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2.7: Cell Transport

Difficulty Level: At Grade Created by: CK-12
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How is a cell 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 a 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. It would be impossible for the cell to maintain homeostasis. Homeostasis means maintaining a stable internal environment. For example, if your body cells have a temperature of 98.6°F, and it is freezing outside, your cells will maintain homeostasis if the temperature of the cells stays the same and does not drop with the outside temperature.

How does the cell ensure it is semipermeable? How does the cell control what molecules enter and leave the cell? The composition of the cell membrane helps to control what can pass through it.

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.

The composition of the cell membrane

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 inside and outside of the cell where water is found. The water-fearing, hydrophobic tails face each other in the middle of the cell membrane, because water is not found in this space. The phospholipid bilayer allows the cell to stay intact in a water-based environment.

An interesting quality of the plasma membrane is that it is very "fluid" and constantly moving, like a soap bubble. 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.  

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.


  • The cell membrane is selectively permeable, meaning only some molecules can get through.
  • The cell membrane is made of a double layer of phospholipids, each with a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail.

Explore More

Use the resources below to answer the following questions.

Explore More I

  1. How is passive transport different from active transport?
  2. What are three types of passive transport? What do these all have in common?
  3. What does the body use iodine for? What kind of transport is necessary to transport this molecule into a cell?
  4. What happens to the receptor complex in receptor mediated endocytosis?

Explore More II

  1. Can proteins in the plasma membrane move around the membrane? Why is this characteristic beneficial to the cell?
  2. What are five functions of the membrane in cells?
  3. What types of lipids are found in plasma membranes? What characteristics do these types of lipids share?


  1. Why is the plasma membrane considered selectively permeable? Why is this important?
  2. Explain the composition of the cell membrane.
  3. Explain the arrangement of phospholipids in the membrane.

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cell membrane Lipid barrier that surrounds the cell; also known as the plasma membrane.
homeostasis Ability of an organism to maintain stable internal conditions, such body temperature, regardless of outside conditions.
hydrophilic Can combine with water (water-loving).
hydrophobic Does not combine with water (water-fearing).
phospholipid Lipid molecule with a hydrophilic (water-loving) head and two hydrophobic (water-hating) tails; makes up the cell membrane.
semipermeable Allowing only certain materials to pass through; characteristic of the cell membrane.
transport protein Protein that assists molecules entering or leaving the cell.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Aug 30, 2016
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