Cell transport made simple

Welcome dear student!

In this short course, you'll learn about cell transport: how things move in and out of a cell to facilitate the many chemical processes that take place in it.

You'll learn about active and passive transport, their mechanisms and utility, along with lots of examples. 

At the end of this course, you'll be able to:

  1. Define active and passive transport.
  2. Distinguish between the different types passive transport mechanisms: diffusion  and osmosis
  3. Explain what occurs in a  cell  in terms of active and passive transport

This is a very hands-on learning course. So gear up for some exciting sessions!


The basics of cell transport

The center of activity: the plasma membrane

For an organism to remain alive, chemicals must be able to move in and out of cells.  To be specific, all this activity occurs across the cell membrane, which is a protective layer that separates the cell from the outside.  

A very thin layer of protein and fat makes up the cell membrane.  It is semipermeable. Some experts prefer to call it "selectively" permeable because it lets certain molecules through but not others. 

So, what are some of the things that go in and out of a cell?  Both plants and animals  take in and release gaseous substances (oxygen, carbon dioxide, water vapor) during respiration, photosynthesis, and excretion.  Intake of nutrients and excretion of waste products are also facilitated by these processes.

Movement of substances across the membrane happens mainly in two ways:

  1. Passive transport: There is no expenditure of energy on the organism's part to move things in and out. Diffusion and osmosis are types of passive transport.
  2. Active transport:  Energy is needed to move substances in and out of the cell.

In the next chapters we'll find out more about these.



Quick check: Why are diffusion and osmosis called "passive"?

Why are diffusion and osmosis called "passive"?

Quick check: Properties of the cell membrane

  • Permeable
  • Lets some substances through but not all
  • Made up of a layer of protein and fats
  • Protects the cell

The cell membrane is....

(3 correct options)

Diffusion and why it is important to organisms


Diffusion is the overall  movement of particles from a region of higher concentration to that of a lower concentration.  We experience diffusion everyday!

Watch the simulation below to learn about the very basics of diffusion. Click each link (1, 2, etc. at the bottom) to learn more. You will have an option to replay the whole thing at the very end.


Drop a pinch of colored powder in water and watch it spread until the color is even.  This is also diffusion. And it stops when  all areas within the solution has an equal concentration of the solute (powder) and the solvent(water).

Diffusion is enormously useful in biological processes. We'll soon see why!

Quick check: What causes molecules to diffuse?

  • Kinetic energy in molecules
  • A concentration gradient of molecules between two areas
  • Air pressure
  • Selective permeability of cell membrane

Quick check: The concentration gradient

  • We say a concentration gradient is present
    when one area has a higher number of molecules in a solution than another area.
  • A steep concentration gradient
    facilitates diffusion.
  • A low concentration gradient
    slows down diffusion.

Diffusion in organisms

In organisms, diffusion takes place across the cell membrane.  Let's look at a few examples:



Iso, Hypo, and Hyper

Two solutions are said to be isotonic when they have an equal concentration of the solute (say a solid) dissolved in the solvent (say, water, or air). 

If a solution has a lower amount of solute than another solution, it is said to be hypotonic. The solution with the higher solute concentration is hypertonic.

What do you think will happen if you keep these two solutions separated by just a selectively permeable membrane? That's right! The solute will diffuse through the membrane from the hypertonic solution (higher concentration) to the hypotonic solution (lower concentration).  Eventually they will become isotonic.

What if the solute is too big to pass through the membrane? The solutions will still want to become isotonic. But how?

 In the next section, we will see an interesting variation.

A special type of diffusion: osmosis

Gaseous substances easily pass through the cell membrane.

What about molecules larger than that?  Let's see what happens in plant cells.

A plant cell  has sucrose in its sap vacuole (which also has a semi-permeable membrane) along with other ions.  Imagine that you've placed the cell in a dilute solution with a much, much lower concentration of sucrose.  Thus, the solution inside the plant cell is hypertonic and the solution on the outside is hypotonic.

Will sugar molecules from the plant cell move to the dilute solution outside so that both solutions can become isotonic? No! That is because these molecules are way too large to penetrate the membrane.

What happens then? Water molecules are much smaller. So instead of  the solute (sucrose) moving out into the dilute solution, water from the dilute (hypotonic) solution diffuses through the membrane till both solutions become isotonic.

Diffusion of water is simply a case of specialized diffusion. It has a special name, osmosis, and it is basically the diffusion of water molecules from a region of  higher water potential (dilute, hypotonic solution where there is more water) to a region of lower water potential (inside of the cell). 

