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Cell Transport

Discusses how the structure of the plasma membrane allows for selective transport.

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Cell Transport: Passive and Active Transport.


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

Controlling the Cell Contents

The "castle wall" of your cells is the cell membrane. Anything that passes into the cell or out of the cell is controlled by the cell membrane. The cell membrane is semipermeable, or selectively permeable, which means that only some molecules can pass through the membrane. Some materials can pass through the membrane while others need to use protein channels. 

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 has two major components:

1. Phospholipids
2. Proteins

License: CC BY-NC 3.0



An interesting quality of the plasma membrane is that it is very "fluid" and constantly moving, like a soap bubble. As a result, mall molecules such as oxygen and carbon dioxide can pass freely through the membrane, but other 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/protein channels. Watch the video below about the cell membrane.



Two types of transport that cells use is passive transport and active transport. The differences between passive transport is :

  • Passive transport uses no energy and moves from areas of high concentrations to low concentrations (with the "concentrations gradient")
  • Active transport uses ATP energy and moves from areas of low concentrations to high concentrations (against the concentration gradient).

As stated above, passive transport is the movement of molecules from high to low concentrations without using energy. There are three types of passive transport that can be used:

1. Diffusion which is the movement of liquids (other than water) and gases from areas of high concentrations to areas of low concentrations. The molecules pass through the cell membrane to establish equilibrium  which is a state of equal balance. Check out the video below to see how diffusion works!

2.. Osmosis is the movement of water from high concentrations to lower concentrations to establish equilibrium. It's simply the diffusion of water. Osmosis occurs through the layers of the cell membrane. Water will continue to move until water concentrations reach equilibrium .

3 Facilitated Diffusion is the movement of larger molecules from high concentrations to low concentrations. Since the the cell membrane is selectively permeable, larger molecules need to move through transport proteins. This will continue until the equilibrium is reached. Check out the figure below to see how facilitated diffusion works. 

License: CC BY-NC 3.0



Active transport is the movement of molecules going from low concentrations to higher concentrations. Since this is against the natural flow of movement,  ATP energy is used to move molecules into or out of the cells

License: CC BY-NC 3.0




Large molecules and cellular fluids can also be taken into or expelled from cells but not with the help of passive or active transport. Instead cells can use endocytosis and exocytosis.

  • Endocytosis ("endo" means "inside" and "cyto" means "cell"): When cells take in large molecules such as proteins, starches, hormones, etc.. by surrounding the particle with the cell membrane, pinching the cell membrane and enclosing the particle in a vesicle. Endocytosis includes phagocytosis and pinocytosis. Look at the diagrams below for phagocytosis of food particles and pinocytosis of cell fluid.

Phagocytosis: "Cell Eating"

License: CC BY-NC 3.0


Pinocytosis: "Cell Drinking"

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  • Exocytosis ("exo" means "outside" and "cyto" means "cell"): Exocytosis is the release of particles by fusing a vesicle with the cell membrane. The membrane opens up and releases the particles out of the cell into the environment. 

License: CC BY-NC 3.0


See Endocytosis and Exocytosis in Action!

Lesson Check:

Lesson checks are used to review the main ideas of the lesson. Click on the link below to complete the lesson check. 


  • cell membrane: Lipid barrier that surrounds the cell; also known as the plasma 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.
  • Passive transport: Uses no energy and moves molecules from areas of high concentrations to low concentrations.
  • Diffusion: The movement of liquids (other than water) and gases from areas of high concentrations to areas of low concentrations.
  • osmosis: The movement of water from high to lower concentrations  without the use of energy.
  • Active transport: uses ATP energy and moves molecules from areas of low to high concentrations.


  • The cell membrane is selectively permeable, meaning only some molecules can get through.
  • The cell membrane is made of a double layer of phospholipids and proteins.
  • Passive transport moves molecules without energy while active transport moves molecules with energy (ATP).
  • Passive transport moves materials from high to low concentrations and active transport moves materials from low to high concentrations.
  • Passive transport includes diffusion and osmosis
  • Cells use endocytosis to take large molecules into cells and release particles using exocytosis. 


Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ License: CC BY-NC 3.0
  3. [3]^ License: CC BY-NC 3.0
  4. [4]^ License: CC BY-NC 3.0
  5. [5]^ License: CC BY-NC 3.0
  6. [6]^ License: CC BY-NC 3.0

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