1. Passive Transport: Diffusion and Osmosis
Section 3.4

2. Objectives SWBAT describe passive transport
SWBAT distinguish between osmosis, diffusion, and facilitated transport.
Main Idea 1: diffusion and osmosis are types of passive transport.
Main Idea 2: some molecules diffuse through transport proteins.

3. Vocabulary Section 3.4 Osmosis (ósmosis) Diffusion (difusión)
Passive transport (transporte pasivo)
Concentration gradient (gradiente de concentración)
Isotonic (isotónica)
Hypertonic (hipertónica)
Hypotonic (hipotónica)
Solute (Soluto)

4. Two Types of Transport
There are two ways of transporting materials across a cell membrane. They are:
Passive Transport – the movement of molecules across a cell membrane without energy input from the cell (the diffusion of molecules across a membrane).
Active Transport – actively drives molecules across the cell membrane from a region of lower concentration to a region of higher concentration. Requires energy input.

5. Diffusion
Diffusion - is the movement of molecules in a fluid or gas from a region of higher concentration to a region of lower concentration.
Concentration gradient – is the difference in the concentration of a substance from one location to another.
Higher concentration
Lower concentration
The egg swelled because water molecules moved from outside the egg, higher water concentration, to inside the egg where there was a lower concentration of water molecules.

6. Diffusion
Diffusion results from the natural motion of particles. Diffusion across a membrane from higher to lower concentrations is part of nature – it is a natural force.
Concentration is defined as the number of molecules of a substance in a given volume.

7. Diffusion
In cells, diffusion is indispensable in moving substances across the cell membrane!
Small lipids (remember our hormone example – aldosterone) and other nonpolar molecules (like carbon dioxide and oxygen) easily diffuse across the membrane.

8. Oxygen Diffusion Example
Most cells continually consume oxygen.
Thus, oxygen concentration is almost always higher outside the cell than inside it.
As a result, oxygen almost always is diffusing from outside the cell, across the cell membrane, to the inside of the cell.
The cell expends no energy.
Oxygen high concentration
Lower oxygen concentration
Oxygen Diffuses

9. Osmosis
Water molecules also diffuse across semipermeable (selectively permeable) membranes from higher concentration to lower concentration, which we call osmosis.
Osmosis – the diffusion of water molecules across a semipermeable membrane (a cell membrane in our case) from higher concentration of water molecules to lower concentration of water molecules.

10. Osmosis – Our Naked Egg Example
In our naked egg experiment, we saw osmosis at work.
First, we saw water move from the vinegar (where there was a higher concentration of water molecules) into the egg (where there was a lower concentration of water molecules. The egg expanded.

11. Osmosis – Our Naked Egg Example
Then, we put the egg in corn syrup. In the corn syrup there was a higher concentration of water molecules in the egg than the corn syrup.
Therefore, the water flowed from the egg, higher concentration, to the corn syrup (lower concentration).

12. Isotonic, Hypertonic, Hypotonic

13. Isotonic, Hypertonic, Hypotonic
Isotonic – a solution is isotonic to a cell if it has the same concentration of dissolved particles, solutes, in the solution outside the cell as inside the cell.
Example: water molecules move into and out of the cell at equal rates because the concentration of dissolved particles is the same outside the cell as inside the cell.
This is a state of equilibrium (remember the word from biochemistry).

15. Isotonic, Hypertonic, Hypotonic
Hypertonic – a solution is hypertonic to a cell if it has a higher concentration of dissolved particles than inside the cell.
Example: if there is a higher concentration of dissolved particles in the solution outside the cell than inside the cell, water will move out of the cell into the solution.
Think of our egg in corn syrup (the corn syrup was hypertonic to the cell. Water from in the cell flowed out of the cell into the corn syrup.

16. When we say solutes, as in “a hypertonic solution has more solutes than a cell,” we are talking about the concentration of dissolved particles (for example, salts, sugars, etc.)

17. Isotonic, Hypertonic, Hypotonic
Hypotonic – the solution outside the cell has a lower concentration of dissolved particles than that inside the cell.
Example: water molecules will diffuse into the cell when the inside of the cell has a higher concentration of dissolved particles and a lower concentration of water molecules than outside of the cell.
Water flowed from the vinegar, where there was a lower concentration of dissolved particles, through the cell membrane to the interior of the egg (where there was a higher concentration of dissolved particles).

19. Facilitated diffusion
Facilitated diffusion – is the diffusion of molecules across a cell membrane through transport proteins.
Facilitated diffusion is for molecules that cannot easily diffuse across a cell membrane.
The word facilitate means to make easier – thus, the transport proteins facilitate (make easier) the passage of some molecules into our out of the cell.

20. Facilitated diffusion
This is still passive transport since the cell does not need to use any of its energy.
There are many types of transport proteins – most types only allow specific molecules to pass through.
The transport proteins act as tunnels through the cell membrane.

21. Question
What will happen to a houseplant if you water it with salt water (a hypertonic solution)?
Why are transport proteins needed in the cell membrane?