get across the membrane. That assistance will come in the form of transport proteins.

Questions:

1. What kinds of things does the cell membrane keep out of the cell?

2. What is another name for the cell membrane?

3. What does “semipermeable” mean?

4. How is the cell like a castle?

5. What is homeostasis?

6. What quality makes phospholipids a good barrier for the outside of the cell?

7. What are the two parts of a phospholipid?

8. Why is it important that the cell membrane is fluid?

9. What molecules can get across the cell membrane?

10. What part of the cell membrane helps molecules get across?