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Cell Processes-Active Transport: Study Guide

Updated on June 7, 2012

Cell Processes

Cells perform a variety of processes in order to be able to function properly. I'd recommend first reading my hub about cell structure in order to properly identify the pieces and parts (organelles) that are in each cell and what they do.

The Cell Membranes Transport Substances

In order for the cell to live, substances such as proteins must be able to enter the cell. This is done through the plasma membrane. The substances needed are found in fluid either inside of the cell or outside of the cell.

  • Intracellular fluid - Fluid inside of the cell
  • Extracellular fluid - Fluid outside of the cell.

The name of extracellular fluid is based on it's location and may sound pretty familiar to you.

  • Interstitial fluid - This is fluid between cells within a tissue
  • Plasma - Fluid within a blood vessel
  • Lymph - Fluid within a lymphatic vessel
  • Cerebrospinal Fluid - Fluid surrounding the brain and spinal cord

How do substances pass through?

Things don't just "float" into the cell. There are ways that a substance moves from one place to another. The first is Diffusion.

Diffusion is the process where solutes move from an area of high concentration to areas of low concentration.

  • Solute: A substance that is dissolved within a fluid. And example would be salt.
  • Concentration: The amount of solute dissolved in a given volume of a solvent
  • Solvent - A fluid in which a solute is dissolved. And example would be water.
  • The solute dissolves in a solvent. Salt dissolves in water.

However, this isn't always how a substance passes into a cell. The membrane must be permeable; it must allow specific substances to pass through. If EVERYTHING were to enter the cell, the cell would die.

Transport Proteins

Along the plasma membrane, transport proteins are embedded, which allow certain substances to pass through. Think of the transport protein as a security gate. Only certain substances with security access may pass through. The transport proteins can be channels/pores or they can be carriers.

  • Channels/pores are tunnels in which a substance can pass through, but have a gate that opens and closes. If the cell needs glucose, and a glucose molecule passes, the gate can open and allow the glucose to enter the cell. If it were a protein molecule, the gate would remain closed.
  • Carriers will have a molecule bind to them. The carrier transporter will then go inside of the cell and release the molecule needed to the inside.
  • Diffusion that involves the aid of these 2 transport proteins is called facilitated transport or facilitated diffusion.


Osmosis is the diffusion of water. Water is the key word, anything else is simply called diffusion. During osmosis, water will move from an area of high concentration of water, to low concentration of water. If you look at the image below, the water molecules are moving to from the right side to the left side. This is because there are less water molecules on the left side.

  • The sugar does not pass because the permeable membrane does not allow it in the picture below.

Osmosis applied to Red Blood Cells

Depending on where your red blood cells are inside of your body, osmosis can have an effect on them. The environment may be either an isotonic solution, a hypotonic solution, or a hypertonic solution.

  • Isotonic solution - The solute concentration outside the cell is the same as the inside of the cell. No water is moved.
  • Hypotonic solution - There is more water outside of the cell than inside. Since osmosis is - again - the flowing of water from an area of high concentration to low concentration, the water flows into the cell, causing the cell to swell up.
  • Hypertonic solution - Higher concentration of water on the inside of the cell than on the outside. The water moves out of the cell, and the cell will shrink.

Active Transport

Active Transport - Energy is used to move substances across a membrane. This form of transport, however, involves transporting substance against a concentration gradient (the concentration gradient is where there is lower concentration to higher concentration.) This can be accomplished via three mechanisms.

  • Pumps - Pumps use ATP to power the movement of substances. The most common pump in a cell is the sodium-potassium pump, which pumps sodium out of the cell and potassium into the cell.
  • Exchangers - These simply swap out molecules. For example, if a cell needs sodium, and has a lot of calcium, they will simply be swapped.
  • Electrical Coupled Transporters - substances are teamed up with some electrons that are being moved across the concentration gradient.

The top picture shows endocytosis. The bottom shows exocytosis.
The top picture shows endocytosis. The bottom shows exocytosis.

Endocytosis and Exocytosis

To bring in larger materials (or to get rid of large materials) for the cell, a membrane will grab the material into a sac (vesicle). Endocytosis is the process of bringing in materials to the cell via these sacs. Exocytosis is the process of part of the cell erupting, and releasing the waste material to the outside of the cell.

  • The following are types of endocytosis: Phagocytosis, Pinocytosis, Receptor-mediated endocytosis.
  • Phagocytosis - The cell will "eat" large particles. Think of white blood cells eating bacteria and killing it. This is an example of phagocytosis.
  • Pinocytosis - The "drinking" of extracellular fluid.
  • Receptor-Mediated endocytosis - This happens when hormones bind to receptors on the plasma membrane. The receptor tells the cell "it's ok, bring in the hormone" and the cell obliges.

The best way to think of endocytosis, in my opinion, is to think of the cell as Pac Man. He gobles up the balls that he needs.

In Closing

This concludes all the processes involving the movement of material inside and outside of the cell. I know it's confusing, but as long as you picture all of this movement along the plasma membrane, it should help with the visual portion. Feel free to leave a comment if you need help, I'll do my best to lend a hand.


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