We'll talk all about osmosis in the next chapter.

Osmosis and why it is important to organisms

Osmosis: A pre-test before you begin

  • Sugar moves out from the concentrated solution into the dilute solution
  • Water moves from the dilute solution into the concentrated solution
  • Nothing. Sugar molecules are to large to pass through the membrane
We take two solutions that have different amounts of sugar dissolved in water. What happens when we place them side by side separated by a selectively permeable membrane?

Water potential and how it triggers osmosis

A dilute solution has more water molecules in it and is said to have a high water potential (WP).

A concentrated solution on the other hand, with fewer number of water molecules, is said to have a low water potential. 

If you were to take a semipermeable membrane to separate two such solutions, water molecules would move across the membrane due to kinetic energy. Take note, that they would actually move in both directions.

However, due to the difference in water potential, a much greater number of water molecules from the dilute solution (high WP) would move into the concentrated solution (low WP).

Thus, osmosis can be defined as the net movement of water molecules from a region of high water potential to that of a lower water potential across a semipermeable membrane.

Test your intuition

  • Bag expands as water enters the bag
  • Bag shrivels as water leaves the bag

A  bag made of Visking tube (it is semipermeable and usually used in dialysis) is filled with a dilute solution of  sugar. Both ends of the bag are then tightly tied. The bag is then placed in a relatively concentrated solution of sugar.  What happens to the bag after a while?

Osmosis in plant and animal cells

A healthy plant that gets enough water thrives.  Water enters through the cell membrane into the sap vacuole, which has a concentrated sugar and salt solution and thus a lower water potential.

Water enters a plant through root hair cells because the surrounding soil has a high water potential. Eventually, the plant cell contains as much water as it can hold. The strong cell wall prevents the cell from bursting.  The cell can then be called turgid.  Turgid cells give ample support to a plant, for example, the soft stems are held upright.

If, however, you keep an animal cell, say red blood cells in distilled water, the osmosis of water into the cell will cause the cell to burst.  This happens because animal cells have no cell wall. This phenomenon is called lysing. 

Osmosis does take place in animal cells but the net water movement is much less. Red blood cells are surrounded by blood plasma, which has more or less the same salt concentration as the cells. Thus, the red blood cells can maintain their shapes in plasma--they neither shrivel nor undergo lysing. 

Can you guess what happens if you keep a plant cell in a concentrated sugar solution? That's right! Water now moves out of the cell!  This is because, the water potential inside the cell is now higher compared to the outside.  The cell eventually shrinks. This state is called plasmolysis.

In dry conditions, where there is very little moisture in the air, there is a higher water potential inside the leaves of a plant.  Water escapes through the stomata (tiny pores on the leaf surface) and eventually the plant wilts.

Video: Putting it all together

This video will help you recapitulate what you learned about osmosis thus far. Enjoy!

Active transport and its role in cell transport

Active transport: a very brief overview

So far, we've learned about the exchange of gaseous substances and the movement of water molecules through the cell membrane.  What about the intake of nutrients?

This happens by means of a mechanism that requires the organism to spend some energy.  The mechanism is called, active transport. 

Why do organisms need to expend energy in active transport? That's because, unlike in diffusion, active transport occurs against a concentration gradient.

Let's learn more about this in the next section!

Active transport in plants and animals

Active transport in plants

Nutrients are often present in a low concentrations in the soil. In fact, cell sap in root hair cells usually has a much higher concentration of these ions (Mg2+, Na+, K+, Ca2+, Cl-).

Nevertheless, the cell needs a constant supply of these nutrients and must collect it from the soil, where their concentration is low.

In this case, the cell uses energy produced in respiration to "pump" these nutrients from the soil into the root hair. This phenomenon, where the cell uses energy to move substances against the concentration gradient is called active transport.

Active transport in animal cells

The small intestine in animals has tiny finger-like projections called villi (singular villus) that assist in absorption of glucose.  These cells have a high rate of respiration that facilitates active transport.

Carrier proteins and high rate of respiration

​Root hair cells and villi cells have special features that are adapted for active transport.  They have many carrier proteins in their cell membranes that carry the nutrients across the membrane.

Additionally, these cells have a very high rate of respiration, which provides the energy for carrier proteins to transport the ions.

Quick check: factors affecting rate of active transport

  • Lack of oxygen would increase the rate of active transport
  • An increase in temperature would decrease the rate of active transport
  • High rate of respiration in root hair cells assists active transport
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